Sedimentology, carbon isotope stratigraphy and micropalaeontology of the Rhaetian Zlambach Formation- Implications for the Dachstein carbonate platform development (Northern Calcareous Alps, Austria)

Citation for published version (APA): Mette, W., Clemence, M-E., Thibault, N., Korte, C., Konrad, B., & Ullmann, C. V. (2019). Sedimentology, carbon isotope stratigraphy and micropalaeontology of the Rhaetian Zlambach Formation-Implications for the Dachstein carbonate platform development (Northern Calcareous Alps, Austria). Sedimentary Geology, 382, 47-60. https://doi.org/10.1016/j.sedgeo.2018.12.009


Introduction
The Late Triassic has been intensively studied in recent years with respect to carbon isotope stratigraphy. Most of these studies (e.g. Hesselbo et al., 2002;Kürschner et al., 2007;Pálfy et al., 2007;Ruhl et al., 2009;Korte and Kozur, 2011;Pálfy and Zajzon, 2012;Felber et al., 2015; see also Korte et al., 2019 for an overview) focussed on the Triassic/Jurassic boundary interval where one of the largest global extinction events of the Phanerozoic is documented and which is characterised by distinct global negative carbon isotope excursions. However, the carbon isotope stratigraphy of the Rhaetian (e.g. Korte et al., 2005;Richoz et al., 2012;Whiteside and Ward, 2011;Mette et al., 2012;Korte et al., 2017;Zaffani et al., 2018) in general and the late Rhaetian pre-extinction interval in particular still lacks sufficient resolution. This is largely due to the fact that there are few continuous Rhaetian sections in open marine settings were the bulk carbonate carbon isotope (δ 13 C carb ) record was not affected by diagenesis or other factors. The only complete Rhaetian δ 13 C carb curves from western Tethyan pelagic environments published so far were reported from Italian sections. Muttoni et al. (2014) published a δ 13 C carb curve of moderate resolution from the southern Alps, showing very large negative and positive excursions which were interpreted as probably caused by mixing of shallow marine carbonate sediments from different sources. Rigo et al. (2015) showed δ 13 C carb and δ 13 C org records of much lower stratigraphic resolution from pelagic deposits of the southern Appennines reaching up to the middle or late Rhaetian (Misikella ultima conodont zone). In the Northern Calcareous Alps, the Rhaetian is represented in continuous successions of intraplatform basinal environments (Kössen Formation) and in oceanic basin successions of the western Tethys (Zlambach Formation). A δ 13 C carb record of low to moderate resolution from the lower to middle Rhaetian part of the Zlambach Formation was reported by Richoz et al. (2012). Subsequent carbon isotopic studies (Mette et al., 2012;Korte et al., 2017) of Rhaetian intraplatform basin

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Sedimentary Geology j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / s e d g e o facies have shown that significant carbon isotope excursions occurred in the late Rhaetian. It is, however, not clear whether these isotope excursions were just the result of regional environmental changes or if they reflect a more significant perturbation of the global carbon cycle (see discussion in Korte et al., 2017). The intention of the present study is therefore to document the carbon isotope record at high resolution as well as the sedimentary succession and palaeoenvironmental changes in the Rhaetian open marine realm of the western Tethys.

