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Tectonostratigraphy of the North Alpine Foreland Basin: correlation of Tertiary depositional cycles and orogenic phases
TECTONOPHYSICS ELSEVIER
Tectonophysics 282 (1997) 223-256
Tectonostratigraphy of the North Alpine Foreland Basin: correlation of Tertiary depositional cycles and orogenic phases W. S i s s i n g h * Faculty of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands
Accepted 30 May 1997
Abstract
Tectonostratigraphic analysis of the depositional history of the North Alpine Foreland Basin indicates that temporal relationships exist between short-term events in the foredeep and the adjacent Alpine orogen. The foredeep can be divided stratigraphically into an Alpine Foredeep (Thanetian-Bartonian), 'Pre-Molasse' Basin (latest Bartonian-earliest Rupelian) and Molasse Basin (Rupelian-Tortonian). Their general and tectonostratigraphic development includes time-significant cycle sets and so-called Cenozoic rift and foredeep (CRF) sequences which are apparently correlative to both foreland and orogen deformation events, as well as to changes in global sea-level. Individual basin-fill sequences and, consequently, general stratigraphic and palaeogeographical aspects of the evolving basin system are thus found to be frequently defined by contemporaneous widely distributed tectonics and eustasy. Successive dominant depositional states (underfilled, steady-state and overfilled episodes) and facies (flysch, Molasse) are correlated with different, concurrently progressing phases of the convergence and subduction history of the European and Apulian plates. Keywords: Alpine foredeep; Alpine orogeny; tectonics; depositional history; general stratigraphy; palaeogeography
1. I n t r o d u c t i o n
The northern foreland basin at the Central and Eastern Alps, referred to as the Molasse Basin, extends from French Savoie (Haute-Savoie) in the west to Lower Austria (Nieder Osterreich) in the east (Fig. 1). The general features of this peripheral basin are well known. The total length of this arcuate basin is some 700 kin. It attains a maximum width of some 150 km in its German part and tapers off towards its western and eastern ends to some 20 km. Seismic interpretations, data from boreholes and information from outcrops of the overthrust units, however, have * Fax: +31-30-253-5030; e-mail; [email protected]
demonstrated that the depositional southern limits of the Molasse Basin extended well beneath the northerly verging thrust sheets of the Alps (see e.g., TriJmpy, 1980, or Ziegler, 1990 for illustrative crosssections). In Austria, Molasse deposits have also been found on top of Austroalpine nappes, where they were affected by thrust movements. As a rule, Molasse deposits accumulated in front of the Alpine thrust front, with depocentres located in proximal positions while only marginally covering the frontal thrusts sheets. The Molasse Basin fill consists predominantly of Oligocene and Miocene syn-orogenic continental and shallow-marine clastics; carbonates play a very subordinate role. The entire Cenozoic series
0040-1951/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0040- 195 1 (97)0022 1-7
W. Sissingh / Tectonophysics 282 (1997) 223-256
224
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Fig. 1. Structural summary map for the area of the North Alpine Foreland Basin.
of the basin attains a maximum thickness of 5000 m near the Alpine thrust front and is generally decreasing rapidly to the north. In Switzerland the distal depositional margin has not been preserved. It was destroyed in conjunction with Late Miocene to Pliocene folding, thrusting and erosion of the Jura Mountains, entailing uplift and erosion of the entire basin. In Germany, erosion of the northern basin margin reflects uplift of the Franconian Platform during Late Miocene and Pliocene times. Similarly, erosion of the northern basin margin in Austria is related to late uplift of the Bohemian Massif. Within the some 300 km long Jura fold-and-thrust belt, Molasse remnants are present in synclines. Jointly, these erosional remnants and those found in the non-folded Swabian and Franconian Jura (Schwfibische and Fr~inkische Alb) show that the northernmost limit of the Molasse Basin was situated some 20-30 km to the north of the present erosional edge (Fig. 1). In the east, crystalline-basement blocks of the Bohemian Massif determine the northern basin geometry.
In combination with dextral strike-slip movements, inversion tectonics uplifted the western Bohemian Massif during the Paleocene and eraplaced the German Landshut-Neu0tting High, an upthrusted, NE-tilted basement block, and the anticlinal Zfirich High in Switzerland (Bachmann et al., 1987; Nachtmann and Wagner, 1987; Schr6der, 1987; Fig. 1). The sediments of this characteristically asymmetric foreland basin are generally underlain by Mesozoic rocks. These rocks have been extensively truncated due to horizontal compressional stresses to which the Alpine foreland was subjected during the Senonian and especially in Paleocene times (Lemcke, 1981; Ziegler, 1990; Bachmann and Mtiller, 1991; Roeder and Bachmann, 1996). Locally, Molasse deposits cover also Palaeozoic and basement rocks. The surface of the entire Molasse Basin is located at present well above sea-level and is subjected to erosion. The strata preserved in the remnant fore-arc foreland basin range in age from Late
W. Sissingh /Tectonophysics 282 (1997) 223-256
Eocene or Early Oligocene to Late Miocene, depending on basin definition. The relatively shallow-water, external Molasse Basin stratigraphically succeeds a generally deepwater, internal Alpine Foredeep, corresponding to a grossly SW-NE-oriented trough. The late Paleocene to Eocene sedimentary succession of the latter comprises a threefold subdivision including shallowmarine limestones at the base, an intermediate series of marls and shales and upper deep-marine turbidite sandstones. The cumulative thickness of these generally superimposed and diachronously deposited units can be hundreds of metres. This part of the basin is largely buried and decapitated by an up to 10-kmthick overburden of thrust sheets. Only a minor portion of it is exposed, mainly in the Subalpine Chains, a region of detached coverfolds and overthrusts south of the Prealps, and also along the northern flank of the crystalline parautochthonous Aar Massif (Fuchs, 1976; Hsti, 1979; Burkhard, 1988). Closure of the Alpine Foredeep proper and first distinct shaping of the Molasse Basin took place during the Late Eocene-Early Oligocene. Together, both parts of the basin constitute a perisutural trench, referred to as the North Alpine Foreland Basin, which evolved along the major compressional zone of orogenic deformation incorporating the megasutural Insubric (Periadriatic) Line. This subvertical structure separates the Central Alps from the Southern Alps. It is the likely result of a large-scale reaction to plate subduction (Schmid et al., 1989). The associated deformation events seem to be (almost) time-equivalent with events in the foredeep. The principal goal of this paper is to present a tectonostratigraphic review of the North Alpine Foreland Basin, paying special attention to the occurrence of depositional sequences and tectonic signatures correlating with the structural development of the radially vergent Alpine orogen. Indications are found for the occurrence of basin-wide sequences with stratigraphic boundaries correlative with eustatic changes in sea-level as well as phases of Alpine tectonism. Similar 'Cenozoic rift and foredeep sequences' (numbered as CRF I to CRF XI) had already been recognized by Sissingh (1998) in the Rhine Graben, the Bresse Graben and the western part of the Motasse Basin. In addition to a palaeogeographical and palaeodepositional overview, an in-
225
terpretative tectonostratigraphic model and a classification scheme for the evolution of the foreland and thrust-wedge system will be proposed. The model relates dynamic aspects of the stratigraphic evolution of the North Alpine Foreland Basin with those of the history of the Alps. Three time-successive basin settings are distinguished: (1) Molasse Basin: Rupelian-Tortonian (2) 'Pre-Molasse' Basin: latest Bartonian-earliest Rupelian (3) Alpine Foredeep: Thanetian-Bartonian The composite foredeep (s.1.) has already been described and analysed in considerable detail at regional and subregional scales, amongst others, (bio)stratigraphically (most notably Berger, 1992), sedimentologically and palaeogeographically (e.g., Btichi and Schlanke, 1977; Lemcke, 1984; Homewood, 1986; Herb, 1988; Homewood et al., 1989; Doppler, 1989; Unger, 1989; Allen and Bass, 1993; Reichenbacher, 1993) and from more integrated stratigraphic and structural geological points of view (e.g., Lemcke, 1974; Trtimpy, 1980; Matter et al., 1980; Homewood et al., 1986; Pfiffner, 1986; Allen et al., 1991; Bachmann and Mtiller, 1991; Roeder and Bachmann, 1996). However, at an interregional scale, the stratigraphic development of the basin has not yet been evaluated in terms of sedimentary sequences reflecting global changes in sea-level as well as tectonic phases of the Alps. Detailed chronostratigraphic control of the relevant geological events in both the foredeep and the orogen is of course crucial for such an enterprise. As it seems, sufficient chronostratigraphic information is available from the foredeep to limit uncertainties in intra-basinal age correlations to acceptable levels of accuracy. Dating of tectonic events in the orogen is often approximate, but the obtained ages are apparently meaningful in the context of integrated foredeep-orogen evolution. With respect to the lithostratigraphy of the different parts of the foredeep, use was made of recently published synthetic overviews, suppressing the names of a bewildering number of local rock-units.
2. Structural setting The structural evolution of the North Alpine Foreland Basin is intimately linked to the Paleocene and Neogene phases of the Alpine orogen which are
W. Sissingh / Tectonophysics 282 (1997) 223-256
226
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Fig. 2. T e c t o n o m a g m a t i c s u m m a r y m a p for the area o f the N o r t h Alpine F o r e l a n d Basin.
related to the convergence of the European (Eurasiatic) Plate with the Apulian (Adriatic) Microplate during the anticlockwise convergence of the African (-Arabian) Plate with Europe (Fig. 2; Platt, 1986; Platt et al., 1989; Coward and Dietrich, 1989). Development of the basin post-dates collision of the orogenic wedge with the Helvetic Shelf, the southern margin of cratonic Europe, and involved its flexural thrust-loaded subsidence. Through time the axis of this foreland basin migrated northward in response to the advance of the Alpine nappe stack. Deflection and subduction of the foreland was accompanied by the development of numerous basin-parallel normal faults along which bending stresses were released at upper crustal levels. Thrust-wedge loading and thickening occurred under largely subaerial conditions. Consequently, thrust-wedge erosion was substantial and provided ample clastic supply to the evolving foreland basin, inducing its further subsidence in response to sediment loading. The sedimentary fill of this Tertiary Alpine Foreland Basin onlaps the Meso-
zoic substrate of the European Foreland in roughly northwestern directions. Following the regional development of the 'Paleocene Unconformity', sedimentation resumed diachronously. The corresponding stratigraphic hiatus becomes increasingly larger towards the north and the west. It represents a major phase of collisional foreland deformation which corresponds to distal foreland buckling in response to the build-up of regional compressional stresses in the foreland. The intra-plate deformations were accompanied by the development of considerable relief, as indicated by the deep truncation of the Mesozoic strata (Trtimpy, 1960; Allen et al., 1991). Today, the Alpine Foredeep is strongly faulted and folded, generally deeply buried under the thrustfold belt and, consequently, badly exposed. For these reasons its stratigraphic record and evolution are comparatively poorly known. From the Late Paleocene till Eocene times, it evolved basically as a remnant oceanic basin of Tethyan origin, its sedimentary fill essentially onlapping a continental mar-
w. Sissingh /Tectonophysics 282 (1997) 223-256
gin (shelf), subjected to a downwarping and bulging of the lithosphere by prograding tectonic loading. During the Early Eocene, the distal European margin, corresponding to the Adula Nappe, started to be overridden by the advancing Alpine nappe system (Schmid et al., 1996; Ziegler et al., 1996). By that time, the oceanic lithosphere in the North Penninic domain had been completely subducted. Towards the end of the Eocene, the mainly deepwater Alpine Foredeep was closed. It was succeeded via a transitional 'Pre-Molasse' Basin - - by the generally shallow-water Molasse Basin, forming the westemmost part of the Paratethys. The eastem parts of the Molasse Basin are relatively little folded but densely faulted. At base-Tertiary level, numerous normal faults with often approximately (N)E-(S)W, basin-parallel orientations, testify here of extensional tectonics perpendicular to the basin axis. In Switzerland, compressional features (folds, inverse faults), roughly parallel to the basin axis, are common (Brink et al., 1992). This part of the basin is characterized by the presence of anticlinal structures, and strike-slip faults. However, along the southern margin of the rather undeformed, so-called Plateau (or Mittelland) Molasse, a narrow Middle Miocene imbrication zone occurs, consisting of closely spaced thrust slices. This Subalpine Motasse zone of deformation is due to proximal overthrusting of the Alpine accretionary wedge (Fig. l). Farther to the west, the thin-skinned Jura fold and thrust belt developed during Middle to Late Miocene times along the northwestern margin of the Molasse Basin; its basal detachment horizon is located either in Triassic evaporites or in the upper crust. Folding and thrusting of the northwest- and north-verging Jura Mountains entailed a commensurate translation and uplift of the Molasse Basin (Laubscher, 1987, 1992). The depositional outline of the western part of the Molasse Basin is possibly reflected by the patchily distributed continental (siderolithic) strata of Middle Eocene age. The presence of these sediments may be due to enhanced block-faulting and differentiation in palaeorelief preceding Late Eocene flooding associated with the demise of the Alpine Foredeep proper and the formation of the Molasse Basin. It seems that the Molasse Basin evolved principally as a post-Eocene flexural foredeep under the influence of Alpine orogenic processes in the collision zone -
-
227
of the European and African plates. Late MiocenePliocene uplift caused basin-wide differential denudation of the younger Molasse series. Uplift and associated erosion was greatest in the southwest, in the region of tectonic shortening in the Jura Mountains. According to Laubscher (1992), and others, the formation and depositional history of the North Alpine Foreland Basin would be mechanically coupled with the coeval outward migration of a curved, broadly SW-NE-oriented foreland bulge. Crestal shifting of this large-scale flexural feature is presumed to have occurred in NW directions till crossing, by Middle to Late Miocene time, the southern extension of the Rhine Graben and the northem Bresse Graben where the Moho discontinuity is found to be shallowest (Fig. 2; Him, 1980). Following Laubscher (1992), the Mid-Miocene migration rate of the forebulge was apparently in the order of 5-10 km per million years. Domal uplift in front of the orogen has also been attributed to buckling of the lithosphere due to compressive stresses transmitted from the orogen into the foreland (Ziegler, 1994). Compressional foreland-stress directions since the Middle Eocene have been predominantly S(S)WN(N)E, interrupted by a period of more or less E - W distension during the Oligocene and with the exception of the Late to post-Miocene period when a regime of roughly N W - S E compression prevailed (Bergerat, 1987; Fig. 2). In the south, the Molasse Basin is defined by the out-of-sequence nappe pile, the primary provenance area of its clastics. During the advance and stacking of this nappe pile, downcutting erosion led to sedimentary loading of the adjoining foreland basin. In western Switzerland, the basin seems to have migrated in N(W) directions at a rate of 7-9 km per million years during the Oligocene and perhaps as little as 2 km per million years in Miocene time. An average basin-migration rate of 5 km per million years has been inferred (Homewood et al., 1986). In eastern Switzerland, basin migration proceeded apparently less rapidly. For this region, average migration rates of about 3 and 2 km per million years during the Late Eocene to Early Oligocene and the post-Early Oligocene, respectively, have been calculated (Pfiffner, 1986). Left-lateral wrench faults crossed the entire basin, generally with N(N)E-
228
w. Sissingh / Tectonophysics 282 (1997) 223-256
S(S)W strikes (Brink et al., 1992). Middle Eocene growth faulting has been identified in the Alpine Foredeep (Pfiffner, 1986). Syn-sedimentary tectonic activity during the Late Eocene has been deduced from the occurrences of conglomerates with locally derived pebbles (Herb, 1988; see also Buchholz, 1989). Anomalous thicknesses of the Oligocene may also be attributed to syn-sedimentary fault activity in the Molasse Basin (Diem, 1986). The depositional history of the North Alpine Foreland Basin must be inherently linked to the contemporaneous formation history of the Alps. The stacking, erosion and lateral movements of the overthrust belt changed the shape and dimensions of the foreland clastic wedge, at least as long as convergent plate boundary processes were active in the orogen. It seems likely that the episodic tectonic evolution - - i.e. peaks of tectonic activity alternating with periods of relative tectonic quiescence - - of an impinging orogenic wedge is recorded in the sequence-stratigraphic architecture of its fringing foredeep. Therefore, recognition of tectonically induced features in the foredeep fill may support and refine concepts concerning the timing of tectonic events in the orogen. Foreland-basin analysis may considerably improve understanding of thrust-wedge evolution. 3. G e n e r a l stratigraphy and depositional history
For the present study, three lithostratigraphic diagrams were compiled (Figs. 3-5). Geographically, these diagrams represent the western, central and eastern parts of the Molasse Basin and their adjacent region in the orogen. They are founded on the crosssectional charts compiled by Gwinner (1978) which were further developed on the basis of data given in Sinclair et al. (1991), Bachmann and Mfiller (1992), Allen and Bass (1993) and others. Steininger et al. (1996) is followed with respect to the correlation of late Tertiary continental and marine stages. For convenience, global standard stages are mostly used. Chronostratigraphic interpretations of rock-units are calibrated with the numerical time-scale of Harland et al. (1990) and combined with the sea-level curves of Haq et al. (1988) and the succession of regional CRF sequences recognized by Sissingh (1998). References to (approx.) maximum thick-
nesses of rock-units and standard biozones (planktonic foraminifera: P; calcareous nannoplankton: NR NN) are given as far as relevant and available. Palaeogeographical maps, presented in Figs. 6 8 for the Priabonian, Rupelian, Chattian to Aquitanian, Burdigalian and Langhian to Tortonian, incorporate information from recent studies and some new names for outstanding structural features. The western parts of these maps are mainly based on Homewood (1986) and Herb (1988) and their eastern parts particularly on Bachmann and Mtiller (1991), Doppler (1989) and Unger (1986, 1989). Incorporation of new data from different regions and timeintervals resulted in a modification of the palaeogeographical maps published by Btichi and Schlanke (1977), Gwinner (1978), Trtimpy (1980) (after H.M. Btirgisser and S. Schlanke), and Lemcke (1984). In the absence of pertinent palinspastic reconstructions, regional palaeopositions of the Alpine thrust front are only shown schematically on all five maps. These maps also show the communications between the Molasse Basin and the European Cenozoic Rift System (cf. Sissingh, 1998). The younger Tertiary clastic rock sequences of the North Alpine Foreland Basin are commonly classified into the following, generally shallow-marine to continental groups (Figs. 3-5): - Upper Freshwater Molasse: Langhian-Tortonian; - Freshwater-Brackish Molasse: late Burdigalian; - Upper Marine Molasse: Burdigalian; - Lower Freshwater Molasse: Chattian-Aquitanian; - Lower Marine Molasse: Rupelian (essentially). These rock-units occur only in the external Molasse Basin; some of them include other facies than their formal name suggests. The basin fill of the older, proximal Alpine Foredeep may be viewed as one single lithostratigraphic group, mainly comprising neritic limestone and sandstone, (hemi)pelagic marl and shale, and deep-water flysch of Thanetian to Bartonian age (Figs. 3-5). On top of this transgressive unit, which gradually encroached on the foreland and reflects rapidly increasing water depths, laterally shaling-out North Helvetic Flysch of 'Priabonian' (latest Bartonian-earliest Rupelian) age was deposited in the western and central re-
W. Sissingh / Tectonophysics 282 (1997) 223-256
gions (Figs. 3 and 4). In the eastern parts of the foreland basin, time-equivalent series contain a substantial succession of sandstone, limestone and marl (Fig. 5). The Priabonian series is here considered as representing a sequence intermediate between the typical fill of the Alpine Foredeep and the Molasse Basin. It was deposited during a period of rising global sea-level, which followed a principal, latest Bartonian glacio-eustatic regression and preceded a pronounced earliest Rupelian transgression (Figs. 3 5; chronostrat, range cf. Herb, 1988, Haq et al., 1988, Buchholz, 1989, and Sinclair et al., 1991, after adopting an earliest Rupelian top for the T4.1T4.3 cycle set, cf. Brinkhuis and Biffi, 1993). The North Helvetic Flysch, a still problematic unit - with respect to age and source area - - consisting of epiclastic turbiditic volcanics, has previously been interpreted as forming part of the Lower Marine Molasse (e.g., Homewood, 1986).
