Upper Triassic, Southern Alps)

Sedimentary Geology 175 (2005) 439 – 457 www.elsevier.com/locate/sedgeo

Research paper

Accommodation/sedimentation development and massive early marine cementation: Latemar vs. Concarena (Middle/Upper Triassic, Southern Alps) Michael SeelingT, Axel Emmerich, Thilo Bechsta¨dt, Rainer Zu¨hlke Geologisch-Pala¨ontologisches Institut, Ruprecht-Karls-Universita¨t Heidelberg, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany Received 26 April 2004; received in revised form 15 September 2004; accepted 29 September 2004

Abstract Massive early marine cementation (MEC) is a major diagenetic feature of some carbonate platforms. The comparison of two Triassic buildups of similar size and preservation but contrasting degree of MEC (Latemar, Dolomites, and Concarena, Lombardic Alps) allows for the investigation of differential cementation on carbonate platforms. The assessment of the response of platform cementation to changes in accommodation/sedimentation led to the identification of fundamental boundary conditions for MEC: (1) the time interval available, i.e. a low rate of creation in accommodation space and (2) low carbonate production. Other important factors are: (3) margin topography (e.g. walled reefs) and (4) effective fluid flow (e.g. wave energy, open and connected cavities provided by rigid frameworks). The Latemar–mainly aggrading and locally retrograding–lacks MEC due to a high AV/SV ratio (i.e. creation/destruction of accommodation in time, AV=dA/dt, vs. changes in sediment supply in time, SV=dS/dt) and a low relief of the reefal margin. In contrast, the Concarena–in the beginning slowly and later rapidly prograding–indicates MEC owing to a low AV/SV ratio and a distinct topography of the reef. Particular features of the Concarena are cement arrangements of lenticular shape and considerable size in the back-reef domain (up to 3 m in diameter and 2 m in height). These arrangements consist of botryoids and isopachous crusts of radiaxial fibrous calcite cements. They represent one of the main components of the platform margin at Concarena. Cathodoluminescence analyses of cements from both platforms quantifies the influence of shallow and deep burial diagenesis and shows that the majority (60–90%) of all cements are of early marine origin. D 2005 Elsevier B.V. All rights reserved. Keywords: Sequence stratigraphy; Cementation; Dolomites; Lombardic Alps; Middle Triassic; Upper Triassic

1. Introduction T Corresponding author. Tel.: +49 6221 544831; fax: +49 6221 545503. E-mail address: [email protected] (M. Seeling). 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2004.09.004

For more than a hundred years, the Southern Alps have been a key area for the study of Triassic carbonate platform development (e.g. Bosellini,

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Despite their similar high degree of preservation and absence of dolomitisation, the Ladinian/Carnian platforms of the Lombardic Alps and especially the Concarena are far less studied than those of the Dolomites. Because of their similar size but contrasting architecture and cementation, Latemar and Concarena are ideally suited for an integrated approach to investigate the controls on massive early marine cementation. Hence, the aims of this study are: (1) to compare the architecture of the Concarena and the Latemar, (2) to investigate zonation of lithofacies along transects through the margins, (3) to assess the link between massive early marine cementation and accommodation/sedimentation development.

1984; Biddle et al., 1992; Jadoul et al., 1992; and references therein). Much research has been carried out on the cementation and diagenesis of Triassic carbonate platforms in this area (e.g. Stoppani, 1858; Assereto and Kendall, 1977; Mutti, 1994). Several studies underline the importance of massive early marine cementation at the upper slope and margin of some Middle to Upper Triassic carbonate platforms (e.g. Grigna: Frisia-Bruni et al., 1989; Marmolada: Russo et al., 2000; Sella: Keim and Schlager, 2001). The precipitation of these early marine cements plays a key role for the stabilisation of the buildups. Furthermore, it contributes a significant portion to the carbonate production (Blendinger, 1994). However, the degree of early marine cementation from one platform to another is highly variable. Other Middle/ Upper Triassic carbonate platforms with minor early marine diagenetic features are among others: Dosso dei Morti (Lombardic Alps: Unland, 1975) and Latemar (Dolomites: Emmerich et al., in press). These differences in early marine cementation of Triassic carbonate platforms have not yet been investigated. In order to shed some light on differential cementation, two carbonate platforms in the Southern Alps were chosen as case studies. In the Dolomites, the Anisian/Ladinian Latemar records platform architecture in primary lithofacies. SE-Asia

The two study areas–Latemar and Concarena–are located in the Southern Alps of Northern Italy (Fig. 1). According to Dercourt et al. (2000, and references therein), the Southern Alps are situated at latitudes of 15–208N on the western termination of the Tethys from Anisian to Carnian (Fig. 1a). Global climate models of Triassic Pangaea suggest strong monsoonal circulation patterns for the western termination of the

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Fig. 1. (a) Simplified palaeogeographic map of the western Tethys during the Middle Triassic (after Dercourt et al., 2000). Abbreviations: LA, Lombardic Alps; Dol, Dolomites; BM, Bohemian Massif; PO, Pindos-Olonos trough; QT, Qiantang-Terrane. Legend of map units/symbols in the upper left corner. (b) Schematic map illustrating platform–basin relationships of the western Dolomites during the Late Anisian/Early Ladinian and of the Lombardic Alps during the Ladinian to Early Carnian. Legend of lithostratigraphic units in the upper right corner, influx of siliciclastic/volcaniclastic turbidites into pelagic basins marked by large arrows. Inset: generalised map of the Southern Alps with an indication of major tectonic lines, asterisks mark the areas of study.

M. Seeling et al. / Sedimentary Geology 175 (2005) 439–457

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Fig. 2. (a) Schematic geological map of the Latemar; legend of map units in the lower right corner; numbers 1 to 3 refer to outcrops described in this study (1: Eggentaler Horn, 2: Erzlahn, 3: Erzlahnscharte). (b) Schematic geological map of the Concarena; legend of map units in the lower right corner; numbers 1 to 4 refer to outcrops described in this study (1: Cima della Bacchetta, 2: Valle del Baione, 3: La Tavola, 4: Pratotondo).

