Use of trace fossils in delineating sequence stratigraphic surfaces (Tertiary Venetian Basin, northeastern Italy)

P/UAU ELSEVIER

Palaeogeography,Palaeoclimatology,Palaeoecology120 (1996) 261-279

Use of trace fossils in delineating sequence stratigraphic surfaces (Tertiary Venetian Basin, northeastern Italy) G. Ghibaudo a, p. Grandesso b, F. Massari b, A. Uchman c a Dipartimento di Scienze della Terra, UniversiM di Torino, Via Accademia delle Scienze 5, 10123 Torino, Italy b Dipartimento di Geologia, Paleontologia e Geofisica, Universit?t di Padova, Via Giotto 1, 35137 Padova, Italy c Institutff~r Palaontologie, Universitat Warzburg, Pleicherwall 1, D-97070 Warzburg, Germany

Received 29 july 1994; revised and accepted 3 May 1995

Abstract

The lowermost third-order sequence of the Venetian Molasse Basin (Upper Chattian-Lower Aquitanian) in the Belluno syncline is composed of a transgressive systems tract, comprising a basal, condensed, glauconitic sand sheet deepening upwards into finer-grained offshore deposits, and a highstand systems tract mainly consisting of prograding, offshore mudstones capped by very fine sandstones representing offshore-transition deposits. An integrated sedimentological, ichnological and micropaleontological approach has allowed the recognition of the internal organization of the sequence, the subdivision into parasequences in absence of an obvious physical expression, and the characterization of significant discontinuity surfaces at both sequence and parasequence scales. The transgressive surface shows a complex geometry of bored and encrusted cavities produced by erosional undercutting and a system of neptunian dykes and sills, with hiatal shell concentrations as infill. Component parasequences in the texturally homogeneous transgressive sand sheet can be identified on the basis of the preservational state of trace fossil assemblages (softground versus firmground conditions) and subtle changes in grain-size and glauconite content. Omission surfaces bounding the parasequences are marked by increased glauconite content, and densely crowded, predominantly vertical or oblique, relatively large, very distinct, thin-walled or unwalled, and uncompacted burrows. The condensed section coincides with highest values in relative abundance of planktic foraminifers and in diversity of benthic foraminifers. Parasequence-bounding flooding surfaces in the muddy offshore portion of the highstand systems tract are recognized as firmground surfaces mantled by thin shell concentrations in a matrix of silty, highly burrowed, glauconitic sand.

1. Introduction

Siliciclastic shelfal parasequences in both transgressive and highstand systems tracts of thirdorder sequences are usually expressed by welldeveloped coarsening- and shallowing-upwards successions bounded by non-depositional flooding surfaces (Mitchum and Van Wagoner, 1991). However, in settings characterized by low rates of accommodation-space creation and sediment 0031-0182/96/$15.00 © 1996Elsevier ScienceB.V. All rightsreserved SSDI 0031-0182(95)00048-8

input, parasequence expression is commonly subtle to obscure (Kidwell, 1993). A number of recent studies indicate that ichnofabric and ichnofacies analysis may significantly contribute in characterizing and differentiating systems tracts within depositional sequences and in recognizing major discontinuity surfaces (e.g., Frey and Howard, 1990; Savrda, 1991; Snedden, 1991; MacEachern et al., 1992a,b). The use of trace fossils is particularly important when primary sedi-

262

G. Ghibaudo et al./Palaeogeography, Palaeoclimatology, Palaeoecology 120 (1996) 261-279

mentary structures are obliterated by bioturbation, and when other sedimentological features, for instance grain size changes, are poorly expressed. This study is an example of how ichnofacies analysis employed in conjunction with sedimentological and micropaleontological evidence can be used both to document the anatomy of a depositional sequence and to define boundaries and internal organization of parasequences in relatively homogeneous siliciclastic nearshore to shelf sediments.

2. Geological setting The Venetian foreland basin of northeastern Italy is bounded to the north by the eastern SouthAlpine chain and to the east by the Dinaric orogenic belt (Fig. l a). The basin developed under the influence of both evolving mountain belts. Two foreland basins of different ages and "polarities" overlap in time and space in the area. These are a Dinaric Paleogene to Lower Miocene basin with NW-trending axes of subsidence, and a SouthAlpine Middle Miocene to Quaternary basin with ENE-trending axes of subsidence. This is reflected by the geometry and ages of the respective clastic

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wedges, and a complex structural pattern resulting from an overlap of deformational effects (Massari et al., 1986). External Dinaric flysch deposits accumulated during the Late Paleocene-Middle Eocene in NW-trending troughs in the eastern SouthAlpine area (Doglioni and Bosellini, 1987) and were mainly deformed in the Late Eocene. The late-orogenic Dinaric molasse in the Venetian foreland basin was deposited from Late Chattian to Langhian. Subsidence during this stage reflected the latest thrusting and progressive outward shift of the Dinaric kinematic front toward the Adriatic foreland. The total thickness of the Dinaric molasse succession in the Adriatic basin, adjacent to the Dinaric thrust front, reached about 4000 m (Miljush, 1973). Although the subsidence pattern was mainly controlled by late Dinaric thrusts, the rapidly rising northern Alpine axial zone represented an important "external" source of terrigenous sediments during this stage, as indicated by the conspicuous contribution from Penninic and Austro-Alpine sources (Massari et al., 1986; Stefani, 1987). It may be inferred that, in addition to a Dinaric "polarity", there was an approximately N-S topographic gradient from the axial Alpine belt to the depositional area and that

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Fig. 1. Geological setting of the study area. (a) Interpretative sketch showing the evolution of the Venetian foreland basin, developing under the influenceof two thrust belts of differentage (slightlymodifiedfrom Doglioni, 1991). (b) Geological sketch map of the Venetian Basin with indication of the study area (box). (c) Detail of the study area; Mas section is asterisked. (d) Geological profile (transect line shown in Fig. lc); UT= Upper Triassic, J= Jurassic, K= Cretaceous, PE= Paleocene-Eocene, M= Upper Chattian-Miocene.