3.1. Alpine Foredeep (Thanetian-Bartonian) The tectono-sedimentary history of the North Alpine Foreland Basin started during the Late Paleocene when a transgressive, deepening succession of Assilina Greensand, Nummulitic Limestone, Globigerina Marls and Flysch was laid down on top the Tertiary cover basal unconformity. The development of this regional unconformity was related to the uplift and erosion of the flexural forebulge in advance of the Alpine thrust belt (Allen et al., 1991). Throughout the Thanetian to Bartonian, a first-order onlapping and overstepping of the northern and northwestern basin margin took place. This process of large-scale diachronous deposition coincided with a northward migration of the shallow- to deep-marine facies belts, including Alpine Flysch with interstratified chaotic wildflysch formations. The basal hiatus increases in magnitude towards the external parts of the basin (Figs. 3-5). The associated foreland uplift led to emergence and subaerial exposure and to the localized deposition of the continental Siderolithic (Bohnerz) Formation. These strata are well known from outcrops in the Jura Mountains and the Helvetic nappes and have also been encountered in wells drilled in central Switzerland. They consist of iron-rich sediments (quartz sandstones, breccia and shales) with, most typically, ferruginous pisolithes
229
filling pockets, cavities and cracks in the karstified Mesozoic series, that have been dated as Lutetian and Bartonian (Wieland, 1976; Weidmann, 1984; Herb, 1988). The formation represents tropical, i.e. climatically hot and wet weathering conditions. Marine flooding of this palaeorelief was generally accompanied by the deposition of a basal Assilina Greensand grading upwards into Nummulitic Limestones. These glauconitic sands are not always developed, limestones may lie directly on top of the angular 'Paleocene Unconformity', either due to non-deposition related to raised palaeotopography or to post-depositional, shallow-submarine erosion of the basal clastics prior to carbonate deposition. The presence of glauconite suggests low net sedimentation rates at essentially shallow-water depths. However, this thin sand unit contains also interbeds of hemipelagic marl indicating an alternation of shallow- and deeper-marine conditions. Overall, areal marine submergence increased in rate during the Eocene and was accompanied by syn-depositional flexural normal faulting as seen in the Helvetic domain of the Central Alps (Herb, 1988; Lihou, 1995; Menkveld-Gfeller, 1995). Throughout the Helvetic domain of the Swiss Alps, Late Paleocene to Eocene deposition of the neritic sediments was controlled by a total of seven main transgressive phases, followed by subsequent regressions causing erosion (Herb, 1988). Only the two oldest can be related to eustatic events. The oldest phase occurred during the early Thanetian (Fliegenspitz Beds: P 3-P 5) and the second (lower part of the Einsiedler Fm.) occurred during the late Thanetian. The other five flooding events occurred during the early mid-Ypresian (upper, main part of the Einsiedler Fm.; with Nummulites planulatus), earliest Lutetian (Gallensis Beds; with N. gallensis), early to middle Lutetian (Btirgen Fm.; about P 10; with N. millecaput and Assilina exponens), latest Lutetian to ?Bartonian (Klimsenhorn Fm.; with N. perforatus gr.) and latest Bartonian to Priabonian (Hohgant Fm.; belonging to the 'Pre-Molasse' Basin fill). The late Ypresian (Einsiedler Fm.) and the middle Lutetian (Btirgen Fm.) sequences indicate regressive conditions. In the Savoie (50 km south of Geneva), correlative marine transgressions at the end of the Ypresian (NP 14, ?P 9) and in Priabonian (NP 18-NP 20) time have been identified in a thrust
W. Sissingh / Tectonophysics 282 (1997) 223-256
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unit by Charollais et al. (1975a). Each transgressive event appeared to follow upon a period of erosion which can be attributed to tectonic activity. In the Helvetic region, north of Interlaken, the Hohgant series includes foreshore to shoreface sandstones. It is underlain by a neritic limestone (Discus Bed) and overlain by similar beds at the top (Discocyclina and Lithothamnium Limestones). According to Breitschmid (1978), four short-term transgressions are recorded in this latest Bartonian to Priabonian (p.p.) cyclothemic sequence. In general terms, deposition of the clastics and carbonates took place in a differentially subsiding, block-faulted basin with palaeomorphology influencing sedimentary facies and submergence events. In the Savoie, discrete Priabonian palaeohighs, capped by fossiliferous algal reefs, have been inferred (Lateltin and Mtiller, 1987). Moreover, as in Switzerland (Herb, 1988), the prevailing S W NE-oriented normal fault system provided significant palaeostructural influences (Pairis and Pairis, 1975). During the Ypresian and Lutetian a shallow-marine gulf extended northward into the region, comprising the Bornes Plateau, the Aravis Chain and the Bauges and Plat6 Massifs. In this basin nummulitic limestones were deposited episodically. Although communication between this basin and the Alpine Foredeep in the east was interrupted by the end of the Lutetian, it became permanently integrated into the Alpine Foredeep during the late Bartonian (Fig. 3; Kerckhove, 1980; Cavelier, 1984). Throughout the Alpine Foredeep Basin the up to 50-m-thick composite clastic-carbonate succession is succeeded by the deeper-marine Bartonian, more or less silty, Globigerina Marls. Diachronous drowning of the transgressive sandstone-carbonate complex can be related to the tectonically induced rapid basin subsidence and an increased input of fine-grained terrigenous clastics, representing the most distal facies of the Alpine Flysch accumulating in the most proximal parts of the basin. On the Helvetic Shelf, Hohgant carbonates of early Priabonian age are overlain with a sharp contact by Globigerina Marls. Such an abrupt lithological change may correspond to increased intra-Priabonian flexural tectonic subsidence, associated with westward migration of thrust-loading. The Globigerina Beds attain thicknesses of up to 400 m in eastern Switzerland. They consist of hemipelagic and pelagic marls
233
characterized by a diversified and rich foraminiferal microfauna (cf. Eckert, 1963). Water depths were mostly in the order of several hundreds of metres as evidenced by very high, upper to middle bathyal P/B ratios (80% to over 90%). In the late Bartonian to Priabonian (P 15, P 16), however, the 'Stadtschiefer' facies has reduced P/B ratios (5080%) indicating shallower though still considerable water depths (Herb, 1988). Alignments of centimetres-long, rod-like foraminiferal tests suggest that during the late Bartonian to early Priabonian (P 15) NNW-SSE-directed currents prevailed in the region of the Aar Massif (Allen et al., 1991). Slump structures and debris flows indicate the presence of a nearby palaeoslope (cf. Lateltin and Mtiller, 1987). The youngest m a r s (Micaceous Marl Slates, Foraminiferal Marls) are found in the Savoie (Fig. 3) where they have been dated as early Rupelian (NP 21, P 18, P 19; Charollais et al., 1975b, 1980; Lateltin and MUller, 1987). Farther south, the basinal Globigerina Marls interfinger with the mega-wedge of the North Penninic to Ultrahelvetic turbidite and wildflysch formations which were deposited in front of the northward advancing orogenic wedge (Homewood, 1983). Extreme tectonic dislocation and disassociation of this major facies belt has so far prevented to establish its suspected multi-sequential development in time and space. It consists predominantly of Paleocene and Eocene conglomerates, sandstones and hemipelagic interbed marls and shales. Included sedimentary, metamorphic and igneous clasts originated from Hercynian basement and Mesozoic cover rocks. Spilitic clasts have been reported from Paleocene flysch. Intra-basinal limestones are present as well. In front of the advancing thrust wedge, deposition occurred at bathyal to abyssal depths in a variety of fan and basinal facies and involved the tectonic-sedimentary emplacements of olisthostromes and wildflysch in the southern Alpine Foredeep. The detrital sources of these deep-water clastics were either intra-basinal submarine or emergent intra-basinal highs. E N E WSW-trending intra-basinal highs, separating the subparallel-trending Ultrahelvetic and North Penninic basins, supplied terrigenous turbidites to both basins. The slopes of the intra-basinal highs were apparently steep enough to cause sediment transport by gravity flow. In the rapidly subsiding Ultrahelvetic
234
W. Sissingh / Tectonophysics 282 (1997) 223-256
basin, which is part of the North Tethyan margin, axial palaeocurrents apparently flowed to the east (Hsti, 1960; Homewood, 1976, 1977; Matter et al., 1980; Homewood and Caron, 1983; Winkler, 1984; Herb, 1988; Wildi, 1988; Caron et al., 1989). The youngest strata of the so-called South Helvetic Flysch are assigned to the early part of the late Priabonian (Herb, 1988). As a whole, the Thanetian to Bartonian succession was deposited in an underfilled foredeep in which the development of accommodation space exceeded the sediment supply. It represents a coherent set of sedimentation cycles consisting of neritic to middle-bathyal deposits which accumulated in a flexural foredeep basin. 3.2. 'Pre-Molasse' Basin (latest Bartonian-earliest Rupelian)
Prior to the accumulation of the various shallowwater Molasse series (s.s.) from the Early Oligocene onwards, the approximately 2000-m-thick North Helvetic Flysch deposited. Initially, northward shaling-out, volcanoclastic sandstones were deposited in the Taveyannaz Turbidite Basin (Fig. 6). The Taveyannaz Sandstones (at least 240 m in eastern Switzerland) accumulated in water depths of at least 500 m at the end of a long period of continued largescale subsidence and onlap of the Helvetic Shelf which had commenced during the Late Paleocene. These sandstones contain up to 90% basaltic andesite detritus (Vuagnat, 1952, 1985). Thicknesses are variable due to syn-depositional changes in local palaeotopography of the Helvetic realm (cf. Lateltin and Mtiller, 1987; Sinclair, 1992). In eastern Switzerland, the Taveyannaz Formation is overlain by some 100 m of mudstone containing dislocated blocks of Taveyannaz Sandstone. It is assumed that this mud sheet was emplaced conformably on top of the grauwacke turbidites, triggered by superficial gravity sliding induced by compressional deformation of the seafloor (Sinclair, 1992). The provenance area of the andesitic detritus is still unknown. Erosion of a volcanic nappe (Vuagnat, 1952, 1985) and redistribution of contemporaneous, subduction-related volcanics (Sawatzki, 1975; see also Martini, 1968) have been suggested. However, neither the remains of such a nappe nor evidence for an Ultrahelvetic volcanic arc
have been found. Nevertheless, the occurrence of angular and fragile volcanic clasts within the Taveyannaz Sandstones excludes extensive reworking in the general area (Homewood and Caron, 1983). Accumulation of mass flow deposits continued in the western Helvetic domain until the earliest Rupelian. These included the proximal turbidites of the relatively polygenetic Val d'Illiez Sandstone (NP 2 1 - N P 23, P 17- P 20) and the Elm Formation (Fig. 6). These contain much less andesitic volcanic detritus and more mudstone than the earlier Taveyannaz Beds (Vuagnat, 1952, 1985; Lateltin and MUller, 1987), and advanced farther to the north. Together with the Matt Formation, which contains slumps indicating syn-depositional slope instability near the thrust front, they represent delta-front turbidites deposited at reduced water depth. Subsequently, the transgressive early Rupelian fish-bearing Engi Shales, the oldest sediments occurring in the Molasse Basin of eastern Switzerland, were deposited (Figs. 3 and 4). In central and western Switzerland, a local Intra-Rupelian Unconformity is present at the contact between the Val d'Illiez Sandstone and the Grigisen Marls (Diem, 1986); this intra-Rupelian gap has not (yet) been recognized farther to the west in the Savoie. Simultaneously with the Taveyannaz Sandstones, ftysch devoid of volcanoclasts and Stockletten Marls (now found in the Helvetic nappes) accumulated in the South Bavarian Turbidite Basin to the east (Fig. 6). Farther north, in the German and Upper Austrian 'Pre-Molasse' Basin, e.g. in the Salzburg Basin, neritic Lithothamnium Limestone (110 m) overlies the fluvial to shallow-marine Basal Sandstone (20 m), laterally grading into the Ampfing Sandstone (70 m) around the Landshut-Neu6tting High. Within the Lithothamnium Limestone two successive northward-transgressing biohermal stages of development of mid- to late Priabonian and late Priabonian to earliest Rupelian age, respectively, are recognized. Each comprises a reefal complex bordered to the north by a lagoon; they are stratigraphically separated by Discocyclina Marl (Buchholz, 1989). As a whole, this composite 'Priabonian' sequence is interpreted as a transitional unit between the Alpine Foredeep and the actual Molasse Basin. In the 'Priabonian' deep-water Stockletten Marls and their time-equivalent shelf deposits of southern
Vii..Sissingh / Tectonophysics 282 (1997) 223-256
235
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T H R U S T FRONT
236
W. Sissingh / Tectonophysics 282 (I 997) 223-256
Germany and Upper Austria, a shallowing trend is not evident (Buchholz, 1989). The shallow-marine Salzburg Basin, which had developed in response to a widespread transgression, was linked to the south with the basin receiving North Helvetic Flysch (Mfiller et al., 1992; Figs. 5 and 6). As tar as known, only a limited amount of flysch-like Molasse deposits (Katzenloch Beds) has accumulated (Hagn, 1978). In the west, the Late Eocene shallow- to deepmarine sediments were deposited on a relatively narrow, ENE-WSW-striking shelf which was offset to the south by the Tavayannaz Turbidite Basin. In the east, sediments accumulated in the relatively wide, shallow- to deep-marine Salzburg Basin which was flanked to the south by the South Bavarian Turbidite Basin (Fig. 6). During the Eocene, the axis of the ENE-WSW-trending, trench-like turbidite basin in Switzerland shifted to the northwest (Herb, 1988), its orientation remaining parallel to the thrust front; intra-basinal clastic transport was directed towards the northeast to east (Radomski, 1961; Martini, 1968; Lateltin and Mfiller, 1987). In the Salzburg Basin, the peninsular Landshut-Neu6tting High acted as a source of the Priabonian shelf sands (Bachmann and Mfiller, 1991); only from the early Rupelian onwards is there evidence for alluvial fans prograding locally from the thrust front into the basin (Fig. 6). Predominantly Priabonian sediments were deposited during a transitional cycle set under partly underfilled and partly steady-state foredeep conditions. They reflect depositional environments which range from deep-water to continental (fluvial) and are characterized by the absence of classical Molasse. 3.3. Molasse Basin (Rupelian-Tortonian) Rupelian. During this period, the Molasse Basin
abruptly deepened and widened. Initial deposition of the Lower Marine Molasse (1500 m) started in the east with the highly organic Fish Shale (P 18, NP 22; 40 m). During a phase of rapid tectonic basin deepening, associated with an eustatically controlled marine incursion from the east, this anoxic deposit developed throughout the badly ventilated stagnant Salzburg Basin (Fig. 6). It drowned the Lithothamnium Limestone platform entirely, but not yet the Landhut-Neu6tting Peninsula. West of this
high, the palaeo-Naab and palaeo-Main rivers supplied already sand to the basin from northern sources (Lemcke, 1985). The predominantly rather deepmarine strata accumulated simultaneously with the orogen-derived turbiditic Deutenhausen Beds (800 m) which dispersed radially from the northwardadvancing thrust front in the south (Fischer, 1979). The western equivalents of the Deutenhausen Beds are the Engi Shales and the Fish Shale. These shales represent the beginning of the Molasse depositional megacycle. The Fish Shale is overlain by the very thin Light Marly Limestone (5 m) and a relatively substantial series of Banded Marl (60 m). Both were deposited under fairly deep-marine conditions and are rich in coccoliths (MUller and Blascke, 1971). This sequence is succeeded by a thick series of openmarine Rupelian Marls (800 m) which are topped by the latest Rupelian to earliest Chattian, partly littoral Baustein Beds (100 m). Accumulation of these alluvial, deltaic and shallow-marine sands is related to the development of a northward prograding influx system (Kronmfiller and Kronmfiller, 1987). During the late Rupelian shallower-marine conditions became more common. In the German Molasse Basin, the distal basin margin experienced a moderate shift to the south, whilst elsewhere the basin widened slightly (Fig. 6). In the east, deposition of predominantly deep-water muds and turbidites still persisted. In the central and western parts of the basin, however, fine-grained shelf deposits of the Horw Shale and the Grisigen Marls give upward way to the deltaic, littoral to fluvial Horw Sandstone (NP 24) and the Bonneville Sandstone. These sediments and the Baustein Beds correspond to partly forced regressive and increasingly high-energy conditions of sedimentation with progradation to the northeast (Figs. 3-5; Diem, 1986). For the terminal Rupelian, a shallow, wave-dominated seaway, with apparently fluctuating salinities and bordered by sandy tropical beaches and paludal shores along the orogen-proximal margin was postulated. This elongated and increasingly narrowing basin was drained axially from the west to the east (Fig. 6). Concurrently, N W SE-oriented clastic shorelines with delta fans migrated eastward in the Swiss region. There is no evidence for tidal activity (Diem, 1986; Homewood, 1986). The end-Rupelian depositional record coincides with a pronounced glacio-eustatic fall in sea-
W. Sissingh /Tectonophysics 282 (1997) 223-256
level (Figs. 3-5; Lemcke, 1983, 1984). This global regression resulted in the local formation of an approximately 500 m deep and 2000 m long canyon, perpendicularly to the thrust front which was incised into its substratum in western Switzerland down to the Val d'Illiez Sandstone (Mayoraz et al., 1988).
Chattian-Aquitanian. Following the end-Rupelian forced regression, the western and central parts of the Molasse Basin were dominated by a continental environment as evidenced by the accumulation of thick, mainly conglomeratic alluvial-fan deposits of the terrigenous Lower Freshwater Molasse (4000 m). Northward-flowing, marginal intra-Alpine streams formed a series of individual and partly laterally interfingering deltaic fans along the Alpine thrust front (Fig. 7). These fans account for the greatest thicknesses of the Lower Freshwater Molasse; north of the Subalpine Molasse this series becomes much thinner. In general, the long axis of the depocentre shifted to the north-northwest during the Chattian and Aquitanian. The thrust wedge-fringing fans were characterized by steep proximal areas, dominated by braided rivers. Downstream, these gave way to relatively low-gradient floodplains typified by meandering rivers. Farther away from the rising mountain belt, lakes and swamps occurred within a fluvial system draining to the east. During the Chattian, a predominantly radial drainage system existed. However, with the increase of clastic inputs, attributed to rapid uplift of the orogen, this system deviated more to the east and thus evolved into a largely longitudinal, east-directed drainage system (Btichi and Schlanke, 1977). This younger, Aquitanian system of meandering rivers was the first to cut a significant pattern of longitudinal valleys in the perialpine region. Local occurrences of marine to brackish microfaunas, typified by ostracods and foraminifers (Trtimpy, 1980), and the deposition of gypsiferous marl (70 m) of late Chattian age are observed along the North Jura Peninsula (Fig. 3; Btichi and Schlanke, 1977; Reggiani, 1989), suggesting for the Savoie region near Switzerland, the existence of saline lakes and playas which were temporarily connected via the Rauracian Depression with the Rhine Graben (cf. Hauber, 1960). At that time the Rhine Graben was characterized by a brackish to marine environment (Sissingh, 1998). Remains of plants and mammals point to rather wet, subtropical conditions.
237
However, the occurrence of gypsiferous horizons and red sediments indicates that the humidity was variable (cf. Reggiani, 1989). Glaciers are assumed to have existed at that time in the Alps (Bergell Mountains; Hantke, 1989), which at this time were already characterized by a considerable topographical relief (Pfiffner, 1986). In the eastern Molasse Basin, the lower Baustein Beds (Schwerd, 1978) were briefly flooded by the sea after the drastic end-Rupelian sea-level drop. This marine transgression, which originated from the east, flooded the Landshut-Neurtting Peninsula. Deep-marine Puchkirchen Turbidites were deposited along the thrust front (Robinson and Zimmer, 1989; Fertig et al., 1991) to the south of a shallow-marine to deltaic area of sedimentation, referred to as the Landshut-Wels Platform, and to the northeast of an intra-Alpine Inntal Basin. The latter was an elongate bay-like depression with marine to limnofluviatile facies extant from the Late Eocene to the Chattian (Fuchs, 1980). In contrast, in the deep-water Molasse Basin the sandy and conglomeratic Lower Puchkirchen Beds (1300 m) accumulated near the Alpine deformation front. At the same time, deltaic to shallow-marine Chattian Sands (1500 m) prograded into this basin from the west. To the west these strata give laterally way to the fluvial and (swampy) lacustrine Lower 'Bunte Molasse' (2000 m; Paulus, 1963), representing a transgressiveregressive cycle (Fig. 5). As a result of a latest Chattian marine regression, a W-E-trending, broad valley was incised into the Chattian Sands southeast of Munich. Although this event caused widespread erosion in shelfal areas, it did not disrupt the accumulation continuity in the deep-water Puchkirchen Turbidite Basin (Bachmann and MUller, 1991). This erosion surface was covered by the transgressive Upper Cyrena Beds and Upper Marls, and 'Aquitanian Sands' (1200 m) which were deposited during a latest Chattian to Aquitanian transgressive-regressive cycle. The by now submerged valley became a submarine canyon through which clastics were transported into the Puchkirchen Turbidite Basin. However, this basin, which was characterized by water depths in the order of 1000 m, received most of its clastics from the Alpine thrust front. These in part coarse clastics were deposited in a system of submarine fans along the steep, unstable southern margin
238
W. Sissingh/Tectonophysics 282 (1997) 223 256
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W. Sissingh/Tectonophysics 282 (1997) 223-256
of the basin (Kollmann and Malzer, 1980; Fertig et al., 1991). During the Aquitanian, the up to 1400m-thick Upper Puchkirchen Beds were deposited in the eastern deep-water basin. This turbidite series is sealed by the latest Aquitanian Fish Shale (250 m), reflecting termination of massif clastic supply to the basin, which at this time was still sufficiently deep for the development of stagnant bottom-water conditions. Also during the Aquitanian, deltaic systems prograded from the west into the deep Puchkirchen Turbidite Basin. To the west of Munich, the continental, brackish and shallow-marine sediments of the Upper 'Bunte Molasse' (1200 m; Paulus, 1963) and the 'Aquitanian Sands' were deposited; similar series occur along the northern fringes of the Puchkirchen Turbidite Basin. A widespread regression at the end of the Aquitanian terminated deposition of the Lower Freshwater Molasse in the entire foredeep, whereas marine sedimentation persisted in the Puchkirchen Turbidite Basin. The Rupelian Lower Marine Molasse and the Chattian-Aquitanian Lower Freshwater Molasse form a first-order upwards-shallowing and coarsening cycle set which commenced with a widespread phase of marine transgression and basin deepening and ended with a non-forced regression giving rise to predominantly continental sedimentation once the volume of supplied sediment was equal or in excess of the accommodation space. However, in the Puchkirchen Turbidite Basin deeper-water conditions were retained till at the end of this cycle
Burdigalian. At the beginning of the Burdigalian, a period characterized by a warm to subtropical climate north of the Alps, a widespread and rapid marine transgression established in the entire Molasse Basin a wave- and tide-controlled, seaway which extended from the Savoie to Upper Austria (Homewood and Allen, 1981; Allen et al., 1985; Faupl and Roetzel, 1987, 1990; Krenmayr, 1991). Marine ingressions flooded the basin from both the west and the east. In certain areas, the top of the underlying Lower Freshwater Molasse was eroded. This erosion surface corresponds to the Base Burdigalian Unconformity which forms the base of the transgressive Upper Marine Molasse. In Upper Austria, this unconformity (Fig. 5) is submarine; here it developed in response to strong basin-axial currents, in con-
239
junction with the latest Aquitanian narrowing of the basin due to emplacement of the Alpine nappes at their present position (P.A. Ziegler, pers. commun., 1996). During the deposition of the Upper Marine Molasse (1300 m), the subsidence axis of the basin shifted further north into the central zone of the present-day Molasse Basin. The sea covered a considerable portion of the future Jura Mountains, and also onlapped the Black Forest Massif. To which extent the sea entered the southern Rhine Graben is unknown. In the east, the basin margin shifted towards the north. In general, the corresponding strata were subsequently eroded. Regional Burdigalian flooding of the perialpine trough resulted in the establishment of an elongate narrow seaway connecting the Western Mediterranean through the Rh6ne Graben in France with the Carpathian Foredeep. The inference of a strongly tide-influenced depositional realm with at least mesotidal regimes (Homewood and Allen, 1981; Homewood et al., 1986; Faupl and Roetzel, 1990) implies that this basin was clearly connected with the world ocean. Tidal modelling suggests that tides propagated into the German and bordering Swiss parts of the Molasse Basin from the east, i.e. from the Indo-Pacific via the Eastern and Central Paratethys until the late Burdigalian (early Ottnangian) or, less likely, from the Eastern Mediterranean through the Slovenian Corridor (cf. Martel et al., 1994; Sztan6, 1994). Water depths reached distally some 100 m, according to palaeontological data (e.g., Berger, 1985). However, the presence of large tidal banks consisting of shelly sandstones with vadose cements, indicates emersion of their crests. Waveripple features also suggest shallow-water depths (Homewood et al., 1986). In the Swiss segment of the basin, maximum water depths were probably in the order of a few tens of metres (cf. Martel et al., 1994). The high-energy tidal regime caused a pronounced differentiation in facies subparallel to the basin axis. Along the central part of the thrust wedge, conglomeratic fan deltas (1000 m) continued to grow at the mouths of northward-discharging intra-Alpine streams (Fig. 7). After a brief regression (deduced from a palaeosol horizon), the late Burdigalian ('Helvetian') interval is characterized by a rise in relative sea-level and by enhancement of clayey deposition and of encroaching cycles of fan deltas and other proximal deposits (cf. Schaad et
~ Sissingh / Teetonophysics 282 (1997) 223-256
240
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GENERAL DIRECTION OF MARINE INCURSION LOCAL DIRECTION OF CLASTIC SUPPLY REGIONAL DIRECTION OF CLASTIC TRANSPORT THRUST FRONT
Fig. 8. Palaeogeographicaldevelopmentsduring the Langhian-Tortonian. al., 1992). Between the fluvial distributary systems, characterized by conglomeratic and sandy channel fills and sheet flows, tidally influenced coastal facies (beaches, tidal sandwaves and channels, intertidal sandflats) dominated. These facies are bordered by a near-shore facies belt (subtidal shoals with megarippies, intershoal swales) which further offshore are replaced by shelly, glauconitic and pebbly sandstones (coquina banks). At the northern margin of the basin, sandy beaches and channelized tidal flats were established along rocky coasts in the Jura (Homewood, 1981; Homewood and Allen, 1981; Homewood et al., 1989); the northern margin of the German Molasse Basin was bordered by shoreline cliffs (Fig. 8). In the eastern part of the Molasse Basin, a mainly west-directed transgression claimed first the area closest to the Alpine nappes, and onlapped from there on the Augsburg Platform, an elevated region which existed throughout the early Burdigalian. Thrust-loaded subsidence controlled a northward shift of the depocentre till near the position of the present basin axis (Figs. 1 and 7).