Tethys (Kutzbach and Gallimore, 1989). This area represents a highly dismembered, passive continental margin with transpressive–transtensive tectonics

(Blendinger, 1985; Doglioni, 1987) and mixed carbonate-clastic sedimentation from the Anisian to Carnian (Fig. 1b).

Litho- and sequence stratigraphy (schematic) Platform

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Base of Anisian: Lehrmann et al. (2002) 247.2 ±0.4

Fig. 3. Middle Triassic litho- and sequence stratigraphic succession and platform-to-basin relationships of the two study areas. In the case of the Latemar area, strata overlying present day topography (as marked by zig-zag line) were projected from immediate surroundings (i.e. Rosengarten and Schlern platforms). Formations in bold letters were studied in detail. Chronostratigraphy after Lehrmann et al. (2002), Mundil et al. (1996, 2003) and Vo¨ro¨s et al. (2003). Biozones, substages and position of Anisian/Ladinian boundary after Brack and Rieber (1993). Sequence stratigraphic subdivision after Gianolla and Jacquin, (1998); for the Anisian, a more detailed scheme of Zu¨hlke (2000) exists.

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After the Permian/Triassic faunal crisis, first carbonate buildups in the Southern Alps reappear in the earliest Anisian and are characterised by microbial carbonates (Flu¨gel, 2002, and references therein). Higher diversified reef communities develop since the Late Anisian, but lack abundant frame-building organisms, such as scleractinians. One example of these Late Anisian reefs is the Latemar (position of Anisian/Ladinian boundary according to Brack and Rieber, 1993), an atoll-like platform with a diameter of approximately 3 km (Fig. 2a). It is entirely made up of the bLatemar limestoneQ (Gaetani et al., 1981), which is part of the Schlern Formation 1 (sensu Brandner et al., 1991; Fig. 3). The transition towards the basin is characterised by slope breccia interfingering with the pelagic Buchenstein Formation (e.g. Viel, 1979). Water depths of the coeval basins during the ultimate stages of Anisian/Ladinian platform evolution range from 800 to 1000 m (Brack and Rieber, 1993). Ladinian/Carnian platform generations record a return of walled reefs with the principal guild being scleractinians capable of resisting wave energy (Flu¨gel, 2002, and references therein). One example of these carbonate platforms is the Concarena in the Val Camonica north of Brescia (Fig. 2b). Correlation with ammonoid and conodont fossil findings in distal slope areas (Balini et al., 2000), indicate a Late Ladinian/Early Carnian age for the Concarena (Fig. 3). Primary lithofacies distribution of the clinostratified platform slope, fossil assemblages of the marginal reef rim and the cyclic arrangement of the at least 800 m thick lagoonal deposits are part of the Esino Limestone (sensu Stoppani, 1858). Clinostratified slope deposits interfinger with basinal sediments of the Wengen and Pratotondo Formation (Fig. 3).

3. Database and methods 3.1. Facies analysis Detailed facies, sedimentological and palaeontological analyses (logging, facies mapping, lateral tracing of physical surfaces, thin sections) were carried out on representative sections and platform margin transects. The facies zonation of the respective margins was investigated at several outcrops (see Fig. 2a and b) and sampled with 2 m increments.

Quantitative microfacies analyses were carried out on polished slabs (up to several dm in size) and thin sections (46cm). Overall, 382 rock specimens and 189 thin sections were collected and analysed. 3.2. Sequence stratigraphy Platform geometries and cementation patterns were analysed with sequence stratigraphic means in order to determine the accommodation/sedimentation (A/S) development in both areas. In this study, we follow the sequence stratigraphic concept of Cross (e.g. Cross and Lessenger, 1998) and Schlager (e.g. 1993). Cross and Lessenger (1998) point out that repeated successions of stratigraphic sequences result from base-level cycles of increasing and decreasing accommodation/sedimentation (A/S) conditions. They also refer to the original definition of Wheeler (1964) where base-level is an bimaginary potentiometric energy surface that describes the changes in energy required to move the earth’s surface up or down through timeQ. The turnaround points from base-level rise-to-fall (maximum A/S) and fall-to-rise (minimum A/S) are correlated across all facies tracts within a basin (e.g. Cross and Lessenger, 1998). Schlager (e.g. 1993) considers the creation/destruction of accommodation in time (AV=dA/dt) and changes in sediment supply in time (SV=dS/dt) as the two principal controls on development of sedimentary systems/ basins. In this case, AV as well as SV represent changes in volume in time (VV=dV/dt). We, however, would like to refine Cross’ A/S conditions (e.g. Cross and Lessenger, 1998) as AV/SV ratio sensu Schlager (1993). The application of this refined definition led to the following fundamental boundary conditions for the relationship between sequence stratigraphy and geometric evolution of carbonate platforms: (1) AV/SV ratios vary on a basin-wide scale. Proximal settings may reveal a higher or seldomly lower ratio of AV/SV than coeval basinal settings. In the following paragraphs, we refer to AV/SV ratios in the shallow marine carbonate platform setting only. (2) If AVV0, erosion can occur. The carbonate platform setting is then mainly governed by extensive subaerial exposure (e.g. dissolution, karst features and meteoric diagenesis). Thus, in the following paragraphs, we refer to AV/SV ratios only, when early marine phreatic diagenesis prevails, i.e. if AVN0. (3) Progradation, aggradation

