Lithofacies, stratigraphy and depositional history of Middle Muschelkalk evaporites (Zeglingen Formation) in northern Switzerland
Swiss Journal of Geosciences volume 116, Article number: 16 (2023)
In northern Switzerland, Middle Muschelkalk evaporites (Zeglingen Formation) were deposited under arid conditions in the southernmost part of the epicontinental Central European Basin during times of reduced inflow of sea water from the Tethyan realm. Because of their marginal position in the basin, direct and detailed correlation of Muschelkalk sediments of northern Switzerland with equivalent strata in interior parts of the basin is not straightforward. Based on detailed sedimentological logging of 640 m of drill cores from ten wells in northern Switzerland, 22 lithofacies and 10 lithofacies associations were distinguished for the Middle Muschelkalk evaporites. High-resolution regional correlations of gamma-ray logs record substantial thickness variation of the evaporites. Locally, dissolution was recognized by visual core examination that could be dated to Middle Triassic times and that was likely related to subsurface fluid flow along deeper seated faults. In combination with the regional thickness variation, the dissolution phenomena suggest a tectonically active depositional setting in the Middle Triassic. Middle Muschelkalk evaporites consist of nine types of mainly auto-cyclic shallowing- or brining-upward mini-cycles which form a correlatable succession of five distinct 4th order cycle-sets. Despite the tectonically active depositional environment, most lithofacies encountered appear to have nearly flat bounding surfaces. Thus, marine transgressions flooded wide areas nearly simultaneously. The corresponding deposits serve as reference levels to tie the peripheral facies of northern Switzerland into the supra-regional context.
In northern Switzerland, the Zeglingen Formation (Anisian, Middle Muschelkalk) consists of evaporites that formed during a period of enhanced subsidence (Wildi et al., 1989; Mazurek et al., 2006). Among those, halite beds up to 170 m thick accumulated (Häring, 2002; Jordan, 2016). These evaporites are of economic as well as geotechnical interest. Middle Muschelkalk halite deposits provide nearly the entire salt supply for Switzerland and have been solution-mined in northern Switzerland since 1837 (Birkhäuser et al., 1987). Due to the high solubility of evaporites, various hydrogeological and geotechnical issues can potentially arise in their surroundings (e.g., Croxton, 2003; Gutiérrez et al., 2015; Gutiérrez, 2016; Desir et al., 2018). In addition to gypsum swelling, increased sinking of the land surface due to solution processes in the subsurface may cause geotechnical challenges under specific hydrogeological conditions (e.g., Meyer, 2001). Many studies, therefore, dealt with the Middle Muschelkalk evaporites and especially the halite deposits in northern Switzerland and adjacent regions and provided explanations on the formation of the evaporites in general and the regional distribution of the halite deposits in particular (Verloop, 1909; Trefzger, 1950; Hauber, 1971, 1980, 1993; Baumann & Stucky, 1984; Dronkert, 1987; Dronkert et al., 1990; Widmer, 1991).
Today’s distribution of halite depends on various factors. Besides sedimentary conditions, evaporite minerals might have been dissolved in the subsurface at various times, shortly after deposition and later (Hovorka, 2000). Dissolution of Middle Muschelkalk halite beds has been described both generally by Simon (2003) and locally for individual deposits in northern Switzerland (Hauber, 1993). Especially for interior parts of the Central European Basin (CEB) in Germany, sedimentology of Middle Muschelkalk evaporites and especially of the halite deposits has been studied in detail (e.g., Schachl, 1954; Wild, 1968, 1973, 1980; Richter-Bernburg, 1977, 1980; Frieg, 1982; Balzer, 2003; Jonischkeit, 2003; Rogowski & Simon, 2005). These studies provided a general lithostratigraphic and sequence stratigraphic subdivision as well as detailed concepts on evaporite formation (Röhling, 2000; Nitsch et al., 2020; Simon et al., 2020a, 2020b).