Around the Eggenburgian-Ottnangnian transition, the entire German Molasse Basin was inundated by a tide-influenced 'epicontinental sea' (Neuhofen Beds). The cliffs, defining the northern basin margin, were cut into Upper Jurassic limestones during this early part of the late Burdigalian when predominantly shallow-marine deposition prevailed. Subsequent cyclical sedimentation occurred in marine to brackish and continental facies during the terminal Burdigalian. The Ottnangian Freshwater-Brackish Molasse may be subdivided into three brackish cycles (resp. with a.o. Baltringen, Grimmelfingen and Kirchberg beds). During the second cycle, tilting of the basin caused the incision of the Graupensand Low and concomitant E - W drainage (Fig. 7). At the end of this cycle, holomarine conditions disappeared for good from the area. A brackish (Kirchberg and Oncophora Beds) and fluviatile (Hochgrat Fan, oldest Younger Jura Conglomerate; Schreiner, 1965; Luterbacher et al., 1992) realm with endemic molluscs was established by a final eastward-directed marine ingression. Farther to the east, the palaeo-
W. Sissingh / Tectonophysics 282 (1997) 223-256
Salzach river continued to form an extended delta (Ortenburg Fan). At the end of the third cycle, the first regional west flowing 'proto-Rh6ne' river system (Glimmersand Fan) developed during the latest Ottnangian or Karpathian (Reichenbacher, 1993). Within the late Ottnangian, as well as at the base and top of the largely westward-directed, progradational Karpathian succession, minor hiatuses and lithological breaks are present. On a regional scale, they delimit and subdivide the Freshwater-Brackish Molasse sequence (Fig. 5; Doppler, 1989; Bachmann and Mtiller, 1991), to which the palaeo-Main and palaeo-Naab rivers contributed fluvial clastics from northern and northeastern sources. Whereas during Chattian to middle Burdigalian times, the basin axial sediment transport was directed towards the east, it reversed towards the end of the Burdigalian, thus establishing a southwestward-directed fluvial system which discharged in the west into a marine basin (Fig. 7; Btichi and Schlanke, 1977; Doppler, 1989; Reichenbacher, 1993). Near the Chambfry Sill in the Savoie, a highenergy tidal system initially prevailed (Fig. 3: Tresserve and Forezan lithosomes). Later on, during the Burdigalian (NN 3, NN 4), a regressive, lowenergy muddy shelf (Montaugier lithosome), associated with a substantial basin narrowing, developed for a brief period. This was followed by the deposition of a transgressive, tide-dominated coastal sequence (Grfsy lithosome) in a sea which persisted into Middle Miocene times (Allen and Bass, 1993). Langhian-Tortonian. During the deposition of the
Upper Freshwater Molasse (over 1500 m), the earlier established basin-wide, ENE-WSW-directed axial drainage pattern prevailed. Along the southern border of the basin, the Alpine thrust front remained active at least from Munich westward, perhaps till into the Pliocene, and alluvial fans continued to be fed with large quantities of gravel from the rising Alps (Bachmann et al., 1982). Adjacent alluvial plains were incised by meandering rivers and channels which left frequently variegated muds and sands. Intra-channel sediments include red-coloured palaeosols, browncoal seams (especially southeast of Regensburg) and freshwater limestones. Floral assemblages indicative of a subtropical to temperate climate were recovered from this Upper Freshwater
241
Molasse (e.g., Gregor et al., 1989). However, the climate fluctuated; although it was relatively warm during the Burdigalian, similar to the Late Oligocene, a subsequent cooling trend is evident (Hantke, 1985; Gregor et al., 1989). A composite and repeatedly changing palaeo-Rh6ne river system, originating in the Eastern Alps (cf. Unger, 1986, 1989), received water from the palaeo-rivers Main, Naab, Salzach and 'Enns' (intermittent; Lemcke, 1984). It discharged into the narrow sea which extended through the Rh6ne basin from the Western Mediterranean (Demarcq and Perriaux, 1984). During this last stage of Molasse deposition, the basin attained in Germany its northernmost extension (Figs. 5-8). In the central and eastem parts of the Molasse Basin, marine or brackish deposits are absent in the Upper Freshwater Molasse. Alluvial fan and plain facies predominated, until the Tortonian. Discontinuity in sedimentation since the preceding period is indicated for this region by the Base Langhian Unconformity. The final filling stage of the basin started abruptly and during the same time interval as in the west (Figs. 3-5). Uplift of the southern spur of the Bohemian Massif (east of Linz) led to a disconnection with the marine Paratethys and to non-saline, fluviatile deposition in the area (Steininger et al., 1986). Post- to Middle Miocene erosion affected the entire basin. As the youngest preserved Molasse is of Tortonian age, the Late Miocene to Pliocene development of the Tertiary basin is poorly known. In eastern France, at the westernmost end of the basin, the partly marine Langhian-Serravallian strata represent NW coastal progradation involving the sudden accumulation of thick-bedded fan-delta sands and conglomerates (Fig. 3: Pont-de-Beauvoisin lithosome and Chamoux Conglomerates). These massive sequences, which are derived from the Alps, occluded the western tide-influenced entrance of the Molasse Basin (Allen and Bass, 1993). As in this area post-Serravallian strata are not preserved, the Late Miocene and Pliocene basin history cannot be reconstructed. Final closure of this marine basin segment was already completed during the Tortonian (Perriaux et al., 1984). Wholesale uplift, causing regional erosion in the foredeep and coinciding with rapid elevation of the Alps, may have started during the latest Miocene
242
~ Sissingh / Tectonophysics 282 (1997) 223-256
and this may be causally related to the termination of Molasse deposition (cf. Lemcke, 1984). During the Pliocene, the eastward-flowing Danube fiver extended northward from Lower Austria and started to drain the Molasse Basin from near its presentday source, the Black Forest. The Late Pliocene Molasse Basin was also intersected by a northwardflowing palaeo-Rhine fiver (which did not yet enter the Rhine Graben) and palaeo-Aare/Doubs fiver that drained westward into the Bresse Graben (Bartz, 1961; Abele, 1977). Consequently, the Upper Marine Molasse to Upper Freshwater Molasse succession forms a sedimentary cycle set analogous with that composed by the Lower Marine Molasse and the Lower Freshwater Molasse as described above. Accordingly, the total depositional record of the North Alpine Foreland Basin can be subdivided into four cycle sets: (1) Late Molasse Cycle Set: Burdigalian-Tortonian; (2) Early Molasse Cycle Set: Rupelian-Aquitanian; (3) 'Pre-Molasse' Cycle Set: latest Bartonianearliest Rupelian; (4) Alpine Foredeep Cycle Set: ThanetianBartonian.
(2) Molasse Basin (Rupelian-Tortonian): Early Post-Collision Episode; (3)'Pre-Molasse' Basin (latest Bartonian-earliest Rupelian): Syn-Collision Episode; (4) Alpine Foredeep (Thanetian-Bartonian): Late Pre-Collision Episode. The defined series of cycle sets fits this succession of geodynamically controlled episodes. Their sequential correspondence with the global sea-level curve is striking, as is the case with the time-significant succession of Cenozoic rift and foredeep sequences (CRF I to CRF XI), discussed by Sissingh (1998) for the Rhine Graben, the Bresse Graben and the western Molasse Basin. The CRF series of the North Alpine Foreland has been slightly modified by including a subdivision of CRF II into four eustatic cycles (a-d; see Haq et al., 1988; Figs. 3-5). The CRF sequence boundaries appear to correlate with both tectonic events and eustatic cycle boundaries. It seems that the second- and third-order fluctuations in global sea-level can be related to changes in intra-plate stress (cf. Cloetingh, 1986). This presumed relationship (Fig. 10) is further documented in the following tectonostratigraphic appraisal. A stratigraphic overview of tectonic events which influenced the development of and depositional processes within the Molasse Basin is given in Fig. 11.
4. Tectonostratigraphic interpretation 4.1. Alpine Foredeep Cycle Set The Tertiary evolution of the Alpine orogen determined the evolution of the North Alpine Foreland Basin (Fig. 9). After a predominantly deep-marine 'Flysch stage' (Fig. 9a,b) development of the basin switched to a 'Molasse stage' (Fig. 9c,d) characterized by the accumulation of mainly shallow-marine and continental strata. This fundamental change in facies occurred when in the Central Alps the Helvetic Shelf became compressionally deformed, extensive thrust-loading of its crust commenced and associated uplift of the orogen initiated massive erosion. The pivotal event occurred as the result of a final stage of intense collisional coupling of Apulia (Adria) and Europe. Thus it is possible to geodynamically subdivide the tectono-sedimentary history of the North Alpine Foreland Basin as follows (cf. Homewood and Lateltin, 1988): (1) 'Post-Molasse' Basin (post-Tortonian): Late Post-Collision Episode;
Sediments assigned to the late pre-collision unit overlap, as a whole, diachronously the 'Paleocene Unconformity' to the Helvetic Shelf. The age of crustal deformation giving rise to the hiatus is intraPaleocene (Ziegler, 1990; Allen et al., 1991). However, when consideration is given to the tectono-sedimentary interpretation of the global curve of eustatic sea-level change, it seems likely that marine flooding started in the early Thanetian. The initiation of this main phase in basin development at the end of the Paleocene orogenic phase coincided with volcanism in the South Penninic realm. This is testified by an early Thanetian bentonite, rich in zircon and apatite (Winkler et al., 1985a). The onset of the main phase in basin development is presumed to have occurred soon after a fundamental change in both the heavymineral content and transport directions of turbidites. In the Penninic Rhenodanubian Flysch Basin, sedi-
W. Sissingh / Tectonophysics 282 (1997) 223-256
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Fig. 9. Geological evolution of the North Alpine Foreland Basin, showing the change of the Alpine Foredeep (a,b) into the Molasse Basin (c,d) due to emplacement of overthrust belt onto passive margin (reconstructions from Einsele, 1992). Time intervals: (a) Senonian; (b) Paleocene-Eocene; (c) Early Oligocene; (d) Middle-Late Miocene.
ment transport direction changed from E - W to W - E in mid-Danian time, near the base of a condensed sequence (NP 3 - N P 8) of pelitic sediment. At the same time, a garnet-dominated heavy-mineral assemblage was replaced by one in which zircon is most abundant (Egger, 1990). The W - E orientation of the basin-axial transport direction, established after an implied uplift in the west, is identical with the
one suggested for the North Penninic-Ultrahelvetic Flysch Basin (Winkler, 1984; Wildi, 1988). The condensed sequence (NP 3 - N P 8) is interpreted by Winkler (1983, 1984) as reflecting a phase of diachronously propagating tectonic activity, resulting in an accentuation of the basinfloor palaeorelief and local changes in current direction. Within the Paleocene and Eocene shelf series
W. Sissingh / Tectonophysics 282 (1997) 223-256
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onlapping the Helvetic Shelf in Switzerland, six main transgressive phases are recognized. The initial Thanetian transgression is interpreted to be eustatically controlled (Herb, 1988). It appears to coincide with the deposition of slope sediments on top of immature fan deposits in the Gurginel-Schlieren Flysch Basin (WinNer, 1984). It corresponds to the beginning of supercycle TA2 of Haq et al. (1988) as well as a subduction-related tectonic pulse associated with syn-orogenic volcanism (Winkler et al., 1985a). According to this interpretation, the development of the North Alpine Foreland Basin commenced at the end of the Paleocene orogenic period during the early Thanetian, i.e. between 59 and 60 Ma ago (cf. Harland et al., 1990). Around this time span, a 'Paleocene Revolution', expressed by lithospheric compression and buckling, causing major intra-plate deformation of the foreland, was associated with a change in plate convergence between Africa-Arabia and Europe from northwest to north (Ziegler, 1990). Post-dating volcanism and tectonics intensified during the latest Thanetian to earliest Ypresian. This is reflected by a second diachronous pelitic deposit with bentonite (latest NP 9 to early NP 11; Winkler et al., 1985a; Egger, 1990) and an abundance of plant debris in the coarse South Penninic Flysch fractions (Caron et al., 1989). Similarly, the later, Middle Eocene transgressions are not only determined by changes in global sea-level. They also reflect syn-sedimentary thrust-loaded subsidence of the Helvetic Shelf, accompanied by normal faulting, while on the adjacent land in the north the Siderolithic Formation accumulated (Herb, 1988) on an erosional surface close to base level. According to Winkler (1984), a second subduction-related tectonic pulse is recognizable within mature fan deposits near the base of the Ypresian. A third and longer-lasting transgression is assumed to have started at the Ypresian-Lutetian transition; it is represented by continental-slope pelites deposited on top of a mature fan complex (WinNer, 1984). All three events are correlative with principal changes in both eustasy and stress regime (Fig. 10). Uplift, faulting and erosion of the land area in the northern foreland and the overall transgression and deepening of the evolving marine Alpine Foreland Basin were related to the advance of the Alpine thrust front and concomitant compression-induced positive lithospheric
deflection or flexuring of the foreland (cf. Allen et al., 1991). Tectonic subsidence was much less than during the deposition of the successive cycle sets (Lemcke, 1974; Homewood et al., 1986). 4.2. 'Pre-Molassse' Cycle Set
During the Priabonian, deposition was strongly controlled by tectonics and sea-level change. S W NE-striking normal faults were active in Switzerland; they accompanied rapid basin subsidence and the deposition of conglomeratic beds within the hemipelagic Globigerina Marl (Herb, 1988). The rapid and widespread onlap of the Hohgant series (seventh transgression of Herb, 1988) and the concurrent progradation of the South Helvetic Flysch, however, were also related to a phased rise in global sea-level after the prominent end-Bartonian drop in sea-level. The three Hohgant cyclothems (Breitschmid, 1978) seem to correspond to the last three eustatic cycles of the Eocene. Tectonism is clearly suggested by the chaotic facies of the volcanoclastic Taveyannaz Sandstone which accumulated apparently in a tectonically active (partly piggy-back) basin adjacent to an evolving accretionary wedge (Kindler, 1990; Sinclair, 1992). In turn, the Tavayannaz Sandstone is covered by wildflysch ('Flysch h lentilles'; Fig. 3), a rock sequence which testifies for strong tangential movements and nappe emplacement (Caron et al., 1989). Its deposition coincided with that of breccias representing massive submarine rock falls in southern Switzerland (Mayoraz, 1995). Slumps are also found in the subjacent Foraminiferal Marls (Lateltin and Mt~ller, 1987). Deepening of the basin ceased and an early 'front runner' thrust is assumed to have propagated into the basin from the south (Allen et al., 1991). The Priabonian andesitic-basaltic mass of Taveyannaz Sandstone (at least 1500 km3; Vuagnat, 1985) was deposited during a syn-collisional phase of intensified thrusting activity. It represents an early stage of thrust-loading of the distal European margin, perhaps contemporaneously, or shortly after, the development of a magmatic arc and prior to the deposition of the Val d'Illiez Sandstone which is typified by a paucity of volcanic detritus. Volcanic material from the Tavayannaz grauwackes has yielded a Lutetian potassium/argon age of 43.5 4- 4.3
w. Sissingh/Tectonophysics 282 (1997) 223-256
Ma (Vuagnat, 1985). Perhaps, this date reflects the time of eruption, an event which may thus have lasted until the latest Bartonian (Fig. 10). Volcanism may have started well before the accumulation of the Tavayannaz Sandstones. Through thrusting of the source area, the resedimented volcanoclastic rocks may have been moved northward, after they were disconnected from their base somewhere in the South Penninic province (cf. Vuagnat, 1985). Still, depending on the significance of the available dating, subaerial volcanic eruption and redeposition of the debris by turbidite currents may have occurred in a relatively quick temporal succession. Wether syncollisional or pre-collisional in origin, the Tavayannaz flysch accumulation can be related to the final closure of a remnant deep-water Tethys basin. After the collision of the orogenic wedge with the Helvetic margin, the source of this flysch was subjected to very rapid uplift and erosion as reflected by the accumulation of the North Helvetic Flysch. The crucial event, which caused the onset of subsidence of the Molasse Basin proper, took probably place in Late Eocene times between 38 and 36 Ma, as indicated by the establishment of the Salzburg Basin and its western equivalents (Figs. 3-5; cf. Harland et al., 1990). Andesite from the Val d'Illiez Sandstones has yielded potassium/argon cq. argon/argon ages of 25.6 q- 1.9 and 27.0-4-2.6 Ma, respectively. These dates correspond to a Chattian episode of metamorphism (Vuagnat, 1985) and of orogenic tectonism and magmatism. Homewood (1986) calculated that tectonic subsidence accelerated dramatically during the 'Priabonian' (to reach some 100-300 m/Ma and 50-100 m/Ma in the west and east, respectively). Average flexural subsidence rates remained high from this 'Eocene Revolution' onward until the Pliocene rebound of the basin (Lemcke, 1974; see below). 4.3. Early Molasse Cycle Set
The initiation of this set of post-collision depositional cycles coincided with a major eustatic marine ingression during the earliest Rupelian (Figs. 3-5). In the east, the typically deep-water Fish Shale was deposited while, concurrently, the Engi Shales were formed in the west. Their deposition was accompanied or shortly followed by diachronous flexural
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subsidence and faulting and a northward shift of the basin axis. After an intra-Rupelian break in the depositional record (Fig. 3), followed by an eustatic onlap event, a large-scale, eustatically induced regression took place. This happened at about the RupelianChattian transition (cf. Lemcke, 1983, 1984). Prior to the further transgressive onlap, a regressive period, probably due to both an eustatic fall in sea-level and tectonic uplift, gave rise to an intra-Rupelian hiatus on top of the Val d'Illiez sandstones in Switzerland. This interpretative tectono-eustatic event occurred close to a presumed mid-Rupelian change in tectonic stress (Fig. 10). The latest Rupelian longterm eustatic fall in sea-level was overprinted by short-term relative rises in sea-level. These rises are thought to be due to thrust-loaded subsidence of the foreland (Diem, 1986). From that event onwards, non-forced regressions are responsible for the accumulation of continental deposits dominating the central and western parts of the basin. In the German region, an eustatic cyclicity is recognized (Bachmann and Mtiller, 1991; Roeder and Bachmann, 1996) and, especially in Switzerland, an intra-Chattian sequence boundary is developed (Fig. 3). This break in sedimentation is followed by rapid onlap due to tectonic subsidence (Burbank et al., 1992). It marks a facies change which includes the onset of localized evaporite deposition as well as a drastic change toward a heavy-mineral assemblage dominated by epidote (e.g., Maurer, 1983). The epidote-dominated assemblage is also known from the upper Chattian of the Bresse Graben (Tchimichkian et al., 1958; Cavelier, 1984). It first appeared in the mid-Chattian and can be attributed to increased uplift rates in the Alpine orogen. As this assemblage extends into eastern France, fluvial sediment transport to the west must have taken place across the Jura (Fig. 7; Debrand-Passard and Courbouleix, 1984). The stratigraphic appearance of this new assemblage follows immediately upon the intra-Chattian tectonic movements and exposure and erosion of crystalline rocks in the orogen (Ftichtbauer, 1967). In the German Molasse Basin, a similar change in scenario is reflected by the transgression of the late Chattian Sands (Fig. 5). Throughout the Molasse Basin, the end of this megacycle is regressive, culminating in subaerial exposure and erosion near the basin margin. It corresponds to the termination of the main
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phase of Rupelian to Aquitanian subsidence in the German Molasse Basin (Jin, 1995). 4.4. Late Molasse Cycle Set