Geological setting
During the late Paleozoic to early Mesozoic, the Hallstatt facies (Northern Calcareous Alps, NCA) was located at the western termination of the Neotethys (Mandl, 2000;Haas et al., 2010;Fig. 1a). In the Late Triassic (Rhaetian), the distal shelf deposits of the western Tethyan margin included the Dachstein platform system in the northwest and the Hallstatt deeper shelf and basin in the southeast (Fig. 1a, b).
The opening of the Neotethys Ocean led to the development of extensional basins and carbonate platforms in the Middle Triassic (Mandl, 2000). A transgressive pulse just before the Carnian/Norian boundary caused an onlap of pelagic limestones over the shallow water carbonates and initiated the rapid growth of the thick Norian carbonate platform (Mandl, 2000;Fig. 1b). In the Hallstatt facies zone (Fig. 1c), the pelagic onlap represents a short time interval and open marine deposits became covered by the prograding carbonate platform of the Dachstein Limestone (Fig. 1b).
In the Rhaetian the Dachstein platform is bordered in the Northwest by an extensive shallow shelf with patch reef deposits (Oberrhaet Limestone) and an intraplatform basin where the Kössen Formation was deposited. The reefs of the southeastern Dachstein platform margin are interfingering with resedimented Norian limestones (Pedata Limestone) and the Rhaetian Zlambach Formation (Fig. 1b, c). The Late Carnian-Norian Pötschen Limestone and the Zlambach Formation represent the basinal part of the open marine deeper Hallstatt shelf in the Neotethys (Mandl, 2000;Richoz et al., 2012). The Zlambach Formation includes a lower 30-40 m thick unit of alternating grey marls ("Untere Zlambach-Schichten"), bioturbated and fossiliferous micricitic limestones, allochthonous arenitic limestones and an upper monotonous unit of dark shales and marls with pyrite nodules and 0,5-1 m thick bioturbated marly limestones ("Obere Zlambach-Schichten") (Krystyn, 1987(Krystyn, , 1991Matzner, 1986). The thickness of the upper unit cannot be determined due to pervasive tectonism. Estimates vary between a minimum of 25 m and a maximum of 100 m.
The bio-and lithostratigraphy of section 1 (~41 m thick; Fig. 2) were studied by Piller (1978), Krystyn (1987) and Weiland (1991). Based on their data and our additional ammonite occurrences, section 1 is attributed to the middle Rhaetian (Vandaites stuerzenbaumi ammonoid Zone) and lower upper Rhaetian (lower C. marshi ammonoid Zone; see Figs. 2, 3). Section 2 (~23 m thick; Fig. 3) is dated as upper Rhaetian (upper C. marshi zone) on the basis of the occurrence of C. marshi. The lithology shows a distinct overlapping interval of~8 m thickness between the uppermost part of section 1 and the lower part of section 2 (Fig. 3). In section 1, the litho-and biostratigraphy data thereby allow to calibrate the high-resolution δ 13 C carb curve reported later in this study (Fig. 3).
In the lowermost part of the Rossmoosgraben composite section (0-11 m interval), attributed to the V. stuerzenbaumi zone, compact clayey limestone beds (10-20 cm) are intercalated with thin light-400 m as well as few macrofauna, including ammonites and shell fragments of pelagic bivalves and brachiopods. The C. ammonitiforme subzone of the C. marshi zone (11-29 m interval) shows stacked thickeningupward sequences (2-4 m) of thicker bioturbated limestone beds (30-40 cm) and light-grey bioturbated marl and dark shale (N1 m) couplets. In the upper third of the section (C. marshi subzone, 29-56 m interval), the Zlambach Formation shows few marly limestone beds and a transition of light-grey marl layers to dark marly shales representing the upper unit of the Zlambach Formation. The light-grey clayey marls are partly laminated in the lower part and become increasingly bioturbated and dark towards the upper part of the C. marshi zone, above 45 m (Fig. 4).

Microfacies analysis
Eighty one samples were taken to characterise the microfacies using the classification scheme of Dunham (1962) and the rock constituents. For a more detailed description of the very fine-grained carbonate types the terminologies of Folk (1959) and Grabau (1904) have been applied. The sampling intervals are indicated in Fig. 4. Based on field observations and detailed microfacies investigations, different lithofacies types have been distinguished.

Analytical procedure
A total of 652 samples were collected for stable isotope analysis and XRF measurements: 339 samples from section 1 and 313 samples from section 2 at vertical sample spacing of~5-10 cm. The resulting composite curve consists of 582 data points in total.

Stable isotope analysis
The bulk rock samples for δ 13 C carb and δ 18 O carb analyses were performed on a ThermoFisher Delta Plus XL isotope ratio mass spectrometer with an analytic precision (1σ) of 0.08‰ for δ 18 O carb and 0.06‰ for δ 13 C carb (Spötl and Vennemann, 2003; Institute of Geology, University of Innsbruck), reported on the V-PDB scale and calibrated against NBS19.

Hand-held XRF measurements
Geochemical composition of the bulk rock powders was measured using a handheld energy dispersive X-Ray Fluorescence (XRF) spectrometer alpha-8000 LZX from Innov-X/Olympus (Dahl et al., 2013) in the department of Geosciences and Natural Resource Management (IGN), University of Copenhagen. Powders were measured directly in their cylindrical containers (Diameter = 3 cm; Height = 6.5 cm; ≥2.5 cm thick layer of powder) by covering the opening with a thin layer of kitchen wrap. Conversion from absorption to concentrations was performed after calibration at stainless steel target and subsequent data reduction was obtained using Innov-X software that included a five-point calibration curve with NIST standards (NIST 2702, NIST 2709, NIST 2710, NIST 2711, NIST 2781. Internal lab calibration of the device is described in Ahm et al. (2017) and the fully detailed recommended procedure for analysis is given in Thibault et al. (2018). Comparison of XRF versus ICP-MS measurements shows that the average relative external uncertainty across all recorded elements is within 13%, and the intrumental error is generally less than 10% (Ahm et al., 2017). A wide range of elements heavier than Na is automatically detected and quantified during XRF analysis.