G. Ghibaudo et aL/Palaeogeography, Palaeoclimatology, Palaeoecology120 (1996) 261-279

transgressions advanced essentially northwards. This may have resulted in the development of an early network of fluvial drainage directed mainly southwards from the rising Alps to the Venetian foreland basin. The Early Miocene paleogeography of the Venetian Basin has been clearly outlined by Stefanini (1915), who described a "periAdriatic gulf", with depositional strike trending approximately E-W in.the Venetian area due to the influence of the Alpine relief discussed above, turning eastward to NW-SE due to the proximity of the Dinaric front. A second stage of molasse development began in the Serravallian a f t e r an abrupt increase in subsidence rate associated'with active thrusting in the eastern South-Alpine chain. The axis of the foredeep shifted rapidly in position and trend and, from the Serravallian onwards, an imbricate stack of overthrusts advanced rapidly towards the SSE in the eastern South-Alpine area and shed large volumes of clastic material into the Venetian Basin. The study area is located in the western part of the Venetian basin (Fig. lb). In this area, the Chattian-Burdigalian molasse deposits crop out in the axes of the NE-SW trending Alpago and Belluno asymmetric synclines, which are part of the South-Alpine thrust front. The local stratigraphy comprises a thick succession of Mesozoic carbonates and Eocene turbidites, unconformably overlain by the Chattian-Burdigalian siliciclastic molasse deposits. The northern flanks of both synclines were overthrust by Mesozoic carbonates along the Belluno thrust fault, while the synclines themselves overthrust southwards onto the Venetian prealpine foothills (Fig. lc). The litho- and chronostratigraphic setting of the Chattian-Burdigalian molasse deposits cropping out in the Alpago and Belluno areas is shown schematically in Fig. 2. The Tertiary molasse deposits addressed in this paper unconformably overlie the Belluno Flysch (Lower-Middle Eocene). They comprise a number of arenaceous and muddy lithostratigraphic units respectively referred to, from base to top, as Alpago Sandstone, Belluno Glauconitic Sandstone, Bastia Siltstone, Orzes Sandstone, Casoni Siltstone, Libano Sandstone and Bolago Marl (Fig. 3). The Chattian-Burdigalian elastic wedge shows a maxi-

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mum thickness in the Alpago area near the N-S trending Fadalto line (Fig. lc) and thins out progressively westward along the Dinaric foreland ramp. A westward increasing condensation of the stratigraphic record along the foreland ramp is also indicated by significant increase in the number and thickness of several glauconitic horizons contained in the Chattian-Burdigalian molasse succession.

3. Sequence stratigraphy of the Chattian-Lower Burdigalian deposits of the Alpago and Belluno synclines Exposures of the Chattian-Burdigalian molasse of the Venetian Basin provide clear evidence of relative sea-level fluctuations. The sequence stratigraphic interpretation of the Chattian-Lower Burdigalian deposits cropping out in the Alpago and Belluno synclines is shown in Fig. 3. The local succession comprises two Type 1 third-order depositional sequences, each represented by well-

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G. Ghibaudo et al./Palaeogeography, Palaeoclimatology, Palaeoecology 120 (1996) 261-279

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flooding surface; H S T = highstand systems tract; G= glauconite concentrations.

defined transgressive and highstand systems tracts (Ferioli et al., 1992, 1994). The lowstand deposits of both sequences are not developed in the outcrop area of the Venetian Basin where only transgressive and highstand shelf sediments crop out. The lowstand deposits of these sequences are likely to be preserved to the south in the subsurface of the Po plain and the northern Adriatic Sea floor. The Lower sequence (Chattian-Lower Aquitanian in age) unconformably overlies the Lower-Middle Eocene Belluno Flysch. The transgressive systems tract of the lower sequence com-

prises the Alpago Sandstone, the Belluno Glauconitic Sandstone and the basal part of Bastia Siltstone, while the highstand deposits correspond to the bulk of the Bastia Siltstone and to the Orzes Sandstones (pro parte). The sequence is thickest (300 m) in the Alpago syncline, corresponding to a local depocentre, and thins westwards (to about 100 m) in the Belluno syncline. The thickness reduction in the Belluno area is due partly to a deeper erosional incision related to the subsequent sequence boundary in this area, and partly to the external position of the area along the Dinaric

G. Ghibaudo et al./Palaeogeography, Palaeoclimatology, Palaeoecology 120 (1996) 261-279

foreland ramp with respect to the main depocentre located in the Alpago region (Fig. 3). The transgressive systems tract deposits of this sequence show significant differences in both facies associations and overall thickness between the Alpago depocentre and the Belluno area. The depocentre is characterized by a 55-m thick succession of aggrading to slightly backstepping, offshore to lower shoreface bioturbated parasequences (Alpago Sandstone), overlying a 20-cm thick, coarse, glauconitic basal transgressive lag. These parasequences are attributed to a retrograding shoreline possibly related to a lobate wavedominated deltaic system. Due to the relatively high sedimentation rate and available accommodation space that characterized this depocentre, parasequences are well-expressed and marked by the vertical stacking of well-defined coarsening- and shallowing-upwards successions bounded by clear flooding surfaces. In contrast, in the Belluno area, located outside the depocentre, the same transgressive systems tract consists of a basal 10-m thick, highly glauconitic, thoroughly bioturbated, apparently homogeneous sandstone interval (Belluno Glauconitic Sandstone), that grades upwards into 24 m of finer-grained, non-glauconitic sediments. The basal highly glauconitic sandstones define a condensed transgressive sand sheet in which parasequences do not have clear physical expression and cannot be detected solely on the basis of their lithological attributes. In both areas, the transgressive systems tract grades upwards into offshore mudstones, which in turn shallow into inner shelf siltstone and, locally (Alpago region), into hummocky cross-stratified lower shoreface sandstones. The shallowing upwards deposits are interpreted as the prograding highstand systems tract of the lower sequence. The upper sequence (Upper Aquitanian-Lower Burdigalian in age) also consists of a transgressive systems tract (Orzes Sandstone pro parte) and a highstand systems tract (Casoni Siltstone and Libano Sandstone) (Fig. 3). The sequence is bounded at the base by an unconformity whose geometry along the approximately E-W trending depositional strike defines two lowstand incised valleys separated by an intermediate interfluve area. The transgressive systems tract deposits con-

265

sist of two areally distinct, relatively thick, timeequivalent estuarine valley fill successions capped, locally, by a pronounced ravinement surface. These deposits are laterally replaced in the interfluve area by a thin (1.5 m) glauconitic, coarse transgressive lag resting directly on the sequence boundary (cf. Fig. 3). The highstand deposits of the upper sequence in both the Belluno and Alpago synclines are represented by a coarsening- and shallowing-upward succession of prodelta to delta front deposits (Casoni Siltstone and Libano Sandstone) that contain abundant macroscopic plant debris and a well-known fauna of odontocetes (Dal Piaz, 1916, 1977). These deposits are capped by a glauconitic transgressive lag marking the base of an overlying sequence. This paper deals with the sedimentology, ichnology, and foraminiferal paleoecology of the lower sequence, with special reference to the small-scale geometry and sedimentary features of the lower transgressive surface (sequence boundary) and criteria for parasequence recognition in relatively homogeneous, fine-grained, siliciclastic shelf deposits. The lower sequence is particularly well exposed along the Cordevole River, near the locality known as Ponte di Mas (Mas section) (Fig. lc). This section has been chosen for its stratigraphic completeness and its particularly well-exposed trace fossils.