The onset of the final major cycle also correlates with a major rise in global sea-level. Similar to the previous set of cycles, marine sedimentation was cyclically replaced by fully continental sedimentation. This may be attributed to eustasy and particularly to the in-filling and closure of the marine corridor east of Linz due to uplift of the Bohemian Spur (Steininger et al., 1986). Within the Burdigalian, a multiple change in facies is significant. In eastern France, the regressive Montaugier lithosome and the associated Intra-Burdigalian Unconformity mark the sudden end of the tide-influenced deposition of the Forezan lithosome (Fig. 3; Allen and Bass, 1993). In Switzerland, a mid-Burdigalian regression is also known at the top of the Luzern Formation (Fig. 4; Schaad et al., 1992). The subsequent 'Helvetian' transgression at the base of the St. Gallen Formation correlates well with the transgressive Gr6sy lithosome of France. The latter directly overlies the Montaugier lithosome and onlaps on the foreland (Fig. 3). In Germany, this event corresponds with the rapid 'Helvetian' (Ottnangian) flooding of the Augsburg Platform (Fig. 5; Bachmann and Mtiller, 1991). Supported by the fact that the Burdigalian is developed as one single transgression-regression cycle (Scholz, 1989), it is most likely that the basin-wide maximum-flooding correlates with the major, tectonic-related condensed section and maximum flooding surface at the base of the Ottnangian (within cycle TB2.1; see Haq et al., 1988). The age of this interregional break within the Burdigalian is close to 19 Ma (cf. Harland et al., 1990). The event is known as the 'Miocene Revolution' (Fig. 10). The latest Burdigalian transgression seems to correlate with the base of the Karpathian interval (Fig. 5: base 'Upper Freshwater-Brackish Molasse' ; Fig. 3: ?base Cognin Mb.). It represents the youngest non-tidal marine influx with brackish inland lakes in the east (Fig. 5). As far as preserved, the post-Burdigalian hemicycle is essentially steady-state regressive, though not without stratigraphic gaps (Z6belein, 1985; Doppler, 1989). One of them is reported from the GermanUpper Austrian Molasse Basin near the 'peneplained
substratum' of the Langhian (transition of the Limnic and Fluviatile Freshwater Beds). At this level, a breccia marker bed (6 m) was identified in the region of the H6rnli Fan (Fig. 8; Btirgisser, 1980, 1984). This so-called Appenzellergranit or Htillistein Horizon (MN 5) has been interpreted to result from an exceptional landslide in the frontal thrust belt which took place some 16 Ma ago. Somewhat earlier, periodic shedding of the torrential Younger Jura Conglomerates started (Fig. 4). As in the case of the Older Jura Conglomerates, their deposition in valleys around the rising Black Forest (Sissingh, 1998) may be due to fluvial unroofing under fluctuating climatic conditions (Schreiner, 1965; Hantke, 1985, 1989). Another hiatus appears to straddle the LanghianSerravallian boundary and a third one is of intra-Serravallian age. These breaks in continental sequences are most likely due to tectonic uplift also affecting the orogen. The younger breaks preceded the deposition of the northern and southern 'Vollschotter' Fans, formed by the palaeo-'Enns' and palaeo-Salzach, respectively (Fig. 8). Bentonites in the Swiss Molasse Basin (15.4-14.4 Ma; Berger, 1992) suggest early Hegau volcanism (14.6-8/7 Ma; Schreiner, 1976) during the Langhian. The Urach 'cryptovolcanism' (15-12 Ma; Neugebauer and Temme, 1981), the meteoritic N6rdlingen or Ries Crater (Pohl et al., 1977; the impact left a 'Brockhorizont' (MN 5) in the basin north of Munich; Hofmann and Hofmann, 1992), and the Steinheim Basin (Reiff, 1977) date from the same time, i.e. 14 or 15 Ma ago (Z6belein, 1985). Doming of the Vosges-Black Forest arch was accompanied by the development of the Kaiserstuhl volcano which was active from about 19 to 13 Ma ago in the southern Rhine Graben (Sissingh, 1998). A temporal tectonomagmatic relationship between Kaiserstuhl, Hegau and Urach volcanism is likely (Fig. 2). Imbrication of the Subalpine Molasse (Fig. 11) and deformation and uplift of the Aar Massif (Burkhard, 1990) occurred also during this Middle Miocene Jura Phase (Laubscher, 1992). Bentonites found in eastern Lower Bavaria are attributed to Miocene (late Badenian?-Sarmatian) volcanism in the Carpathian arc (Unger and Niemeyer, 1985). Overall uplift of the Molasse Basin started at the end of the Miocene, contemporaneously with folding of the Jura Mountains. It induced Pliocene erosion that affected the entire basin. The strongest uplift
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occurred in the west (Lemcke, 1974). As a consequence of this rising and tilting of the proximal foreland, the youngest Molasse deposits (Tortonian) are found only in the eastern part of the perialpine foredeep. The Upper Freshwater Molasse is therefore not preserved in Switzerland west of Bern (Homewood et al., 1986). In southern Germany, the Ottnangian cliffs were uplifted, most of all near the Black Forest (Lemcke, 1984). This 'Pliocene Revolution' of basinal rise and tilting may be related to isostatic adjustment of the lithosphere in answer to the gradually waning thrust movements and related foreland loading. In summary, the tectonostratigraphic history of the North Alpine Foreland Basin includes periods of tectonic activity during which the relatively distal and axial transport regimes of the erosion products received by the foredeep from the bounding provenance areas were changed on both local and regional scales (Figs. 6-8): (1) 'Post-Molasse' Tertiary (post-Tortonian): S-N and E - W transport; (2) Late Molasse Cycle Set: late part (LanghianTortonian) - E - W transport; early part (Burdigalian) - W - E transport; (3) Early Molasse Cycle Set (Rupelian-Aquitanian): W - E transport; (4) 'Pre-Molasse' Cycle Set (latest Bartonianearliest Rupelian): W - E and E - W transport; (5) Alpine Foredeep Cycle Set (Thanetian-Bartonian): W - E and E - W transport; (6) 'Pre-Alpine Foredeep' Tertiary (Danianearliest Thanetian): W - E and E - W transport. The most significant changes in the dispersal system occurred during the earliest Tertiary (midDanian-early Thanetian: local uplift in parts of the Penninic realm; Winkler et al., 1985b; Egger, 1990), during the Rupelian (uplift in the west; Fig. 6) and in the late Burdigalian (uplift in the east; Figs. 7 and 8). During the Priabonian the axial W - E and E - W transport directions were possibly more or less comparable in significance (Fig. 6). In the central-eastern Molasse Basin, during the Pliocene, fluvial transport was principally to the north and east due to the fact that uplift was most pronounced in the west.
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5. Correlations with orogenic events
The following relationships can be envisaged between the development of the North Alpine Foreland Basin and the evolution of the Alpine orogen (partly after Lemcke, 1984): (1) 'Post-Molasse' Tertiary: Continued uplifting and erosion of the Alps. Jura folding. Denudation of the North Alpine Foreland Basin. Continued uplifting of the Central and Western Alps to the west. Late Miocene-Pliocene. (2) Late Molasse Cycle Set: Uplifting of the Central Alps. Major uplifting shifts to the Eastern Alps. Back-thrusting along Insubric Line. Imbrication of External Massifs. Burdigalian-Tortonian. (3) Early Molasse Cycle Set: Uplifting of the Western Alps. Subsidence of the Molasse Basin. Obduction of Austroalpine and Penninic nappes. Rupelian-Aquitanian. (4) 'Pre-Molassse' Cycle Set: Initial development of the Alps and Molasse Basin. Closure of the Alpine Foredeep. Final collision of the orogenic wedge with the Helvetic Shelf. Latest Bartonian-earliest Rupelian. (5) Alpine Foredeep Cycle Set: Development of the Alpine Foredeep and North Alpine Foreland Basin. Gradual closure of the North Penninic trough. Thanetian-Bartonian. A sequence of phases founded on specific thrustbelt features can also be defined. In this assessment of the evolution of the North Alpine Foreland Basin, the model of Sinclair and Allen (1992) is adopted. This model implies during the closure of the North Penninic trough an initial Accretionary Wedge Phase, a period of submarine thrust-wedge development typified by relatively rapid advance of the thrust wedge, dominantly horizontal motions, low exhumation rates and correspondingly only minor erosional unroofing of the orogenic wedge. During this period the Alpine Foredeep and 'Pre-Molasse' sequences were deposited. This initial phase is followed by the Continental Wedge Phase, characterized by increasingly lower rates of frontal advance and increasingly important vertical motions, resulting in the development of a topographical relief and massive erosional unroofing of the orogenic wedge. During this phase the Molasse proper was deposited. The following model for the correlation of cycle sets and thrust-belt
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phases is proposed: Alpine Foredeep Cycle Set: Early Accretionao, Wedge Phase. Gradual closure of the North Penninic trough and subduction of the Briangonnais. Relatively fast, largely submarine wedge migration (possibly after a relatively slow start) and (very) moderate wedge upbuilding, with thrusting in and along a typical turbiditic foredeep and erosional unroofing. Underthrusting, downflexing and block faulting of the European Foreland Plate. ThanetianBartonian. 'Pre-Molasse' Cycle Set: Late Accretionary Wedge Phase. Continued subduction of the Alpine foreland. Pronounced continent-continent collision of Apulia with Europe. Rapid subsidence of the Alpine Foreland Basin. Decreased wedge propagation and increased wedge thickening, exhumation and unroofing. Increasing clastic supply to a partly turbiditic foredeep. Latest Bartonian-earliest Rupelian. Early Molasse Cycle Set: Early Continental Wedge Phase. Decreasing wedge propagation, Insubric back-thrusting, pronounced wedge thickening and exhumation. Frontal thrust-belt steepening along partly turbiditic, marine to continental foredeep. Shortening and imbrication of overthrusted European continental margin. Thrusting and unroofing of Alpine zones. Rupelian-Aquitanian. Late Molasse Cycle Set: Late Continental Wedge Phase. Very slow wedge advance and continued vertical wedge upbuilding along non-turbiditic marine and continental foredeep. Surface deformation of the European Foreland. Continued thrusting (without important back-thrusting) and erosional unroofing of Alpine zones. Burdigalian-Tortonian. 'Post-Late Molasse' Cenozoic: Thrust- Wedge Erosion Phase. Thrust propagation into the Jura domain. Wholesale uplift of the Molasse Basin and Alps. No or only (very) minor horizontal orogenic wedge propagation and thickening. Thrust wedge and foredeep both subjected to denudation. Late MioceneHolocene. General plate-motion directions have changed during these episodes. It appears that during the 'Priabonian' Syn-Collision Episode, or 'Eocene Revolution', and at the beginning of the 'Pliocene Revolution', counterclockwise changes in plate motion took place (Dewey et al., 1989). The 'Miocene Rev-
olution' affected the Alps in 'continental collision' and re-orientated the stress field (Laubscher, 1983, 1987, 1992) as the result of a change in direction of plate movement (Fig. 10; cf. Dewey et al., 1989). The effects of the long-term Palaeogene and Neogene tectonic phases were overprinted by short-lived orogenic pulses which can be correlated with several basin-wide stratigraphic horizons (compare with Fig. 10; mainly after Fuchs, 1980; see also Tollmann, 1966). The following deformation phases are recognized for the Infrahelvetic tectonic complex in eastern Switzerland, underlying the basal thrust of the Helvetic nappes (Fig. 10; summarized from Milnes and Pfiffner, 1977; Pfiffner, 1986): Pizol Phase. Emplacement of exotic Ultrahelvetic strip sheets, associated with the thrusting of the Penninic-Austroalpine nappes on to the internal Helvetic nappes. Early Rupelian. Cavistrau Phase. Emplacement of the allochthonous Subhelvetic units. Rupelian (post-Pizol). Calanda Phase. Main ductile deformation of the entire lnfrahelvetic complex, together with the underlying autochthonous-parautochthonous Flysch, Mesozoic cover and pre-Mesozoic basement (Aar Massif). Rupelian (post-Cavistrau)-Chattian. Ruchi Phase. Development of crenulation cleavage associated with movements on the basal Helvetic thrust (Glarus thrust). Transition AquitanianBurdigalian. For the Helvetic domain of western Switzerland, five deformation phases and their probable onset ages have been deduced from the geometrical relationships of the thrust planes of the major tectonic units (compare with Fig. 10; summarized from Burkhard, 1988): Plaine Morte Phase. Emplacement of Ultrahelvetic units on top of the Wildhorn Nappe. Mid-Priabonian. Prabg Phase. Main internal deformation and emplacement of the Wildhorn and Gellihorn Nappes. Transition Priabonian-Rupelian. Trubelstock Phase. Folding of the basal thrustplane and second deformation of the internal structure in the southern part of the Wildhorn Nappe, correlated with isoclinal folds and a first penetrative schistosity in the southern part of the Doldenhorn Nappe. Transition Rupelian-Chattian.
w. Sissingh/Tectonophysics 282 (1997) 223-256 Kiental Phase. Main deformation and emplacement of the Doldenhorn Nappe together with its crystalline core, the Aar Massif. Mid-Chattian. Grindelwald-Simplon-Rh6ne Phase. Grindelwald: Updoming of the crystalline basement. Formation of the Rawil Depression between the Aar, Gastern and Mt. Blanc-Aiguilles Rouges Massifs. Simplon-Rh6ne: Onset of dextral strike-slip deformation along a major shear zone cutting obliquely through the Helvetic root zone in the Rh6ne valley. The beginning of this phase coincided with the onset of the Ruchi Phase, and (more or less) with the start of rapid cooling following an acme of metamorphism in the Central Alps (23 Ma; Deutsch and Steiger, 1985). Transition Aquitanian-Burdigalian. These orogenic deformation phases are likely to have progressed in intensity, time and space. Apparently, they affected episodically the subsidence of the North Alpine Foreland Basin (cf. Jin, 1995). Their currently accepted ages correlate well (excl. questionable starts), as onset or acme ages, with significant stratigraphic events in the foreland basin (Figs. 3-5 and 10). A similar relationship may be present for the following aspects of the thermal and structural development of the Central Alps along and north of the Insubric Line (compare with Fig. 10; mainly after Steck and Hunziker, 1994): Transition Lutetian-Bartonian. Onset of the Adamello intrusions (42 Ma) and the occurrence of Tavayannaz volcanism (possibly around 43 Ma). Transition Priabonian-Bartonian. Earliest cooling of the Lepontine Gneiss Dome, related to updoming and erosion after the culmination of Tertiary regional metamorphism. Known as the end of the Lepontine Phase (some 38 Ma ago). Rupelian. Onset of dextral transpression associated with strong extension initiating ductile deformation developing into the Simplon Shear Zone. Early S-vergent 'backfolding' deforming the Penninic nappe pile, simultaneously with NW-vergent thrusting of the Aar Massif, the Prealps and the Helvetic nappes (some 35-30 Ma ago). Transition Rupelian-Chattian. Main phase of magmatic intrusions along the Insubric Line (between 31 and 29 Ma). Intrusion of the Biella monzodiorite (30 Ma ago) and Reisenferner tonalite (some 30 Ma ago; Pistotnik, 1980). End of the Adamello magmatic intrusions (29 Ma). Onset of rapid cool-
251
ing (some 30 Ma ago) and main updoming of the Lepontine Gneiss Dome with the intrusion of the Bergell (Bregaglia) tonalites and granodiorites (3229 Ma), concomitant with S-vergent back-folding, back-thrusting and dextral strike-slip movements along the Tonale and Canavese Lines. Known as the onset of the Insubric Phase (some 32-30 Ma ago; pre-Bergell). Chattian. Back-folding (Vanzone, Cressim) along the Insubric Line (some 30-25 Ma ago). Ortler dikes (29 Ma, 26 Ma). Late Chattian. Crystallization of the Novate leucogranite, end of granitoid magmatism (some 25 Ma ago). Beginning of the thickening and slowingdown of the thrust wedge by underplating of the External Massifs (some 25 Ma ago; Pfiffner, 1986; Sinclair et al., 1991). Mid-Burdigalian. Centre of main updoming reached the Simplon region after E-W migration, probably parallel with the westward migration of Apulia, and the development of the dextral Tonale and Canavese Lines (some 20 Ma ago). Transition Serravallian-Tortonian. Formation of the Rhrne-Simplon Line, a low-angle normal fault system which is still active today. This late retrograde manifestation in the pre-existing ductile Simplon Shear Zone coincided with a period of rapid cooling of the Rhrne-Simplon Line (between 12 and 10 Ma ago) and the onset of Jura thrusting (11 Ma ago).
6. Conclusions and perspective The temporal relationship between the sedimentary events in the foredeep and orogenic events in the Alps is summarized in the tectonostratigraphic model proposed in Fig. 10. This chart shows that a number of short-term orogenic events correlate with sequence boundaries in the adjacent North Alpine Foreland Basin. Dating of thermal and structural events within the orogen and sequence-stratigraphic analysis of the sedimentary sequences of the North Alpine Foreland Basin strongly suggest that a stress-induced relationship exists between event originator (or transmitter) and data receiver (or recorder) in both realms. The following bracketed numerical dates of relatively significant and discrete events in the orogen are considered to correlate with
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important moments in the developmental history of the foredeep (Fig. 10): 5-2 Ma: 11 Ma: 20-19 Ma: 22-21 Ma: 25-26 Ma: 30 Ma: 32 Ma: 38 Ma: 60-59 Ma:
Large-scaleuplift of the Alps. 'Pliocene Revolution'. Formation of the Rh6ne-Simplon Line. Onset of Jura thrusting. Updomingof the Simplon region. 'Miocene Revolution'. Onset of the Ruchi Phase. Novate intrusion. End of granitoid magmatism. Onset of Kiental Phase. Main period of magmatic intrusions. Approx. onset of the Pizol and Insubric Phases. End of the Lepontine Phase. Onset of the 'Eocene Revolution'. Onset of the 'Paleocene Revolution'.
It appears that events in the Alpine orogen and depositional cycles within the Alpine foredeep correlate with eustatic events and possible turning points in the regional stress regime (Fig. 10). Contemporaneity of depositional, eustatic and structural events in the foredeep-orogen domain is indicative of a c o m m o n tectonic cause, a hypothesis that requires further analysis with special attention to precise timing, the key to an accurate tectonostratigraphic evaluation of this model on orogen-foredeep relationships. This study presents evidence for a temporal connection between the orogen-foredeep system and the plate tectonics-eustasy interrelationships. Support for this conclusion is given by computer modelling by Sinclair et al. (1991). Without recourse to eustasy or complex viscoelastic models for the lithosphere, regional foreland unconformities (sequence boundaries) could be produced by coupling a thrust wedge advancing on a linear elastic foreland plate with erosion and deposition. Forebulge movements and attendant hiatuses (e.g., Base Burdigalian Unconformity) could be simulated by varying thrust-wedge parameters (rate, slope), while keeping the estimated effective elastic thickness of the foreland plate and sediment transport coefficient constant. Erosion associated with thickening and slowing-down of the forelandwards-moving thrust load caused onlap and offlap, respectively. While rapid advance of the orogenic wedge induced underfilling of the basin and slow propagation promoted overfilling. Modelling suggests that the orogenic activity is able to create foredeep stratigraphy without a link to
eustasy (and to elasticity of the foreland plate). This phenomenon is related to plate motions, the driving force of the Alpine orogeny. Exemplified by coeval foredeep and orogen events, a close kinematic relation of processes exerting a tectono-eustatic control, determined by episodic peaks of tectonic activity alternating with periods of relative tectonic quiescence, may be assumed to underlie the Tertiary evolution of both basin and orogen. These processes also affected the development of the North European Rift System (Sissingh, 1998). Glacio-eustasy inclusive, plate tectonics furnish an overall explanatory theory: acceleration and slowing-down of seafloor-spreading rates cause corresponding transgressional and regressional tendencies, as well as changes in intra-plate stress fields and the palaeoposition of continents.
Acknowledgements This paper is a contribution to the 'PeriTethys Programme': Mapping and modelling of the peritethyan epicratonic basins of Europe, northern Africa and the Middle East (directed by Dr. J. Dercourt, University of R and M. Curie, and Dr. M. Gaetani, University of Milan). Thanks are due to Prof. J.E. Meulenkamp and Dr. R.L.M. Vissers (Utrecht University), Drs. J. Mulock Houwer (Bilthoven) and, most of all, Dr. EA. Ziegler (University of Basel) for discussions and critical comments on an earlier version of the manuscript. This is publication number NSG 950903 of the Netherlands Research School of Sedimentary Geology.
