Record of a dense succession of drowning phases in the Alpstein mountains, northeastern Switzerland: Part II—the Lower Cretaceous Schrattenkalk Formation (late Barremian)
Swiss Journal of Geosciences volume 116, Article number: 1 (2023)
The Schrattenkalk Formation represents a complete succession of Lower Cretaceous shallow-water carbonate platform series cropping out in the Alpstein massif of north-eastern Switzerland. The Schrattenkalk Formation is traditionally divided into two sedimentary units, the “Lower” and the “Upper” Schrattenkalk, separated by the more marly Rawil Member. The “Lower” Schrattenkalk is habitually dated to the late Barremian, while the Rawil Member and the “Upper” Schrattenkalk are dated to the early Aptian. New field observations, however, call the lithostratigraphic dichotomy of the Schrattenkalk into question, as the neritic carbonates are disrupted by several key surfaces associated with karstic episodes and/or transgressive sediments, corresponding to ammonite-rich hemipelagic deposits on the distal shelf. A large number of ammonites were collected in the Drusberg Member as well as rare ammonites from the Schrattenkalk Formation. These ammonites as well as the neritic macrofauna from the Schrattenkalk Formation allow a precise dating of the onset of the Schrattenkalk Formation across the Alpstein massif and its successive phases of progradation. Three successive carbonate bodies and a fourth sedimentary intermediate rock body at the top of the Schrattenkalk platform are defined, based on new biostratigraphic data and updated interpretations of the sequence stratigraphy and geochemical data. The data shows a progressive onset of the Schrattenkalk carbonate platform along the studied transect, following a SE progradation over time. The oldest deposits refer to the upper Barremian T. vandenheckii Zone and the youngest carbonates to the uppermost Barremian M. sarasini Subzone. The new dating of the discontinuity surfaces and key-beds highlight three successive flooding events. The first drowning phase, which correlates with the "Sartousiana" event, dates from the middle late Barremian (upper T. vandenheckii—lower G. sartousiana Zone). The second phase, represented by the Rawil Member, is an incipient drowning, which seems to coincide with the latest Barremian Taxy event (usually reported to the I. giraudi and lowermost M. sarasini zones) according to rare ammonite discoveries. The final demise of the Schrattenkalk platform, situated close to the Barremian-Aptian boundary, is related to an exposure and consecutive drowning event.
The Schrattenkalk limestone, a landscaping element of the Helvetic realm, represents the most imposing lithostratigraphic unit of the Lower Cretaceous Helvetic series. It marks the panorama by its high and white rock walls and its large-scale lapies (karst topography). However, its establishment and evolution are subject of various chronostratigraphic and sedimentological models that are profoundly conflictual. The lithostratigraphic subdivision of the Schrattenkalk Formation has likewise been very fluctuant since the beginning of its investigation.
The Barremian time interval and more particularly the late Barremian-earliest Aptian is considered to have been a period of relative quietness in the evolution of the biosphere (Föllmi, 2012). This period is nested between the latest Hauterivian Faraoni episode (Cecca et al., 1994), the first widely recognized oceanic anoxic event of the Cretaceous and the Selli episode (Coccioni et al., 2003), one of the major phases of paleoenvironmental change of the entire Cretaceous (Föllmi, 2012). The apparent stability within this time-period is argued by lower rates of organic-matter preservation in the oceans and by the maximum extension of the Urgonian carbonate platform on the northern Tethyan margin (Föllmi, 2012). However, the occurrence of numerous thin laminated organic-rich muds (LOM) intervals is known from Barremian sediments of the Tethys (Aguado et al., 2014; Bersezio et al., 2002; Coccioni et al., 1998, 2003, 2006; Föllmi, 2012; Mahanipour & Eftekhari, 2017; Masse & Machhour, 1998; Weissert et al., 1979; Yilmaz et al., 2012) and in the Lower Saxony Basin (Malkoč & Mutterlose, 2010; Mutterlose & Böckel, 1998; Mutterlose & Bornemann, 2000; Mutterlose et al., 2003, 2009, 2010; Pauly et al., 2013). These black shales have been found in abundance in upper lower and lower upper Barremian sedimentary rocks and around the “Barremian/Aptian boundary” interval (Coccioni et al., 2003; Föllmi, 2012; Rieber, 1977; Sprovieri et al., 2006; Talbi et al., 2021; Weissert et al., 1979). Recent studies revised the age of the peri-Vocontian urgonian platforms (Frau et al., 2018a, 2020; Pictet et al., 2019, 2022; Tendil et al., 2018), permitting a better time calibration between neritic carbonate platforms and basinal sedimentary series. Based on these studies, a better synchronicity and timing between platform drownings and anoxic events is enabled.
Oceanic carbonates play a key role in the global carbon cycle as they represent the most important carbon sink (Wissler et al., 2003). In contrast, shallow-water carbonate platforms are particularly sensitive to environmental and oceanographic change (Föllmi et al., 2006, 2007; Wissler et al., 2003). A large amount of work has been dedicated to the Barremian to lowermost Aptian neritic carbonate succession deposited on the Helvetic domain of the north-western Tethyan shelf and more specifically to the sedimentological features of the Schrattenkalk platform due to its well exposed sections (e.g., Bonvallet, 2015; Bonvallet et al., 2019; Briegel, 1972; Burckhardt, 1896; Fichter, 1934; Funk & Briegel, 1979; Heim, 1905; Heim & Baumberger, 1933; Jost-Stauffer, 1993; Kempf, 1966; Lienert, 1965; Staeger, 1944; Wissler, 2001; Wissler et al., 2003). The Schrattenkalk Formation appears to be interspersed with several key surfaces associated to specific orbitolinids- and other fossils-bearing beds (Wissler et al., 2003) and to phosphate-bearing beds onto the platform slope (Bodin et al., 2006a, 2006b; Bonvallet et al., 2019; Föllmi & Gainon, 2008; Pictet et al., 2022; Wissler et al., 2003). Marine phosphate-bearing beds are commonly regarded as the result of drowning episodes. These were associated with increased nutrient input as consequence of intensified chemical weathering on the continent due to warmer and more humid climate conditions (Föllmi et al., 2007). These may coincide in time with large scale changes in the ocean-climate system likely related to increased submarine volcanic activity of the Ontong-Java Plateau, which eventually led to the worldwide early Aptian Selli episode (OAE1a; Coccioni et al., 2003; Erba et al., 2015; Föllmi, 2012).
The current study presents a new biostratigraphical and sedimentological model of the Schrattenkalk Formation in the Alpstein massif. This model is discussed in terms of episodes of environmental change affecting the north western Tethys margin and the associated oceanic anoxic event intervals and other laminated, organic rich muds (LOMs) of latest Barremian age.
2 Geographical and geological setting
The Alpstein massif, situated in north eastern Switzerland, is shared by the cantons of Appenzell Ausserrhoden, Appenzell Innerrhoden, and St.Gallen. The mountain range is located in the north-eastern part of the Helvetic nappes of Switzerland (Fig. 1A). The massif has been thrusted and folded in a north western direction during the Alpine orogenesis (Fig. 1B–D) and detached from its Jurassic substratum along the Säntis Thrust (Pfiffner, 1981). The Cretaceous record presents a well exposed and developed Barremian sedimentary succession composed of a shallow-water carbonate platform series (Urgonian facies) referred as Schrattenkalk Formation, prograding on hemipelagic slope deposits of the Tierwis Formation (Fig. 1B, C; see Pictet et al., 2022).
The delimitation of the hemipelagic limestone and marlstone alternations of the Drusberg Member (i.e., = upper part of the Tierwis Formation) from the neritic carbonates of the Schrattenkalk is problematic due to lateral and temporal transitions from one unit into the other (Bollinger, 1988; Schenk, 1992). The delimitation becomes even more progressive and blurred in the distal domain of the sedimentation area (i.e., further to the southeast; Bollinger, 1988; Heim & Baumberger, 1933). Heim, (1910–1916) already emphasises that the “Drusberg Layers” should be regarded solely as a division based on facies whose “boundary” fluctuates wildly and stratigraphically rises to the south. Nevertheless, the ecological relationship between the Schrattenkalk and the Drusberg series is clear. The Schrattenkalk limestone represents a neritic sedimentary succession passing seaward to the hemipelagic slope deposits of the “Drusberg layers” (Funk & Briegel, 1979; Heim, 1910–1916).