Microfossil analysis
For micropalaeontological analysis 37 samples of shales and marls have been dissolved using a standard laboratory method. Each sample was dried at 80°C and 500 g of sediment was treated with petroleum and 15% hydrogen peroxide. The dissolved residue was washed with sieves of 1.0 mm, 0.3 mm, 0.1 mm and 0.063 mm mesh-size. The residue of prepared samples was completely picked and used for a quantitative ostracod analysis. For the abundance analysis two complete valves or one complete carapace were counted as one specimen. For the population age structure analysis the total number of specimens of each juvenile stage and the adult stage of the species Ogmoconcha amphicrassa (Kristan-Tollmann, 1971) was determined. The taphonomic interpretation of the age structure was based on Whatley (1988) and Boomer et al. (2003). The ecological diversity was evaluated using the Shannon-Wiener diversity index.
were defined. The most important characteristics of the distinguished facies types, together with their interpreted depositional setting, are illustrated in Fig. 4 and summarized in Table 1. For each LF type the corresponding standard microfacies types of Flügel (2004) is quoted.

Elemental geochemical analysis
In order to characterise changes in the chemical composition of the lithofacies described above which are not caused by dilution of CaCO 3 , trace elements were plotted as ratios normalized to Al (e.g. Van der Weijden, 2002). To show variability in the composition of terrigenous input through the deep marine sediments of the Zlambach Formation, we selected those elemental ratios (Ti/Al, Si/Al, Zr/Al and K/Al) which are clearly of terrigenous origin, not depending either on redox or on productivity changes. The elemental ratio of Ca/Al correlates with carbonate content (Fig. 5).
Two significant features in the lithogenic sediment fraction are clearly evident in the XRF data, showing a good correspondence to facies changes through the section. In general, the Ti/Al distribution (Fig. 5) follows the same trend as the ratios of Si/Al, Zr/Al, Ti/Al and Ca/Al through the studied interval. In detail, these ratios are consistently higher in the lower part of the section (0-29 m). In this interval, a total of ten levels (in grey, Fig. 5) with the highest values in Ti/Al, Si/Al, Zr/Al, Ca/Al and a decrease in K/Al are observed reflecting a coarsening of the facies that is also coincident with the occurrence of allodapic limestone beds (LF1-2). Shales and marls are characterised by low Ti/Al, Si/Al, Zr/ Al and Ca/Al values.

Bulk carbonate C-and O-isotope stratigraphy
Carbon isotopic results from section 1 (0-33 m) reveal values that fluctuate gently around an average of 1.5‰, punctuated by a prominent shift at 33 m towards lower values down to 0.2‰ (Fig. 3). This latter negative shift can be correlated with a negative shift in the lower part of section 2 though the shift in section 2 is more progressive and stepwise and only reaches minimum values around 0.5‰ (Fig. 3a, in rectangle). The stratigraphic congruence is also supported by a good correlation for a selection of δ 13 C carb data points at the base and at the top of the overlapping interval between the two sections (R 2 = 0.85) (cross plot in Fig. 3). Datapoints that exhibit very negative values in section 1 have been excluded from this correlation due to potential diagenesis (Fig. 6a, see also Chapter 6.2). According to this integrated stratigraphy, a resulting composite section of the Zlambach Formation at Rossmoosgraben has been constructed by taking data from section 1 up until 33.15 m and all data of section 2 (Fig. 3).
Accordingly, the remainder of section 2 exhibits minimal values of 0.5‰ for the negative carbon isotope excursion (CIE) recorded in the marshi subzone between 33.5 and 41.5 m (Figs. 3,5). This trend is followed by a 6.5 m lasting positive excursion with a sharp base and top and with values returning to an average of 1.8‰ and maximum values of 2‰ between 41.5 and 48.5 m. A second negative excursion is recorded in the last 8 m of the section with values fluctuating around 0.5‰ up until the top of the section at 56 m.
The δ 18 O carb curve (Fig. 7) shows initially negative fluctuating values between a minimum of~−4.0 to a maximum of~−0.5‰ in the V. stuerzenbaumi zone and the lower C. marshi zone (C. ammonitiforme subzone). Then, in the upper C. marshi zone (C. marshi subzone), the oxygen isotope data decrease gradually highlighting a negative trend from~−2.0‰ to~−4.8‰ in average.

Foraminifera
The marls and shales of the Zlambach Formation yield diverse associations of foraminifera and ostracods. The diversity and ecology of foraminifera at the Rosmoosgraben section were investigated by Weiland (1991). Typical platform foraminifera such as Triasina and Aulotortus, which were reported from calciturbidites (Matzner, 1986), were not found in the present study. In our material the most diverse and abundant groups are the Textulariina and Lagenina that represent autochthonous faunal elements (Weiland, 1991). The Lagenina are strongly dominated by Nodosariidae both in abundance and diversity. Oberhausellidae occur in the transgressive intervals of the section (Fig. 8) and are most frequent in the uppermost 5 m of the section. Involutina which was interpreted as an allochthonous faunal element (Weiland, 1991) occurs almost exclusively in the lower part of the section (below 29 m) in low to very low abundance. Relatively high abundance of Involutina is recorded in two samples of the uppermost 5 m of the section. The geochemical proxies for input of allochthonous sediment (Fig. 5) do not, however, indicate any sediment redeposition for this interval. Elements of other groups such as Fusulinina were only rarely found.
The foraminifera assemblages show significant variations in number of specimens and taxonomic diversity (Fig. 3). Very low diversity and abundance values occur in the upper part of the Rossmoosgraben composite section (C. marshi subzone) with minima at 33-34 m, 38-39 m, 44-45 m and in the uppermost 3 m of the section.