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Unger, H.J., 1989. Die Litbozonen der Oberen SiJsswassermolasse Stidostbayerns und ihre vermutlicben zeitlichen ,~quivalente gegen Westen und Osten. Geol. Bavarica 94, 195-237. Unger, H.J., Niemeyer, A., 1985. Die Bentonite in Ostniederbayern: Entstehung, Lagerung, Verbreitung. Geol. Jahrb. Ser. D 71, 3-58. Vuagnat, M., 1952. P6trographie, r6partition et origine des microbrbches du Flysch nord-helvetique. Mat. Carte G6ol. Suisse, N.S. 97, 103 pp, Vuagnat, M., 1985. Les gr~s de Tavayanne et roches similaires: vestiges d'une activit6 magmatique tardi-alpine. Mem. Soc. Geol. Ital. 26, 39-53. Weidmann, M., 1984. Pal6okarst 6ocbne dans l'autochtone chablaisien (VS et VD). Bull. Murithienne 102, 119-127. Wieland, B., 1976. Petrograpbie eoz~iner siderolitischer Gesteine des Helvetikums der Schweiz: ihre Diagenese und schwache Metamorphose. Diss., Univ. Bern, 117 pp. Wildi, W., 1988. 'Btindnerschiefer' and 'Flysch' of the northern margin of Alpine Tethys: turbiditic deep water clastics in extensional and compressional basins. M6m. Soc. G6ol. Fr. N.S. 154, 115-129. Winkler, W., 1983. Stratigraphie, Sedimentologie und Sedimentpetrographie des Schlier-Flysches. Mat. Carte G6ol. Suisse, N.S., 158, 105 pp. Winkler, W., 1984. Palaeocurrents and petrography of the Gurginel-Schlieren Flysch: a basin analysis. Sediment. Geol. 40, 169-189. Winkler, W., Galetti, G., Maggetti, M., 1985a. Bentonite im Gurginel-, Schlieren- und W~igital-Flysch: Mineralogie, Chemismus, Herkunft. Eclogae Geol. Helv. 78, 545-564. Winkler, W., Wildi, W., van Stuijvenberg, J., Caron, C., 1985b. W~igital-Flysch et autres flyschs penniques en Suisse Centrale. Eclogae Geol. Helv. 78, 1-22. Ziegler, EA., 1990. Geological Atlas of Western and Central Europe. Shell Internationale Petroleum Mij. B.V., The Hague, distributed by Geol. Soc. Publ. House, Bath, 2nd ed., 239 pp. Ziegler, EA., 1994. Cenozoic rift system of western and central Europe: an overview. Geol. Mijnbouw 73, 99-127. Ziegler, EA., Schmid, S.M., Pfiffner, A., Sch6nborn, G., 1996. Structure and evolution of the Central Alps and their northern and southern foreland basins. M6m. Mus. Natl. Hist. Nat. 170. 211-233. Z6belein, H.K., 1985. Stratigraphie der n6rdlichen und teils mittleren Vorlandmolasse zwischen Hegau und Isar anhand von 11 Profilen. Mitt. Bayer. Staatssamml. PalS.ontol. Hist. Geol. 25,209-273.
Tectonophysics 282 (1997) 223-256
Tectonostratigraphy of the North Alpine Foreland Basin: correlation of Tertiary depositional cycles and orogenic phases W. S i s s i n g h * Faculty of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands
Accepted 30 May 1997
Abstract
Tectonostratigraphic analysis of the depositional history of the North Alpine Foreland Basin indicates that temporal relationships exist between short-term events in the foredeep and the adjacent Alpine orogen. The foredeep can be divided stratigraphically into an Alpine Foredeep (Thanetian-Bartonian), 'Pre-Molasse' Basin (latest Bartonian-earliest Rupelian) and Molasse Basin (Rupelian-Tortonian). Their general and tectonostratigraphic development includes time-significant cycle sets and so-called Cenozoic rift and foredeep (CRF) sequences which are apparently correlative to both foreland and orogen deformation events, as well as to changes in global sea-level. Individual basin-fill sequences and, consequently, general stratigraphic and palaeogeographical aspects of the evolving basin system are thus found to be frequently defined by contemporaneous widely distributed tectonics and eustasy. Successive dominant depositional states (underfilled, steady-state and overfilled episodes) and facies (flysch, Molasse) are correlated with different, concurrently progressing phases of the convergence and subduction history of the European and Apulian plates. Keywords: Alpine foredeep; Alpine orogeny; tectonics; depositional history; general stratigraphy; palaeogeography
1. I n t r o d u c t i o n
The northern foreland basin at the Central and Eastern Alps, referred to as the Molasse Basin, extends from French Savoie (Haute-Savoie) in the west to Lower Austria (Nieder Osterreich) in the east (Fig. 1). The general features of this peripheral basin are well known. The total length of this arcuate basin is some 700 kin. It attains a maximum width of some 150 km in its German part and tapers off towards its western and eastern ends to some 20 km. Seismic interpretations, data from boreholes and information from outcrops of the overthrust units, however, have * Fax: +31-30-253-5030; e-mail; [email protected]
demonstrated that the depositional southern limits of the Molasse Basin extended well beneath the northerly verging thrust sheets of the Alps (see e.g., TriJmpy, 1980, or Ziegler, 1990 for illustrative crosssections). In Austria, Molasse deposits have also been found on top of Austroalpine nappes, where they were affected by thrust movements. As a rule, Molasse deposits accumulated in front of the Alpine thrust front, with depocentres located in proximal positions while only marginally covering the frontal thrusts sheets. The Molasse Basin fill consists predominantly of Oligocene and Miocene syn-orogenic continental and shallow-marine clastics; carbonates play a very subordinate role. The entire Cenozoic series
0040-1951/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0040- 195 1 (97)0022 1-7
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224
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Fig. 1. Structural summary map for the area of the North Alpine Foreland Basin.
of the basin attains a maximum thickness of 5000 m near the Alpine thrust front and is generally decreasing rapidly to the north. In Switzerland the distal depositional margin has not been preserved. It was destroyed in conjunction with Late Miocene to Pliocene folding, thrusting and erosion of the Jura Mountains, entailing uplift and erosion of the entire basin. In Germany, erosion of the northern basin margin reflects uplift of the Franconian Platform during Late Miocene and Pliocene times. Similarly, erosion of the northern basin margin in Austria is related to late uplift of the Bohemian Massif. Within the some 300 km long Jura fold-and-thrust belt, Molasse remnants are present in synclines. Jointly, these erosional remnants and those found in the non-folded Swabian and Franconian Jura (Schwfibische and Fr~inkische Alb) show that the northernmost limit of the Molasse Basin was situated some 20-30 km to the north of the present erosional edge (Fig. 1). In the east, crystalline-basement blocks of the Bohemian Massif determine the northern basin geometry.
In combination with dextral strike-slip movements, inversion tectonics uplifted the western Bohemian Massif during the Paleocene and eraplaced the German Landshut-Neu0tting High, an upthrusted, NE-tilted basement block, and the anticlinal Zfirich High in Switzerland (Bachmann et al., 1987; Nachtmann and Wagner, 1987; Schr6der, 1987; Fig. 1). The sediments of this characteristically asymmetric foreland basin are generally underlain by Mesozoic rocks. These rocks have been extensively truncated due to horizontal compressional stresses to which the Alpine foreland was subjected during the Senonian and especially in Paleocene times (Lemcke, 1981; Ziegler, 1990; Bachmann and Mtiller, 1991; Roeder and Bachmann, 1996). Locally, Molasse deposits cover also Palaeozoic and basement rocks. The surface of the entire Molasse Basin is located at present well above sea-level and is subjected to erosion. The strata preserved in the remnant fore-arc foreland basin range in age from Late
W. Sissingh /Tectonophysics 282 (1997) 223-256
Eocene or Early Oligocene to Late Miocene, depending on basin definition. The relatively shallow-water, external Molasse Basin stratigraphically succeeds a generally deepwater, internal Alpine Foredeep, corresponding to a grossly SW-NE-oriented trough. The late Paleocene to Eocene sedimentary succession of the latter comprises a threefold subdivision including shallowmarine limestones at the base, an intermediate series of marls and shales and upper deep-marine turbidite sandstones. The cumulative thickness of these generally superimposed and diachronously deposited units can be hundreds of metres. This part of the basin is largely buried and decapitated by an up to 10-kmthick overburden of thrust sheets. Only a minor portion of it is exposed, mainly in the Subalpine Chains, a region of detached coverfolds and overthrusts south of the Prealps, and also along the northern flank of the crystalline parautochthonous Aar Massif (Fuchs, 1976; Hsti, 1979; Burkhard, 1988). Closure of the Alpine Foredeep proper and first distinct shaping of the Molasse Basin took place during the Late Eocene-Early Oligocene. Together, both parts of the basin constitute a perisutural trench, referred to as the North Alpine Foreland Basin, which evolved along the major compressional zone of orogenic deformation incorporating the megasutural Insubric (Periadriatic) Line. This subvertical structure separates the Central Alps from the Southern Alps. It is the likely result of a large-scale reaction to plate subduction (Schmid et al., 1989). The associated deformation events seem to be (almost) time-equivalent with events in the foredeep. The principal goal of this paper is to present a tectonostratigraphic review of the North Alpine Foreland Basin, paying special attention to the occurrence of depositional sequences and tectonic signatures correlating with the structural development of the radially vergent Alpine orogen. Indications are found for the occurrence of basin-wide sequences with stratigraphic boundaries correlative with eustatic changes in sea-level as well as phases of Alpine tectonism. Similar 'Cenozoic rift and foredeep sequences' (numbered as CRF I to CRF XI) had already been recognized by Sissingh (1998) in the Rhine Graben, the Bresse Graben and the western part of the Motasse Basin. In addition to a palaeogeographical and palaeodepositional overview, an in-
225
terpretative tectonostratigraphic model and a classification scheme for the evolution of the foreland and thrust-wedge system will be proposed. The model relates dynamic aspects of the stratigraphic evolution of the North Alpine Foreland Basin with those of the history of the Alps. Three time-successive basin settings are distinguished: (1) Molasse Basin: Rupelian-Tortonian (2) 'Pre-Molasse' Basin: latest Bartonian-earliest Rupelian (3) Alpine Foredeep: Thanetian-Bartonian The composite foredeep (s.1.) has already been described and analysed in considerable detail at regional and subregional scales, amongst others, (bio)stratigraphically (most notably Berger, 1992), sedimentologically and palaeogeographically (e.g., Btichi and Schlanke, 1977; Lemcke, 1984; Homewood, 1986; Herb, 1988; Homewood et al., 1989; Doppler, 1989; Unger, 1989; Allen and Bass, 1993; Reichenbacher, 1993) and from more integrated stratigraphic and structural geological points of view (e.g., Lemcke, 1974; Trtimpy, 1980; Matter et al., 1980; Homewood et al., 1986; Pfiffner, 1986; Allen et al., 1991; Bachmann and Mtiller, 1991; Roeder and Bachmann, 1996). However, at an interregional scale, the stratigraphic development of the basin has not yet been evaluated in terms of sedimentary sequences reflecting global changes in sea-level as well as tectonic phases of the Alps. Detailed chronostratigraphic control of the relevant geological events in both the foredeep and the orogen is of course crucial for such an enterprise. As it seems, sufficient chronostratigraphic information is available from the foredeep to limit uncertainties in intra-basinal age correlations to acceptable levels of accuracy. Dating of tectonic events in the orogen is often approximate, but the obtained ages are apparently meaningful in the context of integrated foredeep-orogen evolution. With respect to the lithostratigraphy of the different parts of the foredeep, use was made of recently published synthetic overviews, suppressing the names of a bewildering number of local rock-units.
2. Structural setting The structural evolution of the North Alpine Foreland Basin is intimately linked to the Paleocene and Neogene phases of the Alpine orogen which are
W. Sissingh / Tectonophysics 282 (1997) 223-256
226
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Fig. 2. T e c t o n o m a g m a t i c s u m m a r y m a p for the area o f the N o r t h Alpine F o r e l a n d Basin.
related to the convergence of the European (Eurasiatic) Plate with the Apulian (Adriatic) Microplate during the anticlockwise convergence of the African (-Arabian) Plate with Europe (Fig. 2; Platt, 1986; Platt et al., 1989; Coward and Dietrich, 1989). Development of the basin post-dates collision of the orogenic wedge with the Helvetic Shelf, the southern margin of cratonic Europe, and involved its flexural thrust-loaded subsidence. Through time the axis of this foreland basin migrated northward in response to the advance of the Alpine nappe stack. Deflection and subduction of the foreland was accompanied by the development of numerous basin-parallel normal faults along which bending stresses were released at upper crustal levels. Thrust-wedge loading and thickening occurred under largely subaerial conditions. Consequently, thrust-wedge erosion was substantial and provided ample clastic supply to the evolving foreland basin, inducing its further subsidence in response to sediment loading. The sedimentary fill of this Tertiary Alpine Foreland Basin onlaps the Meso-
zoic substrate of the European Foreland in roughly northwestern directions. Following the regional development of the 'Paleocene Unconformity', sedimentation resumed diachronously. The corresponding stratigraphic hiatus becomes increasingly larger towards the north and the west. It represents a major phase of collisional foreland deformation which corresponds to distal foreland buckling in response to the build-up of regional compressional stresses in the foreland. The intra-plate deformations were accompanied by the development of considerable relief, as indicated by the deep truncation of the Mesozoic strata (Trtimpy, 1960; Allen et al., 1991). Today, the Alpine Foredeep is strongly faulted and folded, generally deeply buried under the thrustfold belt and, consequently, badly exposed. For these reasons its stratigraphic record and evolution are comparatively poorly known. From the Late Paleocene till Eocene times, it evolved basically as a remnant oceanic basin of Tethyan origin, its sedimentary fill essentially onlapping a continental mar-
w. Sissingh /Tectonophysics 282 (1997) 223-256
gin (shelf), subjected to a downwarping and bulging of the lithosphere by prograding tectonic loading. During the Early Eocene, the distal European margin, corresponding to the Adula Nappe, started to be overridden by the advancing Alpine nappe system (Schmid et al., 1996; Ziegler et al., 1996). By that time, the oceanic lithosphere in the North Penninic domain had been completely subducted. Towards the end of the Eocene, the mainly deepwater Alpine Foredeep was closed. It was succeeded via a transitional 'Pre-Molasse' Basin - - by the generally shallow-water Molasse Basin, forming the westemmost part of the Paratethys. The eastem parts of the Molasse Basin are relatively little folded but densely faulted. At base-Tertiary level, numerous normal faults with often approximately (N)E-(S)W, basin-parallel orientations, testify here of extensional tectonics perpendicular to the basin axis. In Switzerland, compressional features (folds, inverse faults), roughly parallel to the basin axis, are common (Brink et al., 1992). This part of the basin is characterized by the presence of anticlinal structures, and strike-slip faults. However, along the southern margin of the rather undeformed, so-called Plateau (or Mittelland) Molasse, a narrow Middle Miocene imbrication zone occurs, consisting of closely spaced thrust slices. This Subalpine Motasse zone of deformation is due to proximal overthrusting of the Alpine accretionary wedge (Fig. l). Farther to the west, the thin-skinned Jura fold and thrust belt developed during Middle to Late Miocene times along the northwestern margin of the Molasse Basin; its basal detachment horizon is located either in Triassic evaporites or in the upper crust. Folding and thrusting of the northwest- and north-verging Jura Mountains entailed a commensurate translation and uplift of the Molasse Basin (Laubscher, 1987, 1992). The depositional outline of the western part of the Molasse Basin is possibly reflected by the patchily distributed continental (siderolithic) strata of Middle Eocene age. The presence of these sediments may be due to enhanced block-faulting and differentiation in palaeorelief preceding Late Eocene flooding associated with the demise of the Alpine Foredeep proper and the formation of the Molasse Basin. It seems that the Molasse Basin evolved principally as a post-Eocene flexural foredeep under the influence of Alpine orogenic processes in the collision zone -
-
227
of the European and African plates. Late MiocenePliocene uplift caused basin-wide differential denudation of the younger Molasse series. Uplift and associated erosion was greatest in the southwest, in the region of tectonic shortening in the Jura Mountains. According to Laubscher (1992), and others, the formation and depositional history of the North Alpine Foreland Basin would be mechanically coupled with the coeval outward migration of a curved, broadly SW-NE-oriented foreland bulge. Crestal shifting of this large-scale flexural feature is presumed to have occurred in NW directions till crossing, by Middle to Late Miocene time, the southern extension of the Rhine Graben and the northem Bresse Graben where the Moho discontinuity is found to be shallowest (Fig. 2; Him, 1980). Following Laubscher (1992), the Mid-Miocene migration rate of the forebulge was apparently in the order of 5-10 km per million years. Domal uplift in front of the orogen has also been attributed to buckling of the lithosphere due to compressive stresses transmitted from the orogen into the foreland (Ziegler, 1994). Compressional foreland-stress directions since the Middle Eocene have been predominantly S(S)WN(N)E, interrupted by a period of more or less E - W distension during the Oligocene and with the exception of the Late to post-Miocene period when a regime of roughly N W - S E compression prevailed (Bergerat, 1987; Fig. 2). In the south, the Molasse Basin is defined by the out-of-sequence nappe pile, the primary provenance area of its clastics. During the advance and stacking of this nappe pile, downcutting erosion led to sedimentary loading of the adjoining foreland basin. In western Switzerland, the basin seems to have migrated in N(W) directions at a rate of 7-9 km per million years during the Oligocene and perhaps as little as 2 km per million years in Miocene time. An average basin-migration rate of 5 km per million years has been inferred (Homewood et al., 1986). In eastern Switzerland, basin migration proceeded apparently less rapidly. For this region, average migration rates of about 3 and 2 km per million years during the Late Eocene to Early Oligocene and the post-Early Oligocene, respectively, have been calculated (Pfiffner, 1986). Left-lateral wrench faults crossed the entire basin, generally with N(N)E-
228
w. Sissingh / Tectonophysics 282 (1997) 223-256
S(S)W strikes (Brink et al., 1992). Middle Eocene growth faulting has been identified in the Alpine Foredeep (Pfiffner, 1986). Syn-sedimentary tectonic activity during the Late Eocene has been deduced from the occurrences of conglomerates with locally derived pebbles (Herb, 1988; see also Buchholz, 1989). Anomalous thicknesses of the Oligocene may also be attributed to syn-sedimentary fault activity in the Molasse Basin (Diem, 1986). The depositional history of the North Alpine Foreland Basin must be inherently linked to the contemporaneous formation history of the Alps. The stacking, erosion and lateral movements of the overthrust belt changed the shape and dimensions of the foreland clastic wedge, at least as long as convergent plate boundary processes were active in the orogen. It seems likely that the episodic tectonic evolution - - i.e. peaks of tectonic activity alternating with periods of relative tectonic quiescence - - of an impinging orogenic wedge is recorded in the sequence-stratigraphic architecture of its fringing foredeep. Therefore, recognition of tectonically induced features in the foredeep fill may support and refine concepts concerning the timing of tectonic events in the orogen. Foreland-basin analysis may considerably improve understanding of thrust-wedge evolution. 3. G e n e r a l stratigraphy and depositional history
For the present study, three lithostratigraphic diagrams were compiled (Figs. 3-5). Geographically, these diagrams represent the western, central and eastern parts of the Molasse Basin and their adjacent region in the orogen. They are founded on the crosssectional charts compiled by Gwinner (1978) which were further developed on the basis of data given in Sinclair et al. (1991), Bachmann and Mfiller (1992), Allen and Bass (1993) and others. Steininger et al. (1996) is followed with respect to the correlation of late Tertiary continental and marine stages. For convenience, global standard stages are mostly used. Chronostratigraphic interpretations of rock-units are calibrated with the numerical time-scale of Harland et al. (1990) and combined with the sea-level curves of Haq et al. (1988) and the succession of regional CRF sequences recognized by Sissingh (1998). References to (approx.) maximum thick-
nesses of rock-units and standard biozones (planktonic foraminifera: P; calcareous nannoplankton: NR NN) are given as far as relevant and available. Palaeogeographical maps, presented in Figs. 6 8 for the Priabonian, Rupelian, Chattian to Aquitanian, Burdigalian and Langhian to Tortonian, incorporate information from recent studies and some new names for outstanding structural features. The western parts of these maps are mainly based on Homewood (1986) and Herb (1988) and their eastern parts particularly on Bachmann and Mtiller (1991), Doppler (1989) and Unger (1986, 1989). Incorporation of new data from different regions and timeintervals resulted in a modification of the palaeogeographical maps published by Btichi and Schlanke (1977), Gwinner (1978), Trtimpy (1980) (after H.M. Btirgisser and S. Schlanke), and Lemcke (1984). In the absence of pertinent palinspastic reconstructions, regional palaeopositions of the Alpine thrust front are only shown schematically on all five maps. These maps also show the communications between the Molasse Basin and the European Cenozoic Rift System (cf. Sissingh, 1998). The younger Tertiary clastic rock sequences of the North Alpine Foreland Basin are commonly classified into the following, generally shallow-marine to continental groups (Figs. 3-5): - Upper Freshwater Molasse: Langhian-Tortonian; - Freshwater-Brackish Molasse: late Burdigalian; - Upper Marine Molasse: Burdigalian; - Lower Freshwater Molasse: Chattian-Aquitanian; - Lower Marine Molasse: Rupelian (essentially). These rock-units occur only in the external Molasse Basin; some of them include other facies than their formal name suggests. The basin fill of the older, proximal Alpine Foredeep may be viewed as one single lithostratigraphic group, mainly comprising neritic limestone and sandstone, (hemi)pelagic marl and shale, and deep-water flysch of Thanetian to Bartonian age (Figs. 3-5). On top of this transgressive unit, which gradually encroached on the foreland and reflects rapidly increasing water depths, laterally shaling-out North Helvetic Flysch of 'Priabonian' (latest Bartonian-earliest Rupelian) age was deposited in the western and central re-
W. Sissingh / Tectonophysics 282 (1997) 223-256
gions (Figs. 3 and 4). In the eastern parts of the foreland basin, time-equivalent series contain a substantial succession of sandstone, limestone and marl (Fig. 5). The Priabonian series is here considered as representing a sequence intermediate between the typical fill of the Alpine Foredeep and the Molasse Basin. It was deposited during a period of rising global sea-level, which followed a principal, latest Bartonian glacio-eustatic regression and preceded a pronounced earliest Rupelian transgression (Figs. 3 5; chronostrat, range cf. Herb, 1988, Haq et al., 1988, Buchholz, 1989, and Sinclair et al., 1991, after adopting an earliest Rupelian top for the T4.1T4.3 cycle set, cf. Brinkhuis and Biffi, 1993). The North Helvetic Flysch, a still problematic unit - with respect to age and source area - - consisting of epiclastic turbiditic volcanics, has previously been interpreted as forming part of the Lower Marine Molasse (e.g., Homewood, 1986).