The “Schratten-Kalk” was mentioned for the first time by Studer (1834) describing the vast limestone fields of the southern slopes of the Schrattenflue Mountain (Canton Lucerne, Switzerland, Fig. 2). The Schrattenflue itself draws the origin of its name from the local language in which “schratten” means lapies (karst plateau). The Schrattenkalk term was popularized in the whole Helvetic domain by Heim, (1910–1916), prevailing on the French Urgonian limestone of d’Orbigny, (1847–1849) (see Funk & Briegel, 1979; Schenk, 1992). Funk and Briegel (1979) raised the question of whether it is permissible to compare the Schrattenkalk with the French Urgonian Formation. Following the definition worked out by the Colloquium on the Lower Cretaceous (Rat, 1965), the urgonian denomination is restricted to Lower Cretaceous limestone facies, which contain rudist shells of the genus Toucasia Munier-Chalmas, 1873. Such a definition allows equivalence between the Urgonian Formation and the Schrattenkalk Formation. The “Schrattenkalk series” were originally subdivided by Kaufmann (1867) into two units, which share nothing with their current lithostratigraphic nomenclature (Fig. 2). Nowadays, the Schrattenkalk Formation is conventionally subdivided into a “Lower Schrattenkalk” unit and an “Upper Schrattenkalk” unit sensu Heim, (1910–1916; i.e., = lower and upper rudists-bearing limestone of Kaufmann, 1867; Fig. 2), separated by the “Orbitulina Beds” (Kaufmann, 1867) or “Lower Orbitulina Beds” (Burckhardt, 1896), today referred as Rawil Member.
These “Orbitulina Beds” were reported for the first time by Kaufmann (1867) at the locality of Lopperberg (Canton of Nidwalden) where he described an intercalated marly bundle inside the Schrattenkalk series (Fig. 2). This intercalation is characterised by blue to yellow, iron-rich, sandy marlstones and limestones, which he named “Lower Orbitulina Beds” due to the mass occurrence of the benthic foraminifera Palorbitolina lenticularis (Blumenbach, 1805). Lienert (1965) designated these layers in the Säntis area as “Middle Schrattenkalk”. Schenk (1992) described the presence of abundant plant remains such as wood fragments. She introduced the Rawil Member to define the Lower Orbitolina Beds from the Wildhorn nappe of the Bernese Oberland, which represent more distal environments of the Helvetic domain compared to the Lower Orbitolina Beds type-series of central and eastern Switzerland. The Rawil Member and the Lower Orbitolina Beds are considered to be largely synchronous due to their microfaunal association (Lienert, 1965). The geographic distribution of the Rawil Member and the Lower Orbitolina Beds is limited to the proximal platform. A dichotomy of the Schrattenkalk series seems therefore only possible in this domain.
Briegel (1972) formally defined the Schrattenkalk series as a formation (Fig. 2). Bollinger (1988) divided the Schrattenkalk Formation in ascending order in a Schrattenkalk Member and a Grünten Member (Bollinger, 1988; the latter corresponding to the Upper Orbitolina Beds; Burckhardt, 1896; Fichter, 1934). Nevertheless, the glauconitic echinodermic series of the Grünten Member are considered as devoid of rudist shells and contains phosphatic beds. It does not fit the definition of the Urgonian facies accepted by the Colloquium on the Lower Cretaceous (Rat, 1965). Thus, the Grünten Member was moved to the base of the overlying Garschella Formation by Linder et al. (2006) (Fig. 2). Funk et al. (1993) introduced the Lower and Upper Schrattenkalk members to name the two superposed cliffs constituting the formation (Fig. 2). Föllmi et al. (2007) extended the Rawil Member to the whole Helvetic domain, replacing the Lower Orbitolina Beds type-series of central and eastern Switzerland (Fig. 2).
The establishment and evolution of the Schrattenkalk platform are subject of various chronostratigraphic and sedimentological models. Until recently the dating of the Schrattenkalk Formation was mainly based on the neritic foraminifera biostratigraphy of the family Orbitolinidae (Lienert, 1965; Schroeder et al., 1968, 2002, 2007). The use of these neritic microfossils led to profoundly conflictual chronostratigraphic datings of the Schrattenkalk Formation [Fig. 3; e.g., Clavel et al., 2014 versus Bonvallet et al. (2019), as well as Schroeder et al. (2007) versus Föllmi, (2008)]. Platform-to-basin correlations as well as correlations between mountain ranges of the Helvetic/Dauphinois domain were also used to date indirectly the Schrattenkalk Formation [e.g., Bodin et al., (2006a, 2006b) versus Clavel et al., (2014)]. A peculiarly condensed bed, called Chopf Bed by Briegel (1972), was identified in the Drusberg Member in Vorarlberg (Bollinger, 1988; Heim & Baumberger, 1933), in eastern Switzerland (Bodin et al., 2006a, 2006b; Briegel, 1972; Heim, 1910–1916; Lienert, 1965; Oberholzer, 1933; Pictet et al., 2022; Wissler et al., 2003) and in central Switzerland (Fichter, 1934; Staeger, 1944). Bodin et al. (2006a) dated the Chopf Bed in its type locality as transitional between the T. vandenheckii and the G. sartousiana zones. They attributed this bed to the maximum flooding surface (MFS) Ba3 of more or less comparable age. Based on a sequence stratigraphic interpretation of the Tierwis section, Bodin et al. (2006a) attributed the marlstone interval at the transition of the Drusberg Member to the Schrattenkalk Formation to the MFS Ba3, de facto attributing a maximal age for the installation of the Schrattenkalk platform in the Säntis area (Fig. 3). Nevertheless, the relationship between the Chopf Bed in distal platform environments and the facies change within the shallow-water urgonian succession remained unclear (Bonvallet et al., 2019). Clavel et al., (2014, p. 45) cast doubt on this hypothesis supported by two very poorly preserved Barremian ammonites figured by Bodin et al., (2006b, fig. 3E and F). Based on observations made in the French Subalpine chains (Haute-Savoie department), Clavel et al. (2014) suggest that the Schrattenkalk Formation arose during the earliest Barremian. Nevertheless, the use of a variety of tools as micro- and macro-fossils (e.g., Arnaud & Arnaud-Vanneau, 1991; Arnaud et al., 1998; Arnaud-Vanneau, 1980; Clavel et al., 1986, 2007, 2014; Frau et al., 2018a; Masse, 1976, 1995; Masse & Fenerci-Masse, 2008, 2017, 2018; Masse & Humbert, 1976; Masse et al., 2020) or sequence stratigraphic correlations and geochemistry (Bonvallet et al., 2019; Frau et al., 2018a, 2018b; Huck & Heimhofer, 2015; Huck et al., 2011, 2013; Wissler, 2001; Wissler et al., 2003) did not help to resolve all the existing conflicts of time correlation within the Schrattenkalk Formation.
3 Material and methods
The methods used for this study are essentially the same as described for the first part of the study of Pictet et al. (2022).
We carried out a sedimentological and palaeontological field study of the Schrattenkalk Formation based on sections in four reference areas (Tierwis-Grauchopf, Langtal-Säntis, Altmann and Litten-Chreialp—Tesel). The collected or reconsidered macrofossils are stored in the Naturmuseum St.Gallen (NMSG, ex-coll. Museum Heiden and Coll. Kürsteiner), in the inatura—Erlebnis Naturschau Dornbirn (Austria), in the Eidgenössische Technische Hochschule Zürich (ETHZ) and in the Musée cantonal de géologie of Lausanne (MGL, Coll. Föllmi).
Discontinuity surfaces are indicated on the lithological logs and figures and are numbered from D1 to D17. These surfaces were correlated with the sequence stratigraphic scheme of Arnaud (2005), derived from the Barremian stratotype of Angles (SE France). The sequence stratigraphic interpretation is given on the right side of the lithological logs. Sequence boundaries (SB) are indicated by red full lines, maximum flooding surfaces (MFS) by blue dot lines and transgressive surfaces (TS) by green dash-dot lines.
Geochemical measurements from the Tierwis-Grauchopf section were performed by Wissler (2001) and Bonvallet (2015). We used the carbon-isotope segments B1 to A1 (negative, stable or positive trend), that correspond to the nomenclature of Wissler, (2001) and Wissler et al., (2002, 2003) for the Barremian and lowermost Aptian stages.
The ammonite biozonation used in this work bases on the zonal scheme of the Tethys realm proposed at the 6th International Meeting of the IUGS Lower Cretaceous “Kilian Group” (Reboulet et al., 2018).