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and retrogradation of carbonate platforms reflect different AV/SV ratios. Progradation occurs when AV/SVb1, aggradation occurs when AV/SV=1 and the platform retrogrades when AV/SVN1. 3.3. Cements and cathodoluminescence In the following chapters, we refer to massive early cementation (MEC) as a marine phreatic diagenetic process recorded by isopachous crusts and/or botryoids forming cementstones sensu Wright (1992) in sheltered environments (e.g. primary cavities, interreef voids, etc.). Wright’s definition of a cementstone is ba limestone [composed] almost totally of fibrous cement (commonly replaced and/or recrystallised), in which grains or in situ biogenic material do not constitute a frameworkQ. Isopachous crusts of MEC are made up of radiaxial-fibrous calcite and are thought to reflect primary features (e.g. Kendall, 1985). Cathodoluminescence techniques were used to identify different generations of cements from both platforms, especially to separate early marine to shallow burial cements (e.g. radiaxial fibrous cements, dog-tooth cement) from burial cements (e.g. zoned blocky calcites; cf. cement stratigraphy of the Northern Calcareous Alps: Zeeh and Bechsta¨dt, 1994). The application of cathodoluminescence was necessary in order to estimate the amount of early marine cements with respect to whole rock cementation. A Leica/Leitz DM RP microscope equipped with Leica N Plan 2.5/0.07 P and Pl Fluotar 5/0.12 P lenses was combined with a Citl cold cathode apparatus of the type CCL 8200 mk3. Thin sections were placed on a tray controlled by X-Y manipulators in a vacuum chamber with an upper window for microscopic observations. An electron beam was deflected on the sections by means of an obliquely arranged gun. A beam voltage of 20 kV and a current of 400 to 600 AA was used. Images were acquired using a Leica DFC 480 digital camera.

4. Latemar 4.1. Previous research The Latemar is a model Mesozoic platform (primary lithofacies, geometries, cyclostratigraphic

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interior, etc.) and hence has been studied in detail (e.g. Gaetani et al., 1981; Zu¨hlke et al., 2003; and references therein). Although most of the past studies focus on the stacking patterns of the lagoonal deposits (e.g. Goldhammer and Harris, 1989), more recent studies also deal with the reef and slope (e.g. Emmerich et al., in press, and references therein). 4.1.1. Platform development Platform cycles of the cyclostratigraphic interior consist of decimetre- to metre-thick beds recording shallowing-upward conditions (e.g. Egenhoff et al., 1999, and references therein) with a sub-Milankovitch frequency (Zu¨hlke et al., 2003). An alternative model of cycle generation is proposed by Blendinger (2004), where the origin of the cyclic stacking pattern is thought to be related to diagenesis in a hydrothermal field. In contrast, research on the reef of the Latemar is comparatively rare (Gaetani et al., 1981; Harris, 1993). These studies emphasise the importance of microbial encrustations; recent research (Emmerich et al., in press) shows the affinity to Anisian fossil assemblages of the Dolomites but with a distinctly higher diversity. The results of the studies on the slope of the Latemar by Harris (1994) are in accordance with Bosellini’s bLadinianQ model (1984) where late stage progradation is observed. Emmerich et al. (in press) have proposed a differentiated concept of the Latemar slope. Depending on the aspect, aggradation, progradation and backstepping of the margin occurs simultaneously. Even contrasting sedimentological settings (erosional, bypass and depositional sensu Schlager and Ginsburg, 1981) are simultaneously present at different exposures. In contrast to Harris’ model (1994), Emmerich et al. (in press) report an initially much larger platform with backstepping during later stages of platform evolution. 4.1.2. Bio- and sequence stratigraphy Biostratigraphic data from the lagoonal interior indicate a stratigraphic range of the preserved Latemar buildup from the upper Reitzi- until the uppermost Secedensis/basal Curionii-zone (Zu¨hlke et al., 2003, and references therein; Fig. 3). According to Gianolla et al. (1998), the Anisian and Ladinian marks a major 1st-order transgressive–regressive (T–R) cycle. Strong increase in accommodation during Anisian and Early

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Ladinian times allowed the deposition of great sediment thicknesses and forced carbonate platforms to backstep in the domain of the Southern Alps (Gianolla and Jacquin, 1998). The peak transgression occurred in the Secedensis /Curionii boundary interval (bChiesense grooveQ sensu Brack and Rieber, 1993; Gianolla and Jacquin, 1998). This sequence stratigraphic surface is represented by a thin condensed micritic layer in the basinal Buchenstein Formation documenting a very slow sedimentation rate (e.g. Bagolino section, Lombardic Alps) and by drowning/ backstepping of coeval Schlern Formation 1 carbonate platforms in the Dolomites (drowning: Cernera; Brack and Muttoni, 2000; local backstepping: Latemar; Emmerich et al., in press). At the moment it is still a matter of debate whether the Schlern Formation 1 belongs to the 3rd-order L1 (Brandner et al., 1991), La1 (Gianolla and Jacquin, 1998) or An5 (Zu¨hlke, 2000) sequence (bsequenceQ sensu Van Wagoner et al., 1988). All authors define the basal sequence boundary at a subaerial unconformity on top of the Contrin Formation. At Latemar, the TST corresponds to the lower part of the succession (Lower Edifice/Lower Platform Facies: Gaetani et al., 1981; Goldhammer and Harris, 1989; Egenhoff et al., 1999). According to our observations, the MFS at Latemar corresponds to the peak transgression recorded by local retreat of the platform margin in the upper part of the cyclic succession. The HST and upper sequence boundary of L1/La1/An5 is not preserved in the present day Latemar succession and is defined differently on a basin-wide scale by each one of the above mentioned authors. 4.1.3. Diagenesis and cementation Diagenesis and cementation of the Latemar are investigated in detail by Dunn (1991) and Harris (1993). Dunn (1991) investigates the differing styles of cementation in the bLower EdificeQ and the cyclic part of the Latemar succession and describes the highest amount of cementation in the tepee belt (N50%) and in vadose crusts capping cycles of the lagoonal interior (40%). However, it is also stressed that field, petrographic and isotopic evidence does not support a significant role for freshwater diagenesis. In spite of frequent subaerial exposure of the Latemar platform top during the deposition of the cyclic facies, seawater is still the dominant diagenetic solution. Dunn (1991)