4. Methods employed Both sedimentologic features and trace fossils were examined mainly on fresh and weathered surfaces in the field. Only selected trace fossils have been studied on polished slabs. Percentages of glauconite were determined by visual comparison charts. These observations were supplemented by a qualitative and quantitative analysis of the foraminiferal assemblages of the Bastia Siltstone. Palaeoecological interpretations based on foraminifers may be somewhat biased by the moderate to poor preservational state. Results concerning the relative percentage of suborders of benthic foraminifers are based on determinations at the species level using 300 or more individuals. This allowed the determination of Fischer ~ diversity index

266

G. Ghibaudoet al./Palaeogeography, Palaeoclimatology. Palaeoecology 120 (1996) 261-279

derived graphically with the Murray diagram (Murray, 1991).

5. The Upper Chattian-Lower Aquitanian sequence The Upper Chattian-Lower Aquitanian sequence in the Mas section (Belluno area) consists of a basal glauconitic sheet-like sandstone forming a prominent body (Belluno Glauconitic Sandstone) and an upper, essentially muddy, unit (Bastia Siltstone) (Figs. 3 and 17). These units are assigned to the transgressive (Belluno Glauconitic Sandstone and lower part of Bastia Siltstone) and highstand (upper part of Bastia Siltstone) systems tracts of the depositional sequence. 5.1. The transgressive systems tract

The transgressive systems tract in the Mas section is 34 m thick and comprises a sandy glauconitic unit (Belluno Glauconitic Sandstone) overlain by a silty unit (lower part of Bastia Siltstone) (Fig. 17). These deposits rest, with a well defined transgressive surface, on the Belluno Flysch of Ypresian age (Figs. 3 and 4). The latter consists of a basin-plain facies association of thin- to medium-bedded, laminated (Tce, Tde) carbonate sand-mud turbidite couplets alternating with hemipelagic muds. Although not evident on outcrop scale, the contact between the Belluno Flysch and

Belluno Glauconitic Sandstone corresponds, on a regional scale, to a slight angular unconformity. The geometric and sedimentologic characteristics of the transgressive surface and the litho- and biofacies of the overlying transgressive deposits will be discussed separately in the following sections. 5.1.1. The transgressive surface

The basal transgressive surface truncates older strata on a regional scale. In detail, the transgressive surface is an uneven erosional contact, the geometry and relief of which are determined by differential erosion of lithologies in the underlying Belluno Flysch turbidites (Fig. 5). The transgressive surface shows a complex erosional geometry characterized by up to 30-cm deep, irregularlyspaced, prominent cavities produced by erosional undercutting, and local small neptunian dykes and sills infilled with highly glauconitic and fossiliferous sandstone containing sparse, small chert and quartzite pebbles (Fig. 6). The uppermost turbidite sandstone bed (20 cm thick) is reduced to isolated remnants and is undercut at the expense of the less resistant subjacent hemipelagic marls; these

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Fig. 4. Belluno Glauconitic Sandstone in the Mas section. Basal transgressive contact on Ypresian turbidites of the Belhino Flysch visible on the left. Encircledman for scale.

Fig. 5. Detail of the transgressive surface at the base of the Belluno GlauconiticSandstone. The irregularityof the contact, with both conformableand unconformablerelationships to the host substrate, is highlighted by the relationships between the highly glauconitic transgressive lag and the uppermost layers of the Belluno Flysch. The truncation of the topmost turbidite layer (light-coloured) results from both bioerosion and longlasting mechanical attack by waves. Note the complexity of cavities infilled with the fossiliferouslag and the overhanging walls.

G. Ghibaudo et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 120 (1996) 261-279

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Fig. 6. Schematicsmall-scalegeometryand ichnofabricof the transgressivesurface at the base of the Belluno Glauconitic Sandstone. The neptunian dyke followsa small fault affectingthe flysch. are largely removed from the underside of the turbidite sandstone bed so as to leave an overhanging portion in relief (mushroom-like relief), which forms the r o o f of fiat cavities aligned parallel to the turbidite bed surfaces. Cavities produced by erosional undercutting are 15-30 cm deep and 1-3 m long, and their roofs, pavements and walls are bored and encrusted. Borings are present in both the resistant sandstone bed and in the marly bed. Borings in the sandstone bed are determined as Gastrochaenolites (clavate borings 1.5-2.5cm in diameter and 3-4 cm long) (Fig. 7) and Trypanites (up to 2 cm long and 3 mm in diameter); the marly bed is characterized by small hemispherical borings (2-6 mm in diameter and 2-3 m m deep). The encrusting fauna is restricted to the resistant sandstone bed and includes Ostrea, serpulids and balanids. Densely crowded Spongeliomorpha burrows occur at the bases of the glauconitic sandstone infiUs of the cavities (Fig. 8). This ichnogenus

Fig. 7. Borings of Gastrochaenolitesin the topmost sandstone layer of the Belluno Flysch, infilled with glauconitic sediment of the transgressive lag. Base of the Belluno Glauconitic Sandstone. Coin is 21 mm in diameter. occurs also at the base of the uppermost turbidite bed along the sandstone/marl interface. Neptunian dykes and sills (Fig. 6), which conduct glauconitic

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G. Ghibaudo et al./Palaeogeography, Palaeoclimatology, Palaeoecology 120 (1996) 261-2 79

Fig. 8. Spongeliomorpha at the base of the Belluno Glauconitic Sandstone. Coin is 24 m m in diameter.