In order to avoid reproducing the profound nomenclatural confusion concerning the different carbonate units composing the Schrattenkalk Formation and affine units, the successive limestone units are named from bottom to top as S1 to S4-Gr. Each Schrattenkalk unit herein corresponds to an individual prograding platform body, which is characterised by its own petrographic and/or macrofaunal content. Schrattenkalk units are separated by flooding surfaces or flooding intervals such as the Rawil Member.
4 The key sections
The sections at Tierwis-Grauchopf, Langtal-Säntis, Altmann and Litten-Chreialp—Tesel are described following a northwest-southeast platform-to-basin transect.
4.1 The Tierwis-Grauchopf section
The transition from the Drusberg Member to the light-grey shallow-water carbonates of the Schrattenkalk Formation is progressive, occurring on a thickness of ca. 5 m. In this section, the neritic carbonate series of the latter only present the units S1 and S3 with from bottom to top:
A 100 m-thick cliff-forming unit S1 representing the 60% of the thickness of the whole formation. The cliff is massive, without marked stratification (Heim, 1905; Fig. 4A). Its base consists of a bioclastic facies and contains some oysters-bearing beds (Fig. 4B). The middle part is fairly coralliferous (Fig. 4C). Its top contains abundant rudist shells dominated by the Barremian rudists species Agriopleura blumenbachi (Studer, 1834) and A. marticensis (d’Orbigny, 1847) (Fig. 4D; see also Bollinger, 1988). The top of the Schrattenkalk unit S1 shows a very strongly karstified surface covered with a red palaeosol containing reworked pebbles and fragments of rudist shells (Fig. 4E; exposure surface E.s. on Fig. 5A);
A 25 m-thick Orbitolina-rich intercalation (Fig. 5A—Rawil Mb) of brownish, thin and well bedded limestones (Heim, 1905). This unit starts with a fossiliferous basal limestone containing abundant internal moulds of gastropods belonging to Harpagodes pelagi (Brongniart, 1821) and numerous rudist shells of Requieniidae like Toucasia carinata Matheron, 1842 and Requienia ammonia (Goldfuss, 1832). Above the basal limestone, brackish, grey marlstones and clayey-limestones (Fig. 5B) and shore deposits extremely rich in charophytes and tree trunks (Fig. 5C; Bonvallet et al., 2019) are deposited. Upwards, follows coral-bearing beds (Fig. 5D; see Baron-Szabo, 2021) grading upward into rudist-bearing beds, dominated by a requieniid-monopleurid assemblage. Near the top of the unit, a bioturbated firmground (D14) is observed (Fig. 5E). The surface is overlain by marlstones (Fig. 5A –O. l. bed) filling the bioturbations of the underlying firmground with a bioclastic sediment almost exclusively composed by the foraminifera Palorbitolina lenticularis (Blumenbach, 1805). Echinoids as Pygaulus desmoulinsi Agassiz, 1847 and Heteraster oblongus (Brongniart, 1821) are common (Figs. 5F and 6B);
A 25 m-thick Schrattenkalk unit S3 (Figs. 5A and 7A), which only represents 1/5 of the thickness of the formation. Unit S3 is well bedded (Heim, 1905) and very rich in rudist shells of Requieniidae (Fig. 7B). The terminal surface shows an irregular topography (Fig. 7A) of epikarstic appearance (Fig. 7C);
The irregular and corroded surface is covered by iron and phosphate crusts. The cavities are filled with an undated echinodermic limestone containing abundant phosphoclasts and pebbles of Schrattenkalk facies (Fig. 7C). The whole is capped by grey to brown glauconitic marlstones attributed to the Sellamatt Beds (Selun Member; Fig. 7A) of middle Albian age (Föllmi & Ouwehand, 1987).
Carbon and oxygen stable isotope measurements were performed in the Schrattenkalk Formation by Wissler (2001) (Fig. 8). The Schrattenkalk Formation was later resampled for higher resolution by Bonvallet (2015) and Bonvallet et al. (2019) who also measured the phosphorus content (Fig. 8).
The carbon stable-isotope record from the Schrattenkalk Formation in Tierwis-Grauchopf section exhibits five successive trends characterising the carbon isotope segments B3 to A1 following the nomenclature of Wissler (2001) but differing to his interpretation of the individual segments (in gray):
-Segment B3, first “stable” and then bearing decreasing values within the Drusberg Member and lowermost Schrattenkalk beds (refer to Pictet et al., 2022, fig. 5 for a more detailed curve of segments B1 to B3);—B4 with increasing values starting inside the lowermost Schrattenkalk beds and then stabilizing by around 2.7‰ and a top marked by a drop of 2‰ occurring just below the karstic surface D13;—B7 is characterized by a positive excursion to higher δ13C values then stagnating by around 3.4‰;—B8 with a decreasing trend toward more negative values up to 2.8‰; A1 with increasing values up to 3.6‰. The lack of statistically significant correlation between δ13C and δ18O values (covariance R2 = 0.11; Bonvallet, 2015, pp. 85 and 89) for the section of Tierwis-Grauchopf excludes a diagenetic modification of the primary carbonate C isotope signature.
The phosphorus (P) content shows three intervals inside the Schrattenkalk series with:—a general upward decrease trend across the unit S1, ending by strong fluctuations in the rudists-bearing beds;—increasing values in the overlying Rawil Member (Bonvallet et al., 2019; Stein et al., 2012a);—a new decrease in the unit S3;—a pronounced peak starting a little below the terminal karstified surface of the Schrattenkalk, due to karstic infiltrations from the overlying Garschella Formation (Bonvallet et al., 2019).
4.2 The Langtal-Säntis section
Like in the Tierwis-Grauchopf section, the uppermost beds of the Drusberg Member still contain ammonites from the T. vandenheckii Zone (Fig. 9B; Pictet et al., 2022). These beds seem to extend stratigraphically further upward than in the Tierwis-Grauchopf section before they pass progressively, over a distance of approx. 5 m, into the Schrattenkalk unit S1 (Fig. 9C). At Langtal-Säntis, the unit S1 is only about 10 m-thick, in contrast to a thickness of 100 m in Tierwis-Grauchopf. The unit S1 ends with a noticeable discontinuity surface D12 (Fig. 9D). Above this surface, the Schrattenkalk unit S2 starts with thin-bedded bioclastic and oolitic series before grading into a massive limestone cliff with a thickness of about a 100 m. The unit S2 is overlain by the Rawil Member and the Schrattenkalk unit S3 (Fig. 9B).
4.3 The Altmann section
The Schrattenkalk Formation progressively arises from the Drusberg Member, representing the G. sartousiana Subzone (Pictet et al., 2022). The Schrattenkalk unit S1 and its type fauna is missing here as, according to the ammonites, it sedimented under the Drusberg facies. Therefore, the Schrattenkalk cliff presents the successive units S2 to S4-Gr with from bottom to top:
A 116 m-thick massive unit S2 starting with 20 m of obliquely stratified limestones (Fig. 10A–C). A juvenile specimen of Barremitidae related to the genus Barremites or Montanesiceras was found in the scree by Christian Klug (Paläontologisches Institut und Museum Zürich; Fig. 11A). A fragment of a large ammonite was found in the lower third of the unit S2 by T. Kempf (Lienert, 1965, p. 22). Lienert reported this fragment to a possible “precursor” to the “Parahoplitidae” found in the “Upper” Schrattenkalk of Brunnen (Canton of Schwytz). The specimen was unfortunately not found in the collections of the Swiss Federal Institute of Technology Zurich (ETHZ) and so probably never collected.
A 20 m-thick interval with two tender bundles, corresponding to the Rawil Member (Fig. 10C).
A 28 m-thick, better stratified limestone unit S3 (Figs. 10C).
A 3 m-thick, stratified limestone unit S4-Gr richer in echinoid debris and presenting glauconite grains (Fig. 10C and D), indicating a transition to the overlying Garschella Formation.
Geochemical analyses of the stable carbon and oxygen isotopes were performed by Wissler (2001, p. 109) but the sample spacing is too loose for supporting any interpretation.
4.4 The Litten-Chreialp—Tesel section
The Schrattenkalk Formation was never really studied in detail in this part of the Alpstein massif.
Our observations were made along the anticline, mostly in Litten-Chreialp section situated on the eastern side of the Tristen Sattel (Coordinates 2′745′7347;1′232′270 system CH1903 + /LV95) and along the path from Eggsteihalde (coord.: 2′745′347;1′231′654) to Chridegass (coord.: 2′645′318;1′231′981).