Nonetheless, the understanding of the formation and distribution of the halite deposits as well as the processes that enabled or prevented their preservation is only fragmentary. So far, investigations in northern Switzerland have only been of a local character. Furthermore, due to its rather marginal position in the CEB, Mesozoic strata in northern Switzerland often show different facies and thicknesses than their equivalents in interior parts of the basin, making direct correlations complicate (e.g., Reisdorf et al., 2011; Jordan, 2016). This also applies for the Middle Muschelkalk and thus, only first attempts have been made to correlate the German lithostratigraphic subdivision in the basin center with the marginal facies in northern Switzerland (Simon et al., 2020a). In contrast to lithostratigraphy, sequence-stratigraphic correlation of the Middle Muschelkalk in northern Switzerland with the CEB has not yet been done. Furthermore, a detailed study of the factors fostering preservation or dissolution of halite beds in northern Switzerland is still pending. Given the recent extensive deep drilling campaign by Nagra ("National Cooperative for the Disposal of Radioactive Waste") and new findings from interior parts of the CEB, a re-evaluation of the marginal facies in northern Switzerland is necessary.
Based on new and re-visited drill cores from the Nagra, this study addresses the temporal and spatial facies development of Middle Muschelkalk sediments in northern Switzerland. It is the purpose of this study (1) to analyze lithofacies and lithofacies associations of Middle Muschelkalk sediments, (2) to introduce a sequence-stratigraphic framework, (3) to present a conceptional model of evaporite formation and dissolution and (4) to compare the new findings and, if possible, to relate them with recent studies in interior parts of the CEB.
2 Geological setting
2.1 Study area
The study area is located in central northern Switzerland and covers the three potential deep geological repository sites for the disposal of radioactive waste (Fig. 1). It was affected by the Alpine orogeny and thereby a foreland basin (Swiss Molasse Basin) and a foreland fold-thrust belt (Folded Jura) formed during the Oligo- to Miocene. The décollement of the thin-skinned Folded Jura is located in the evaporites of the Triassic, in the eastern part of the Folded Jura mainly in the evaporites of the Middle Muschelkalk and in particular in the halite beds of the Middle Muschelkalk (Laubscher, 1961; Jordan, 1994; Schori, 2021). The study area is located north of the Internal Folded Jura, but the potential deep geological repository sites ‘Jura Ost’ and ‘Nördlich Lägern’ are localized in the External Folded Jura domain ("Vorfaltenzone" sensu Diebold et al., 2006) and the Mesozoic strata are sheared off as well. Due to the Alpine orogeny, the Mesozoic and the underlying basement in the study area are gently dipping roughly towards south (Thury et al., 1994).
The basement is locally highly dissected. Parts of the study area are located on a deep, WSW–ENE oriented Permo-Carboniferous (half-)graben (North Swiss Permo-Carboniferous Basin or “Constance-Frick Trough”; Diebold & Naef, 1990; Madritsch et al., 2018), which is dissected by SSW–NNE and WNW–ESE striking faults. The studied wells are located in different positions with respect to the North Swiss Permo-Carboniferous Basin (Fig. 1): wells within the potential deep geological repository sites ‘Jura Ost’ and ‘Nördlich Lägern’ are located mainly above the trough center. Wells directly lying north of ‘Jura Ost’, wells within ‘Zürich Nordost’ as well as Schlattingen-1 are located in the area of the northern trough-shoulder. The borehole Benken is located on a crystalline high, Marthalen-1, Trüllikon-1 and Schlattingen-1 drilled 56.1 m, 13.6 m, and 78 m of Permian sediments, respectively, before reaching the crystalline basement.