applies cathodoluminescence techniques and identifies up to three shallow to late burial cement generations within spar filled voids between isopachous crusts of early marine diagenetic origin. Harris (1993) observes early marine cements mainly in the boundstone facies of the platform margin where radiaxial or radial fibrous calcite typically makes up 15–30% of the rock volume. Only in a few cases, cements may constitute 50–70% of total rock volume—an observation supported by recent studies (Emmerich et al., in press-a). A detailed quantitative analysis of the Latemar cements has not yet been published. 4.2. Platform development The description and interpretation of the architecture of the Latemar platform and its platform to basin transitions is shown using the example of the Eggentaler Horn transect (locations 1–3 in Fig. 2a; Fig. 4a–c). The buildup of the lagoon-, reef- and slope-facies of the Schlern Formation 1 at Eggentaler Horn starts on top of an initially emersed part of the Contrin Formation (area (A) in Fig. 4b). A lagoonal facies conformably overlies this palaeo-karst surface and forms an isolated btowerQ-like structure (B). The contact of the slope-facies to the SE (left-hand side in Fig. 4b) is erosional, as displayed by the unconformity cutting right down to the formation boundary between Contrin Formation and Schlern Formation. Area (C) cannot safely be attributed to neither the lagoon- nor slope-facies due to missing sedimentary structures; it most probably is a remnant part of outer lagoon or reefal domains. (C) is in turn overlain by lagoonal facies (D). Towards the upper part of the Eggentaler Horn section, lagoonal facies is absent and two units within the slope are separated by major erosional unconformities mapped around the Erzlahnscharte (for further details refer to Emmerich et al., in press). The common feature of all platform–basin transitions at Eggentaler Horn is the missing reefal margin apart from the small area around the Erzlahnscharte (Fig. 4c). Only the central part of the reef-facies characterised by massive microbial encrustations is preserved at this locality (locality 3 in Fig. 2a). This area is therefore being referred to as bmicrobial ridgeQ. The Eggentaler Horn transect witnesses at least four stages of major collapse of the platform margin,

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SE

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Fig. 4. Platform architecture of the Latemar. Legends on the respective figures; for further explanations refer to the text. (a) Panoramic view of the SE flank of Eggentaler Horn with an indication of pictured areas in (b) and (c). (b) Magnification of the middle part of the Eggentaler Horn transect with an interpretation of sedimentary structures. Letters (A) to (D) correspond to different stages of platform development. (c) Magnification of the upper part of the Eggentaler Horn transect (i.e. Erzlahnscharte, locality 3 in Fig. 2b) with an interpretation of sedimentary structures. The reef-facies displaying massive microbial encrustations is well exposed.

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which remove large quantities of outer lagoon- and reef-facies and bypass sediment to the lower slope. The platform margin subsequently backsteps and the slope reveals mainly bypass/erosional characteristics during the development at this exposure. The NE area of the Latemar records more depositional and less erosional characteristics on the slope, but erosional intervals are still common as indicated by a distinct timeline of an erosional slope at for instance the Cresta De Do Peniola (Fig. 2a; Emmerich et al., in press). 4.3. Margin configuration The general organisation of the margin at Latemar is documented using the description and interpretation of the Erzlahnscharte area (location 3 in Fig. 2a; Fig. 6a). Other locations reveal a slightly different biotic content/lithofacies arrangement and have been discussed in Emmerich et al. (in press) in detail. They show the importance of calcisponges (inozoans, sphinctozoans), Porostromata and Microproblematica for the organisation of the reefal margin. The outcrops of the reef-facies at Erzlahnscharte–with a width of 20-30 m–belong to the back reef and reef-crest domain (Fig. 6a). A particular feature of the back reef at this locality is the presence of bTubiphytesQ multisiphonatus thrombolites sensu Riding (1991) of up to 0.7 m in height and 3 m in diameter (Emmerich et al., in press). For a more detailed description of the first finding of this species outside its type locality (Hydra, Greece, Carnian bPantokrator LimestoneQ) the reader should refer to Emmerich et al. (in press). The reef crest displays massive microbial encrustations with minor abundance of scleractinian framestones. The reef has a low topography with strata gently sloping towards the centre of the lagoonal platform top and to the upper slope. Evidence for subaerial emersion is absent, suggesting that the reef always remains submerged. The palaeobathymetrically most elevated part of the Latemar platform is the tepee belt, located between back reef and lagoon (Egenhoff et al., 1999; Fig. 6a). 4.4. Carbonate cementation As mentioned in previous studies, major volumes of carbonate cement occur mainly in the platform

margin at Latemar. The palaeobathymetrically most elevated part of the Latemar platform–the tepee belt– is exposed to subaerial conditions during certain periods of platform development (Goldhammer and Harris, 1989; Egenhoff et al., 1999) and is governed by vadose to marine-phreatic cementation. The tepeefacies in the cyclic succession of the Latemar is characterised by the presence of pisoids, red internal sediments, dissolution features, isopachous radiaxialfibrous cements and less commonly small botryoids within the central fissure of the tepees (Goldhammer and Harris, 1989; Egenhoff et al., 1999; Fig. 7a). The principal characteristics of the tepee belt are: (1) it is a very narrow part of the margin, (2) it is not present during all stages of platform evolution and (3) cements are mainly restricted to the central cavity between the inclined flanks. Only one location on the uppermost slope reveals abundant early marine cements (Schenon, Fig. 2a). This setting is characterised by cavities between blocks of a proximal talus fan facilitating the formation of bEvinospongiaeQ-like crusts (Fig. 7b). This restricted occurrence of massive cementation precludes its volumetric importance during the development of the entire platform. The absence of significant volumes of MEC at Latemar, however, does not allow for ruling out early lithification of the platform margin. In contrast, the occurrence of abundant megabreccia at the entire slope and neptunian dykes at two localities at Latemar stresses the importance of early lithification. A large portion of the carbonate deposits at Latemar records synsedimentary tectonic activities because of their brittle behaviour due to early cementation (cf. Kerans et al., 1986). Our investigations on cement succession at Latemar are generally in accordance with the scheme of Dunn (1991). The majority of all cements are of early marine phreatic origin (60–80%), burial cements play a minor role only. Burial cements can be subdivided in at least two generations, locally maybe three. The isopachous crusts representing early marine cement precipitation are non-luminescent and are followed by sparry calcitic cement. The first generation of spar cement is dully luminescent and displays a sharp to transitional contact towards the isopachous cements. The second main generation of spar cement consists of several bright luminescent layers of calcite (Fig. 8a–c).