3.1. Alpine Foredeep (Thanetian-Bartonian) The tectono-sedimentary history of the North Alpine Foreland Basin started during the Late Paleocene when a transgressive, deepening succession of Assilina Greensand, Nummulitic Limestone, Globigerina Marls and Flysch was laid down on top the Tertiary cover basal unconformity. The development of this regional unconformity was related to the uplift and erosion of the flexural forebulge in advance of the Alpine thrust belt (Allen et al., 1991). Throughout the Thanetian to Bartonian, a first-order onlapping and overstepping of the northern and northwestern basin margin took place. This process of large-scale diachronous deposition coincided with a northward migration of the shallow- to deep-marine facies belts, including Alpine Flysch with interstratified chaotic wildflysch formations. The basal hiatus increases in magnitude towards the external parts of the basin (Figs. 3-5). The associated foreland uplift led to emergence and subaerial exposure and to the localized deposition of the continental Siderolithic (Bohnerz) Formation. These strata are well known from outcrops in the Jura Mountains and the Helvetic nappes and have also been encountered in wells drilled in central Switzerland. They consist of iron-rich sediments (quartz sandstones, breccia and shales) with, most typically, ferruginous pisolithes
229
filling pockets, cavities and cracks in the karstified Mesozoic series, that have been dated as Lutetian and Bartonian (Wieland, 1976; Weidmann, 1984; Herb, 1988). The formation represents tropical, i.e. climatically hot and wet weathering conditions. Marine flooding of this palaeorelief was generally accompanied by the deposition of a basal Assilina Greensand grading upwards into Nummulitic Limestones. These glauconitic sands are not always developed, limestones may lie directly on top of the angular 'Paleocene Unconformity', either due to non-deposition related to raised palaeotopography or to post-depositional, shallow-submarine erosion of the basal clastics prior to carbonate deposition. The presence of glauconite suggests low net sedimentation rates at essentially shallow-water depths. However, this thin sand unit contains also interbeds of hemipelagic marl indicating an alternation of shallow- and deeper-marine conditions. Overall, areal marine submergence increased in rate during the Eocene and was accompanied by syn-depositional flexural normal faulting as seen in the Helvetic domain of the Central Alps (Herb, 1988; Lihou, 1995; Menkveld-Gfeller, 1995). Throughout the Helvetic domain of the Swiss Alps, Late Paleocene to Eocene deposition of the neritic sediments was controlled by a total of seven main transgressive phases, followed by subsequent regressions causing erosion (Herb, 1988). Only the two oldest can be related to eustatic events. The oldest phase occurred during the early Thanetian (Fliegenspitz Beds: P 3-P 5) and the second (lower part of the Einsiedler Fm.) occurred during the late Thanetian. The other five flooding events occurred during the early mid-Ypresian (upper, main part of the Einsiedler Fm.; with Nummulites planulatus), earliest Lutetian (Gallensis Beds; with N. gallensis), early to middle Lutetian (Btirgen Fm.; about P 10; with N. millecaput and Assilina exponens), latest Lutetian to ?Bartonian (Klimsenhorn Fm.; with N. perforatus gr.) and latest Bartonian to Priabonian (Hohgant Fm.; belonging to the 'Pre-Molasse' Basin fill). The late Ypresian (Einsiedler Fm.) and the middle Lutetian (Btirgen Fm.) sequences indicate regressive conditions. In the Savoie (50 km south of Geneva), correlative marine transgressions at the end of the Ypresian (NP 14, ?P 9) and in Priabonian (NP 18-NP 20) time have been identified in a thrust
W. Sissingh / Tectonophysics 282 (1997) 223-256
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unit by Charollais et al. (1975a). Each transgressive event appeared to follow upon a period of erosion which can be attributed to tectonic activity. In the Helvetic region, north of Interlaken, the Hohgant series includes foreshore to shoreface sandstones. It is underlain by a neritic limestone (Discus Bed) and overlain by similar beds at the top (Discocyclina and Lithothamnium Limestones). According to Breitschmid (1978), four short-term transgressions are recorded in this latest Bartonian to Priabonian (p.p.) cyclothemic sequence. In general terms, deposition of the clastics and carbonates took place in a differentially subsiding, block-faulted basin with palaeomorphology influencing sedimentary facies and submergence events. In the Savoie, discrete Priabonian palaeohighs, capped by fossiliferous algal reefs, have been inferred (Lateltin and Mtiller, 1987). Moreover, as in Switzerland (Herb, 1988), the prevailing S W NE-oriented normal fault system provided significant palaeostructural influences (Pairis and Pairis, 1975). During the Ypresian and Lutetian a shallow-marine gulf extended northward into the region, comprising the Bornes Plateau, the Aravis Chain and the Bauges and Plat6 Massifs. In this basin nummulitic limestones were deposited episodically. Although communication between this basin and the Alpine Foredeep in the east was interrupted by the end of the Lutetian, it became permanently integrated into the Alpine Foredeep during the late Bartonian (Fig. 3; Kerckhove, 1980; Cavelier, 1984). Throughout the Alpine Foredeep Basin the up to 50-m-thick composite clastic-carbonate succession is succeeded by the deeper-marine Bartonian, more or less silty, Globigerina Marls. Diachronous drowning of the transgressive sandstone-carbonate complex can be related to the tectonically induced rapid basin subsidence and an increased input of fine-grained terrigenous clastics, representing the most distal facies of the Alpine Flysch accumulating in the most proximal parts of the basin. On the Helvetic Shelf, Hohgant carbonates of early Priabonian age are overlain with a sharp contact by Globigerina Marls. Such an abrupt lithological change may correspond to increased intra-Priabonian flexural tectonic subsidence, associated with westward migration of thrust-loading. The Globigerina Beds attain thicknesses of up to 400 m in eastern Switzerland. They consist of hemipelagic and pelagic marls
233
characterized by a diversified and rich foraminiferal microfauna (cf. Eckert, 1963). Water depths were mostly in the order of several hundreds of metres as evidenced by very high, upper to middle bathyal P/B ratios (80% to over 90%). In the late Bartonian to Priabonian (P 15, P 16), however, the 'Stadtschiefer' facies has reduced P/B ratios (5080%) indicating shallower though still considerable water depths (Herb, 1988). Alignments of centimetres-long, rod-like foraminiferal tests suggest that during the late Bartonian to early Priabonian (P 15) NNW-SSE-directed currents prevailed in the region of the Aar Massif (Allen et al., 1991). Slump structures and debris flows indicate the presence of a nearby palaeoslope (cf. Lateltin and Mtiller, 1987). The youngest m a r s (Micaceous Marl Slates, Foraminiferal Marls) are found in the Savoie (Fig. 3) where they have been dated as early Rupelian (NP 21, P 18, P 19; Charollais et al., 1975b, 1980; Lateltin and MUller, 1987). Farther south, the basinal Globigerina Marls interfinger with the mega-wedge of the North Penninic to Ultrahelvetic turbidite and wildflysch formations which were deposited in front of the northward advancing orogenic wedge (Homewood, 1983). Extreme tectonic dislocation and disassociation of this major facies belt has so far prevented to establish its suspected multi-sequential development in time and space. It consists predominantly of Paleocene and Eocene conglomerates, sandstones and hemipelagic interbed marls and shales. Included sedimentary, metamorphic and igneous clasts originated from Hercynian basement and Mesozoic cover rocks. Spilitic clasts have been reported from Paleocene flysch. Intra-basinal limestones are present as well. In front of the advancing thrust wedge, deposition occurred at bathyal to abyssal depths in a variety of fan and basinal facies and involved the tectonic-sedimentary emplacements of olisthostromes and wildflysch in the southern Alpine Foredeep. The detrital sources of these deep-water clastics were either intra-basinal submarine or emergent intra-basinal highs. E N E WSW-trending intra-basinal highs, separating the subparallel-trending Ultrahelvetic and North Penninic basins, supplied terrigenous turbidites to both basins. The slopes of the intra-basinal highs were apparently steep enough to cause sediment transport by gravity flow. In the rapidly subsiding Ultrahelvetic
234
W. Sissingh / Tectonophysics 282 (1997) 223-256
basin, which is part of the North Tethyan margin, axial palaeocurrents apparently flowed to the east (Hsti, 1960; Homewood, 1976, 1977; Matter et al., 1980; Homewood and Caron, 1983; Winkler, 1984; Herb, 1988; Wildi, 1988; Caron et al., 1989). The youngest strata of the so-called South Helvetic Flysch are assigned to the early part of the late Priabonian (Herb, 1988). As a whole, the Thanetian to Bartonian succession was deposited in an underfilled foredeep in which the development of accommodation space exceeded the sediment supply. It represents a coherent set of sedimentation cycles consisting of neritic to middle-bathyal deposits which accumulated in a flexural foredeep basin. 3.2. 'Pre-Molasse' Basin (latest Bartonian-earliest Rupelian)
Prior to the accumulation of the various shallowwater Molasse series (s.s.) from the Early Oligocene onwards, the approximately 2000-m-thick North Helvetic Flysch deposited. Initially, northward shaling-out, volcanoclastic sandstones were deposited in the Taveyannaz Turbidite Basin (Fig. 6). The Taveyannaz Sandstones (at least 240 m in eastern Switzerland) accumulated in water depths of at least 500 m at the end of a long period of continued largescale subsidence and onlap of the Helvetic Shelf which had commenced during the Late Paleocene. These sandstones contain up to 90% basaltic andesite detritus (Vuagnat, 1952, 1985). Thicknesses are variable due to syn-depositional changes in local palaeotopography of the Helvetic realm (cf. Lateltin and Mtiller, 1987; Sinclair, 1992). In eastern Switzerland, the Taveyannaz Formation is overlain by some 100 m of mudstone containing dislocated blocks of Taveyannaz Sandstone. It is assumed that this mud sheet was emplaced conformably on top of the grauwacke turbidites, triggered by superficial gravity sliding induced by compressional deformation of the seafloor (Sinclair, 1992). The provenance area of the andesitic detritus is still unknown. Erosion of a volcanic nappe (Vuagnat, 1952, 1985) and redistribution of contemporaneous, subduction-related volcanics (Sawatzki, 1975; see also Martini, 1968) have been suggested. However, neither the remains of such a nappe nor evidence for an Ultrahelvetic volcanic arc
have been found. Nevertheless, the occurrence of angular and fragile volcanic clasts within the Taveyannaz Sandstones excludes extensive reworking in the general area (Homewood and Caron, 1983). Accumulation of mass flow deposits continued in the western Helvetic domain until the earliest Rupelian. These included the proximal turbidites of the relatively polygenetic Val d'Illiez Sandstone (NP 2 1 - N P 23, P 17- P 20) and the Elm Formation (Fig. 6). These contain much less andesitic volcanic detritus and more mudstone than the earlier Taveyannaz Beds (Vuagnat, 1952, 1985; Lateltin and MUller, 1987), and advanced farther to the north. Together with the Matt Formation, which contains slumps indicating syn-depositional slope instability near the thrust front, they represent delta-front turbidites deposited at reduced water depth. Subsequently, the transgressive early Rupelian fish-bearing Engi Shales, the oldest sediments occurring in the Molasse Basin of eastern Switzerland, were deposited (Figs. 3 and 4). In central and western Switzerland, a local Intra-Rupelian Unconformity is present at the contact between the Val d'Illiez Sandstone and the Grigisen Marls (Diem, 1986); this intra-Rupelian gap has not (yet) been recognized farther to the west in the Savoie. Simultaneously with the Taveyannaz Sandstones, ftysch devoid of volcanoclasts and Stockletten Marls (now found in the Helvetic nappes) accumulated in the South Bavarian Turbidite Basin to the east (Fig. 6). Farther north, in the German and Upper Austrian 'Pre-Molasse' Basin, e.g. in the Salzburg Basin, neritic Lithothamnium Limestone (110 m) overlies the fluvial to shallow-marine Basal Sandstone (20 m), laterally grading into the Ampfing Sandstone (70 m) around the Landshut-Neu6tting High. Within the Lithothamnium Limestone two successive northward-transgressing biohermal stages of development of mid- to late Priabonian and late Priabonian to earliest Rupelian age, respectively, are recognized. Each comprises a reefal complex bordered to the north by a lagoon; they are stratigraphically separated by Discocyclina Marl (Buchholz, 1989). As a whole, this composite 'Priabonian' sequence is interpreted as a transitional unit between the Alpine Foredeep and the actual Molasse Basin. In the 'Priabonian' deep-water Stockletten Marls and their time-equivalent shelf deposits of southern
Vii..Sissingh / Tectonophysics 282 (1997) 223-256
235
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236
W. Sissingh / Tectonophysics 282 (I 997) 223-256
Germany and Upper Austria, a shallowing trend is not evident (Buchholz, 1989). The shallow-marine Salzburg Basin, which had developed in response to a widespread transgression, was linked to the south with the basin receiving North Helvetic Flysch (Mfiller et al., 1992; Figs. 5 and 6). As tar as known, only a limited amount of flysch-like Molasse deposits (Katzenloch Beds) has accumulated (Hagn, 1978). In the west, the Late Eocene shallow- to deepmarine sediments were deposited on a relatively narrow, ENE-WSW-striking shelf which was offset to the south by the Tavayannaz Turbidite Basin. In the east, sediments accumulated in the relatively wide, shallow- to deep-marine Salzburg Basin which was flanked to the south by the South Bavarian Turbidite Basin (Fig. 6). During the Eocene, the axis of the ENE-WSW-trending, trench-like turbidite basin in Switzerland shifted to the northwest (Herb, 1988), its orientation remaining parallel to the thrust front; intra-basinal clastic transport was directed towards the northeast to east (Radomski, 1961; Martini, 1968; Lateltin and Mfiller, 1987). In the Salzburg Basin, the peninsular Landshut-Neu6tting High acted as a source of the Priabonian shelf sands (Bachmann and Mfiller, 1991); only from the early Rupelian onwards is there evidence for alluvial fans prograding locally from the thrust front into the basin (Fig. 6). Predominantly Priabonian sediments were deposited during a transitional cycle set under partly underfilled and partly steady-state foredeep conditions. They reflect depositional environments which range from deep-water to continental (fluvial) and are characterized by the absence of classical Molasse. 3.3. Molasse Basin (Rupelian-Tortonian) Rupelian. During this period, the Molasse Basin
abruptly deepened and widened. Initial deposition of the Lower Marine Molasse (1500 m) started in the east with the highly organic Fish Shale (P 18, NP 22; 40 m). During a phase of rapid tectonic basin deepening, associated with an eustatically controlled marine incursion from the east, this anoxic deposit developed throughout the badly ventilated stagnant Salzburg Basin (Fig. 6). It drowned the Lithothamnium Limestone platform entirely, but not yet the Landhut-Neu6tting Peninsula. West of this
high, the palaeo-Naab and palaeo-Main rivers supplied already sand to the basin from northern sources (Lemcke, 1985). The predominantly rather deepmarine strata accumulated simultaneously with the orogen-derived turbiditic Deutenhausen Beds (800 m) which dispersed radially from the northwardadvancing thrust front in the south (Fischer, 1979). The western equivalents of the Deutenhausen Beds are the Engi Shales and the Fish Shale. These shales represent the beginning of the Molasse depositional megacycle. The Fish Shale is overlain by the very thin Light Marly Limestone (5 m) and a relatively substantial series of Banded Marl (60 m). Both were deposited under fairly deep-marine conditions and are rich in coccoliths (MUller and Blascke, 1971). This sequence is succeeded by a thick series of openmarine Rupelian Marls (800 m) which are topped by the latest Rupelian to earliest Chattian, partly littoral Baustein Beds (100 m). Accumulation of these alluvial, deltaic and shallow-marine sands is related to the development of a northward prograding influx system (Kronmfiller and Kronmfiller, 1987). During the late Rupelian shallower-marine conditions became more common. In the German Molasse Basin, the distal basin margin experienced a moderate shift to the south, whilst elsewhere the basin widened slightly (Fig. 6). In the east, deposition of predominantly deep-water muds and turbidites still persisted. In the central and western parts of the basin, however, fine-grained shelf deposits of the Horw Shale and the Grisigen Marls give upward way to the deltaic, littoral to fluvial Horw Sandstone (NP 24) and the Bonneville Sandstone. These sediments and the Baustein Beds correspond to partly forced regressive and increasingly high-energy conditions of sedimentation with progradation to the northeast (Figs. 3-5; Diem, 1986). For the terminal Rupelian, a shallow, wave-dominated seaway, with apparently fluctuating salinities and bordered by sandy tropical beaches and paludal shores along the orogen-proximal margin was postulated. This elongated and increasingly narrowing basin was drained axially from the west to the east (Fig. 6). Concurrently, N W SE-oriented clastic shorelines with delta fans migrated eastward in the Swiss region. There is no evidence for tidal activity (Diem, 1986; Homewood, 1986). The end-Rupelian depositional record coincides with a pronounced glacio-eustatic fall in sea-
W. Sissingh /Tectonophysics 282 (1997) 223-256
level (Figs. 3-5; Lemcke, 1983, 1984). This global regression resulted in the local formation of an approximately 500 m deep and 2000 m long canyon, perpendicularly to the thrust front which was incised into its substratum in western Switzerland down to the Val d'Illiez Sandstone (Mayoraz et al., 1988).
Chattian-Aquitanian. Following the end-Rupelian forced regression, the western and central parts of the Molasse Basin were dominated by a continental environment as evidenced by the accumulation of thick, mainly conglomeratic alluvial-fan deposits of the terrigenous Lower Freshwater Molasse (4000 m). Northward-flowing, marginal intra-Alpine streams formed a series of individual and partly laterally interfingering deltaic fans along the Alpine thrust front (Fig. 7). These fans account for the greatest thicknesses of the Lower Freshwater Molasse; north of the Subalpine Molasse this series becomes much thinner. In general, the long axis of the depocentre shifted to the north-northwest during the Chattian and Aquitanian. The thrust wedge-fringing fans were characterized by steep proximal areas, dominated by braided rivers. Downstream, these gave way to relatively low-gradient floodplains typified by meandering rivers. Farther away from the rising mountain belt, lakes and swamps occurred within a fluvial system draining to the east. During the Chattian, a predominantly radial drainage system existed. However, with the increase of clastic inputs, attributed to rapid uplift of the orogen, this system deviated more to the east and thus evolved into a largely longitudinal, east-directed drainage system (Btichi and Schlanke, 1977). This younger, Aquitanian system of meandering rivers was the first to cut a significant pattern of longitudinal valleys in the perialpine region. Local occurrences of marine to brackish microfaunas, typified by ostracods and foraminifers (Trtimpy, 1980), and the deposition of gypsiferous marl (70 m) of late Chattian age are observed along the North Jura Peninsula (Fig. 3; Btichi and Schlanke, 1977; Reggiani, 1989), suggesting for the Savoie region near Switzerland, the existence of saline lakes and playas which were temporarily connected via the Rauracian Depression with the Rhine Graben (cf. Hauber, 1960). At that time the Rhine Graben was characterized by a brackish to marine environment (Sissingh, 1998). Remains of plants and mammals point to rather wet, subtropical conditions.