2.2 Paleogeography and lithostratigraphy
During the Permian and Early Triassic, the relief formed during Variscan times was eroded while continental siliciclastics accumulated (Weitenau and Dinkelberg formations; Thury et al., 1994; Leiber & Bock, 2013). A peneplain formed and northern Switzerland became part of the epicontinental Central European Basin (CEB). The CEB started to form during latest Carboniferous to early Permian in the area of the North Sea (Northern and Southern Permian Basin; Bachmann et al., 2008; van Wees et al., 2000). During Middle Triassic times, the CEB was located in the subtropics, roughly between 20° and 35° N (Gaetani et al., 2000; Stampfli & Borel, 2002; Röhling et al., 2020) and, consequently, arid conditions prevailed (Szulc, 2008a; Röhling et al., 2020). For the study area, recent palynological investigations gave evidence for arid climate from early Illyrian to Fassanian (Hochuli et al., 2020). During Muschelkalk times, the study area was located in a position between the CEB in the north and the Burgundian Basin in the southwest. South of the study area, a landmass and elevated regions characterized by a thin Triassic cover in the area of the present-day extern massifs of the Alps separated the Tethyan realm in the south from the CEB (Fig. 2). During Early Triassic times, marine connections between the CEB and the Tethyan realm established via East Carpathian and Silesian-Moravian gateways, which are presumed to have been tectonically controlled (Szulc, 2000). These gateways allowed marine incursions into the basin (Szulc, 2000; Bachmann et al., 2010; Röhling et al., 2020). Faunal migration indicates the later existence of another gateway in the southwest of the CEB (Klug et al., 2005; Wagner, 1956; Kozur, 1974; Hagdorn, 1985; Urlichs & Mundlos, 1985). Its location, however, is still under debate. Based on palynological studies, the first connection between the Tethyan realm and the southwestern part of the CEB can be dated to the Early Anisian (Lower Muschelkalk; Feist-Burkhardt et al., 2008; Götz & Gast, 2010; Götz & Feist-Burkhardt, 2012).
During the Anisian (Bithynian), the southern margin of the CEB was affected by a transgression, which first led to the deposition of carbonates and marls of the Lower Muschelkalk (Szulc, 2000; Götz & Gast, 2010; Franz et al., 2021). An eustatic sea-level fall during early Illyrian times (Haq et al., 1988; Ogg et al., 2016) and perhaps also tectonic movements in the area of the marine gateways led to a restricted water exchange between the CEB and the Tethyan realm (Bachmann et al., 2010). Subsequently, up to 220 m thick evaporites accumulated in the study area and large parts of the CEB during Middle Muschelkalk times (Fig. 2; Wild, 1968; Dronkert et al., 1990; Ziegler, 1990; Szulc, 2000; Jordan, 2016; Nitsch & Hagdorn, 2020). In Switzerland, the Muschelkalk represents a phase of increased tectonic subsidence compared to the overall Mesozoic subsidence path (Wildi et al., 1989; Leu et al., 2001; Wetzel et al., 2003; Mazurek et al., 2006). Consequently, some thickness variations of the Middle Muschelkalk and especially of the halite beds are explained with synsedimentary tectonics (Hauber, 1971, 1993; Widmer, 1991).
In northern Switzerland, the evaporites of the Middle Muschelkalk constitute the Zeglingen Formation (Jordan, 2016). Since a formal subdivision on member level is still outstanding, traditionally four units are distinguished (Widmer, 1991; Hauber, 1971, 1993; Jordan, 2016). From young to old: (1) «Dolomitzone» ("Dolomite Zone"); (2) «Obere Sulfatzone» ("Upper Sulfate Zone"); (3) «Salzlager» ("Salt Beds"); and (4) «Untere Sulfatzone» ("Lower Sulfate Zone"). For the study area, Dronkert et al. (1990) described the lithofacies of the Middle Muschelkalk, interpreted its formation in periodically desiccated hypersaline pools and lagoons, and established a detailed subdivision of the «Obere Sulfatzone». Roughly at the same time, Widmer (1991) studied Middle Muschelkalk sediments in northwestern Switzerland in cores drilled for halite exploration. He also described the lithofacies and established a detailed lithostratigraphy of the Middle Muschelkalk in this area. The presence of the second, upper halite deposit in northwestern Switzerland led Widmer (1991) to define two cycles of (i) halite deposition, (ii) partial dissolution of the halite under sabkha environments followed by (iii) a transgression and deposition of anhydrite beds. Accordingly, he divided the «Salzlager» and the «Obere Sulfatzone» into a lower and an upper unit, respectively, consisting of (i) halite beds («Salzschichten»), (ii) dissolution breccias and sabkha sediments («Brekzien»), and (iii) anhydrite beds («Anhydritschichten»). The transition between these two saline cycles and the dolomites of the «Dolomitzone» was classified as so-called «Übergangsschichten» (“Transitional Beds”) and «Dolomit-dominierte Schichten» (“Dolomite dominated Beds”). In this study, the informal units «Untere Sulfatzone», «Untere Salzschichten», «Obere Sulfatzone», «Obere Salzschichten», and «Dolomitzone» are used in the rank of a member. Out of genetic reasons, Widmer (1991) grouped the interval between the «Untere Salzschichten» and the «Obere Brekzien» into the «Salzlager». This study does not follow this allocation. The informal subunits of the «Salzlager» and the «Obere Sulfatzone» of Widmer (1991), however, are used in the rank of a informal horizon (Fig. 3), but with the «Übergangsschichten» and the «Dolomit-dominierte Schichten» grouped together.