sand into the underlying turbidite deposits, are up to 80 cm long and 5 cm thick. Similar to the undercut cavities, walls of dykes and sills are bored by small Gastrochaenolites borings and also burrowed by small Thalassinoides. Crowding and juxtaposition of these borings and burrows inside the walls of dykes and sills locally lead to total replacement of turbidite marls by glauconitic sandstone. The Thalassinoides burrows are uncompacted and display sharp boundaries, presumably reflecting firmground conditions. The borings were produced in cracks and crevices on hardened rock, and subsequently infilled with glauconitic sediment. Interestingly, they were produced within an unusual cryptic setting. As a whole, the trace fossil assemblages of the transgressive surface may be compared to the Trypanites and Glossifungites ichnofacies (Seilacher, 1977; Ekdale et al., 1984; Frey et al., 1990; Bromley and Asgaard, 1991 ). The Trypanites ichnofacies, in particular, is typical of shallowmarine, high-energy environments with hard substrates, and includes borings such as Trypanites, Gastrochaenolites and hemispherical pits. The Glossifungites ichnofacies (Ekdale et al., 1984) is represented by Spongeliornorpha and partly by Thalassinoides. Well-preserved scratch marks of the former ichnotaxon indicate firmground substrates (Ftirsich et al., 1981). In conclusion, the transgressive surface may be interpreted as a hardground/firmground complex.

5.1.2. The sandy glauconitic unit The Belluno Glauconitic Sandstone is 11 m thick (Fig. 4) and can be subdivided, from base to top, into three main facies: (1) shell-rich, gravelly glauconitic sandstones; (2) glauconitic sandstones characterized by alignment of bioclasts; (3) bioturbated glauconitic sandstones. The overall glauconitic content decreases from 95% in the basal part to 15% in the upper part. The high glauconite and fossil content testify to a prolonged period of low sedimentation rate and the condensed character of the transgressive sand sheet. The main lithologic and ichnologic features of Belluno Glauconitic Sandstone are shown in Fig. 9. Facies 1: Shell-rich, gravelly glauconitic sandstones. This facies directly overlies the transgressive surface and ranges in thickness from 15 to 35 cm depending on the depth of infilled scours and cavities in the underlying turbidite substrate. It consists of a richly bioclastic, medium-grained glauconitic sandstone with sparse, rounded pebbles of chert and quartzite, up to 4 cm long, and angular clasts of the underlying turbidite sandstone up to 8 cm long (Figs. 6 and 10). The matrix of the gravelly, bioclastic sand is a well-sorted, almost pure glaucarenitic sandstone (~95% glauconite). In the lower part of the unit, the bioclastic fraction, which represents as much as 40% of the rock, is densely packed and consists mainly of disarticulated and/or fragmented valves of thin-shelled bivalves associated with relatively common shark teeth. The upper 6-10 cm of this facies are dominated by epifaunal elements, mainly pectinids, associated with rare gastropods. Bivalves occur with both disarticulated and articulated/closed valves; the latter are partially abraded, but not bored or encrusted, and most of them are horizontal, with both concave-up and convex-up orientations (Fig. 10). Biogenic structures were not observed. This coarse-grained, shell-rich, glauconitic basal facies is interpreted as a transgressive lag developed at the base of a transgressive sand sheet. Transgressive lags typically represent considerable time and multiple reworking by various processes, chiefly storm waves and currents that lead to sediment bypassing and winnowing (Fiirsich and

G. Ghibaudo et al./Palaeogeography, Palaeoclimatology, Palaeoecology 120 (1996) 261-279 5

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Fig. 10. Facies 1 (lower half of the photo) and facies 2 (upper half of the photo) in the BellunoGlauconitic Sandstone. Facies 1 exhibits tightly packed shells mixed with exoticpebbles (chert and quartz, visible at the right corner) and represents an amalgamation of storm layers in connection with very low sedimentation rate. The overlying facies 2, which is characterized by shell pavements that commonly include articulated bivalves (mostly pectinids), is interpreted as resulting from isolated storm events (event concentrations in the sense of Kidwell).

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Fig. 9. Schematiclog of Belluno Glauconitic Sandstone showing cyclicitydefinedby vertical changes in ichnofossildistribution, textural features, and glauconiticcontent. Oschmann, 1993; Kidwell, 1993). The irregularity of the transgressive surface, including bored and encrusted cavities, testifies to a long period of omission and erosion. The overlying skeletal concentration may be defined as a hiatal concentration in the sense of Kidwell (1991, 1993), which originated from amalgamation of events during a period of slow net sedimentation. Owing to waterdepth and ecological/taphonomic changes over the period o f condensation, hiatal concentrations comprise assemblages from more than one environment (Kidwell, 1993). The upward change of the benthic community from infauna- to epifaunadominated may reflect shell production and accu-

mulation exceeding the rate of sedimentation, which may lead to successive colonization by epifaunal suspension feeders and a restriction of the number of deposit feeders (taphonomic feedback). This is expected to be a frequent case in somewhat turbulent shelf areas during the transgressive stage, where the wave/current pattern and reduced sediment supply prevent a continuous and high sedimentation rate (Fi~rsich, 1978).

Facies 2: Glauconitic sandstones with aligned bioclasts. This facies consists of a 68-cm thick interval of medium-grained glauconitic sandstones (up to 95% glauconitic grains in the lower part, decreasing to about 80% in the upper part) characterized by the recurrence of shell pavements (Figs. 6 and 10). The glauconitic sandstones are well sorted and homogenized by bioturbation [mostly biodeformational structures and rare Thalassinoides-Ophiomorpha transitional forms indicated below as ?Thalassinoides B (see taxonomic comments in the section on facies 3) up to 1 cm in diameter], displaying the highest index of bioturbation (ii5) according to the scale of Droser and Bottjer (1989) and Bottjer and Droser (1991). The shell pavements are 5-15 cm apart and