Ostracods
Ostracods from the Zlambach Formation were studied by Kollmann (1960Kollmann ( , 1963, Bolz (1971) and Kristan-Tollmann (1970, 1971a, 1971b, 1973. These studies were, however, mainly focussed on the taxonomy of material from different sections and the palaeoecology of the ostracods is therefore still not well-known.
The present study includes a detailed quantitative, taphonomic and diversity analysis of 37 ostracod assemblages from the Rossmoosgraben section. The ostracod assemblages recovered so far from the Rossmoosgraben section include 98 species. The number of species in individual assemblages varies between 4 and 49. The Bairdioidea and Healdiidae represent N95% of the ostracod specimens, the remainder consist of Cytheroidea, Cytherellidae, Cyprididae and Polycopidae.
Seven samples yielded a high number of specimens of Ogmoconcha amphicrassa (Kristan-Tollmann, 1971a, 1971b and could therefore be used for a detailed age structure analysis. All these assemblages show an age structure of a low-energy thanatocoenosis according to Whatley (1988) and Boomer et al. (2003) indicating little or no post-mortem transport. All other samples with moderate specimen numbers of O. amphicrassa (Kristan-Tollmann, 1971a, 1971b) yielded at least a few adults and show moderate abundance of larger juveniles. A typical low-energy taphocoenosis, which indicates long-distance transport and would be expected from samples of distal turbidites, is characterised by the absence of adult specimens and strong predominance of small juveniles. Such ostracod associations were not found. The preservation of shells is good to very good and abrasive structures were rarely observed. Heavily ornamented shallow- water taxa such as Dicerobairdia or Nodobairdia and also the shallow-water genus Cytherelloidea which are typical for shallow platform environments are absent or very rare.
Furthermore typical shallow platform ostracod genera like Lutkevichinella which was reported from the bedded Dachsteinkalk facies  were never found in the Zlambach beds. Other carbonate platform faunal elements which are reported from the allochthonous fossil deposits of the Fischerwiese (Matzner, 1986;Krystyn, 1991) such as coral fragments, bryozoans, calcisponges or calcareous algae are also missing or very rare in the present material.

Ostracod abundance and diversity
The ostracod assemblages have been analysed with respect to total and relative abundance of the taxa and ecologic diversity. The results show some significant changes both in ostracod abundance and diversity (Fig. 3). The assemblages from the lower part of the section (0-32 m) show abundances between 45 and 608 specimens and diversity values between 1.75 and 2.85 with strong variations. At 33 m the data reflect an abrupt decline to very low abundance (from 108 to 10 specimens) and diversity (Index: 2.47 to 1.03). This negative shift is followed upwards by a diversity increase at 36 m to 2.6 and another decrease at 38 m to 1.47 and low abundance (27 specimens). Another increase of diversity and abundance occurs at 41 m (172 specimens/ 1.95). At 45 m occurs the next minimum in abundance (6 specimens) and relative low diversity (1.5). The values increase again between 46 m and 49 m. Strong and short-term fluctuations in abundance and moderate diversity are recorded from the uppermost 3 m of the section.
Variations in ostracod and foraminifera abundance and diversity data show a remarkable coincidence with the changes of the carbon isotope curve. The first strong negative shift of ostracod abundance and diversity occurs at the same level where the first negative carbon isotope excursion starts. The interval of the negative isotope excursion (33 m to 41 m) and the subsequent positive shift (41 m to 45 m) is characterised by fluctuations between moderate and very low abundance and diversity values of the ostracods. At the top of the section (53.5 m to 56 m) occur strong short-term changes of ostracod abundance.