A several m-thick pure dolomite, the so-called «Dolomitzone», forms the uppermost part of the Middle Muschelkalk. The «Dolomitzone» partially contains anhydrite nodules and mainly consists of intertidal microbial laminites (Dronkert et al., 1990; Widmer, 1991; Hauber, 1993; Simon et al., 2020a). After deposition of the «Dolomitzone», a relative rise of sea-level induced the sedimentation of the carbonates of the Upper Muschelkalk. The boundary between Middle and Upper Muschelkalk is defined with the transition of pure dolomite to (dolomitic) limestone (Merki, 1961; Pietsch et al., 2016; Simon et al., 2020b). This transition is heterochronous on basinal scale. To the north, in Baden-Württemberg, the facies-defined boundary occurs stratigraphically a few meters deeper (Simon et al., 2020a). In the well Ühlingen-2, in the north of the study area, the so-called «Hornsteinbank», forming the base of the Upper Muschelkalk in northern Baden-Württemberg (Hagdorn et al., 2020; Simon et al., 2020a, 2020b), already lies 6 m below the Middle/Upper Muschelkalk boundary (Sawatzki et al., 2005).
2.3 Stratigraphic framework and age assignments
The assignment of chronometric ages of Middle Triassic sediments in the study area is subject to several difficulties:
1) The correlation of global Middle Triassic biostratigraphic units with chronometric ages has not yet been finally resolved. Ages of individual horizons vary between studies, in particular due to different methods of radiometric dating (see Menning, 2020 for a summary of different chronometric ages).
2) For the Triassic of the CEB itself, no radiometric age data is available due to the lack of appropriate sediments (Menning, 2020). Therefore, the regional stratigraphic scale must be correlated with the global stratigraphic scale first.
3) Although the Muschelkalk of the interior CEB is biostratigraphically well structured (Hagdorn, 2020 and references therein), the endemic fauna in the CEB complicates a continuous biostratigraphic correlation with the Tethyan realm that has been achieved only for few horizons (Brack et al., 1999; Kozur, 1999; for a synopsis see Menning, 2020). For the intervals in between, only estimates have been made, for instance based on cycles. In the case of the Middle Muschelkalk, the high salinity led to a lack of index fossils that precludes biostratigraphic classification. Magnetostratigraphic data are hardly available for the Muschelkalk of the CEB (Menning, 2020).