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5. Concarena 5.1. Previous research The Concarena carbonate platform is one of the least studied carbonate platforms in the Southern Alps. Most of the scarce information is derived from two studies (Rossetti, 1966; Brack, 1984). Much more work has so far been carried out on the surrounding areas (e.g. Assereto and Casati, 1965; Frisia-Bruni et al., 1989; Balini et al., 2000). 5.1.1. Platform development Rossetti (1966) is the first to recognise the lateral transition from platform facies (Esino Limestone) to distal slope (Pratotondo Formation). Six of his sections covering an interval from Buchenstein Formation to Lozio Shale (Fig. 6) enable a basic geometric reconstruction of the Concarena platform. Brack (1984) describes the progradation of the upper part of the Concarena succession.

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and basin-ward progradation of shorelines. The Triassic of the Lombardic Alps is further subdivided into 3rd-order sequences sensu Van Wagoner et al. (1988). Following the different schemes, the Concarena platform belongs to the Lad3/Car1 sequences (Gianolla and Jacquin, 1998) or the L2–C1 sequences (Gaetani et al., 1998). Both sequence stratigraphic subdivisions are based on a compilation of data from different regions of the Lombardic Alps. Up to now, no sequence stratigraphic framework has been established for the Concarena platform. 5.1.3. Diagenesis and cementation bEvinospongiaeQ- or bGrogoolithQ structures within the Esino Limestone at Concarena are briefly mentioned by Rossetti (1966). Basic diagenetic features of the platform top like botryoids and pisoids are described by Brack (1984). However, existing studies of the Concarena area lack detailed information on diagenesis and/or cementation. 5.2. Platform development

5.1.2. Bio- and sequence stratigraphy In the literature, no biostratigraphic data is reported from the Esino Limestone of the Concarena platform. However, lateral correlation of age-indicative intervals from coeval, adjacent basins is possible. These basins are infilled by sediments pertaining to two successive formations: Wengen and Pratotondo Formation (Rossetti, 1966; Brack, 1984; Balini et al., 2000; Fig. 3). Age-diagnostic fossils (ammonoids, conodonts, daonellids) are known from several units of the Wengen Formation (e.g. Assereto and Casati, 1965; Balini et al., 2000) indicating a Longobardian age. The overlying Pratotondo Formation contains a Late Longobardian/Early Julian conodont fauna with the Ladinian/Carnian boundary located approximately 35 m above the base of the Pratotondo Formation (Balini et al., 2000). The Concarena buildup is therefore ascribed to the latest Ladinian/earliest Carnian. The sequence stratigraphic framework for the Lombardic Alps is provided by Gaetani et al. (1998) and Gianolla and Jacquin (1998). The Esino Limestone of the Concarena platform belongs to the 2nd Triassic 1st-order T–R cycle and is part of its regressive phase. In the Lombardic Alps, this regressive phase is documented by decreasing water depths

The onset of platform development of Concarena is obscured, but the eastern part of the platform shows the contact between Esino Limestone and underlying Wengen Formation. Approximately 800 m of lagoonal facies with shallowing upward cycles are preserved in the northwestern part of the platform (Fig. 5a). Stacking patterns of 1:5 are common in the lower part of the lagoon where metre-scale, mainly sub- and intertidal cycles prevail. The upper part of the lagoon is characterised by a distinct decrease of bed and cycle thicknesses (Fig. 5b: Valle del Baione view of Cima della Bacchetta). In this part of the succession, inter- to supratidal horizons with tepees, pisoids, vadose and phreatic cements are frequent. MEC in the lagoon cause early lithification and stabilisation of sediment. An exact differentiation of these two different stages in platform development is difficult as the lagoon-facies records a slow transition between these two intervals. The platform margin shows moderate progradation during the first platform stage, whereas the second platform stage is characterised by pronounced progradation (Fig. 5a and c). The maximum thickness of the Concarena carbonate platform slope is preserved in its eastern part (La Tavola area, location 3, Fig. 2b) with a succession of at least 1500 m thick stacked

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M. Seeling et al. / Sedimentary Geology 175 (2005) 439–457 Cma. della Bacchetta (2549m)

a

N

S

Cma. dei Ladrinai (2403m)

Esino Lst Lagoon-facies

1

Esino Lst Reef-facies

Esino Lst Slope-facies Valle del Baione (~ 2000m)

1

2

Studied outcrops Hiking path 81A Progradation of platform top

b NW

c

Monte Vaccio (2265m)

~ 2450m

Second platform stage

Esino Lst - Lagoon-facies agoon-facies Second platform stage SE

Esino Lst Reef-facies

First platform stage Esino Lst - Slope-facies

Spluga (2050m)

~ 2050m

Fig. 5. Platform architecture of the Concarena. Legends on the respective figures; for further explanations refer to the text. (a) Drawing of the NW flank of Cima della Bacchetta with an interpretation of geometric platform–basin relationships and an indication of studied outcrops. (b) Enlargement of the lagoonal succession. Note the two distinct stages of lagoonal development. (c) Panoramic view of the NW flank of the Monte Vaccio area (also Fig. 2a) with an interpretation of geometric platform–basin relationships. Reefal margin of the second stage of platform evolution shows pronounced progradation over a depositional slope.

debris flows. Slope-facies is preserved at all sides of the Concarena buildup: large-scale clinoforms are well visible at its eastern and southern termination (Fig. 5c). The geometries of the platform–basin transition indicate depositional characteristics sensu Schlager and Ginsburg (1981). Climbing progradation of megabreccia clinoforms and intercalation with the basinal Wengen Formation (volcaniclastics) characterise the slope-basin relationship in the La Tavola area (eastern termination of Concarena; location 3 in Fig. 2b). The thickness and amount of carbonate debrisflows increases upward in these adjacent basinal parts. The relatively high amount of clastic input into the basin and the moderate progradation of slope clinoforms determine the base relationships of the platform in this area. In the Pratotondo area (southern

margin of Concarena; location 4 in Fig. 2b) the geometric configuration of clinoforms is different from the La Tavola area. Slope strata are younger with respect to La Tavola, showing thinner clinoforms and flatten out to well bedded, dm-thick, dark micritic limestones (Pratotondo Limestone). Most likely, poorly oxygenated conditions develop, preventing distinct bioturbation. 5.3. Margin configuration Large parts of the margin are exposed around the summit of Cima della Bacchetta, being the topographically and stratigraphically highest part of the Concarena (location 1 on hiking path 81A; Figs. 2b and 5a). Similar to the tepee belt at Latemar, a zone of