The Drusberg Member, outcropping well in Litten-Cheialp, progressively passes upward to the Schrattenkalk series, which present the successive units S2 to S4-Gr with from bottom to top (Fig. 12A):
A 100 m-thick limestone unit S2, presenting an alternance of beds, dominated by bioclastic facies, grading to a massive cliff (Fig. 12B, D and E). The base of the unit S2 yielded the echinoid Pygaulus desmoulinsi Agassiz, 1847, while the upper and massive part contains abundant rudist shells belonging to Agriopleura marticensis (d’Orbigny, 1847) (Fig. 12C) and the gastropod Harpagodes pelagi (Brongniart, 1821);
An about 4 m-thick Rawil Member with a tender interval composed of thin-bedded clayey limestones grading upward to white massive carbonates;
A 30 m-thick Schrattenkalk unit S3 (Fig. 12D and E), in which the rudist fauna consist only of representatives of the family Requieniidae. The top surface looks like a discrete bioturbated firmground;
A 17 m-thick unit S4-Gr (Figs. 12D–E and 13C–D) made of fine, light grey to grey and weakly glauconitic echinodermic limestones. The base is composed of a 2 m-thick bioclastic limestone, presenting a cross-stratification. Its top surface shows a discrete bioturbated firmground. This surface is overlain by a more marly and strongly bioturbated 1.5 m-thick intercalation. This marlstone is rich in orbitolinids, such as Palorbitolina lenticularis (Blumenbach, 1805) (Fig. 12F), in echinoids such as Heteraster peroni Ficheur, 1900 (Fig. 6C) and contains abundant small-sized rudist shells of Requieniidae. The unit ends by a well-visible firmground D16 injected with a quartz sand.
This uppermost discontinuity surface marks a strong sedimentological change toward more friable and dark-coloured series which are attributed to the “middle” Cretaceous Garschella Formation.
4.5 South-eastern mountain range, close to the Rhine valley (Stauberen-Hoher Kasten-Kamor)
In this range, the Schrattenkalk series is represented by a single limestone cliff (Fig. 14B).
The Schrattenkalk and Garschella formations were mostly observed along the road from Rüthi to the Kamor (Schlatt-Mätzenacker, Fig. 14A, coord.: 2′759′093;1′241′164 to 2′758′902;1′241′083).
The sedimentary succession measures around 140 m-thick and is represented by the units S3 and the Grünten Member with, from bottom to top:
The lower two thirds of the unit S3 are composed of bioclastic, mostly echinodermic cross-stratified limestones (Figs. 14A, D and E). In the upper third of the unit, these bioclastic limestones grade into rudist-bearing beds dominated by the Requienidae, a typical assemblage of the unit S3. The unit ends by a corrugated and bored hardground (D15; Figs. 14F and G).
The surface of the unit S3 is overlain by the Grünten Member (Garschella Formation). It is much coarser-grained echinodermic grainstones, associated to glauconite, orbitolinids (Fig. 14H) and large corals (e.g., in Bergli section). The unit ends with a corrugated firmground D16.
A 1 m-thick marly-limestone with corals grading upward to a very fractured, coarse-grained echinodermic limestone of the Brisi Member.
5.1 Onset and development of the Schrattenkalk platform in the Alpstein massif
The successive thrust sheets composing the Alpstein massif of the Helvetic Säntis nappe are most likely inherited from tilted blocks as discussed in Zerlauth et al. (2014) and Pictet et al. (2022). As in the case of the Tierwis Formation (Bodin et al., 2006b; Pictet et al., 2022), synsedimentary tectonics most likely had an impact on the development and spatial distribution of the different carbonate bodies forming the Schrattenkalk platform.
5.1.1 Tilted block of Tierwis-Grauchopf
Based on carbon-stable isotopes analyses (Fig. 8) and their correlations with the basinal record (see also Wissler et al., 2002), Wissler (2001) attributed the onset of the Schrattenkalk platform at the Tierwis-Grauchopf section to the upper part of the B3 geochemical segment which he correlated with the late T. vandenheckii Zone. Above, Wissler attributed the Schrattenkalk unit S1 to the uppermost B3, B4 and B5 geochemical segments, which he correlated with the late T. vandenheckii Zone and the G. sartousiana Zone. Nevertheless, we notice that Wissler et al. (2003) situate the segments B4 and B5 below the Chopf Bed in the Alvier section, a bed dated by ammonites in Bodin et al. (2006a) from the transition between the T. vandenheckii and the G. sartousiana zones. Thus, the segments B4 and B5 belong to the T. vandenheckii Zone. The Rawil Member and the Schrattenkalk unit S3 were attributed to the isotopic segments B6 to A2 and correlated with the I. giraudi to D. forbesi zones. At the opposite, on the basis of the orbitolinids foraminifers and sequence stratigraphic interpretations, Bonvallet et al. (2019) correlated our Schrattenkalk unit S1 with the T. vandenheckii to M. sarasini zones, while the Rawil and the “upper” Schrattenkalk unit were correlated with the lower Aptian series. For our part, we date the units S1 to S4-Gr using macrofossils assemblages, containing ammonites, rudists, echinoids and gastropods.
The ammonite sampling in the upper Drusberg series of the Tierwis-Grauchopf section indicates the T. vandenheckii Zone (Pictet et al., 2022), which is in accordance with Wissler, (2001). Based on very tenuous observations made on thin sections, Bonvallet, (2015) and Bonvallet et al. (2019) suggested the presence of a major discontinuity surface with meteoric characters at the top of the Drusberg Member. Our field observations show a progressive onset of the Schrattenkalk Formation which rises from the Drusberg Member without any sedimentary interruption. Bonvallet, (2015) and Bonvallet et al. (2019) attributed their exposure surface to the important regressive marine phase close to the boundary between the early and late Barremian. This regression would have led to the emersion of the hemipelagic sediments in the proximal part of the slope and to the deposition of a lowstand systems tract in distal slope. However, the ammonites sampling in the Tierwis section allows to locate the boundary between the early and late Barremian 21 m below the base of the Schrattenkalk Formation, at the first firmground surface (firmground 1 in Pictet et al., 2022, fig. 5). We assume that the overlying thick limestone dominated-unit forming the base of the Dursberg Member could represent the lowstand systems tract deposits of the sequence B3 whose age is indicated by the presence of Toxancyloceras sp. (see Pictet et al., 2022, p. 21).
The occurrence of the Barremian rudist genus Agriopleura Kühn, 1932 in the uppermost metres of the Schrattenkalk unit S1 is in accordance with an age not younger than the G. provincialis Subzone, since the last occurrence datum of the genus Agriopleura in the western Tethys is situated at the top of this subzone (Léonide et al., 2008, 2012; Masse & Fenerci-Masse, 2011; Masse et al., 2020). The co-occurrence of the species Agriopleura blumenbachi (Studer, 1834) helps to restrict this dating to the T. vandenheckii Zone (Masse & Fenerci-Masse, 2015; Masse et al., 2020). The paleontological dating differs slightly from the geochemical attribution of Wissler (2001) for the Tierwis-Grauchopf type section by excluding the G. sarousiana Zone. The transition between the Schrattenkalk unit S1 and the Rawil Member shows a very strongly karstified surface covered with a red paleosol containing reworked pebbles and fossils. The layer is passing upward into a brackish facies and into shore deposits extremely rich in charophytes and tree trunks. The Rawil Member and the overlying Schrattenkalk limestones contain a rudist fauna composed exclusively by species of the Family Requienidae (mostly Toucasia and Requienia spp.). The Rawil Member and the Schrattenkalk unit S2 are dated in the literature as early Aptian (e.g., Bollinger, 1988; Bonvallet et al., 2019; Lienert, 1965; Renevier, 1877, 1881, 1890; Schenk, 1992; Stein et al., 2012a), an age attribution that will be discussed later. At this point of the discussion, the rudist assemblage is not in contradiction with this lower Aptian age. Following these facts, a long-lasting exposure event is inferred, corresponding to our Schrattenkalk unit S2. By the way, this long exposure event of the proximal platform could be explained by the tectonic movements of the block of Tierwis-Grauchopf.
5.1.2 Tilted block of Langtal–Säntis
The series of the Tierwis Formation are very similar to those of the Tierwis section. On the other hand, the Schrattenkalk Formation is markedly different. The Schrattenkalk unit S1 is only about 10 m-thick, ending by a noticeable discontinuity surface D12 (Fig. 9). Above this surface, a thick and massive Schrattenkalk unit S2 is present (Fig. 9) followed by the Rawil Member and the Schrattenkalk unit S3 which are also thick.