Carbonate microfacies
The LF 1,2 and 3 represent distal calciturbiditic deposits as indicated by frequent occurrence of elements of platform origin (peloids, shallow-reef benthic foraminifera), together with typical pelagic fossil elements (radiolarians, sponge spicules and thin-shelled benthic foraminifera and ostracods). The identified shallow-water bioclasts are characteristic elements of subtidal-to-intertidal environments and are encountered in the surrounding Dachstein carbonate platform. The peloids are typical representatives of the platform, such as lagoon or open shelf environments. Occurrence of deep-water fauna and the finer grain-size lithofacies support the distal to very distal and low density calciturbiditic nature of the resedimented limestone layers and exclude their misinterpretation as shallower tempestites. The observations of shallow-water fauna also distinguish the studied calciturbidite deposits from bottom current deposits. This finding is supported by the absence of grains with iron staining or ferro-manganese coatings observed in contourites. The poor sorting of the lithofacies in medium-grained sequences is an additional typical feature of calciturbidites. The sedimentologic features and fossil assemblages of the Zlambach Formation thus point to a distal toe-of-slope depositional environment, in the slope to basin transitional zone (Table 1) which is in concert with earlier sedimentologic studies of the Zlambach Formation (Matzner, 1986).
The lithology and interpretation of microfacies, as well as the biostratigraphy of the Rossmoosgraben section, which exposes the middle and upper Rhaetian of the Zlambach Formation, are shown in Fig. 4. Calciturbidite is the most characteristic facies in the lower part of the section (0-29 m; V. stuerzenbaumi zone and C. ammonitiforme subzone) followed by predominant pelagic deposits in the C. marshi subzone (29-56 m). The calciturbidites display m-scale coarsening upward sequences with medium (LF1, LF2) and fine-grained (LF3, LF4) intervals, representing well-distinguished sedimentary layers. The resedimented limestones become thicker and bioturbated upwards and the most prominent calciturbidite bed K (Figs. 3, 4) marks the transition between the calciturbidite-dominated deposits of the lower section and pelagic deposits of the upper part of the section.

Geochemical results
The geochemical trends observed in the lower part of the section, from 0 to 29 m (Fig. 5), reflect ten levels showing a coarsening of the facies and a strong increase of Ti/Al, Si/Al, Zr/Al and Ca/Al ratios that is coincident with the occurrence of calciturbidites (LF1-2). The positive peaks of Ti/Al, Si/Al, Zr/Al and Ca/Al ratios (Fig. 5) indicate maxima in fluvial sediment input. These latter previously interpreted as the basal part of turbiditic sequences are followed by a rise in K/Al values, whereupon the facies becomes finer grained mudstones. Minima in Ti/Al, Si/Al, Zr/Al and Ca/Al values are reached in coincidence with the change to pelagic deposits (LF4) above 29 m, with six intercalated distal turbidites (LF3; Fig. 5).
Regional patterns of variation in terrigenous proxy records can provide information about sources and fluxes of detrital material within the Hallstatt Basin. These changes are broadly aligned with facies changes. The calciturbidites present a stacking architecture that allows drawing stratigraphic sequences (Fig. 8). This sequence stratigraphic feature is typically used to recognise patterns in relative sea-level change (De Graciansky et al., 1998;Church and Coe, 2003).

Carbon isotope record
The carbon isotope record preserved in the marine rock record (δ 13 C carb ) is strongly depending on the partitioning between the marine carbonate carbon and organic pools (e.g. Scholle and Arthur, 1980;Weissert, 1989). Whereas positive shifts in the δ 13 C carb are generally related to enhanced marine burial of sedimentary organic relative to inorganic carbon (Jenkyns, 2010), negative CIEs might be related to reduced organic productivity, diminished organic carbon burial or processes remobilizing 13 C depleted carbon such as volcanism or methane hydrate release (e.g. Menegatti et al., 1998;Hesselbo et al., 2002;Tanner, 2010). Shallow water carbonate sediments deposited in situ are not reliable recorders of global δ 13 C carb as they are known to be affected by short-term changes in sea level, which subject the deposits to influences of meteoric diagenesis during which time the δ 13 C carb can be altered substantially (Swart, 2008). A more diagenetically stable environment is found on the slopes of carbonate platforms, at depths below sea-level changes, where mixtures of pelagic and platformderived sediments are deposited. In these location the δ 13 C carb of carbonate sediments, although subjected to closed-system diagenesis, are not substantially altered from their original δ 13 C carb values.
In the present study, the cross-plot of all carbon and oxygen isotope data of bulk samples from Rossmoosgraben (Fig. 6a) reveal a low covariance between δ 13 C carb and δ 18 O carb (R 2 = 0.19) suggesting that a diagenic impact, at least for one of the isotope systems, is only minor. A number of data points of section 1 appeared to stand out of the cloud with more negative values in both isotopic systems but particularly in δ 13 C (Fig. 6a). These values corresponding to the very pronounced negative excursion expressed in section 1 above 35.5 m (Fig. 3), we did not take them into account in our composite and preferred values of section 2 for this stratigraphic interval. The cross-plot between δ 18 O and δ 13 C of section 2 also appears to highlight potential alteration for values above 52.4 m (Fig. 6b). This is also in concert with the stronger fluctuating δ 13 C values at this stratigraphic level, a feature that is often caused by intermittent diagenetic impacts. Diagenesis, however, appears insufficient to explain the two δ 13 C negative excursions of the marshi zone.
We also note that high-frequency fluctuations are more common in the lower part of the Rossmoosgraben δ 13 C carb record (0-29 m), suggesting alteration of the primary carbon-isotope signature within calciturbiditic sequences characterised by significant increased detrital fraction (Fig. 7). The regressive phases in this stratigraphic interval have substantially superimposed the effect of diagenetic processes on the carbonates such as 13 C-depleted cements (Allan and Matthews, 1982;Joachimski, 1994;Sattler et al., 2005;Immenhauser et al., 2008). Moreover, the carbon isotopic composition of our carbonates fall in the range of 2.3‰ to~0.5‰ minimum, typical for pelagic and hemipelagic records (e.g., Jarvis et al., 2001;Muttoni et al., 2014).
The δ 18 O carb values show strong fluctuations in relation to lithofacies changes. Lighter δ 18 O carb values characterise marly beds, whereas heavier δ 18 O carb values are related to limestone beds, and this is wellreflected in the correlation between %CaCO 3 and δ 18 O carb (Fig. 6c) We also note that a slightly less significant correlation can also be observed between %CaCO 3 and δ 13 C carb (Fig. 6d). These observations could reflect a reduced diagenetic impact in calciturbidites (Fig. 7).