Because of the various, sometimes contradictory, correlation options that arise from the above-mentioned aspects, studies and assumptions used and made for age attributions are briefly described: The Pelsonian–Illyrian boundary at 244.5 Ma can be correlated with the Schaumkalk Member of the uppermost Jena Formation (uppermost Pelsonian/Paragondolella bifurcata conodont zone; Kozur, 1999; lowermost Illyrian/Judicarites sp. in Schreyerites abichi ammonoid subzone; Brack et al., 1999). This correlation is supported by new palynological data obtained in the study area (Hochuli et al., 2020). Consequently, for the top of Lower Muschelkalk in Germany, an age of 244.4 Ma can be assumed, following scheme B in Menning (2020). In interior parts of the CEB, the Middle/Upper Muschelkalk boundary can be placed in the uppermost part of the Paraceratites trinodosus ammonoid zone (early Illyrian) based on immigrated conodonts from the Tethyan realm (Kozur, 1999). Kozur (1999) further describes a change towards a conodont fauna similar to that at the base of the Tethyan Kellnerites felsoeoersensis ammonoid zone within the atavus ceratite zone in the lowest part of the Central European Upper Muschelkalk. Thus, an age of 243.6 Ma of the top of P. trinodosus zone can be assumed for the German Middle–Upper Muschelkalk boundary. Consequently, the Middle Muschelkalk evaporites (Karlstadt Formation–Diemel Formation in Germany; «Orbicularis-Mergel»–«Dolomitzone» in northern Switzerland) must have formed in a relative short period between the Pelsonian–Illyrian boundary and uppermost part of the P. trinodosus zone within around 800 kyr. All correlations of global biostratigraphic with chronometric age data follow the CA-TIMS time scale in Menning (2020). Alternatively using the time scale of Ogg et al. (2016) provide discrepancies of ca. 0.9 Myrs for top of the Jena Formation (~ 243.5 Ma) and 0.8 Myrs for the Middle–Upper Muschelkalk boundary (~ 242.8 Ma).
3 Material and methods
3.1 Sedimentological logging and facies analysis
This study is based on a total of 640 m of cores recovered from 10 wells in northern Switzerland drilled by the Nagra since the 1980s (Fig. 1, Table 1). In each well, the entire Zeglingen Formation was logged sedimentologically at a scale of 1:50 or in greater detail. For all sites, planar core photographs were available. For the boreholes drilled since 2019, in addition 360° scans of washed and marked cores made by the mud logging team directly after core recovery. Minor alterations of the core material since its extraction resulted in an increase in color contrast and thus better visibility of the sedimentary structures in some cases, whereas in other cases reduced contrast resulted in blurring of the structures. Macroscopic logging of the cores, therefore, was performed on both 360° core scans and cores of the new boreholes, and only on the cores for the older boreholes. In the case of missing core sections due to previous sampling, the 360° core scans provide the only option for logging the sedimentary structures.
Based on lithology, sedimentary structures, texture, and fabric the sediments of the Middle Muschelkalk examined in this study were grouped into lithofacies (LF). Typical and recurring successions of different LF were afterwards subsumed to lithofacies associations (LFA), allowing further interpretation of the spatial and temporal evolution of the depositional system.
Unless otherwise mentioned, all meter specifications in this study are given in core depth and measured depth (MD, depth along hole).
3.2 Sequence stratigraphy
Due to the very dynamic spatial and temporal evolution of the depositional setting including early solution of particular lithologies and the very flat topography of the depositional area, vertical facies transitions do not necessarily occur between the adjacent facies belts shown in Fig. 7 (for details see Sect. 4.2). Therefore, the facies law by Walther (1894) is only of limited validity for the studied sediments. Sedimentary cycles were delineated based on the detailed facies analysis. Besides the previously determined LFA, the identification of truncation and, especially, of desiccation horizons was also of major importance for this purpose, particularly where the LFA above and below such horizons are identical, that is particularly the case in halite beds.
3.3 Gamma-ray log analysis
In this study, the lithofacies are mainly distinguished by means of the sulfate texture, which strongly depends on the depositional environment. Due to the rapid lithofacies changes in an evaporite succession and the small-scale differentiation of the depositional area, most boundaries of lithostratigraphic members cannot be considered isochronous per se. In contrast, variations of the clay content are well recorded in gamma-ray log and depend on supraregional factors such as climate, tectonic processes in the source area, etc. that simultaneously affect the whole depositional area. The purely lithostratigraphic subdivision is, therefore, complemented by gamma-ray log units (GR units), which are considered to be more or less isochronous in the study area and its surroundings, as they may represent "extrabasinal" signals, except locally occurring dissolution intervals. Therefore, gamma-ray logs (GR logs) of the Middle Muschelkalk were correlated for all boreholes of which the cores were logged in this study. For a regional overview, additional wells without detailed sedimentological core logging were included in the correlation (Schlattingen-1, Stetten-M, Trüllikon-1, Ühlingen-2). Digital log data were provided by Nagra, the gamma-ray log of Ühlingen-2 was digitized from Sawatzki et al. (2005), the one of Stetten-M from Rogowski and Simon (2005). Laterally clearly correlatable intervals of logs being distinctly delimited from above and below then were grouped into GR units, which subsequently were subdivided into different gamma-ray log subunits (GR subunits). In none of the boreholes in the study area all GR units are present. Thus, for the synopsis of GR (sub)units in Fig. 3, sections taken from different boreholes were combined to a composite gamma-ray log. All GR units are designated with abbreviations written in capital letters. The GR subunits are numbered sequentially from base to top within a GR unit (Fig. 3). In order to include the halite beds in regional correlations, two GR units were distinguished, which are defined purely lithologically and not by gamma-ray log (Fig. 3): MM.2 and MM.5, which correspond to the «Untere Salzschichten» and the «Obere Salzschichten» and are not considered isochronous, unlike the other GR units.