237
However, the occurrence of gypsiferous horizons and red sediments indicates that the humidity was variable (cf. Reggiani, 1989). Glaciers are assumed to have existed at that time in the Alps (Bergell Mountains; Hantke, 1989), which at this time were already characterized by a considerable topographical relief (Pfiffner, 1986). In the eastern Molasse Basin, the lower Baustein Beds (Schwerd, 1978) were briefly flooded by the sea after the drastic end-Rupelian sea-level drop. This marine transgression, which originated from the east, flooded the Landshut-Neurtting Peninsula. Deep-marine Puchkirchen Turbidites were deposited along the thrust front (Robinson and Zimmer, 1989; Fertig et al., 1991) to the south of a shallow-marine to deltaic area of sedimentation, referred to as the Landshut-Wels Platform, and to the northeast of an intra-Alpine Inntal Basin. The latter was an elongate bay-like depression with marine to limnofluviatile facies extant from the Late Eocene to the Chattian (Fuchs, 1980). In contrast, in the deep-water Molasse Basin the sandy and conglomeratic Lower Puchkirchen Beds (1300 m) accumulated near the Alpine deformation front. At the same time, deltaic to shallow-marine Chattian Sands (1500 m) prograded into this basin from the west. To the west these strata give laterally way to the fluvial and (swampy) lacustrine Lower 'Bunte Molasse' (2000 m; Paulus, 1963), representing a transgressiveregressive cycle (Fig. 5). As a result of a latest Chattian marine regression, a W-E-trending, broad valley was incised into the Chattian Sands southeast of Munich. Although this event caused widespread erosion in shelfal areas, it did not disrupt the accumulation continuity in the deep-water Puchkirchen Turbidite Basin (Bachmann and MUller, 1991). This erosion surface was covered by the transgressive Upper Cyrena Beds and Upper Marls, and 'Aquitanian Sands' (1200 m) which were deposited during a latest Chattian to Aquitanian transgressive-regressive cycle. The by now submerged valley became a submarine canyon through which clastics were transported into the Puchkirchen Turbidite Basin. However, this basin, which was characterized by water depths in the order of 1000 m, received most of its clastics from the Alpine thrust front. These in part coarse clastics were deposited in a system of submarine fans along the steep, unstable southern margin
238
W. Sissingh/Tectonophysics 282 (1997) 223 256
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W. Sissingh/Tectonophysics 282 (1997) 223-256
of the basin (Kollmann and Malzer, 1980; Fertig et al., 1991). During the Aquitanian, the up to 1400m-thick Upper Puchkirchen Beds were deposited in the eastern deep-water basin. This turbidite series is sealed by the latest Aquitanian Fish Shale (250 m), reflecting termination of massif clastic supply to the basin, which at this time was still sufficiently deep for the development of stagnant bottom-water conditions. Also during the Aquitanian, deltaic systems prograded from the west into the deep Puchkirchen Turbidite Basin. To the west of Munich, the continental, brackish and shallow-marine sediments of the Upper 'Bunte Molasse' (1200 m; Paulus, 1963) and the 'Aquitanian Sands' were deposited; similar series occur along the northern fringes of the Puchkirchen Turbidite Basin. A widespread regression at the end of the Aquitanian terminated deposition of the Lower Freshwater Molasse in the entire foredeep, whereas marine sedimentation persisted in the Puchkirchen Turbidite Basin. The Rupelian Lower Marine Molasse and the Chattian-Aquitanian Lower Freshwater Molasse form a first-order upwards-shallowing and coarsening cycle set which commenced with a widespread phase of marine transgression and basin deepening and ended with a non-forced regression giving rise to predominantly continental sedimentation once the volume of supplied sediment was equal or in excess of the accommodation space. However, in the Puchkirchen Turbidite Basin deeper-water conditions were retained till at the end of this cycle
Burdigalian. At the beginning of the Burdigalian, a period characterized by a warm to subtropical climate north of the Alps, a widespread and rapid marine transgression established in the entire Molasse Basin a wave- and tide-controlled, seaway which extended from the Savoie to Upper Austria (Homewood and Allen, 1981; Allen et al., 1985; Faupl and Roetzel, 1987, 1990; Krenmayr, 1991). Marine ingressions flooded the basin from both the west and the east. In certain areas, the top of the underlying Lower Freshwater Molasse was eroded. This erosion surface corresponds to the Base Burdigalian Unconformity which forms the base of the transgressive Upper Marine Molasse. In Upper Austria, this unconformity (Fig. 5) is submarine; here it developed in response to strong basin-axial currents, in con-
239
junction with the latest Aquitanian narrowing of the basin due to emplacement of the Alpine nappes at their present position (P.A. Ziegler, pers. commun., 1996). During the deposition of the Upper Marine Molasse (1300 m), the subsidence axis of the basin shifted further north into the central zone of the present-day Molasse Basin. The sea covered a considerable portion of the future Jura Mountains, and also onlapped the Black Forest Massif. To which extent the sea entered the southern Rhine Graben is unknown. In the east, the basin margin shifted towards the north. In general, the corresponding strata were subsequently eroded. Regional Burdigalian flooding of the perialpine trough resulted in the establishment of an elongate narrow seaway connecting the Western Mediterranean through the Rh6ne Graben in France with the Carpathian Foredeep. The inference of a strongly tide-influenced depositional realm with at least mesotidal regimes (Homewood and Allen, 1981; Homewood et al., 1986; Faupl and Roetzel, 1990) implies that this basin was clearly connected with the world ocean. Tidal modelling suggests that tides propagated into the German and bordering Swiss parts of the Molasse Basin from the east, i.e. from the Indo-Pacific via the Eastern and Central Paratethys until the late Burdigalian (early Ottnangian) or, less likely, from the Eastern Mediterranean through the Slovenian Corridor (cf. Martel et al., 1994; Sztan6, 1994). Water depths reached distally some 100 m, according to palaeontological data (e.g., Berger, 1985). However, the presence of large tidal banks consisting of shelly sandstones with vadose cements, indicates emersion of their crests. Waveripple features also suggest shallow-water depths (Homewood et al., 1986). In the Swiss segment of the basin, maximum water depths were probably in the order of a few tens of metres (cf. Martel et al., 1994). The high-energy tidal regime caused a pronounced differentiation in facies subparallel to the basin axis. Along the central part of the thrust wedge, conglomeratic fan deltas (1000 m) continued to grow at the mouths of northward-discharging intra-Alpine streams (Fig. 7). After a brief regression (deduced from a palaeosol horizon), the late Burdigalian ('Helvetian') interval is characterized by a rise in relative sea-level and by enhancement of clayey deposition and of encroaching cycles of fan deltas and other proximal deposits (cf. Schaad et
~ Sissingh / Teetonophysics 282 (1997) 223-256
240
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Fig. 8. Palaeogeographicaldevelopmentsduring the Langhian-Tortonian. al., 1992). Between the fluvial distributary systems, characterized by conglomeratic and sandy channel fills and sheet flows, tidally influenced coastal facies (beaches, tidal sandwaves and channels, intertidal sandflats) dominated. These facies are bordered by a near-shore facies belt (subtidal shoals with megarippies, intershoal swales) which further offshore are replaced by shelly, glauconitic and pebbly sandstones (coquina banks). At the northern margin of the basin, sandy beaches and channelized tidal flats were established along rocky coasts in the Jura (Homewood, 1981; Homewood and Allen, 1981; Homewood et al., 1989); the northern margin of the German Molasse Basin was bordered by shoreline cliffs (Fig. 8). In the eastern part of the Molasse Basin, a mainly west-directed transgression claimed first the area closest to the Alpine nappes, and onlapped from there on the Augsburg Platform, an elevated region which existed throughout the early Burdigalian. Thrust-loaded subsidence controlled a northward shift of the depocentre till near the position of the present basin axis (Figs. 1 and 7).
Around the Eggenburgian-Ottnangnian transition, the entire German Molasse Basin was inundated by a tide-influenced 'epicontinental sea' (Neuhofen Beds). The cliffs, defining the northern basin margin, were cut into Upper Jurassic limestones during this early part of the late Burdigalian when predominantly shallow-marine deposition prevailed. Subsequent cyclical sedimentation occurred in marine to brackish and continental facies during the terminal Burdigalian. The Ottnangian Freshwater-Brackish Molasse may be subdivided into three brackish cycles (resp. with a.o. Baltringen, Grimmelfingen and Kirchberg beds). During the second cycle, tilting of the basin caused the incision of the Graupensand Low and concomitant E - W drainage (Fig. 7). At the end of this cycle, holomarine conditions disappeared for good from the area. A brackish (Kirchberg and Oncophora Beds) and fluviatile (Hochgrat Fan, oldest Younger Jura Conglomerate; Schreiner, 1965; Luterbacher et al., 1992) realm with endemic molluscs was established by a final eastward-directed marine ingression. Farther to the east, the palaeo-
W. Sissingh / Tectonophysics 282 (1997) 223-256
Salzach river continued to form an extended delta (Ortenburg Fan). At the end of the third cycle, the first regional west flowing 'proto-Rh6ne' river system (Glimmersand Fan) developed during the latest Ottnangian or Karpathian (Reichenbacher, 1993). Within the late Ottnangian, as well as at the base and top of the largely westward-directed, progradational Karpathian succession, minor hiatuses and lithological breaks are present. On a regional scale, they delimit and subdivide the Freshwater-Brackish Molasse sequence (Fig. 5; Doppler, 1989; Bachmann and Mtiller, 1991), to which the palaeo-Main and palaeo-Naab rivers contributed fluvial clastics from northern and northeastern sources. Whereas during Chattian to middle Burdigalian times, the basin axial sediment transport was directed towards the east, it reversed towards the end of the Burdigalian, thus establishing a southwestward-directed fluvial system which discharged in the west into a marine basin (Fig. 7; Btichi and Schlanke, 1977; Doppler, 1989; Reichenbacher, 1993). Near the Chambfry Sill in the Savoie, a highenergy tidal system initially prevailed (Fig. 3: Tresserve and Forezan lithosomes). Later on, during the Burdigalian (NN 3, NN 4), a regressive, lowenergy muddy shelf (Montaugier lithosome), associated with a substantial basin narrowing, developed for a brief period. This was followed by the deposition of a transgressive, tide-dominated coastal sequence (Grfsy lithosome) in a sea which persisted into Middle Miocene times (Allen and Bass, 1993). Langhian-Tortonian. During the deposition of the
Upper Freshwater Molasse (over 1500 m), the earlier established basin-wide, ENE-WSW-directed axial drainage pattern prevailed. Along the southern border of the basin, the Alpine thrust front remained active at least from Munich westward, perhaps till into the Pliocene, and alluvial fans continued to be fed with large quantities of gravel from the rising Alps (Bachmann et al., 1982). Adjacent alluvial plains were incised by meandering rivers and channels which left frequently variegated muds and sands. Intra-channel sediments include red-coloured palaeosols, browncoal seams (especially southeast of Regensburg) and freshwater limestones. Floral assemblages indicative of a subtropical to temperate climate were recovered from this Upper Freshwater
241
Molasse (e.g., Gregor et al., 1989). However, the climate fluctuated; although it was relatively warm during the Burdigalian, similar to the Late Oligocene, a subsequent cooling trend is evident (Hantke, 1985; Gregor et al., 1989). A composite and repeatedly changing palaeo-Rh6ne river system, originating in the Eastern Alps (cf. Unger, 1986, 1989), received water from the palaeo-rivers Main, Naab, Salzach and 'Enns' (intermittent; Lemcke, 1984). It discharged into the narrow sea which extended through the Rh6ne basin from the Western Mediterranean (Demarcq and Perriaux, 1984). During this last stage of Molasse deposition, the basin attained in Germany its northernmost extension (Figs. 5-8). In the central and eastem parts of the Molasse Basin, marine or brackish deposits are absent in the Upper Freshwater Molasse. Alluvial fan and plain facies predominated, until the Tortonian. Discontinuity in sedimentation since the preceding period is indicated for this region by the Base Langhian Unconformity. The final filling stage of the basin started abruptly and during the same time interval as in the west (Figs. 3-5). Uplift of the southern spur of the Bohemian Massif (east of Linz) led to a disconnection with the marine Paratethys and to non-saline, fluviatile deposition in the area (Steininger et al., 1986). Post- to Middle Miocene erosion affected the entire basin. As the youngest preserved Molasse is of Tortonian age, the Late Miocene to Pliocene development of the Tertiary basin is poorly known. In eastern France, at the westernmost end of the basin, the partly marine Langhian-Serravallian strata represent NW coastal progradation involving the sudden accumulation of thick-bedded fan-delta sands and conglomerates (Fig. 3: Pont-de-Beauvoisin lithosome and Chamoux Conglomerates). These massive sequences, which are derived from the Alps, occluded the western tide-influenced entrance of the Molasse Basin (Allen and Bass, 1993). As in this area post-Serravallian strata are not preserved, the Late Miocene and Pliocene basin history cannot be reconstructed. Final closure of this marine basin segment was already completed during the Tortonian (Perriaux et al., 1984). Wholesale uplift, causing regional erosion in the foredeep and coinciding with rapid elevation of the Alps, may have started during the latest Miocene
242
~ Sissingh / Tectonophysics 282 (1997) 223-256
and this may be causally related to the termination of Molasse deposition (cf. Lemcke, 1984). During the Pliocene, the eastward-flowing Danube fiver extended northward from Lower Austria and started to drain the Molasse Basin from near its presentday source, the Black Forest. The Late Pliocene Molasse Basin was also intersected by a northwardflowing palaeo-Rhine fiver (which did not yet enter the Rhine Graben) and palaeo-Aare/Doubs fiver that drained westward into the Bresse Graben (Bartz, 1961; Abele, 1977). Consequently, the Upper Marine Molasse to Upper Freshwater Molasse succession forms a sedimentary cycle set analogous with that composed by the Lower Marine Molasse and the Lower Freshwater Molasse as described above. Accordingly, the total depositional record of the North Alpine Foreland Basin can be subdivided into four cycle sets: (1) Late Molasse Cycle Set: Burdigalian-Tortonian; (2) Early Molasse Cycle Set: Rupelian-Aquitanian; (3) 'Pre-Molasse' Cycle Set: latest Bartonianearliest Rupelian; (4) Alpine Foredeep Cycle Set: ThanetianBartonian.
(2) Molasse Basin (Rupelian-Tortonian): Early Post-Collision Episode; (3)'Pre-Molasse' Basin (latest Bartonian-earliest Rupelian): Syn-Collision Episode; (4) Alpine Foredeep (Thanetian-Bartonian): Late Pre-Collision Episode. The defined series of cycle sets fits this succession of geodynamically controlled episodes. Their sequential correspondence with the global sea-level curve is striking, as is the case with the time-significant succession of Cenozoic rift and foredeep sequences (CRF I to CRF XI), discussed by Sissingh (1998) for the Rhine Graben, the Bresse Graben and the western Molasse Basin. The CRF series of the North Alpine Foreland has been slightly modified by including a subdivision of CRF II into four eustatic cycles (a-d; see Haq et al., 1988; Figs. 3-5). The CRF sequence boundaries appear to correlate with both tectonic events and eustatic cycle boundaries. It seems that the second- and third-order fluctuations in global sea-level can be related to changes in intra-plate stress (cf. Cloetingh, 1986). This presumed relationship (Fig. 10) is further documented in the following tectonostratigraphic appraisal. A stratigraphic overview of tectonic events which influenced the development of and depositional processes within the Molasse Basin is given in Fig. 11.
4. Tectonostratigraphic interpretation 4.1. Alpine Foredeep Cycle Set The Tertiary evolution of the Alpine orogen determined the evolution of the North Alpine Foreland Basin (Fig. 9). After a predominantly deep-marine 'Flysch stage' (Fig. 9a,b) development of the basin switched to a 'Molasse stage' (Fig. 9c,d) characterized by the accumulation of mainly shallow-marine and continental strata. This fundamental change in facies occurred when in the Central Alps the Helvetic Shelf became compressionally deformed, extensive thrust-loading of its crust commenced and associated uplift of the orogen initiated massive erosion. The pivotal event occurred as the result of a final stage of intense collisional coupling of Apulia (Adria) and Europe. Thus it is possible to geodynamically subdivide the tectono-sedimentary history of the North Alpine Foreland Basin as follows (cf. Homewood and Lateltin, 1988): (1) 'Post-Molasse' Basin (post-Tortonian): Late Post-Collision Episode;
Sediments assigned to the late pre-collision unit overlap, as a whole, diachronously the 'Paleocene Unconformity' to the Helvetic Shelf. The age of crustal deformation giving rise to the hiatus is intraPaleocene (Ziegler, 1990; Allen et al., 1991). However, when consideration is given to the tectono-sedimentary interpretation of the global curve of eustatic sea-level change, it seems likely that marine flooding started in the early Thanetian. The initiation of this main phase in basin development at the end of the Paleocene orogenic phase coincided with volcanism in the South Penninic realm. This is testified by an early Thanetian bentonite, rich in zircon and apatite (Winkler et al., 1985a). The onset of the main phase in basin development is presumed to have occurred soon after a fundamental change in both the heavymineral content and transport directions of turbidites. In the Penninic Rhenodanubian Flysch Basin, sedi-
W. Sissingh / Tectonophysics 282 (1997) 223-256
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Fig. 9. Geological evolution of the North Alpine Foreland Basin, showing the change of the Alpine Foredeep (a,b) into the Molasse Basin (c,d) due to emplacement of overthrust belt onto passive margin (reconstructions from Einsele, 1992). Time intervals: (a) Senonian; (b) Paleocene-Eocene; (c) Early Oligocene; (d) Middle-Late Miocene.
ment transport direction changed from E - W to W - E in mid-Danian time, near the base of a condensed sequence (NP 3 - N P 8) of pelitic sediment. At the same time, a garnet-dominated heavy-mineral assemblage was replaced by one in which zircon is most abundant (Egger, 1990). The W - E orientation of the basin-axial transport direction, established after an implied uplift in the west, is identical with the
one suggested for the North Penninic-Ultrahelvetic Flysch Basin (Winkler, 1984; Wildi, 1988). The condensed sequence (NP 3 - N P 8) is interpreted by Winkler (1983, 1984) as reflecting a phase of diachronously propagating tectonic activity, resulting in an accentuation of the basinfloor palaeorelief and local changes in current direction. Within the Paleocene and Eocene shelf series
W. Sissingh / Tectonophysics 282 (1997) 223-256
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W. Sissingh/ Tectonophysics 282 (1997) 223-256
onlapping the Helvetic Shelf in Switzerland, six main transgressive phases are recognized. The initial Thanetian transgression is interpreted to be eustatically controlled (Herb, 1988). It appears to coincide with the deposition of slope sediments on top of immature fan deposits in the Gurginel-Schlieren Flysch Basin (WinNer, 1984). It corresponds to the beginning of supercycle TA2 of Haq et al. (1988) as well as a subduction-related tectonic pulse associated with syn-orogenic volcanism (Winkler et al., 1985a). According to this interpretation, the development of the North Alpine Foreland Basin commenced at the end of the Paleocene orogenic period during the early Thanetian, i.e. between 59 and 60 Ma ago (cf. Harland et al., 1990). Around this time span, a 'Paleocene Revolution', expressed by lithospheric compression and buckling, causing major intra-plate deformation of the foreland, was associated with a change in plate convergence between Africa-Arabia and Europe from northwest to north (Ziegler, 1990). Post-dating volcanism and tectonics intensified during the latest Thanetian to earliest Ypresian. This is reflected by a second diachronous pelitic deposit with bentonite (latest NP 9 to early NP 11; Winkler et al., 1985a; Egger, 1990) and an abundance of plant debris in the coarse South Penninic Flysch fractions (Caron et al., 1989). Similarly, the later, Middle Eocene transgressions are not only determined by changes in global sea-level. They also reflect syn-sedimentary thrust-loaded subsidence of the Helvetic Shelf, accompanied by normal faulting, while on the adjacent land in the north the Siderolithic Formation accumulated (Herb, 1988) on an erosional surface close to base level. According to Winkler (1984), a second subduction-related tectonic pulse is recognizable within mature fan deposits near the base of the Ypresian. A third and longer-lasting transgression is assumed to have started at the Ypresian-Lutetian transition; it is represented by continental-slope pelites deposited on top of a mature fan complex (WinNer, 1984). All three events are correlative with principal changes in both eustasy and stress regime (Fig. 10). Uplift, faulting and erosion of the land area in the northern foreland and the overall transgression and deepening of the evolving marine Alpine Foreland Basin were related to the advance of the Alpine thrust front and concomitant compression-induced positive lithospheric
deflection or flexuring of the foreland (cf. Allen et al., 1991). Tectonic subsidence was much less than during the deposition of the successive cycle sets (Lemcke, 1974; Homewood et al., 1986). 4.2. 'Pre-Molassse' Cycle Set
During the Priabonian, deposition was strongly controlled by tectonics and sea-level change. S W NE-striking normal faults were active in Switzerland; they accompanied rapid basin subsidence and the deposition of conglomeratic beds within the hemipelagic Globigerina Marl (Herb, 1988). The rapid and widespread onlap of the Hohgant series (seventh transgression of Herb, 1988) and the concurrent progradation of the South Helvetic Flysch, however, were also related to a phased rise in global sea-level after the prominent end-Bartonian drop in sea-level. The three Hohgant cyclothems (Breitschmid, 1978) seem to correspond to the last three eustatic cycles of the Eocene. Tectonism is clearly suggested by the chaotic facies of the volcanoclastic Taveyannaz Sandstone which accumulated apparently in a tectonically active (partly piggy-back) basin adjacent to an evolving accretionary wedge (Kindler, 1990; Sinclair, 1992). In turn, the Tavayannaz Sandstone is covered by wildflysch ('Flysch h lentilles'; Fig. 3), a rock sequence which testifies for strong tangential movements and nappe emplacement (Caron et al., 1989). Its deposition coincided with that of breccias representing massive submarine rock falls in southern Switzerland (Mayoraz, 1995). Slumps are also found in the subjacent Foraminiferal Marls (Lateltin and Mt~ller, 1987). Deepening of the basin ceased and an early 'front runner' thrust is assumed to have propagated into the basin from the south (Allen et al., 1991). The Priabonian andesitic-basaltic mass of Taveyannaz Sandstone (at least 1500 km3; Vuagnat, 1985) was deposited during a syn-collisional phase of intensified thrusting activity. It represents an early stage of thrust-loading of the distal European margin, perhaps contemporaneously, or shortly after, the development of a magmatic arc and prior to the deposition of the Val d'Illiez Sandstone which is typified by a paucity of volcanic detritus. Volcanic material from the Tavayannaz grauwackes has yielded a Lutetian potassium/argon age of 43.5 4- 4.3
w. Sissingh/Tectonophysics 282 (1997) 223-256