Comparison with other Middle to Late Rhaetian δ 13 C carb records
As stated in the introduction there is presently no other highresolution C carb record of the Middle-Late Rhaetian interval which could be directly compared with the present data. The Rhaetian C carb curve of the sections in the southern Alps of northern Italy (Muttoni et al., 2014) shows strong fluctuations. They found oscillations of 3‰ in C carb at the Italcementi quarry section and 3‰ in C carb respectively at the Brumano section. These strong variations were explained by the latter authors by mixing of shallow water carbonates from different sources. Although no statistical analysis of carbon and oxygen isotope data were provided the latter authors stated that these isotope data would represent original signals. Moreover the Rhaetian sections in the Southern Alps are not dated biostratigraphically due to the lack of ammonites and conodonts and the isotope record is therefore not comparable with the present data. Another C carb record was reported from Rhaetian sediments at the Pignola-Abriola section in southern Italy (Rigo et al., 2015) which were deposited in the deep marine Lagonegro Basin. This isotope record is of at least ten times lower stratigraphic resolution and focussing on the Norian-Rhaetian boundary and not the Middle Rhaetian. It is also not clear if these data were affected by diagenesis as statistical analysis of the isotope data is lacking. The δ 13 C carb data in Rigo et al. (2015) are thus also not directly comparable with the present isotope results. In contrast to the present results the δ 13 C carb curve in Rigo et al. (2015) shows several minor fluctuations of 1‰ δ 13 C carb or less and a long-term positive trend throughout the Rhaetian. Therefore a direct comparison of the δ 13 C carb curve from southern Italy with the present results is not possible.
The carbon isotope trend of Zaffani et al. (2018) for Northern Italy is based on bulk organic matter. A negative excursion is observed in the middle of the Rhaetian and this was correlated by these authors with a negative wiggle of the low resolution dataset of Whiteside and Ward (2011) provided for British Columbia. The biostratigraphic control, however, is poor. Because of both, the low resolution of the data and the poor biostratigraphic control of the literature dataset(s), it is difficult to correlate these isotope trends with our negative carbon isotope excursion. The stratigraphic part of the negative δ 13 C excursion(s) in the lower part of the Rhaetian presented by Zaffani et al. (2018) is not covered in our record.

Establishing a sequence stratigraphic framework for the Rossmoosgraben deposits
The facies characteristics of the succession exposed in Rossmoosgraben suggest a long-term deepening trend in the early Rhaetian to early Hettangian interval which was related to the opening of the western ("Ligurian") Tethys and corresponds to the so-called "Rhaetian" transgression of central Europe (De Graciansky et al., 1998;Hallam, 2001) establishing marine connections between the Tethyan and Boreal oceans along the proto-Atlantic seaway.
The lithofacies and geochemical proxies have revealed the presence of numerous calciturbidites that present a stacking architecture with sequences of carbonate beds that become progressively thicker (Fig. 8).
Turbid flows are the result of slope instability and therefore, the most likely explanation for such sequences is that of a sea-level control. Accordingly, a medium, third-order (sensu De Graciansky et al., 1998) regressive-transgressive sea level cycle is known to have occurred from the V. stuerzenbaumi zone to C. marshi zone. At the base of the transgressive tract, a sequence boundary (SB2) records the abrupt change in lithology from the predominance of calciturbidites to more pelagic facies.
The base of each fourth-order cycle is delineated by sequence boundaries (SB 1, SB 2) and reflected by maxima in the Ti/Al, Si/Al and Zr/Al values (coarse detrital fraction, see Fig. 5), whereas the end of the transgression, represented by a maximum flooding surface (MFS) corresponds to the highest relative proportion of the K/Al ratio (fine detrital fraction). The lowest part of the section (V. stuerzenbaumi zone) corresponds to the end of the regression of cycle 1 and terminates at 11 m.
The stratigraphic episodes of elevated K/Al ratios (Fig. 5) of the 16 "low-order" or minor cycles can be explained either by changes in the contribution of clay minerals or by changing ratios of fluvial versus aeolian sediment supply due to rapid sea-level changes and/or rapid, possibly orbital climatic variations that influenced sediment supply and loading on the slope. Within the 16 "low-order" or minor cycles, levels attributed to aeolian dust plumes are dominant at the base of each cycle whereas components transported by fluvial input are dominant at the top of each cycle.