3.4 Quantification and timing of halite dissolution
Dissolution of halite occurred locally at different times within the study area. Dissolution leads to a net loss of volume and thus to the formation of characteristic terrain forms such as sinkholes or subrosion depressions (Hovorka, 2000; Warren, 2016). If dissolution occurs during the deposition of sediments, these depressions can be filled by higher sediment thicknesses compared to the surrounding area. Consequently, positive thickness anomalies within the sediment stack overlying residual horizons can be used to indirectly date the dissolution processes. When estimating the original thickness of the dissolved salt, this can be compared with the dimensions of such sediment-filled subrosion depressions in order to (a) perform a plausibility test and (b) to be able to identify polyphase dissolution processes. Such analysis was carried out for dissolution phenomena encountered in the two wells Marthalen-1 and Trüllikon-1 (Sect. 7.4).
To determine the original pre-dissolution thickness of the halite beds, the drilled salt thicknesses on a regional scale were compared, especially in cases without signs of considerable dissolution; this provides a rough estimate of the original salt thickness in domains affected by salt dissolution. Relating the extrapolated (hypothetical) general trend of original salt thickness and the actual thickness allows approximating the amount of dissolution. Another common approach is to determine the original content of impurities (anhydrite, siliciclastics, carbonates) and relate this to the thickness of an interval of undissolved residues. For this purpose, salt purity values were collected from regional literature and unpublished data of the Swiss saltworks (Schweizer Salinen AG and former Schweizerische Sodafabrik Zurzach) to evaluate original salt purity. The data presented below are based on macroscopic logging of drill cores. For the two most recent exploration wells of the Schweizer Salinen AG petrophysical calculations of the sediment petrography were carried out in addition to these geological estimates and they are listed here as well:
Widmer (1991): 72–86% salt purity (macroscopic estimate)
Hauber (1993): ~ 75% salt purity (macroscopic estimate)
Exploration well S156, 2019 (Schweizer Salinen AG): Petrophysical 80%, macroscopic 84%
Exploration well S157, 2019 (Schweizer Salinen AG): Petrophysical 85%, macroscopic 78%
Brine field Bäumlihof, 4th construction phase 2018, 8 wells (Schweizer Salinen AG): 62.0–78.4%, Ø 72.0% (macroscopic estimate)
Brine field Grosszinggibrunn, 2nd construction phase 2013, 10 wells (Schweizer Salinen AG): 73.1–88.0%, Ø 80.2% (macroscopic estimate)
Based on this data, a salt purity of approximately 80% can be assumed as the best guess for the «Untere Salzschichten». A salt purity of 70% is considered as minimum whereas salt beds with only 15% of insolubles (85% salt purity) is considered as very pure.
4 Facies analysis
The evaporites of the Zeglingen Formation examined in this study comprise 22 lithofacies (LF; Table 2, Figs. 4, 5, 6). They were assigned into five groups: (1) layered sulfates and carbonates, (2) nodular to layered sulfates, (3) conglomerates, breccias and chaotic deformed sulfates, (4) halite, and (5) clay and marl. The lithofacies scheme presented in this study incorporates different previously established schemes (Dronkert et al., 1990; Widmer, 1991), and integrates new observations. Furthermore, 10 genetically linked lithofacies associations (LFA) are introduced, comprising typical and recurrent successions of different LF and defining the depositional system of the Zeglingen Formation (Fig. 7). The deduced environments range from perennial lagoons to supratidal sabkhas, from carbonate depositional settings to halite-bearing sedimentary systems. In addition to one LFA representing sheet floods, two LFA are assigned to dissolution processes at different times.