Ma (Vuagnat, 1985). Perhaps, this date reflects the time of eruption, an event which may thus have lasted until the latest Bartonian (Fig. 10). Volcanism may have started well before the accumulation of the Tavayannaz Sandstones. Through thrusting of the source area, the resedimented volcanoclastic rocks may have been moved northward, after they were disconnected from their base somewhere in the South Penninic province (cf. Vuagnat, 1985). Still, depending on the significance of the available dating, subaerial volcanic eruption and redeposition of the debris by turbidite currents may have occurred in a relatively quick temporal succession. Wether syncollisional or pre-collisional in origin, the Tavayannaz flysch accumulation can be related to the final closure of a remnant deep-water Tethys basin. After the collision of the orogenic wedge with the Helvetic margin, the source of this flysch was subjected to very rapid uplift and erosion as reflected by the accumulation of the North Helvetic Flysch. The crucial event, which caused the onset of subsidence of the Molasse Basin proper, took probably place in Late Eocene times between 38 and 36 Ma, as indicated by the establishment of the Salzburg Basin and its western equivalents (Figs. 3-5; cf. Harland et al., 1990). Andesite from the Val d'Illiez Sandstones has yielded potassium/argon cq. argon/argon ages of 25.6 q- 1.9 and 27.0-4-2.6 Ma, respectively. These dates correspond to a Chattian episode of metamorphism (Vuagnat, 1985) and of orogenic tectonism and magmatism. Homewood (1986) calculated that tectonic subsidence accelerated dramatically during the 'Priabonian' (to reach some 100-300 m/Ma and 50-100 m/Ma in the west and east, respectively). Average flexural subsidence rates remained high from this 'Eocene Revolution' onward until the Pliocene rebound of the basin (Lemcke, 1974; see below). 4.3. Early Molasse Cycle Set
The initiation of this set of post-collision depositional cycles coincided with a major eustatic marine ingression during the earliest Rupelian (Figs. 3-5). In the east, the typically deep-water Fish Shale was deposited while, concurrently, the Engi Shales were formed in the west. Their deposition was accompanied or shortly followed by diachronous flexural
247
subsidence and faulting and a northward shift of the basin axis. After an intra-Rupelian break in the depositional record (Fig. 3), followed by an eustatic onlap event, a large-scale, eustatically induced regression took place. This happened at about the RupelianChattian transition (cf. Lemcke, 1983, 1984). Prior to the further transgressive onlap, a regressive period, probably due to both an eustatic fall in sea-level and tectonic uplift, gave rise to an intra-Rupelian hiatus on top of the Val d'Illiez sandstones in Switzerland. This interpretative tectono-eustatic event occurred close to a presumed mid-Rupelian change in tectonic stress (Fig. 10). The latest Rupelian longterm eustatic fall in sea-level was overprinted by short-term relative rises in sea-level. These rises are thought to be due to thrust-loaded subsidence of the foreland (Diem, 1986). From that event onwards, non-forced regressions are responsible for the accumulation of continental deposits dominating the central and western parts of the basin. In the German region, an eustatic cyclicity is recognized (Bachmann and Mtiller, 1991; Roeder and Bachmann, 1996) and, especially in Switzerland, an intra-Chattian sequence boundary is developed (Fig. 3). This break in sedimentation is followed by rapid onlap due to tectonic subsidence (Burbank et al., 1992). It marks a facies change which includes the onset of localized evaporite deposition as well as a drastic change toward a heavy-mineral assemblage dominated by epidote (e.g., Maurer, 1983). The epidote-dominated assemblage is also known from the upper Chattian of the Bresse Graben (Tchimichkian et al., 1958; Cavelier, 1984). It first appeared in the mid-Chattian and can be attributed to increased uplift rates in the Alpine orogen. As this assemblage extends into eastern France, fluvial sediment transport to the west must have taken place across the Jura (Fig. 7; Debrand-Passard and Courbouleix, 1984). The stratigraphic appearance of this new assemblage follows immediately upon the intra-Chattian tectonic movements and exposure and erosion of crystalline rocks in the orogen (Ftichtbauer, 1967). In the German Molasse Basin, a similar change in scenario is reflected by the transgression of the late Chattian Sands (Fig. 5). Throughout the Molasse Basin, the end of this megacycle is regressive, culminating in subaerial exposure and erosion near the basin margin. It corresponds to the termination of the main
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phase of Rupelian to Aquitanian subsidence in the German Molasse Basin (Jin, 1995). 4.4. Late Molasse Cycle Set
The onset of the final major cycle also correlates with a major rise in global sea-level. Similar to the previous set of cycles, marine sedimentation was cyclically replaced by fully continental sedimentation. This may be attributed to eustasy and particularly to the in-filling and closure of the marine corridor east of Linz due to uplift of the Bohemian Spur (Steininger et al., 1986). Within the Burdigalian, a multiple change in facies is significant. In eastern France, the regressive Montaugier lithosome and the associated Intra-Burdigalian Unconformity mark the sudden end of the tide-influenced deposition of the Forezan lithosome (Fig. 3; Allen and Bass, 1993). In Switzerland, a mid-Burdigalian regression is also known at the top of the Luzern Formation (Fig. 4; Schaad et al., 1992). The subsequent 'Helvetian' transgression at the base of the St. Gallen Formation correlates well with the transgressive Gr6sy lithosome of France. The latter directly overlies the Montaugier lithosome and onlaps on the foreland (Fig. 3). In Germany, this event corresponds with the rapid 'Helvetian' (Ottnangian) flooding of the Augsburg Platform (Fig. 5; Bachmann and Mtiller, 1991). Supported by the fact that the Burdigalian is developed as one single transgression-regression cycle (Scholz, 1989), it is most likely that the basin-wide maximum-flooding correlates with the major, tectonic-related condensed section and maximum flooding surface at the base of the Ottnangian (within cycle TB2.1; see Haq et al., 1988). The age of this interregional break within the Burdigalian is close to 19 Ma (cf. Harland et al., 1990). The event is known as the 'Miocene Revolution' (Fig. 10). The latest Burdigalian transgression seems to correlate with the base of the Karpathian interval (Fig. 5: base 'Upper Freshwater-Brackish Molasse' ; Fig. 3: ?base Cognin Mb.). It represents the youngest non-tidal marine influx with brackish inland lakes in the east (Fig. 5). As far as preserved, the post-Burdigalian hemicycle is essentially steady-state regressive, though not without stratigraphic gaps (Z6belein, 1985; Doppler, 1989). One of them is reported from the GermanUpper Austrian Molasse Basin near the 'peneplained
substratum' of the Langhian (transition of the Limnic and Fluviatile Freshwater Beds). At this level, a breccia marker bed (6 m) was identified in the region of the H6rnli Fan (Fig. 8; Btirgisser, 1980, 1984). This so-called Appenzellergranit or Htillistein Horizon (MN 5) has been interpreted to result from an exceptional landslide in the frontal thrust belt which took place some 16 Ma ago. Somewhat earlier, periodic shedding of the torrential Younger Jura Conglomerates started (Fig. 4). As in the case of the Older Jura Conglomerates, their deposition in valleys around the rising Black Forest (Sissingh, 1998) may be due to fluvial unroofing under fluctuating climatic conditions (Schreiner, 1965; Hantke, 1985, 1989). Another hiatus appears to straddle the LanghianSerravallian boundary and a third one is of intra-Serravallian age. These breaks in continental sequences are most likely due to tectonic uplift also affecting the orogen. The younger breaks preceded the deposition of the northern and southern 'Vollschotter' Fans, formed by the palaeo-'Enns' and palaeo-Salzach, respectively (Fig. 8). Bentonites in the Swiss Molasse Basin (15.4-14.4 Ma; Berger, 1992) suggest early Hegau volcanism (14.6-8/7 Ma; Schreiner, 1976) during the Langhian. The Urach 'cryptovolcanism' (15-12 Ma; Neugebauer and Temme, 1981), the meteoritic N6rdlingen or Ries Crater (Pohl et al., 1977; the impact left a 'Brockhorizont' (MN 5) in the basin north of Munich; Hofmann and Hofmann, 1992), and the Steinheim Basin (Reiff, 1977) date from the same time, i.e. 14 or 15 Ma ago (Z6belein, 1985). Doming of the Vosges-Black Forest arch was accompanied by the development of the Kaiserstuhl volcano which was active from about 19 to 13 Ma ago in the southern Rhine Graben (Sissingh, 1998). A temporal tectonomagmatic relationship between Kaiserstuhl, Hegau and Urach volcanism is likely (Fig. 2). Imbrication of the Subalpine Molasse (Fig. 11) and deformation and uplift of the Aar Massif (Burkhard, 1990) occurred also during this Middle Miocene Jura Phase (Laubscher, 1992). Bentonites found in eastern Lower Bavaria are attributed to Miocene (late Badenian?-Sarmatian) volcanism in the Carpathian arc (Unger and Niemeyer, 1985). Overall uplift of the Molasse Basin started at the end of the Miocene, contemporaneously with folding of the Jura Mountains. It induced Pliocene erosion that affected the entire basin. The strongest uplift
w. Sissingh /Tectonophysics 282 (1997) 223-256
occurred in the west (Lemcke, 1974). As a consequence of this rising and tilting of the proximal foreland, the youngest Molasse deposits (Tortonian) are found only in the eastern part of the perialpine foredeep. The Upper Freshwater Molasse is therefore not preserved in Switzerland west of Bern (Homewood et al., 1986). In southern Germany, the Ottnangian cliffs were uplifted, most of all near the Black Forest (Lemcke, 1984). This 'Pliocene Revolution' of basinal rise and tilting may be related to isostatic adjustment of the lithosphere in answer to the gradually waning thrust movements and related foreland loading. In summary, the tectonostratigraphic history of the North Alpine Foreland Basin includes periods of tectonic activity during which the relatively distal and axial transport regimes of the erosion products received by the foredeep from the bounding provenance areas were changed on both local and regional scales (Figs. 6-8): (1) 'Post-Molasse' Tertiary (post-Tortonian): S-N and E - W transport; (2) Late Molasse Cycle Set: late part (LanghianTortonian) - E - W transport; early part (Burdigalian) - W - E transport; (3) Early Molasse Cycle Set (Rupelian-Aquitanian): W - E transport; (4) 'Pre-Molasse' Cycle Set (latest Bartonianearliest Rupelian): W - E and E - W transport; (5) Alpine Foredeep Cycle Set (Thanetian-Bartonian): W - E and E - W transport; (6) 'Pre-Alpine Foredeep' Tertiary (Danianearliest Thanetian): W - E and E - W transport. The most significant changes in the dispersal system occurred during the earliest Tertiary (midDanian-early Thanetian: local uplift in parts of the Penninic realm; Winkler et al., 1985b; Egger, 1990), during the Rupelian (uplift in the west; Fig. 6) and in the late Burdigalian (uplift in the east; Figs. 7 and 8). During the Priabonian the axial W - E and E - W transport directions were possibly more or less comparable in significance (Fig. 6). In the central-eastern Molasse Basin, during the Pliocene, fluvial transport was principally to the north and east due to the fact that uplift was most pronounced in the west.
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5. Correlations with orogenic events
The following relationships can be envisaged between the development of the North Alpine Foreland Basin and the evolution of the Alpine orogen (partly after Lemcke, 1984): (1) 'Post-Molasse' Tertiary: Continued uplifting and erosion of the Alps. Jura folding. Denudation of the North Alpine Foreland Basin. Continued uplifting of the Central and Western Alps to the west. Late Miocene-Pliocene. (2) Late Molasse Cycle Set: Uplifting of the Central Alps. Major uplifting shifts to the Eastern Alps. Back-thrusting along Insubric Line. Imbrication of External Massifs. Burdigalian-Tortonian. (3) Early Molasse Cycle Set: Uplifting of the Western Alps. Subsidence of the Molasse Basin. Obduction of Austroalpine and Penninic nappes. Rupelian-Aquitanian. (4) 'Pre-Molassse' Cycle Set: Initial development of the Alps and Molasse Basin. Closure of the Alpine Foredeep. Final collision of the orogenic wedge with the Helvetic Shelf. Latest Bartonian-earliest Rupelian. (5) Alpine Foredeep Cycle Set: Development of the Alpine Foredeep and North Alpine Foreland Basin. Gradual closure of the North Penninic trough. Thanetian-Bartonian. A sequence of phases founded on specific thrustbelt features can also be defined. In this assessment of the evolution of the North Alpine Foreland Basin, the model of Sinclair and Allen (1992) is adopted. This model implies during the closure of the North Penninic trough an initial Accretionary Wedge Phase, a period of submarine thrust-wedge development typified by relatively rapid advance of the thrust wedge, dominantly horizontal motions, low exhumation rates and correspondingly only minor erosional unroofing of the orogenic wedge. During this period the Alpine Foredeep and 'Pre-Molasse' sequences were deposited. This initial phase is followed by the Continental Wedge Phase, characterized by increasingly lower rates of frontal advance and increasingly important vertical motions, resulting in the development of a topographical relief and massive erosional unroofing of the orogenic wedge. During this phase the Molasse proper was deposited. The following model for the correlation of cycle sets and thrust-belt
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phases is proposed: Alpine Foredeep Cycle Set: Early Accretionao, Wedge Phase. Gradual closure of the North Penninic trough and subduction of the Briangonnais. Relatively fast, largely submarine wedge migration (possibly after a relatively slow start) and (very) moderate wedge upbuilding, with thrusting in and along a typical turbiditic foredeep and erosional unroofing. Underthrusting, downflexing and block faulting of the European Foreland Plate. ThanetianBartonian. 'Pre-Molasse' Cycle Set: Late Accretionary Wedge Phase. Continued subduction of the Alpine foreland. Pronounced continent-continent collision of Apulia with Europe. Rapid subsidence of the Alpine Foreland Basin. Decreased wedge propagation and increased wedge thickening, exhumation and unroofing. Increasing clastic supply to a partly turbiditic foredeep. Latest Bartonian-earliest Rupelian. Early Molasse Cycle Set: Early Continental Wedge Phase. Decreasing wedge propagation, Insubric back-thrusting, pronounced wedge thickening and exhumation. Frontal thrust-belt steepening along partly turbiditic, marine to continental foredeep. Shortening and imbrication of overthrusted European continental margin. Thrusting and unroofing of Alpine zones. Rupelian-Aquitanian. Late Molasse Cycle Set: Late Continental Wedge Phase. Very slow wedge advance and continued vertical wedge upbuilding along non-turbiditic marine and continental foredeep. Surface deformation of the European Foreland. Continued thrusting (without important back-thrusting) and erosional unroofing of Alpine zones. Burdigalian-Tortonian. 'Post-Late Molasse' Cenozoic: Thrust- Wedge Erosion Phase. Thrust propagation into the Jura domain. Wholesale uplift of the Molasse Basin and Alps. No or only (very) minor horizontal orogenic wedge propagation and thickening. Thrust wedge and foredeep both subjected to denudation. Late MioceneHolocene. General plate-motion directions have changed during these episodes. It appears that during the 'Priabonian' Syn-Collision Episode, or 'Eocene Revolution', and at the beginning of the 'Pliocene Revolution', counterclockwise changes in plate motion took place (Dewey et al., 1989). The 'Miocene Rev-
olution' affected the Alps in 'continental collision' and re-orientated the stress field (Laubscher, 1983, 1987, 1992) as the result of a change in direction of plate movement (Fig. 10; cf. Dewey et al., 1989). The effects of the long-term Palaeogene and Neogene tectonic phases were overprinted by short-lived orogenic pulses which can be correlated with several basin-wide stratigraphic horizons (compare with Fig. 10; mainly after Fuchs, 1980; see also Tollmann, 1966). The following deformation phases are recognized for the Infrahelvetic tectonic complex in eastern Switzerland, underlying the basal thrust of the Helvetic nappes (Fig. 10; summarized from Milnes and Pfiffner, 1977; Pfiffner, 1986): Pizol Phase. Emplacement of exotic Ultrahelvetic strip sheets, associated with the thrusting of the Penninic-Austroalpine nappes on to the internal Helvetic nappes. Early Rupelian. Cavistrau Phase. Emplacement of the allochthonous Subhelvetic units. Rupelian (post-Pizol). Calanda Phase. Main ductile deformation of the entire lnfrahelvetic complex, together with the underlying autochthonous-parautochthonous Flysch, Mesozoic cover and pre-Mesozoic basement (Aar Massif). Rupelian (post-Cavistrau)-Chattian. Ruchi Phase. Development of crenulation cleavage associated with movements on the basal Helvetic thrust (Glarus thrust). Transition AquitanianBurdigalian. For the Helvetic domain of western Switzerland, five deformation phases and their probable onset ages have been deduced from the geometrical relationships of the thrust planes of the major tectonic units (compare with Fig. 10; summarized from Burkhard, 1988): Plaine Morte Phase. Emplacement of Ultrahelvetic units on top of the Wildhorn Nappe. Mid-Priabonian. Prabg Phase. Main internal deformation and emplacement of the Wildhorn and Gellihorn Nappes. Transition Priabonian-Rupelian. Trubelstock Phase. Folding of the basal thrustplane and second deformation of the internal structure in the southern part of the Wildhorn Nappe, correlated with isoclinal folds and a first penetrative schistosity in the southern part of the Doldenhorn Nappe. Transition Rupelian-Chattian.
w. Sissingh/Tectonophysics 282 (1997) 223-256 Kiental Phase. Main deformation and emplacement of the Doldenhorn Nappe together with its crystalline core, the Aar Massif. Mid-Chattian. Grindelwald-Simplon-Rh6ne Phase. Grindelwald: Updoming of the crystalline basement. Formation of the Rawil Depression between the Aar, Gastern and Mt. Blanc-Aiguilles Rouges Massifs. Simplon-Rh6ne: Onset of dextral strike-slip deformation along a major shear zone cutting obliquely through the Helvetic root zone in the Rh6ne valley. The beginning of this phase coincided with the onset of the Ruchi Phase, and (more or less) with the start of rapid cooling following an acme of metamorphism in the Central Alps (23 Ma; Deutsch and Steiger, 1985). Transition Aquitanian-Burdigalian. These orogenic deformation phases are likely to have progressed in intensity, time and space. Apparently, they affected episodically the subsidence of the North Alpine Foreland Basin (cf. Jin, 1995). Their currently accepted ages correlate well (excl. questionable starts), as onset or acme ages, with significant stratigraphic events in the foreland basin (Figs. 3-5 and 10). A similar relationship may be present for the following aspects of the thermal and structural development of the Central Alps along and north of the Insubric Line (compare with Fig. 10; mainly after Steck and Hunziker, 1994): Transition Lutetian-Bartonian. Onset of the Adamello intrusions (42 Ma) and the occurrence of Tavayannaz volcanism (possibly around 43 Ma). Transition Priabonian-Bartonian. Earliest cooling of the Lepontine Gneiss Dome, related to updoming and erosion after the culmination of Tertiary regional metamorphism. Known as the end of the Lepontine Phase (some 38 Ma ago). Rupelian. Onset of dextral transpression associated with strong extension initiating ductile deformation developing into the Simplon Shear Zone. Early S-vergent 'backfolding' deforming the Penninic nappe pile, simultaneously with NW-vergent thrusting of the Aar Massif, the Prealps and the Helvetic nappes (some 35-30 Ma ago). Transition Rupelian-Chattian. Main phase of magmatic intrusions along the Insubric Line (between 31 and 29 Ma). Intrusion of the Biella monzodiorite (30 Ma ago) and Reisenferner tonalite (some 30 Ma ago; Pistotnik, 1980). End of the Adamello magmatic intrusions (29 Ma). Onset of rapid cool-
251
ing (some 30 Ma ago) and main updoming of the Lepontine Gneiss Dome with the intrusion of the Bergell (Bregaglia) tonalites and granodiorites (3229 Ma), concomitant with S-vergent back-folding, back-thrusting and dextral strike-slip movements along the Tonale and Canavese Lines. Known as the onset of the Insubric Phase (some 32-30 Ma ago; pre-Bergell). Chattian. Back-folding (Vanzone, Cressim) along the Insubric Line (some 30-25 Ma ago). Ortler dikes (29 Ma, 26 Ma). Late Chattian. Crystallization of the Novate leucogranite, end of granitoid magmatism (some 25 Ma ago). Beginning of the thickening and slowingdown of the thrust wedge by underplating of the External Massifs (some 25 Ma ago; Pfiffner, 1986; Sinclair et al., 1991). Mid-Burdigalian. Centre of main updoming reached the Simplon region after E-W migration, probably parallel with the westward migration of Apulia, and the development of the dextral Tonale and Canavese Lines (some 20 Ma ago). Transition Serravallian-Tortonian. Formation of the Rhrne-Simplon Line, a low-angle normal fault system which is still active today. This late retrograde manifestation in the pre-existing ductile Simplon Shear Zone coincided with a period of rapid cooling of the Rhrne-Simplon Line (between 12 and 10 Ma ago) and the onset of Jura thrusting (11 Ma ago).
6. Conclusions and perspective The temporal relationship between the sedimentary events in the foredeep and orogenic events in the Alps is summarized in the tectonostratigraphic model proposed in Fig. 10. This chart shows that a number of short-term orogenic events correlate with sequence boundaries in the adjacent North Alpine Foreland Basin. Dating of thermal and structural events within the orogen and sequence-stratigraphic analysis of the sedimentary sequences of the North Alpine Foreland Basin strongly suggest that a stress-induced relationship exists between event originator (or transmitter) and data receiver (or recorder) in both realms. The following bracketed numerical dates of relatively significant and discrete events in the orogen are considered to correlate with
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important moments in the developmental history of the foredeep (Fig. 10): 5-2 Ma: 11 Ma: 20-19 Ma: 22-21 Ma: 25-26 Ma: 30 Ma: 32 Ma: 38 Ma: 60-59 Ma:
Large-scaleuplift of the Alps. 'Pliocene Revolution'. Formation of the Rh6ne-Simplon Line. Onset of Jura thrusting. Updomingof the Simplon region. 'Miocene Revolution'. Onset of the Ruchi Phase. Novate intrusion. End of granitoid magmatism. Onset of Kiental Phase. Main period of magmatic intrusions. Approx. onset of the Pizol and Insubric Phases. End of the Lepontine Phase. Onset of the 'Eocene Revolution'. Onset of the 'Paleocene Revolution'.
It appears that events in the Alpine orogen and depositional cycles within the Alpine foredeep correlate with eustatic events and possible turning points in the regional stress regime (Fig. 10). Contemporaneity of depositional, eustatic and structural events in the foredeep-orogen domain is indicative of a c o m m o n tectonic cause, a hypothesis that requires further analysis with special attention to precise timing, the key to an accurate tectonostratigraphic evaluation of this model on orogen-foredeep relationships. This study presents evidence for a temporal connection between the orogen-foredeep system and the plate tectonics-eustasy interrelationships. Support for this conclusion is given by computer modelling by Sinclair et al. (1991). Without recourse to eustasy or complex viscoelastic models for the lithosphere, regional foreland unconformities (sequence boundaries) could be produced by coupling a thrust wedge advancing on a linear elastic foreland plate with erosion and deposition. Forebulge movements and attendant hiatuses (e.g., Base Burdigalian Unconformity) could be simulated by varying thrust-wedge parameters (rate, slope), while keeping the estimated effective elastic thickness of the foreland plate and sediment transport coefficient constant. Erosion associated with thickening and slowing-down of the forelandwards-moving thrust load caused onlap and offlap, respectively. While rapid advance of the orogenic wedge induced underfilling of the basin and slow propagation promoted overfilling. Modelling suggests that the orogenic activity is able to create foredeep stratigraphy without a link to
eustasy (and to elasticity of the foreland plate). This phenomenon is related to plate motions, the driving force of the Alpine orogeny. Exemplified by coeval foredeep and orogen events, a close kinematic relation of processes exerting a tectono-eustatic control, determined by episodic peaks of tectonic activity alternating with periods of relative tectonic quiescence, may be assumed to underlie the Tertiary evolution of both basin and orogen. These processes also affected the development of the North European Rift System (Sissingh, 1998). Glacio-eustasy inclusive, plate tectonics furnish an overall explanatory theory: acceleration and slowing-down of seafloor-spreading rates cause corresponding transgressional and regressional tendencies, as well as changes in intra-plate stress fields and the palaeoposition of continents.
Acknowledgements This paper is a contribution to the 'PeriTethys Programme': Mapping and modelling of the peritethyan epicratonic basins of Europe, northern Africa and the Middle East (directed by Dr. J. Dercourt, University of R and M. Curie, and Dr. M. Gaetani, University of Milan). Thanks are due to Prof. J.E. Meulenkamp and Dr. R.L.M. Vissers (Utrecht University), Drs. J. Mulock Houwer (Bilthoven) and, most of all, Dr. EA. Ziegler (University of Basel) for discussions and critical comments on an earlier version of the manuscript. This is publication number NSG 950903 of the Netherlands Research School of Sedimentary Geology.
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