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early marine cementation is present at the transition from lagoon to back reef. However, this zone is characterised by lenticular cement structures of considerable size (2–3 m across and 1–2 m in height). The situation at Latemar is paralleled once again, the main features of the back reef at Concarena are bTubiphytesQ multisiphonatus framestones forming thrombolites of up to 1.5 m in height and 4 m in lateral extent interfingering with wacke-/packstones of the back reef lagoonal setting. Apart from bTubiphytesQ multisiphonatus associated with isopachous cement crusts (Fig. 6b) the only other components are fine-grained bioclasts (e.g. pelecypods, gastropods), peloids, sessile encrusting foraminifers, porostromate algae and micritised worm tubes. In contrast to Latemar, Concarena’s reef is characterised by the abundance of in situ scleractinian coralline framestones representing the Tethys wide reappearance of scleractinians towards the end of the Ladinian (Flu¨gel, 2002). The scleractinian colonies are up to 3–4 m in height and 10 m in diameter, interfingering each other and building a laterally continuous rim within the 15–60 m broad reefal margin. Framestones of this fossil assemblage are characterised by Margarosmilia sp.-like corallites with a diameter of up to 15 mm. Trapped sediment is rare, components being mainly peloids and grains with micritic envelopes, only in few cases foraminifers and algae. The scleractinian rim is rich in primary cavities and vugs, lined by radiaxial fibrous cements. Towards the upper slope, dm-sized talus blocks are increasing in size and abundance. These deposits are mainly stabilised by synsedimentary cements. At Concarena–in contrast to the Latemar–bTubiphytesQ multisiphonatus thrombolites occur up to the transition to the foreslope. These thrombolites exhibit the same features as in the back reef environment. 5.4. Carbonate cementation At Concarena, the distribution of carbonate cements and cementstones follows the zonation of facies belts: (1) the lagoon is characterised by vadose cements indicating frequent subaerial exposure, (2) the transition towards the back reef by botryoids and isopachous crusts, (3) the reef by several generations of marine phreatic cements and (4) the uppermost slope by bEvinospongiaeQ-like isopachous crusts.

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Pisoids and oncoids are common in the upper part of the lagoonal succession. Another diagenetic feature is brecciation with infiltration of fine-grained microcrystalline dolomite in a red argillaceous matrix related to near-surface processes (bterra rossaQ Assereto and Kendall, 1977; Mutti, 1994). Some localities contain botryoids covered by fine-grained peloidal sediment pointing towards an early marine origin (Fig. 8d). Some samples contain dripstone cements, symmetric and asymmetric meniscus cements—indicators for inter- to supratidal conditions with frequent exposure. The striking feature of the transition from the lagoon towards back reef settings are the cement arrangements of lenticular shape (convex-upward, decimetre to metre size in diameter and height; Fig. 7c). The base of each cement lens is formed by cm- to dm-thick botryoids closely linked to microbial crusts. Massive isopachous cements cover the dark coloured botryoids (Fig. 7c–e). These isopachous crusts are formed entirely by up to 5 mm thick alternating light and dark layers of fibrous calcite. Each cementation phase consists of small mm-sized calcite fibres showing undulous extinction with radiaxial fibrous pattern. Isopachous crusts and botryoids are indicators for synsedimentary marine cementation (e.g. Kendall, 1985; Frisia-Bruni et al., 1989). At Concarena, these precipitates form cementstones (sensu Wright, 1992) in lagoon/back reef settings. With respect to the other facies belts, the reefal margin of the Concarena platform has the lowest volumetrical content of early cement. Cementation mainly took place within the bTubiphytesQ multisiphonatus zone and the scleractinian rim. The coralline part of the reef exhibits features such as large open cavities, precipitation of radiaxial fibrous calcite (layers with a thickness of up to 3 mm) together with late blocky calcite inside small vugs and/or coral branches. Like in other facies belts, the reefal margin also shows early cementation and later recrystallisation of components. The occurrence of massive, grey isopachous crusts–consisting of several mm to cm-thick layers of fibrous calcite–is also a common feature of reef front/uppermost slope settings. Multiple generations of these crusts form globose structures ranging up to several dm in size (Fig. 7f). These structures are known as bRiesenoolithQ, bGrogoolithQ and/or

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Fig. 6. Schematic reef model of the Latemar (a) and Concarena (b) indicating the three main zones (reef front, reef crest and back reef) and the relationship with the lagoonal succession. Legend in the lower part of the figure. The common feature of both margin models is the bTubiphytesQ multisiphonatus zone in back-reef settings. Vertical exaggeration, not to scale. (a) Latemar’s reef is always below sea level, the bathymetrically highest point of its margin is the tepee belt separating the back reef from the lagoonal interior of the platform (also Egenhoff et al., 1999). Organisms with more delicate/branching growth are found beneath the mean wave base, whereas the more or less wave resistant area within the surf zone is marked mainly by algae and minor corals. (b) The reef of the Concarena is always below sea level, the bathymetrically highest point of its margin is the lagoon. This is indicated by the geometrical relationship between reef and lagoon and the absence of emersive features at the reef (abundant at the lagoon). The walled, wave resistant area within the surf zone is marked by corals. The position of the mean wave base is unclear. bTubiphytesQ multisiphonatus thrombolites also occur on the uppermost slope.

bEvinospongiaeQ (e.g. Frisia-Bruni et al., 1989; Russo et al., 2000). Under crossed polars, the fibrous calcite shows undulous extinction with radiaxial fibrous patterns.