4.1 Lithofacies (LF)
The characteristics of all 22 LF can be found in Table 2. To describe breccias, the non-genetic, purely descriptive nomenclature of Morrow (1982) was used: packbreccias are clast-supported, floatbreccias are matrix-supported; crackle breccias denote fabrics with only minor relative displacement of fragments, mosaic breccia fragments are largely but not wholly displaced, and rubble breccias show a fabric in which fragments do not match any more.
In some cases, it is difficult to classify nodular anhydrites with thin seams as chicken-wire per se due to the phase transition from gypsum to anhydrite. For example, similar textures may result from the alteration of selenite crystals (Warren, 2016). If chicken-wire-like textures could not be clearly assigned otherwise, they were classified as chicken-wire and assigned to LF 5.
4.2 Lithofacies associations (LFA)
4.2.1 LFA 1—Supratidal sabkha and mudflats
LFA 1 mainly consists of displacive grown intrasediment evaporites. Such evaporites may be nodular (LF 5, LF 6) or enterolithic (LF 8). Nodular and enterolithic sulfates may gradually merge, commonly combined with ductile deformation of the sulfates (LF 7). Some marly to anhydritic strata within the halite beds contain hypidiomorphic to idiomorphic halite crystals, interpreted to result from capillary evaporation as well (LF 20). Monomictic rubble breccias interpreted as shrinkage breccias (LF 12) occur occasionally. Argillaceous layers (LF 21, LF 22) often exhibit mudcracks and are then assigned to LFA 1.
Mudcracks, anhydrite nodules, and enterolithic anhydrite are typical phenomena of sabkhas (e.g., Warren & Kendall, 1985; Kendall, 2010). Moreover, displacive intrasediment growth of evaporites due to capillary evaporation is the key criterion for sabkhas (Warren, 1991, 2016). Ductile-deformed anhydrite of LF 7 is interpreted as the result of hydration of the sediment, possibly accompanied by minor dissolution processes. Hydration, dissolution, and growth of nodules in short temporal succession are considered to have caused the often chaotic appearance of these sediments. Breccias formed by shrinkage and swelling processes due to multiphase hydration and dehydration of gypsum and carbonates within soil horizons indicate varying vertical extent of the capillary fringe, controlling the formation of the displacive intrasediment evaporites. Mudcracks occurring repeatedly and extending deep into the sediment finally provide clear evidence for prolonged or repeated desiccation of the sediment surface. Sedimentary sequences of LFA 1 are repeatedly truncated by muddy sheet floods or transgressions, thereafter inter- to subtidal sulfates or carbonates accumulated. In sediments above the truncation surfaces, anhydrite nodules or even 'chicken-wires' are also frequently present, indicating again desiccation and capillary evaporation. Sheet floods, storms and wind are considered the main suppliers of non-evaporitic sediment onto sabkha flats (Kendall, 2010).
LFA 1 occurs throughout the entire Zeglingen Formation, but mainly within the «Untere Brekzien» and «Obere Brekzien».
4.2.2 LFA 2—Intertidal microbial mats
Wavy to crinkly laminated sulfates and carbonates (LF 1) are the most frequent lithofacies occurring in the studied cores. These sediments often occur together with intercalated flat pebbles and chip conglomerates (LF 11). The laminated layers sometimes show small-scale mudcracks, which provide clear evidence for desiccation.