Discussion of micropalaeontological results
The taphonomic analysis and taxonomic composition of the microfossil assemblages suggest that they were not significantly affected by redeposition. Therefore the suggestion of Krystyn (1991) and Richoz and Krystyn (2015) that the microbenthic assemblages from marly beds of the Zlambach Formation were generally transported from the Dachstein platform has to be rejected. According to the results of the microfacies and XRF analysis sediment redeposition from the platform was largely confined to the carbonate beds (see Chapters 4.2. and 6.2.).
According to Weiland (1991) the foraminifera suggest a predominance of favourable environmental conditions which were interrupted by short intervals of high organic input and low oxygen concentrations indicated in the lowermost part of the section and above 29 m by the predominance of infaunal taxa. The foraminifera associations from the upper part of the section (above 29 m) were interpreted to reflect alternations between normal and low oxygen conditions (Weiland, 1991). The predominance of nodosariid foraminifera in the present material is in accordance with a deep neritic environment. The relatively high abundance of Oberhausellidae at the top of the section may be either due to great water depths or short periods of oxygen deficiency (Clemence and v. Hillebrandt, 2013). A detailed palaeoecologic analysis of foraminifera from the upper part of the section (above 29 m) (Konrad, 2014) demonstrated a significant drop in diversity at 41-45 m where the positive carbon isotope excursion occurs (Figs. 2,4) indicating stressed environmental conditions. The high species diversity of the ostracods and absence of deep-water as well as typical shallowwater taxa is also suggestive of a deep neritic environment.
There is a coincidence between the first negative carbon isotope excursion and the decrease in ostracod and foraminifera abundance and diversity at the base of section 2, around 33.5 m as well as strong fluctuations in the abundance and diversity of ostracods and foraminifera in the intervals 34-45 m and 52-56 m (Fig. 3) that suggests an environmental perturbation in the C. marshi ammonoid subzone.
In summary most ostracod and foraminifera associations of the Rossmoosgraben section were not affected by redeposition and they reflect a predominance of stable and favourable conditions at the sea floor during the middle Rhaetian (V. stuerzenbaumi zone) and early late Rhaetian (lower C. marshi zone; C. ammonitifome subzone). For the upper C. marshi zone the benthic microfossil data are indicative of significant environmental deterioration.