5.1.3 Tilted block of Altmann, Tesel and Frümsner Alp
The Drusberg Member differs from the situation in the previous blocks. The Drusberg Member is thicker and stratigraphically extending upward at least up to the G. sarstousiana Subzone according to the ammonite data (Pictet et al., 2022). Beds corresponding to the Schrattenkalk unit S1 from the previous and more internal sections are here represented by a 2 m-thick succession of light grey, hard limestones beds, intercalated in the middle of the Drusberg Member (Fig. 10B). This limestone bundle locally shows some bioclastic flows constituted of neritic material remobilised from the carbonate platform. The echinoid Pygaulus desmoulinsi Agassiz, 1847, collected in the basal beds of the unit S2, and the presence of Harpagodes pelagi (Brongniart, 1821) in the upper beds, indicates that this unit is not older that the G. sartousiana Zone (see discussion in Pictet, 2021). The presence of the rudist genus Agriopleura in the upper beds indicates that the unit S2 cannot be younger than the G. provincialis Subzone but not older either, as the species Agriopleura blumenbachi (Studer, 1834) is no longer present. Thus, the faunal association of the unit S2 corresponds to the G. sartousiana and G. provincialis subzones (according to the biostratigraphy of Agriopleura species; Masse & Fenerci-Masse, 2015; Masse et al., 2020).
An interesting fragment of ammonite labelled “Altmann” is stored in the Naturmuseum St.Gallen. This fragment belongs to the genus Kutatissites (Fig. 11B). Unfortunately, the original layer of the find is not indicated on the label. The fragment has a grey matrix that resembles the Drusberg facies and devoid of glauconite grains. This genus is restricted to the latest Barremian M. sarasini Zone and earliest Aptian D. oglanlensis Zone (see Frau et al., 2018a). This age is incompatible with the faunas known from the Drusberg Member and the Schrattenkalk unit S2. As a result, it is extremely unlikely that this ammonite comes from the Drusberg Member. On the other hand, the Rawil Member in this more distal setting of the platform area begins to show some clayey limestone beds, from which this specimen could have its origin.
In this south eastward situated part of the transect, the unit S4-Gr appears for the first time with a thickness of only a few metres and visually hardly differs from the Schrattenkalk unit S3 below. The echinodermic and slightly glauconitic limestone is nearly devoid of any rudist shells, and therefore should correspond to the “Schrattenkalk—Echinodermic breccia” of Heim (1905) and to the “glauconitischer Schrattenkalk (Grüntenschichten)” of Heim and Baumberger (1933, p. 212). In the Tesel section, this sedimentary unit starts to differ more clearly from the underlying Schrattenkalk unit S3 by its thin-bedded limestone beds and by the appearance of a Palorbitolina-rich marly level in its lower third. However, it also contains small-sized rudist shells of Toucasia Munier-Chalmas, 1873. The discovery of the echinoid Heteraster peroni Ficheur, 1900 in this Orbitolina-bearing layer in the Tesel section allows to date this unit to the late early Aptian (Clavel et al., 2014). The appearance of this echinoid genus seems to be a morphological adaptation to the late early Aptian drowning episode, which postdates the last urgonian deposits all around the Vocontian basin (e.g., Frau et al., 2018a; Pictet, 2016; Pictet et al., 2015, 2019).
According to Heim and Baumberger (1933), the Drusberg Member is very thick in the most south eastern anticlinal where the Frümsner Alp and Alp Wis outcrops are situated, reaching about 50 m and more (Fig. 14B). Following the sections, a green glauconitic limestone and associated phosphatic conglomerate is observed at the base or near the middle of the Drusberg Member (e.g., Heim & Baumberger, 1933), overlaying a well-developed firmground. The collected ammonites date from the late T. vandenheckii Zone—G. sartousiana Subzone and allow to attribute this glauconitic bed to the Chopf Bed (Fig. 14C; Pictet et al., 2022). Due to their position above the Chopf Bed, the overlying upper Drusberg marlstones correspond to the Lower Hurst Beds of the Alvier region (Briegel, 1972), which we labelled D2 in Pictet et al. (2022). Ammonites from the G. sartousiana Subzone date this unit (Pictet et al., 2022) which likely extends upward to the lower M. sarasini Zone toward the Rhine valley, where the Schrattenkalk platform unravels in the Drusberg Member. Heim (1905) and Heim and Baumberger (1933) noticed that the Rawil Member is absent in the eastern part of the Alpstein massif but were not able to explain this absence. In our opinion, the absence in the eastern regions can be explained by the later south eastward progradation of the Schrattenkalk platform over deeper environments. The Rawil Member is represented by sedimentary series of marly hemipelagic facies (Upper Hurst Beds; Briegel, 1972), integrated in the Drusberg Member, whereas the Schrattenkalk Formation is represented only by its younger part (unit S3). A half shell of a large Martelites martelli Conte, 1989 (Fig. 11C) was collected in 1973 in Brau (Bezau, Austria), 25 km east from the Alpstein massif, which original level of this ammonite is unknown. This latest Barremian ammonite supports a younger age of the upper Drusberg series in the eastern Rhin valley.
The neritic macrofauna collected in unit S3, like in the tilted block of Langtal—Säntis, does not allow differentiation between a late Barremian and an early Aptian age. To resolve this uncertainty, further studies of the ammonite fauna of hemipelagic sedimentary series equivalent to the Rawil and the “upper” Schrattenkalk members are needed, e.g. in the Werdenberg massif (canton of St.Gallen) and the Brienzergrat (cantons of Berne, Lucerne and Obwalden). In the meantime, a review of the few ammonites collected in the Schrattenklak Formation of central Switzerland is given in chapter 5.3.
5.1.4 Summary of the field observations about the Schrattenkalk Formation in the Alpstein massif
In most proximal platform settings of the Alpstein massif like in Tierwis, the onset of the first carbonate platform progradation phase (forming the unit S1) starts in the late Barremian (T. vandenheckii Zone). The second progradation phase (unit S2) is missing there. Instead, the third progradation phase (unit S3) and together with the transgressional Rawil Member are superposed directly on the unit S1, separated by a conspicuous karstic surface and associated shore deposits. In the middle ranges of the Alpstein massif like in the Langtal-Säntis bloc, the three platform progradation phases (units S1, S2, and S3) are superimposed vertically and from northwest to southeast (see Fig. 15), with a very reduced unit S1. Units S2 and S3 are separated by a remarkable brown to grey marlstone-limestone bundle, characteristic for the Rawil Member. In the eastern range like in Litten-Chreialp and Teselalp, the first carbonate platform progradation phase (unit S1) is completely replaced by the distal shelf Drusberg facies and the phosphate and glauconite-bearing Chopf Bed. This Bed itself is overlain by the upper series of the Drusberg Member, corresponding to the Lower Hurst marls of the Alvier region (Briegel, 1972).
A more echinodermic and slightly glauconitic carbonate unit S4-Gr is observed atop the Schrattenkalk succession in the eastern sections. It marks a new progradation phase of the platform, separated from the unit S3 by the intermediary of a firmground surface (D15).
5.2 Lithostratigraphic implications
For more than a century the Schrattenkalk Formation has been thought to be composed of two units, the “Lower” and the “Upper” Schrattenkalk, separated by the more marly Rawil Member. New field observations question this dichotomy. A first bimodal lithostratigraphic cut of the Schrattenkalk Formation was introduced by Kaufmann (1867) following the absence versus the presence of rudist shells (Fig. 2). It was quickly abandoned in favour of the Lower and Upper Schrattenkalk units in the sense of Heim, (1910–1916), separated by the “Orbitolina marls”. The lithostratigraphic subdivision of the Schrattenkalk Formation was criticised by Bollinger, (1988), at least for the eastern Swiss region. He proposed to abolish the terms “lower” and “upper” and to keep only a Schrattenkalk Member (= Schrattenkalk Formation in the current sense; see also Jost-Stauffer, 1993). Such a dichotomy is also problematic in the most proximal areas where the Rawil Member presents purely carbonated facies (Schenk, 1992), forming in total an enormous uniform rock wall.