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mostly consist of both disarticulated and articulated pectinids associated with rare scaphopods (Dentalium) and gastropods. Epifauna predominates over infauna. Valves are mostly subhorizontal, both concave-up and concave-down, although some are randomly oriented. Such shell pavements are interpreted as event concentrations (in the sense of Kidwell, 1991, 1993), produced by brief episodes of hardpart concentration, probably as a result of storms. The vertical transition from hiatal concentration (facies 1) to bioclastic eventconcentration deposits may reflect a slight increase in the rate of sediment supply sufficient to preserve discrete event beds following deposition of the basal transgressive lag. Sedimentation rate, however, remained extremely low, as evidenced by the very high glauconite content of the sandy matrix. Facies 3." Bioturbated glauconitic sandstone. Facies 3 is 9.5 m thick and consists of bioturbated glauconitic sandstones characterized overall by an upward decrease in glauconite content (range 50-15%), and a slight fining-upward trend. These trends suggest a progressive increase in sedimentation rate, as well as a progressive deepening. The sediments are totally bioturbated and still display the highest index of bioturbation (ii5). Allochthonous plant debris sparsely occur and may indicate influx pulses of wood during the transgression (e.g., Savrda et al., 1993). Rare remains of vertebrates (vertebrae and ribs of odontocetes) are also typical of this condensed facies. Before treating the ichnofossil assemblages, some remarks are needed concerning the trace fossils of the Thalassinoides group. Our ichnofossil assemblages include burrows or burrow systems which show hybrid characters, and particularly forms which may be regarded as transitional between the ichnogenera Thalassinoides and Ophiomorpha. The problem is not yet taxonomically resolved by an appropriate terminology, so we have chosen an informal designation: relatively large Thalassinoides are Thalassinoides A, and chiefly vertical or oblique, cylindrical, rarely branched, endichnial burrows 0.7-2.5 cm in diameter are ?Thalassinoides B. Unwalled, thin- and thick-walled forms occur in the same population of burrows of this type. Occasionally, the exterior of the wailed burrows is partially covered with

poorly preserved pelletoid knobs. We could not find convincing criteria for taxonomic separation of the forms lumped in ?Thalassinoides B. The vertical orientation and rare peloidal knobs on the exterior of some burrows resemble Ophiomorpha. On the other hand, the smooth burrows resemble Thalassinoides, but they are much more rarely branched and smaller than Thalassinoides A, which is the typical form of this ichnogenus. Thalassinoides occurs in different types of substrates of different consistency and the presence/ absence of its wall is controlled by the consistency of the sediment. Probably the walled burrows were produced in softgrounds and the unwalled burrows in firmgrounds. It can be expected that in firmground substrate, tracemakers do not need to protect their burrows against collapse by construction of the wall. The producers of this form were probably crustaceans, which are known to be prone to very strange adaptations (Bromley, 1990), and to be able to change their habit of life from deposit feeders to suspension feeders. The morphology of their burrows may change strongly in different substrates over short distances and display features of different ichnotaxa, including Thalassinoides-Ophiomorpha transitional forms (e.g. Kern and Warme, 1974). The facies 3 appears relatively homogeneous at first sight. Detailed analysis, however, reveals that it is organized into small-scale, shallowing-upward cycles bounded by flooding surfaces. The evidence of this internal cyclicity is rather subtle and essentially based on the character of ichnofabrics and covariations in glauconite content and grain size (Fig. 9). Individual cycles range from 1.6 to 5.2 m thick and exhibit a slight upward increase in grain size (from silty fine sandstones at the base to finemedium sandstone at the top) and glauconite content, although small-scale fluctuations also occur (Fig. 9). Cycles are characterized by a welldeveloped Cruziana ichnofacies indicating openshelf conditions ranging from below fair-weather wave base to deeper, quieter offshore waters (Ekdale et al., 1984). The lower and middle parts of individual cycles are homogeneous, densely bioturbated with a mottled background texture. They occasionally con-

G. Ghibaudo et al./Palaeogeography, Palaeoclimatology, Palaeoecology 120 (1996) 261-279

tain rare and randomly dispersed small, thinshelled cardiids, ostreids and pectinids, fragments of thicker-shelled bivalves, ahermatypic corals, echinoid spines, scaphopods (Dentalium), gastropods, and rare dispersed small (up to 1 cm) pebbles. Discrete trace fossils are poorly recognizable. They are usually compacted burrows of small size, represented by Teichichnus, rare vertical/subvertical ?Thalassinoides B (1 cm in diameter), occasional Macaronichnus segregatis with 1.5 mm thick walls enriched in glauconitic grains (Fig. 11 ), rare Thalassinoides A and Planolites (the latter 2 mm in diameter), and very rare Terebellina (3-5 mm in diameter) (Fig. 12). Thalassinoides A, ?Thalassinoides B and Teichichnus are locally

271

reworked by small secondary burrows of Planolites. Some burrows of Thalassinoides A and ?Thalassinoides B are superimposed in the vertical plane and form the teichichnoid structure. Very similar Thalassinoides was illustrated by Frey and Seilacher (1980). With the transition from the lower to the upper part of each cycle, trace fossils become more distinct, only slightly compacted, larger and more vertical. They consist of increasingly abundant Macaronichnus segregatis, vertical, oblique and horizontal backfilled ?Thalassinoides B (up to 2 cm in diameter) (Fig. 13), and large Teichichnus (up to 14 cm wide and 4 cm high). The upper part of the cycles is characterized by a few generations of crowded and juxtaposed burrows which may be interpreted as representing a pre-omission suite. These consist of large Teichichnus, ?Thalassinoides B, and large Thalassinoides A (up to 5 cm in diameter and 20 cm in length). Crosscutting relationships are common: ?Thalassinoides B and Teichichnus are crosscut by Planolites (Fig. 14); Thalassinoides A is crosscut by Teichichnus, ?Thalassinoides B, Planolites, Phycosiphon incertum (Figs. 15 A,B). Locally up to three generations of mutually cross-

Fig. 11. Macaroniehnus segregatis (arrow) in facies 3 of the Belluno Glauconitic Sandstone. Note concentration of glauconite, heavy minerals and biotite flakes near burrow walls. Coin is 21 mm in diameter.

Fig. 12. Terebellina (arrowed) in facies 3 of the Belluno Glauconitic Sandstone. Scale bar subdivided in cm.

Fig. 13. Horizon of pectinid shells in facies 3 of the Belluno Glauconitic Sandstone. ?Thalassinoides B (7") displays meniscate backfill. Coin is 24 mm in diameter.