Basin development
The basin development is summarized in Fig. 8. The middle and upper Rhaetian succession encountered in the Rossmoosgraben section shows a regressive-transgressive trend. The lower part of the section (0-29 m; V. stuerzenbaumi zone, C. ammonitiforme subzone), shows a predominance of distal toe-of-slope facies. The lower part of this interval (0-11.5 m) represents a first phase of platform progradation which is followed upwards (11.5-15.5 m) by a retrogradation as indicated by the decrease of calciturbidites and first occurrence of deep-water foraminifera (Oberhausellidae). The interval between 15.5 m and 29 m shows an upward increasing input of detrital carbonate reflecting a second platform progradation. In the upper part of the section (29-56 m; C. marshi subzone) calciturbiditic beds become successively thinner and much less frequent indicating platform retrogradation. The uppermost 8 m of the section are devoid of any calciturbidite which may be either due to increasing distance of the source area or to a final cessation of platform margin growth. Krystyn et al. (1991) suggested a drowning of the Dachstein reef and platform margin during the middle Rhaetian (V. stuerzenbaumi zone). The sequence stratigraphic results and occurrence of abundant and thick calciturbidites with platform derived components in the lower part of the Rossmoosgraben section (Fig. 8), however, suggest that the platform margin growth continued at least until the late Rhaetian (lower C. marshi zone, C. ammonitiforme subzone).
Our results also show a stratigraphic coincidence of significant carbon isotope excursions, rapid depletion of ostracod and foraminifera abundance and diversity and the disappearance of calciturbidites. This coincidence may be purely accidental. In our view this interpretation seems however improbable. We suggest a causal link of the sedimentological, micropalaeontological and geochemical changes reflecting an environmental perturbation and ecological change in the Hallstatt facies zone. In our view the most plausible interpretation is that this change was related to the cessation of the Dachstein platform margin growth. We are aware that this interpretation has to be confirmed by further extensive research in particular with regard to the biostratigraphy of the Dachstein platform and reef carbonates.
Within this framework of long-term facies changes, short-term facies changes can also be seen which may reflect minor sea-level or orbital climatic changes. The 16 "low order" cycles show microfacies types LF1 and 2 at the top with a high amount of platform-derived microfossils and peloids indicative of highstand-shedding intervals (Reijmer et al., 1992;Schlager et al., 1994). The stacking pattern of these cycles would be consistent with a control by Milankovitch periodicities (pers. comm. J. Pálfy). Shoaling upward cycles are a general feature of the Dachstein platform development. The famous Norian "Lofer cycles" were originally interpretated as deepening−upward units (Fischer, 1975) but later studies came to the conclusion that they were predominantly affected by periodic sea level drops (Goldhammer et al., 1990;Satterley, 1996;Enos and Samankassou, 1998;Haas et al., 2007). Minor sea level oscillations with short-term platform exposure of the Dachstein platform were also recorded by variations in turbidite composition of the upper Norian Pedatakalk Formation and the Pötschenkalk Formation of the NCA (Reijmer and Everaars, 1991;Reijmer et al., 1994). A shallowing−upward calciturbidite cyclicity, similar to that observed in the Zlambach Formation, was reported from Rhaetian basinal deposits of the Lombardy Basin in the southern Alps (Burchell et al., 1990). Transgressive-regressive cycles in late Rhaetian intraplatform basin deposits (Kössen Formation) also reflect periodic sea level drops (Golebiowski, 1989;Holstein, 2004) as highlighted by gamma ray spectroscopy results (Mette et al., 2016). Although Milankovitch periodicities were identified in Late Triassic calciturbidites of the Pedatakalk and Pötschenkalk Formations (e.g. Goldhammer et al., 1990;Reijmer et al., 1994) other factors such as tectonic control or autocyclic processes were also suggested (e.g. Goldhammer et al., 1990;Satterley and Brandner, 1995;Satterley, 1996) to play an important role for the cyclicity in Late Triassic platform deposits of the Alps.
A water depth of at least 300-500 m or even deeper has been estimated for the Hallstatt Basin during the Norian by Kenter and Schlager (2009) based on the geometry of clinoforms at the Dachstein carbonate platform slope. The latter authors stated, however, that the water depth may have decreased significantly during the Rhaetian due to terrigenous input in the Hallstatt Basin and may have also been changed by syndepositional movements between the platform and the basin. The benthic microfossil assemblages indicate that the depositional depth of the Zlambach Formation was probably not N300 m because typical bathyal ostracod genera with long spines such as Acanthoscapha or Nemoceratina, are absent or very rare. The high species diversity (at least 98 species) of the ostracod fauna would also be in better accordance with deep subtidal conditions than with a bathyal environment (Benson, 1988).

Conclusions
The Zlambach Formation exposed at Rossmoosgraben section is an open marine regressive-transgressive succession of marl-limestone alternations which was deposited in a toe−of−slope to basin environment and ranges in age from Middle to Late Rhaetian.
Sequence boundaries, transgressive surfaces and maximum flooding surfaces of 3rd and 4th order sequences and related to long-term sea level changes are recorded by detrital proxies indicating maxima and minima of terrestrial sediment input and by abrupt changes in the input of detrital carbonate from the platform. Minor sea level fluctuations are indicated by subordinate coarsening− and thickening− upward cycles characterised by distal calciturbidites which were derived from the Dachstein carbonate platform and slope.
The occurrence of platform derived calciturbidites in the late Rhaetian (C. marshi zone) is in contrast to the results of earlier studies indicating a drowning and cessation of Dachstein reef and platform margin growth during the middle Rhaetian (V. stuerzenbaumi zone). The present results are indicative of a final cessation of platform margin growth during the upper C. marshi zone.
Carbon isotope data of the Rossmoosgraben section show a 1.0‰ negative excursion and a subsequent 1.3‰ positive excursion in the upper C. marshi zone which is followed upwards by another gradual negative trend towards the top of the section. These results suggest that significant changes in carbon isotope ratios in the late Rhaetian were not confined to the intraplatform environments (Mette et al., 2012;Korte et al., 2017) but occurred as well in the open marine realm of the western Tethys.
The micropalaeontogical results show significant changes in abundance and diversity of ostracods and foraminifera in stratigraphic coincidence with the carbon isotope excursions in the upper Zlambach Formation (upper C. marshi zone) suggesting some important perturbations of marine environments in the western Tethys during the late Rhaetian.
The stratigraphic coincidence between the carbon isotope excursions, impoverishment of benthic fossils associations and the disappearance of calciturbidites suggests a causal link with the termination of the Dachstein platform margin reef growth.