More recently, Wissler, (2001) and Wissler et al. (2003) documented the Schrattenkalk Formation within a section along the western flank of the Zuestoll peak, in the Churfirsten, 10 km south of our working area (coord: 2′739′870/1′224′320 to 2′739′950/1′224′770). They identified three sedimentary cycles (P1, P2 and P3) inside the Schrattenkalk Formation that reflect three phases of platform progradation separated by exposure and/or transgressive facies. Such sediments could potentially be interpreted as “Orbitolina beds” separating some Schrattenkalk cliff-forming units despite different stratigraphical positions. Following Huck et al., (2013, p. 171: “Palorbitolina mass occurrences are interpreted as regional expressions of an increasing nutrient influx related to accelerated hydrological cycling, presumably combined with an oscillating sea-level”) orbitolinid beds thus can occur at various stratigraphical levels within the Barremian stage. Wissler et al. (2003) correlated the discontinuity surface observed between their platform progradation cycles P1 and P2 with the glauconitic Chopf Bed and the second transgression phase observed at the base of their cycle P3 with the Rawil Member.
Our biostratigraphic data as well as bedding patterns, facies architecture and bounding surfaces in the Schrattenkalk Formation suggest that the carbonate production mode is a typical ramp-type platform with low-angle oblique clinoforms. Because of the lack of frame-building organisms, the sedimentation is dominated by bioclastic carbonates with stratal patterns like those described from siliciclastic shelves, a common feature of the urgonian platforms (Hunt & Tucker, 1993). The series related to the Schrattenkalk platform sensu stricto were developed in a succession of three carbonate bodies (S1 to S3, Fig. 15). These lenses are arranged following a progradation phase (S1 to S2), followed by an aggradation-progradation phase (S2 to S3). Wissler’s studies of the Churfirsten (Wissler, 2001; Wissler et al., 2003) allow us to affirm that the Churfirsten and the Alpstein mountain ranges present the same Schrattenkalk platform succession with three platform progradation phases, each of contemporaneous age.
The question arises if cartographers are able to differentiate between the successive marly bundles intercalated in the Schrattenkalk Formation, although specialised researchers are fighting about their age attribution and stratigraphic position. We suggest to be very careful when attributing a lithostratigraphic rank to the cliff units composing the Schrattenkalk Formation.
The topmost unit S4-Gr rises problems in term of lithostratigraphy. The unit is a light-coloured limestone visually very similar to the Schrattenkalk Formation in the Altmann sections. It is characterised by a more echinodermic and slightly glauconitic limestone. Rudist shells of the genus Toucasia Munier-Chalmas, 1873 are even observed in the Tesel section. Because of its lithological similarity with the Schrattenkalk limestone, Heim (1905) named this unit “Schrattenkalk-Echinodermenbreccie”. Heim and Baumberger (1933, p. 212) called it “glauconitischer Schrattenkalk (Grüntenschichten)” and noticed that this unit was a possible lateral equivalent of the Grünten Member. In facts, the unit S4-Gr thickens and enriches in orbitolinids towards the Rhein valley and presents no more rudist shells. It becomes mappable south eastward from the Tesel section, from where it becomes clear that this unit is contemporaneous with the Grünten Member of central and western Swiss Alps. For reminder, Linder et al. (2006) excreted the Grünten Member from the Schrattenkalk Formation because of the presence of phosphate-bearing beds. However, up to now, no phosphatic beds were observed in our unit S4-Gr of this region. The Grünten Member is considered as late early Aptian in age by Linder et al. (2006), a dating which is in agreement with the biostratigraphical markers of our unit. Moreover, the unit S4-Gr is attributed to the Grünten Member by Funk et al. (2000). However, in the Alpstein, the unit S4-Gr presents intermediate sedimentological and paleontological characters between the Schrattenkalk Formation ant the Garschella Formation, which makes it difficult to classify in term of lithostratigraphy, at least for its most proximal occurrences.
5.3 Key beds and ammonite bio-events
A succession of marker-surfaces and beds observed within or at the boundaries of the Schrattenkalk Formation is associated or correlated with: (i) positive excursions in oceanic phosphorus burial; (ii) notable carbon isotope fluctuations with negative and following positive excursion; (iii) condensed intervals sometimes combined with important phosphate authigenesis; (iv) ammonite accumulations and bio-events. These surfaces and beds are interpreted as the result of drowning episodes. Our revised calibration substantially modifies the dating of the drowning phases within the upper Barremian Schrattenkalk Formation. Three events of regional significance are recognized.
5.3.1 “Sartousiana” event
The “Sartousiana” event was discussed in Pictet et al. (2022) regarding to slope environments. We discuss here how this event affects the neritic platform environments of the Schrattenkalk Formation.
This event is associated to the discontinuity surface D12 (Figs. 9, 10, 14), a remarkable sedimentary event, which constitutes a well-marked time-line crossing in our platform-to-basin transect (Figs. 15, 16). In the distal platform setting, this same discontinuity surface D12 is associated to a glauconite and phosphate-rich conglomerate, the Chopf Bed (Fig. 14C). This bed is situated inside the Drusberg Member and corresponds to the uppermost T. vandenheckii Zone and lowermost G. sartousiana Zone according to Bodin et al. (2006a). Landwards like in Litten-Chreialp and Altmann sections, the Chopf Bed overlays the clinoform packages of the Schrattenkalk unit S1 (Fig. 10B). In shallower environments represented by the Langtal-Säntis section, the discontinuity D12 overlies a 10 m-thick unit S1, which is incorporated within the base of the Schrattenkalk cliff. The discontinuity surface presents a sharp lithological break associated with a prominent facies change large enough to be observed from afar (Fig. 9D). In the Tierwis-Grauchopf area, representing the most proximal platform setting, the discontinuity D12 is combined with the discontinuity D13 atop the unit S1, which represents the total thickness of the lower cliff. The discontinuity surface is a pronounced karstic surface, developed only along the north-western range (Fig. 4E). The surface records a drop of 2‰ in δ13C just below it (Fig. 8). This negative isotopic peak must be regarded as the diagenetic imprint of the exposure. It cannot be correlated with any negative excursion in the pelagic record contrary to the interpretation of Stein et al., (2012a, 2012b) and Bonvallet et al. (2019). The macrofauna, present on both sides of the exposure surface, indicates that the entire G. sartousiana Zone is missing in this area.
5.3.2 Taxy event
The discontinuity D13 (Figs. 5, 8, 9, 10, 12) corresponds in the Alpstein massif to a regional exposure of the proximal carbonate platform associated with the last occurrence of the rudist genus Agriopleura. This pseudotermination of the genus Agriopleura was pointed out on the various urgonian carbonate platforms bordering the Vocontian basin and called the “Agriopleura event” by Masse and Fenerci-Masse (2013a, b, 2015) and Masse et al. (2020). This event is assigned to the G. provincialis–H. feraudianus subzonal boundary (Frau et al., 2018a). Orbitolinid foraminifera were also strongly impacted with a drastic diversity decrease (Clavel et al., 2014). Echinoids equally present a turn-over with the replacement of the neritic species Heteraster couloni (Agassiz, 1839) by H. oblongus (Brongniart, 1821) and the hemipelagic species Toxaster seynensis (Lambert, 1920) by T. colleignoi (Sismonda, 1843) (see Clavel et al., 2014). Following Masse and Fenerci-Masse (2015) and Masse et al. (2020), this biological crisis of the carbonate platform coincides in time with climatic cooling, increasing sea-water fertility, modifications in deep-sea bottom circulation, platform perturbations including exposure and basin margin instability.
The discontinuity surface D13 indicates a remarkable sedimentary change from coral and rudist-bearing Urgonian-type series to a mixed siliciclastic–carbonate depositional system (Rawil Member = Lower Orbitolina Beds; Marne à Heteroceras of Paquier, 1900pro parte; Vire à Heteroceras “VH” of Ferry, 2017). This sedimentary change constitutes a well-marked stratigraphical interval visible through our platform-to-basin transect (Figs. 15, 16). The series of the Rawil Member very often present a transgressive system tract composed of several superposed shallowing-upward parasequences showing features of exposure at their top (Bonvallet et al., 2019; Stein et al., 2012a). In the Tierwis-Grauchopf sections, two prominent marly episodes are recorded. The first level situated one metre above the exposure surface (Figs. 5A–C) is here characterized by a mass occurrence of the gastropod Harpagodes pelagi (Brongniart, 1821) and by charophytes. The second marly layer at the top of the Rawil Member overlays a strongly bioturbated surface, which is characterized by the mass occurrence of Palorbitolina lenticularis (Blumenbach, 1805) in association with the echinoid Heteraster oblongus (Brongniart, 1821) (Figs. 5E–F and 6B). Between the two marly layers, typical urgonian series with rich coral carpets and rudist-bearing carbonates are intercalated. Stein et al., (2012a, pp. 955–956) observed these two transgressive episodes on a larger scale across the Helvetic domain, characterised by circalittoral, deeper-water assemblages and the abundance of echinoderms. They interpret the upper marly episode as the maximum flooding surface, which is in accordance with the appearance of a diversified echinoderm fauna in the Tierwis-Grauchopf section. Two successive transgressive parasequences are also recorded in the Jura domain (Pictet, 2021; Pictet et al., 2019). In the latter area, a lower charophytes-bearing marly intercalation contains brackish to freshwater facies and yielded abundant microfossils of latest Barremian age. An upper marly layer with abundant Heteraster oblongus (Brongniart, 1821) represents open-marine facies.