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G. Ghibaudo et al./Palaeogeography, Palaeoclimatology, Palaeoecology 120 (1996) 261-279

Fig. 14. Teichichnus (Te) and Planolites (Pl) in facies 3 of the Belluno Glauconitic Sandstone. Scale bar 15 cm long.

cutting Thalassinoides A may be recognized. Sediments displaying such large and welldeveloped burrows in the upper part of the cycles are systematically richer in glauconite, which is mostly concentrated in the second- or thirdgeneration burrows. Moreover macrofossils, if present, tend to be concentrated into shell pavements. They are mostly represented by horizontally oriented, unfragmented, disarticulated or articulated valves of pectinids, that are locally associated with other bivalves, gastropods, ahermatypic corals and scaphopods (Dentalium). Trace fossils at the top of cycles are densely crowded. Pre-omission, omission, and postomission suites can be recognized. The preomission suite is represented by burrows of

Planolites, Phycosiphon incertum, Macharonichnus segregatis, Teichichnus, thick-walled Thalassinoides A and ?Thalassinoides B. These burrows were produced in softground. The omission suite is represented by thin-walled and unwalled ?Thalassinoides B presumably produced in progressively stiffer sediment. The post-omission suite comprises thin- and thick-walled Thalassinoides A and ?Thalassinoides B, whose unwalled form also penetrates into the firm substrate. Interpretation of facies 3. Most of the recognized trace fossils belong to the Cruziana ichnofacies. Their dimensions and preservation vary upwards within individual cycles. Randomly distributed, poorly preserved and compacted members of this ichnofacies occur throughout each cycle. However,

A

B Fig. 15. Large uncompacted burrows of the pre-omission suite, in totally bioturbated background of glauconitic sandstone, infilled with overlying lighter sediment. Facies 3, Belluno Glauconitic Sandstone. A. Thalassinoides A (Th), Planolites (PI) and Macaronichnus segregatis (Ms). Coin is 24 mm in diameter. B. Thalassinoides A (Th) and Planolites (PI), produced after Thalassinoides A. Coin is 21 mm in diameter.

in the topmost part of cycles associated with omission, they are overprinted by larger, more vertical, more distinctive and less compacted burrows. These burrows should be discussed in the context of the Glossifungites ichnofacies because they are referred to firmground. However, the Glossifungites ichnofacies is generally described as consisting of U- or tear-shaped pseudo-borings, densely crowded branching dwelling burrows, and protrusive spreiten in some burrows, which may occur in different proportions (Ekdale et al., 1984). This characteristic ichnofacies does not strictly

G. Ghibaudoet al./Palaeogeography, Palaeoclimatology, Palaeoecology 120 (1996)261-279

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Fig. 16. Exampleof post-omissionsuite: ?Thalassinoides B of an earlier generation, infilled with glauconitic sediment, is cross-cut by ?Thalassinoides B of a later generation, infdled with post-omissionlighter,non-glauconiticsediment.Facies3, Belluno Glauconitic Sandstone. Base of parasequence. Glauconitic sediment was probably advected upwards from the top of underlyingparasequence.Coinis 24 mmin diameter. occur in our omission suite. ?Thalassinoides B is densely crowded, but displays rare branchings. In any case, common occurrence of this form without wall suggests cohesive substrates. In conclusion, although the examined trace fossils fit better in the Cruziana ichnofacies, the evidence of firm substrate below the omission surfaces indicates an ichnofacies which may be regarded as transitional between the Cruziana and Glossifungites ichnofacies. The concentration of glauconite in uppermost parts of cycles is thought to be produced during the omission stage, as a result of sediment starvation. The glauconitic sediment was subsequently piped downwards through large burrows extending several decimetres below the surface. Postomission burrows occurring below the omission surface can be recognized by their infill of lightgrey, non-glauconitic sediment which was likely the first sediment covering the omission surface. Burrows with glauconitic infill may also occur above the omission surface (Fig. 16), and reflect post-omission upward advection of glauconitic sediment from below the omission surface. Trace fossil generations of the lower and middle parts of the cycles are produced in a soft substrate and consequently are poorly preserved, compacted, and their margins are indistinct. The slow-down and halt in sedimentation at the top of cycles

273

(omission stage) resulted in an enhanced cohesiveness of the substrate and, occasionally, in development of a firmground as a result of decreased saturation by water. Increased cohesiveness of sediment probably eliminated smaller soft-bottom burrowers, such as the producers of Planolites and Macaronichnus. Moreover, due to the more cohesive substrate, horizontal reworking of sediment became more difficult and less efficient, thereby promoting vertical burrowing. Thalassinoides A produced in the pre-omission stage was commonly crosscut by the unwalled form of ?Thalassinoides B, produced in firmground cohesive substrate. In the condensed sediments, nutrients are quickly exploited and, with increased cohesiveness, sediments become more difficult to rework. In such conditions crustacean producers of ?Thalassinoides B tend to produce more vertical burrows. They might also change their mode of life and become suspension feeders. Suspension-feeding activity of callianassid crustaceans was discussed by Bromley (1990). The burrows in the omission suite are generally unwalled. The behaviour of the ?Thalassinoides B producers is particularly characteristic: in cohesive substrates they do not reinforce the walls of their burrows and their tunnels are only slightly compacted. Probably, the opportunistic ?Thalassinoides B producers were partly suspension feeding during the omission stage. During this stage the burrows at the top of cycles, especially ?Thalassinoides B, acted as traps for glauconite. Reconstruction of tiering patterns in the lower to middle parts of the cycles is very difficult. In these parts of cycles, which were characterized by slow but continuous aggradation and persistent softground conditions, most of the shallower burrow systems of Thalassinoides A and the upper parts of the vertical components of burrow systems of ?Thalassinoides B were probably obliterated by deep, smaller burrows as they shifted upwards with sediment accretion. In the uppermost parts of cycles, on the contrary, the tiering pattern was probably preserved, i.e. not obliterated by the shifting tiers, in response to the omission stage and associated firmground production. Hence, the larger burrows of later generations and shallow parts of the vertical burrows had a chance to be preserved. This was probably an additional factor

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G. Ghibaudo et aL /Palaeogeography, Palaeoelimatology, Palaeoecology 120 (1996) 261-279 I

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G. Ghibaudo et al./Palaeogeography, Palaeoclimatology, Palaeoecology 120 (1996) 261-279

influencing the observed predominance of larger and vertical forms in the uppermost part of cycles. The reestablishment of soft substrates with the resumption of sedimentation above the omission surface (which also represents the flooding surface of the following cycle) is indicated by the replacement of suspension feeders by deposit feeders producing small, dominantly horizontal, compacted burrows. The cycles recognized within the Belluno Glauconitic Sandstone may be interpreted as backstepping parasequences of a relatively condensed transgressive sand sheet. On the basis of the general fine-grained lithology, the thorough bioturbation with complete obliteration of primary sedimentary structures, and the presence of the Cruziana ichnofacies, these parasequences are inferred to represent offshore-transition successions. Their recognition as parasequences in the sense of Van Wagoner et al. (1990) is justified by their internal structure, because they develop as shallowing upward cycles above flooding surfaces. As discussed above, individual parasequences cannot be readily differentiated on purely lithological basis. In such homogeneous deposits trace fossil assemblages and their preservation are particularly useful for recognizing discontinuity surfaces and internal organization of parasequences.