Under cathodoluminescence, botryoidal cements– restricted to lagoonal areas of the Concarena platform–show patchy, dull luminescence. The common radiaxial fibrous cements–forming isopachous cement

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Fig. 7. Cements at Latemar and Concarena. (a) Sample from the central cavity between the inclined flanks of tepees showing multiple generations of cement and terra rossa (Latemar, tepee-facies, Erzlahnspitze area; scale bar is 1 cm with subdivisions of 2 mm). (b) Photograph of polished slab from talus blocks with isopachous crusts (Latemar, breccia of slope-facies, Schenon; scale bar is 1 cm with 2 mm subdivisions). (c) MEC at the lagoon in the cement zone. Lower part of the lens-like structure is entirely made up of botryoids whereas the upper part contains isopachous crusts (Concarena, Cima della Bacchetta, scale bar in cm and inches). (d) Enlargement of a botryoid from (c) (approx. 3 cm in length; Concarena, Cima della Bacchetta, scale bar in cm with subdivision of 5 mm). (e) MEC at the lagoon in the cement zone (Concarena, Cima della Bacchetta, scale bar in cm and inches). (f) MEC at the uppermost slope (Concarena, Cima della Bacchetta, scale bar in cm and inches).

crusts in lagoon/back-reef and slope settings–are nonluminescent. Shallow burial cements like dog-tooth cement are common in all facies belts of the Concarena. They often cover early radiaxial fibrous calcite and show insignificant or dull luminescence

with multiple, bright luminescing outer rims. Their equant crystals can be easily distinguished from acicular crystals of radiaxial fibrous calcite. Dogtooth cement is followed by blocky calcite filling most of the remaining porosity and late fractures. A

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Fig. 8. Photomicrographs of cements at Latemar and Concarena. (a) Isopachous cement crust from the tepee-facies at Latemar. The boundary between the first generation of cements (1: isopachous crusts; marine phreatic) and burial cements (2) is well visible (transmitted light; Latemar, Erzlahnspitze area, scale bar is 1 mm with subdivisions of 200 Am). (b) The same thin section of (a) under crossed polars. Cement phase 2 of (a) can be subdivided into two generations. (c) The same thin section of (a) under cathodoluminescence. Burial cements (3) show bright luminescence. Boundary between 1 and 2 is faint. (d) Botryoidal cement from the lagoonal facies covered by peloids (Concarena, Cima della Bacchetta, scale bar is 1 mm with subdivisions of 200 Am). (e) Burial cements of sample (d) under transmitted light (scale bar is 1 mm with subdivisions of 200 Am). (f) The same thin section of (e) under cathodoluminescence. Five generations of cement are distinguishable by their differing degrees of luminescence.

succession of three to five cement generations with different degree of luminescence is observed (Fig. 8d– e). In a few cases, red luminescing crystals of deep burial saddle dolomite follows zoned blocky calcite. The cement succession of the Concarena is in accordance with schemes from other Esino Limestone platforms (e.g. Grigna and Valle Brembana; FrisiaBruni et al., 1989). In lagoonal and back-reef domains of the Concarena, early marine cements represent up to 90% of the total cement volume in these rocks. The reefal margin of the Concarena platform contains lesser amounts of early marine cements, values around 60% are common. The slope of the Concarena platform contains similar amounts of cement like lagoonal/back reef areas (up to 90% with respect to whole rock cementation).

6. Discussion 6.1. Comparison of platform development Despite laterally traceable cycles of the lagoonal interior—the Latemar reveals a complex pattern of

different, even contrasting sedimentological characteristics lacking a uniform evolution in space and time. The development of the slope-facies at Latemar is governed by synsedimentary tectonics at the southwestern, western and west–northwestern parts of the platform where the platform margin is mutilated. Other outcrops are not influenced by tectonics and show mainly aggradation or at one locality even faint progradation (Emmerich et al., in press). The bLower EdificeQ, building up directly from the Contrin ramp, records platform retreat due to multiple collapses on the slope, margin and even platform top. Autocyclic processes of collapse–fast accommodation fill–oversteepening–collapse together with synsedimentary tectonics result in a mainly aggrading platform (AV/ SV=1). These observations together with local retrogradation (AV/SVN1 and increasing) during the last stages of platform development indicate that the carbonate factory at Latemar is stretched to its limits (cf. Emmerich et al., 2005). The Concarena carbonate platform shows a less differentiated, but two-phased development. This evolution of the Concarena carbonate platform is controlled by a pronounced decrease in the AV/SV

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ratio triggering distinct progradation (AV/SVb1). Ladinian/Carnian boundary sections in the area indicate a fall in relative sea level (Gaetani et al., 1998), slowing down the creation of accommodation space in the Lombardic Basin significantly. The combination of this relative sea-level fall with a compartmentalisation of basins (Assereto and Casati, 1965; Brack, 1984) and subsequent local development of starved basinal conditions decreased carbonate production (extrinsic factors sensu Schlager, 1993). This indicates that the decline in AV is more important for the progradation than the decline in SV. The Latemar and Concarena carbonate platforms are case studies for the sensitive interplay between carbonate production (S) and accommodation (A). Schlager et al. (1994) report that during times of positive AV (=increasing accommodation) carbonate platforms produce an excess of carbonate sediment (positive SV). Much of this carbonate is shed into surrounding basin areas (highstand shedding, Schlager et al., 1994), forming a large amount of prograding gravity flow deposits on the slope (e.g. first platform stage of Concarena and entire Latemar slope). During times of decreasing accommodation and carbonate production thin-bedded carbonate turbitide deposits derived from the slope are common (e.g. second platform stage of Concarena; not present at Latemar). 6.2. Comparison of the reefal margins Detailed microfacies and palaeontological investigations at the margins of the two Middle and Upper Triassic carbonate platforms show that bTubiphytesQ multisiphonatus is an important reef builder. Apart from previous findings at Aggtelek (Hungary), Hydra (Greece) and Latemar (Italy) the Concarena is only the fourth locality where this enigmatic microproblematicum occurs (for a detailed list refer to Emmerich et al., in press). In contrast to all other locations where bTubiphytesQ multisiphonatus thrombolites are found in the back-reef area (e.g. Erzlahnscharte, Latemar), at Concarena they are also part of the reef front to uppermost slope. Additionally, the size and abundance of bTubiphytesQ multisiphonatus bioconstructions at Concarena exceeds observations from all previously described localities, it is one of the main constituents of its reefal margin. Up to now, the occurrence of this biota is observed in the central Tethys area only. This