Sedimentary series corresponding to the Rawil Member were originally assigned to the lowermost part of the Aptian stage by Renevier (1877, 1881, 1890) based on the macrofaunal assemblage, an age followed until today. Following Renevier, “Pterocera” pelagi (Brongniart, 1821), Heteraster oblongus (Brongniart, 1821), Orbitolina lenticularis (Blumenbach, 1805) and “Reqienia Lonsdali” (Sowerby, 1837) are typical species of its Rhodanian stage (Renevier, 1877; i.e., = early Aptian) previously described in the Perte-du-Rhône (Jura mountains, Renevier, 1855). The age of this Jura fauna is revised in Pictet et al., (2016, 2019). The early Aptian age of the Rawil Member was later supported by the cooccurrence of ammonites from the Chartreuse and Vercors massifs (French Subalpine ranges; Arnaud et al., 1998; Gidon, 1952; Moret & Deleau, 1960) referred to Ancyloceras gr. matheronianum Orbigny, 1842 and Deshayesites gr. weissi (Neumayr & Uhlig, 1881) but never described and only partially figured. Both specimens are discussed in Frau et al., (2018a, p. 243) and attributed to the latest Barremian. In the Helvetic domain, a few ammonites from the Rawil Member and from the “upper” Schrattenkalk are cited in the literature. Fichter (1934) signalled a very worn fragment in the Rawil Member of the Bauen-Brisi area, which unfortunately is missing in his material and considered as lost or never collected. Lienert (1965, pp. 78 and 128) reports a Procheloniceras sp. from the upper Schrattenkalk of Brunnen, stored in the collections of ETHZ. The genus Procheloniceras Spath, 1923 is present as well in the uppermost Barremian than in the lowermost Aptian record. Lienert (1965, p. 22) also mentions large Aptian Parahoplites specimens from the upper Schrattenkalk of Brunnen. A corresponding 35 cm large counter print of an ammonite from Brunnen was found in the ETHZ collections (Fig. 17; n coll eth27789). This very evolute to nearly advolute specimen shows backward inclined and straight ribs as well as relatively flattened sides, which are typical for the genus Martelites Conte, 1989. This genus characterises the uppermost Barremian M. sarasini Subzone, in accordance with the co-occurrence of the genus Procheloniceras Spath, 1923. Furthermore, Staeger (1944) collected in the “upper Schrattenkalk” of Alp Breitenfeld, in the Brienzer Rothorn massif, a complete specimen of Procheloniceras seminodosum (Sinzow, 1906) (presently assigned as P. sp. pending the specimen review; Fig. 18). Finally, Jost-Stauffer (1993) collected, at the top of the “upper Schrattenkalk”, a fragment of an ammonite on the Schöngutsch section in the Brienzer Rothorn summit (Fig. 18). This fragment was originally determined by R. Busnardo in 1993 as Deshayesites sp. (Jost-Stauffer, 1993, p. 35). This fragment shows a typical rectangular to squared cross-section and ribbing style with oblique orientation on the venter that allows to revise it as a part of an Heteroceratidae indet. All these ammonites’ discoveries point to a latest Barremian (M. sarasini ammonite Subzone) for the”upper” Schrattenkalk. By the way, a major ammonoid turnover at the base of the Imerites giraudi Zone was reported by Vermeulen (2005), characterized by the diversification of Heteroceratidae and a gradual replacement of the Hemihoplitidae and Barremitidae by the diversifying Ancyloceratidae.
Thus, all these paleontological data point to an uppermost Barremian age for the Rawil Member. These observations are in accordance with the worldwide reported occurrence of laminated, organic-rich muds in basinal environments during the latest Barremian. These latest Barremian LOMs were particularly studied in the la Bédoule section and named as “Taxy Episode” by Föllmi (2012).
5.3.3 Final demise of the Schrattenkalk platform
An important unconformity is located between the Schrattenkalk and the Garschella formations indicating the end of the rudist-bearing shallow-water carbonate platform sedimentation. This unconformity as well as the overlaying sediments show a strong differentiation along the platform-to-basin transect. In distal parts of the platform, like in Schlatt-Mätzenacker and Tesel Alp, the Schrattenkalk Formation ends with a discrete corrugated and bored hardground topped by the lower Aptian Grünten Member or by the unit S4-Gr (Fig. 14F–G). In the Altmann section, the Schrattenkalk Formation is topped by a reduced—unit S4-Gr and/or by a reduced upper Aptian Brisi Member by the intermediary of a shallow to a deep unconformity. In proximal platform parts, like in Langtal and Tierwis, the Schrattenkalk Formation ends with a very irregular and corroded surface, associated to locally brecciated epikarstic cavities, and karstic fissures (Fig. 7A, C). The top of the Schrattenkalk Formation is missing due to extensive and repeated erosion during later successive eustatic cycles. The Schrattenkalk is capped by the middle Albian Sellamatt Beds (Selun Member, Garschella Formation). Thus, the discontinuity surface shows a more and more corroded surface toward the land, passing from a bored hardground to a strongly karstified surface. The surface is covered by younger sediments toward the proximal platform, comprising an increasing time duration of the associated hiatus. This sedimentary story seems to occur throughout the Helvetic domain (Föllmi, 2008).
The onset of the final drowning phase of the Schrattenkalk Formation cannot be dated precisely in the Alpstein massif. However, the distal platform sections like Tesel Alp, where the hiatus has the least duration, indicate that platform drowning occurred before the deposition of the upper lower Aptian unit S4-Gr/Grünten Member. The previously discussed age arguments for the Rawil Member and above Schrattenkalk unit S3, as well as the rare ammonites from central Switzerland, indicate that the Schrattenkalk demise occurred during or just after the M. sarasini Zone. We therefore conclude that it occurred somewhere between the latest Barremian M. sarasini Zone and the middle early Aptian.
The Urgonian carbonate platform demise is regarded as of global importance by Föllmi (2008). It is recorded all along the peri-Vocontian platforms by a subaerial exposure like on the North Provence platform (Léonide et al., 2012, 2014; Masse & Fenerci-Masse, 2011), the Languedoc platform (Pictet et al., 2015), the Vercors platform (Arnaud-Vanneau et al., 2005; Moss & Tucker, 1996), the Jura platform (Charollais et al., 1994; Pictet et al., 2016, 2019) and the Helvetic platform (Bonvallet et al., 2019; Pictet, 2016). Because its previous dating from the middle early Aptian, the final demise has been believed to be the result of an important phase of paleoenvironmental change related with the early Aptian oceanic anoxic event (OAE 1a: “Selli event”; e.g., Föllmi, 2008; Föllmi & Gainon, 2008; Schlager & Philip, 1990; Wissler et al., 2003). Age control obtained by Föllmi (2008) and Föllmi and Gainon (2008) in the Helvetic domain suggested that the onset of platform demise slightly preceded OAE 1a. This delay was regarded as the expression of a lead-lag effect. Based on cyclostratigraphic calibration of basinal sections, Huck et al. (2011) have established a time gap of about 300 kyr between the Helvetic platform drowning and OAE1a. Thus, these authors suggested that the causal relationship between these two events had to be re-considered. However, the recent revised dating of Urgonian carbonate platforms show that the demise predates the OAE 1a of at least 1 Myr (Frau et al., 2020). Frau et al. (2020) suggested that this large-scale exposure event may have resulted from a relative sea-level fall of high amplitude, resulting from a short-lasted global cooling phase associated with glacio-eustasy.