5.1.3. The silty unit A schematic log of the Bastia Siltstone, which shows trace fossil distribution, abundance of foraminiferal groups and Fischer • diversity index, is presented in Fig. 17. The lower Bastia Siltstone (24 m thick) is thought to represent the remaining portion of the transgressive systems tract and consists of bioturbated, predominantly muddy sediments showing an overall fining upwards trend (fig. 17). Relative to the underlying transgressive sand sheet, the glauconite content of the lower Bastia Siltstone decreases drastically to a few scattered grains. Parasequences may be still identified in this portion of the transgressive systems tract. However, their number, thickness and stacking pattern cannot be precisely identified due to their very fine-grained lithology and locally poor expo-

275

sure. They are represented by subtle successions some metres thick, coarsening upwards from muddy, bioturbated, coarse silts to very finegrained sandstones. The gradual upward lithologic change in individual cycles, moreover, is coincident with changes in trace fossil assemblages which include different ichnotaxa of the Cruziana ichnofacies. The lower muddy portion of cycles are characterized by poorly preserved, small Teichichnus, Planolites and Phycosiphon incertum, whereas slightly coarser-grained upper parts are characterized by large Teichichnus, Thalassinoides A, ?Thalassinoides B and rare Rosselia. This vertical change may reflect an upward transition from calm, deeper offshore conditions to more agitated and possibly shallower conditions characterized by relatively unstable substrates. These bioturbated, fine-grained, subtle coarsening-upwards successions are interpreted to represent the distal, offshore equivalents of the better-defined, coarsergrained shelf parasequences developed in the lower part of the transgressive systems tract.

5.2. The maximum flooding surface The Bastia Siltstone as a whole displays an initially slightly fining-upward trend (lower Bastia Siltstone), which then changes to a slightly coarsening-upwards trend (upper Bastia Siltstone). The horizon of trend reversal is interpreted to be the maximum flooding surface. Indeed, at the top of the inferred transgressive systems tract, a 50-cmthick bed of fine-grained, completely bioturbated (ii5), glauconitic sandstone crops out (Fig. 17). This bed is characterized by about 20% glauconite, rare quartz granules and a relatively abundant macrofauna. Macrofossils include articulated, thin-shelled, infaunal elements, disarticulated pectinid valves, scaphopods (Dentalium), small gastropods, fish teeth, sparse ahermatypic corals and plant debris. Trace fossil assemblage is diverse and includes large, uncompacted Thalassinoides A and ?Thalassinoides B abundant Planolites, and less common Teichichnus and Phycosiphon incertum. This assemblage is thought to represent a preomission suite. The omission suite, suggesting firmground conditions, is typically represented by unwalled ?Thalassinoides B, extending downward

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into the underlying muddy silts and infilled with highly glauconitic sands. Characteristics of this bed (high glauconite content, sparse siliciclastic granules, omission burrows, shell concentrations, fish teeth) indicate a prolonged period of very slow sedimentation or non-deposition combined with possible winnowing, glauconite formation, intense burrowing and increased cohesiveness of the sediment (firmground?). The preservation of small plant debris and increased abundance of Phycosiphon incertum and Planolites suggest lowered oxygenation below the sediment interface. These characteristics, together with the mid-cycle position of this unit, match the expected attributes of maximum flooding surfaces as defined by Loutit et al. (1988). This unit is therefore interpreted to represent the maximum flooding surface of the sequence, separating the underlying overall fining-upwards transgressive deposits from the overlying coarseningupwards highstand deposits. This interpretation is supported by both ichnofossil associations and patterns of foraminiferal abundance and diversity within the muddy silt deposits immediately underlying and overlying the candidate maximum flooding surface (Fig. 17). Ichnofossils in the immediately underlying deposits consist almost exclusively of abundant, small Teichiehnus and rare ?Thalassinoides B (Cruziana ichnofacies), while the immediately overlying deposits are characterized by Phycosiphon incertum and Planolites (Zoophycos ichnofacies). Both ichnofacies suggest a soft substrate and lowenergy, relatively deep-water conditions within an onshore-offshore profile. The Zoophycos ichnofacies, in particular, may indicate a poorly oxygenated environment (Ekdale et al., 1984). The assemblage of benthic foraminifers in these fine-grained mid-cycle deposits is characterized by the maximum abundances of Triplasia sp. (3.5%), Karreriella siphonella exilis (5.0%), Uvigerina eocaena-mexicana group (11.5%), U. galloway (4.5%), Cibicidoides pachyderma (5.0%) and Anomalinoides alazanensis (4.5%). A slight decrease in the degree of bottom oxygenation is suggested by the relative abundance of Uvigerina (28%) and of members of the Bolivina-Brizalina group (17%), which are known to abound in

poorly oxygenated environments (Borsetti et al., 1986; Van der Zwaan and Jorissen, 1991). In addition, mid-sequence deposits exhibit the highest values in relative abundance of planktic foraminifers and in diversity of benthic foraminifers (Fig. 17). These characteristics are considered typical of condensed sections (Loutit et al., 1988). The interval corresponds to maximum depths, with an indication of outer-shelf depositional environments (cfr. Wright, 1978). In conclusion, the identification of this interval as a condensed section is clearly supported by the foraminiferal assemblages, maximum values of Fischer a diversity index, as well as sedimentological and ichnofabric data.