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might point to (1) endemism and/or (2) specific environmental conditions, which are required for the growth of bTubiphytesQ multisiphonatus (e.g. composition of seawater, temperature, nutrient supply, marine currents, etc.). A further difference to Latemar’s reef is the abundance of in situ scleractinian coralline framestones at Concarena (see also Section 5.3). This feature is the main reason behind the difference in topography of the respective reefal margins (microbially dominated reef at Latemar vs. walled reef at Concarena; Fig. 6a and b). The scleractinian bioconstructions at Concarena are responsible for the wave-resistant nature and abundant cavities of its reef. 6.3. Massive early marine cementation (MEC) At Latemar, massive early marine cementation (MEC) plays a minor role—it is only present at one locality on the upper slope (see Section 4.4). The insignificant distribution of MEC owes mainly to the Anisian nature of this reef. It is made up to a large extent of low-growing, encrusting organisms. These construct a small reefal margin with only small and isolated voids and a low topography inhibiting effective fluid pumping. Another cause might be the close relationship between carbonate production and MEC (Lighty, 1985). According to Lighty (1985), high amounts of cementation generally occur in reef environments that have low carbonate production/ framework accumulation rates (low SV). It is, however, more precise to link MEC to a low AV/SV ratio. Again, this means a decrease in creation of accommodation space (AV; i.e. total subsidence plus sealevel rise; e.g. Jervey, 1988; Posamentier et al., 1988) combined with low carbonate production rates (SV). A high rate of creation of accommodation space together with a high rate of sedimentation/production shortens the amount of time available for marine phreatic diagenesis–and the formation of e.g. botryoids–as the sediments more quickly reach the realm of shallow marine burial diagenesis. As the Latemar reveals very high rates of total subsidence/net carbonate accumulation (650–800 m/Ma; Zu¨hlke et al., 2003; Emmerich et al., 2005), it is obvious that MEC must be low. MEC is much more likely to develop in Late Ladinian/Carnian rimmed platforms like the Concarena. The reefal margins of these buildups are

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characterised by an increased amount in rigid bioconstructors (e.g. scleractinians; Flu¨gel, 2002 and references therein). Another feature is the change in morphology towards walled, wave resistant margins enabling effective fluid flow through inter-reef pore spaces. According to Ginsburg et al. (1971), James et al. (1976) and Marshall (1986) only seaward margins of walled reefs show MEC related to environmental factors such as high-energy conditions supplying effective pumping of marine fluids. Isolation of primary voids through encrusting organisms and subsequent infill with peloids impedes MEC. In the case of the Concarena, the local development of starved basinal conditions decrease carbonate production (extrinsic factors sensu Schlager, 1993), possibly a further important factor to favour MEC. Additional major players in the cementation of carbonate platforms are physicochemical and -biological processes and aspects (e.g. alkalinity, pH, microbial activity). Most of them–however–are hard to assess on fossil carbonate platforms and hence are not investigated within the context of this study. The excellent porosity of many modern hydrocarbon reservoirs in carbonates is tied to basins undergoing rapid total subsidence at times of high sea level (Moore and Haydari, 1993) leaving insufficient time for pervasive cementation. It is, however, more precise to link this diagenetic evolution of porosityprone, MEC-poor platforms to periods of fast sealevel rise and/or rapid total subsidence (i.e. high AV). Nevertheless, a certain amount of cementation is necessary in order to counterbalance compaction and to preserve porosity. Leaving burial cementation aside, porosity seems best to be preserved in a certain window of creation of accommodation space balanced by carbonate production allowing enough cementation to stabilise the platform. Consequently, the link between the AV/SV ratio and MEC allows constraining this early diagenetic development to platform types with certain geometries: (1) aggradational or retrogradational platforms are unlikely to develop features of MEC, (2) platforms with progradational characteristics are prone to MEC. This hypothesis is confirmed if it is tested against other Triassic platforms in the western Tethys area. Boni et al. (1994) and Climaco et al. (1997) describe strikingly similar features from bpathologically progradingQ (Bosellini, 1989) Upper Triassic platforms

of Calabria (Southern Italy). Like at Concarena, the last stages of platform development are governed by a slow-down in accommodation increase in combination with the development of anoxic conditions in the surrounding basins. Strong similarities also exist with platforms of the Northern Calcareous Alps (Brandner and Resch, 1981; Henrich and Zankl, 1986; Zeeh et al., 1995) where MEC is always linked to progradation and reefs with a high degree of primary porosity.

7. Conclusions The comparison of the Triassic carbonate buildups of Latemar (Late Anisian/Early Ladinian, Dolomites) and Concarena (Late Ladinian/Early Carnian, Lombardic Alps) stresses the importance of bTubiphytesQ for the development of Triassic carbonate platforms. The degree and style of cementation vary strongly at Latemar and Concarena. The tepee belt at Latemar is governed by meteoric diagenesis, whereas the cement zone at Concarena is dominated by early marine phreatic cements. The comparison of the Latemar and the Concarena lead to the identification of several boundary conditions for MEC: (1) abundant open and connected cavities provided by e.g. rigid frameworks of reef building organisms or inter-particle space of talus breccias, (2) effective fluid flow mechanisms like wave activity in combination with matching platform margin morphology (walled reefs), (3) slow increase in accommodation (low AV) prolonging the time interval of early marine phreatic diagenesis (i.e. AV/ SVV1), (4) low carbonate production/framework accumulation rates (low SV) prolonging the time available for early marine phreatic diagenesis and fluid flow permeability.

Acknowledgements The authors would like to express special thanks to F. Jadoul, University of Milano (Italy), F. Berra and G. B. Siletto, Servizio Geologico Regione Lombardia (Italy) and P. Brack, ETH Zu¨rich (Switzerland). We also thank K. Carrie`re for improving the English of our manuscript. This project has been financed in part

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by the bDeutsche ForschungsgemeinschaftQ (funding M. Seeling; Project Be-641/33), the bStudienstiftung des deutschen VolkesQ (funding A. Emmerich) and the International Postgraduate Programme in geosciences at the University of Heidelberg (Germany). To L. Keim, A. Immenhauser and an anonymous reviewer we owe important comments on the manuscript.

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