5.4 Preludes to the OAE1a event
Föllmi (2012) and students considered the Barremian time interval and more particularly the late Barremian-earliest Aptian to have been a period of relative stability in the evolution of the biosphere. These conditions allowed the development of extensive Urgonian platforms on the north-western Tethyan margin, which were particularly favourable during the late Barremian (Bonvallet et al., 2019). Following Bodin et al. (2006b), Godet et al. (2010), Föllmi and Godet (2013) and Bonvallet et al. (2019), a long-term decreasing trend in detrital and phosphorus contents took place during the late Barremian as weathering rates lowered on the adjacent continents and nutrient levels decreased in the surrounding seas, which enabled the late Barremian pulse in the development of the Urgonian shallow-water carbonate platform.
However, the hypothesis that the late Barremian-early Aptian time interval had been a period of relative stability in the evolution of the biosphere is challenged by the revised dating of the peri-Vocontian platforms (e.g., Frau et al., 2018a; Pictet, 2021; Pictet et al., 2019) and by this study. Furthermore, organic rich shales were reported worldwide in the upper Barremian sedimentary series while they are nearly absent in the lowermost Aptian records. A series of black shales were observed in the T. vandenheckii, G. sartousiana and I. giraudi-M. sarasini zones respectively in the Lower Saxony Bassin (Hoffmann & Mutterlose, 2011; Malkoč & Mutterlose, 2010; Mutterlose & Böckel, 1998; Mutterlose & Bornemann, 2000; Pauly et al., 2013), in the central Tethys (Cecca & Landra, 1994; Frau et al., 2016; Landra et al., 2000; Sprovieri et al., 2006), partially in the Vocontian basin area like in La Bédoule (SE France; Frau et al., 2016; Masse & Fenerci-Masse, 2011; Masse & Machhour, 1998; Stein et al., 2012b) and in southern Spain (Aguado et al., 2014). Further black shales occurrences were also observed in the central and northern Atlantic as observed in DSDP and ODP sites (Stein et al., 1988; Weissert, 1981). Thus, a succession of short and repeated periods of dysaerobic to anaerobic conditions, associated to the deposition of a succession of pelagic organic-rich deposits took place during the late Barremian with particularly enhanced occurrences during the G. sartousiana Zone and the I. giraudi–M. sarasini zones.
It appeared that, in the Alpstein mountains, the Schrattenkalk Formation is interspersed with two sedimentary events associated to higher phosphorus values and to mixed photozoan—heterozoan assemblages, marked by significant quantities of flat orbitolinids and annelids (Bonvallet et al., 2019; Wissler et al., 2003), the “Sartousiana” event and the Taxy event respectively. The “Sartousiana” event is dated to the uppermost T. vandenheckii Zone and lowermost G. sartousiana Subzone (Pictet et al., 2022). The Taxy event is dated from the I. giraudi and lower M. sarasini zones (Frau et al., 2016; Masse & Fenerci-Masse, 2011). This dating is in accordance with the few ammonites of latest Barremian age collected in the “upper” Schrattenkalk and distal counterparts. The “Sartousiana” and the Taxy events are interpreted as the result of drowning phases, which both may coincide in time with large scale changes in the ocean-climate system.
In fact, the latest Barremian-early Aptian time interval is globally characterized by large fluctuations in palaeoceanographic, paleoenvironmental, and palaeobiological conditions (Aguado et al., 2014). Increased seafloor spreading and an increased submarine volcanic activity of the Ontong-Java Plateau led to a high sea level and intense volcanic outgassing (e.g., Erba et al., 2015). This contributed to the onset of a widespread greenhouse climate and to a major perturbation in the global carbon cycle (Aguado et al., 2014; Erba, 1994; Föllmi, 2012; Huck et al., 2013; Larson, 1991; Leckie et al., 2002; Mehay et al., 2009; Skelton, 2003; Tejada et al., 2009). These conditions induced high primary productivity, favouring the largely distributed deposition of organic-rich sediments. It culminated in the worldwide early Aptian Selli episode (OAE1a; Coccioni et al., 2003; Erba et al., 2015; Föllmi, 2012) and, together with a dense succession of “mid-Cretaceous” oceanic anoxic events, representing a major phase of paleoenvironmental change (Aguado et al., 2014; Arthur et al., 1990; Erba, 1994; Jenkyns, 1980, 2003, 2010; Larson & Erba, 1999; Leckie et al., 2002).
We revised the dating of the Schrattenkalk Formation in the Alpstein massif and analysed bedding patterns, the facies architecture, and bounding surfaces of the formation. Based on these data, we suggest that the carbonate production mode is a typical ramp-type platform with low-angle oblique clinoforms. The stratal patterns are dominated by bioclastic carbonates which deposited in the same way as in siliciclastic shelves, a common feature of the urgonian platforms. In contrast to the actual subdivision of the Schrattenkalk into a lower and an upper part, separated by the Rawil Member, we postulate that the Schrattenkalk series developed in a succession of three carbonate platform progradation phases, each offset being bounded by an exposure surface and subsequent drowning, while in the distal shelf, condensed ammonites-bearing series have been deposited. The drowning episodes could potentially be interpreted as successive Orbitolina-rich beds. The development phases of the platform started with a progradation phase (units S1 to S2) followed by an aggradation-progradation phase (units S2 to S3). The onset of the Schrattenkalk carbonate platform is progressive, showing a SE-directed progradation over time. The oldest neritic deposits of our platform-to-basin transect correspond to the T. vandeheckii Zone. The younger carbonates are attributed with uncertainties to the M. sarasini Subzone. The lower platform interruption is related to the “Sartousiana” event by correlation with the Chopf Bed on the platform slope. The Rawil Member marks the second platform carbonate interruption and is linked to the Taxy event. Ammonites with uncertain stratigraphic position in the Alpstein mountains suggest that the Rawil Member could date to the latest Barremian (H. feraudianus to M. sarasini subzones). This age attribution is affined by ammonite discoveries from the “upper” Schrattenkalk of central Switzerland. Due to our observations, the commonly used bimodal nomenclature of the Schrattenkalk cliffs should be used with caution.
The overlying unit S4-Gr is morphologically similar to the Schrattenkalk limestone in the Alpstein massif and presents a facies intermediate between the Schrattenkalk Formation and the Grünten Member. The unit S4-Gr locally contains small-sized rudist shells of the genus Toucasia Munier-Chlamas, 1873 and differs from the Schrattenkalk Formation by the presence of glauconite grains and a higher echinodermic content. It appears south eastward from the Altmann section. The unit S4-Gr dates from the late early Aptian and thus represents the oldest sedimentary deposits of this stage in the Alpstein massif.
The Barremian time interval and more particularly the late Barremian was considered to have been a period of relative quietness in the evolution of the biosphere, allowing the development of extensive Urgonian platforms. However, organic rich shales were worldwide reported in the upper Barremian series and mostly reported from the G. sartousiana and I. giraudi-lower M. sarasini ammonite-zones. These age intervals are in accordance with a few ammonites of late Barremian age observed in the field or stored in museum collections. The “Sartousiana” and the Taxy events are interpreted as the result of drowning periods which may coincide with large scale changes of the ocean-climate system. The latest Barremian time interval is globally characterized by large fluctuations in palaeoceanographic, paleoenvironmental, and palaeobiological conditions which predated the worldwide and dense succession of oceanic anoxic events of the “mid-Cretaceous” period.
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The authors warmly thank Dr. Toni Bürgin, Director of the Naturmuseum St.Gallen, for his financial support for the realization of this study. The first author is grateful to the Musée cantonal de géologie of Lausanne for having funded its last fieldtrips and the paleontological plaster copies deposited in the Musée cantonal de geology of Lausanne as well as for funding the publication fees. The authors greatly acknowledge the curators of geoscience collections Dr. Toni Bürgin (Naturmuseum St.Gallen), Dr. J. Georg Friebe (Inatura Museum of Dornbirn), Prof. Dr. Alfons Berger (University of Bern), Dr. Iwan Stössel (Swiss Federal Institute of Technology Zurich) and Dr. Walter Etter (Naturhistorisches Museum Basel) for providing access to the collections or helping us in locating specimens. Dr. Jean-Pierre Masse (Aix-Marseille University) is thanked for his opinion on the rudist shells. The reviewers Serge Ferry and Olivier Kempf are acknowledged for their constructive comments, which have helped to improve this manuscript.
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Pictet, A., Tschanz, K. & Kürsteiner, P. Record of a dense succession of drowning phases in the Alpstein mountains, northeastern Switzerland: Part II—the Lower Cretaceous Schrattenkalk Formation (late Barremian). Swiss J Geosci 116, 1 (2023). https://doi.org/10.1186/s00015-022-00430-z