5.3. The highstand systems tract The inferred highstand systems tract of the sequence is 50 m thick and is represented by the upper part of the Bastia Siltstone. This interval shows an overall upward coarsening trend expressed by the transition from a lower silty part to a thin interval of fine-grained sandstones. The silty deposits are homogeneous due to thorough bioturbation, and contain sparse, small, thinshelled bivalves and gastropods. Trace fossils, mainly represented by Phyeosiphon incertum and Planolites, are typical of the Zoophycos ichnofacies. They suggest relatively deep water, softground, low-energy offshore conditions with possible low oxygenation (Ekdale et al., 1984). Shallowing-upward parasequences are not recognizable in these uniformly fine-grained offshore deposits. However, they cannot be ruled out because the presence of covered intervals prevents continuous, detailed field observations. These deposits show at least three possible discontinuity surfaces, 4-14 m apart, in the lower, better exposed portion. Such surfaces are expressed by thin (<20cm) horizons of glauconitic coarse silt or very fine sand (2-3% glauconite). They are intensely bioturbated (Phycosiphon incertum, Planolites, Thalassinoides A and ?Thalassinoides B interpreted as pre-omission suite) and are characterized by shell pavements consisting of small, thin-shelled, articulated infaunal bivalves, disarticulated pectinids, rare scaphopods (Dentalium) and

G. Ghibaudo et al./Palaeogeography, Palaeoclimatology, Palaeoecology 120 (1996) 261-279

small gastropods, locally accompanied by fish teeth and plant debris. ?Thalassinoides B burrows are piped downwards several centimeters in the underlying finer-grained deposits, and typically infilled with glauconite-rich granular material. Such surfaces are interpreted as omission surfaces and considered to be the physical expression of flooding surfaces in a distal offshore environment, punctuating the highstand progradation. Parasequences in such a distal offshore setting would only be recorded by flooding events without clear expression of intervening shallowing-upwards successions. Omission stages, however, could produce, via sufficiently long periods of sediment starvation, faunal concentrations, glauconite, and intensely burrowed horizons with evidence of increased cohesiveness of sediments (firmgrounds?). The shallowing trend of the homogeneous, muddy portion of the highstand systems tract is clearly recorded by the upward decrease in the relative abundance of planktic foraminifers and in the ~ diversity index (Fig. 17), although benthic forms specifically indicative of inner shelf setting are missing. The regressive trend is truncated by the upper sequence boundary, corresponding to the erosional base of an estuarine valley fill (Fig. 3). The coarser-grained deposits characterizing the upper part of the highstand systems tract are 7.5 m thick and consist of a faintly layered alternation of thin- to medium-bedded (<20 cm), totally bioturbated, homogeneous, very fine sand and silt. Recognizable ichnotaxa include Teichichnus, Phycosiphon incertum and Planolites referable to the Cruziana ichnofacies. The assemblage indicates slightly higher-energy and higher oxygenation at the sea floor with respect to the underlying finergrained deposits. Coarser-grained texture, foraminiferal content, and ichnofacies all indicate a slight shallowing-upward trend with respect to the underlying offshore sediments. The bioturbated, thin-bedded sand-silt couplets are thought to have been deposited in a storm-dominated offshoretransition shelf environment. They are thought to represent the distal equivalents of well-developed, lower shoreface hummocky cross-stratified deposits developed in the upper part of the same pro-

277

grading highstand systems tract several kilometers to the east in the Alpago area (Fig. 3).

6. Conclusions

(1) The lowermost third-order sequence of the Venetian molasse basin (Upper Chattian-Lower Aquitanian) consists of a transgressive systems tract that comprises a basal, condensed, glauconitic sand sheet deepening upwards into finer-grained offshore deposits, and a highstand systems tract mainly consisting of prograding, fine-grained offshore to offshore-transition deposits. (2) The transgressive surface coincides with the sequence boundary and shows a complex geometry of bored and encrusted cavities produced by erosional undercutting and a system of neptunian dykes and sills. The Trypanites and Glossifungites ichnofacies are well developed, indicating that the transgressive surface may be interpreted as a hardground/firmground complex. This surface is overlain by a glaucarenitic, bioclastic, transgressive lag, representing a hiatal shell concentration, passing upwards into a homogeneous, thoroughly bioturbated, condensed glauconitic sand sheet. (3) The glauconitic, transgressive sand sheet represents a condensed deposit in an offshoretransition setting, and is characterized by thorough bioturbation and quite uniform texture. Although component parasequences have no obvious physical expression, they can be identified on the basis of the preservational state of trace fossil assemblages (softground versus firmground conditions) and to a lesser extent by slight verticalchanges in grain size and glauconite content. Discontinuity surfaces bounding the parasequences are marked by an increased amount of glauconite. Enhanced consistency of the substrate and sediment starvation during the omission stage are indicated by a distinctive suite of densely crowded burrows, which are predominantly vertical or oblique, relatively large, very distinct, thin-walled or unwalled, and uncompacted. (4) The maximum flooding surface corresponds to a glauconite-rich omission surface with sparse macrofossils, rare fish teeth and sparse siliciclastic granules, which indicate a prolonged period of

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slow sedimentation or non-deposition with seafloor winnowing and increase in sediment cohesiveness. The condensed section, including the maximum flooding surface, coincides with highest values in relative abundance of planktie foraminifers and in diversity of benthic foraminifers. (5) Parasequences in homogeneous, highstand, muddy offshore deposits are only recorded by flooding surfaces expressed by thin, glauconiterich, omission horizons. No clear expression of intervening coarsening- and shallowing-upward successions is recognized. (6) This study has shown that trace fossils may be one of the most significant tools for recognizing otherwise poorly expressed sequence-stratigraphic surfaces. In addition, their use as indicators of palaeobathymetric trends, energy of the environment, and substrate properties proved very valuable for differentiating systems tracts within depositional sequences.

Acknowledgments We are grateful to Franz Fiirsich, Ru Smith, Finn Surlyk for careful review of the manuscript. We are greatly indebted to Charles Savrda for his patient, extremely accurate and critical review, allowing remarkable improvement of the manuscript. F. Todesco is acknowledged for preparation of drawings, and C. Brogiato for care in execution of photographs. Field and laboratory work was financially supported by the Centro di Studio C.N.R. per la Geodinamica Alpina (Padova), Italian Ministry of Education (M.U.R.S.T.) and C.N.R.-C.S. Geodinamica Catene Collisionali, Torino.

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