The wavy to crinkly laminated sulfates and carbonates are interpreted as microbial-laminated sediments as also indicated by the presence of fenestrae and isolated 'elephant-skin' and 'upturned-margin' structures. Within the «Obere Sulfatzone» laminated anhydrite dominates, within the «Dolomitzone» laminated dolomite, the latter often showing layer-parallel chert nodules. Biofilms forming gypsum deposits and sulfate stromatolites are known from modern and historic sabkha and saltern systems (Vogel et al., 2009; Taher, 2014; Strohmenger & Jameson, 2018) as well as from ancient evaporite successions (Rouchy & Monty, 1981; Allwood et al., 2013; García-Veigas et al., 2015).
LFA 2 occurs throughout the entire «Untere Sulfatzone» and «Obere Sulfatzone». The «Dolomitzone» consists mainly of intertidal sediments of this LFA, as already noted by Dronkert et al. (1990) and Widmer (1991).
4.2.3 LFA 3—Channel deposits
As already noted by Dronkert et al. (1990), small scours filled with arenites indicate at least small-scale channeling within the Zeglingen Formation. Recognizing channels in cores is difficult; in particular, the differentiation from sheet floods appears arbitrary in some cases. In general, clast-supported conglomerates (LF 11) were interpreted as channel fills whereas matrix-supported ones (LF 11 (occasionally), LF 13) were interpreted as sheet floods. Some collapse breccias may result from gully wall collapse (Dronkert et al., 1990). LFA 3 mainly occurs intercalated within subtidal to intertidal successions of carbonates and sulfates.
4.2.4 LFA 4—Subtidal carbonates
LFA 4 consists of thin- to medium-bedded dolomites without signs of desiccation (LF 3b). In most cases, they do not display recognizable internal structures (mudstones or arenites) but occasionally normal grading or flaser stratification indicate transport. In some cases, beds of LFA 4 show erosive bases. Often, such dolomite layers are intercalated with sulfate layers of similar appearance (LF 3a). If carbonate predominates, these sediments are assigned to LFA 4.
Sediments of this LFA are interpreted to have formed under subaqueous conditions in water undersaturated with respect to CaSO4. LFA 4 occurs at the base of the «Untere Sulfatzone» within the gradual transition from the dolomitic «Unterer Dolomit» of the Kaiseraugst Formation to the sulfates of the Zeglingen Formation. Only few carbonate layers occur between the top of the «Untere Sulfatzone» and the top of the «Obere Anhydritschichten». In the «Übergangsschichten» and «Dolomit-dominierte Schichten», however, LFA 4 occurs more frequently again. There, layers of marl (LF 21) are increasingly intercalated.
4.2.5 LFA 5—Subtidal sulfates
LFA 5 consists of laminated to thin-bedded sulfates (LF 2, LF 3a) and selenitic sulfate layers (LF 4), both interpreted as subaqueous sediments, which can be formed in several ways (Warren, 2016): "In situ" sulfates may precipitate at the brine-surface and build rafts before sinking to the bottom of the brine (cumulates) or nucleate at the sediment surface (bottom nucleated). In addition, normal grading, intraclasts, isolated ripples and slumps indicate syndepositional mechanical reworking. More or less planar-laminated sulfates (LF 2) may accumulate from pelagic rain (cumulates) or may represent distal evaporite turbidites, both subaqueous phenomena. Cumulates are typical for (seasonally) meromictic brines due to changes in the chemistry in the upper water body that may lead to mineralogical changes of the cumulates and, therefore, to lamination (Kendall, 2010; Warren, 2016). Massive anhydrite (LF 3a) is seen as the result of resedimentation or "in situ" formation. In both cases, recrystallization and gypsum–anhydrite transformation may erase subtle sedimentary structures. The lack of distinct dissolution surfaces or any sign of desiccation indicates formation under subaqueous conditions. LF 4 ('selenitic sulfate layers') shows bottom-nucleated crystals, which form at the bottom of holomictic water masses (Kendall, 2010). Normal grading, intraclasts, isolated ripples and slumps indicate syndepositional mechanical reworking. Especially, sediments of LF 2 and LF 3 are frequently folded due to slumping (Fig. 8 A). Slumping may occur at very low slope angles (< 1°) due to seismic activity (Field et al., 1982; García-Tortosa et al., 2011) and is reported from subaqueous evaporites of the Dead Sea (Alsop & Marco, 2013).
LFA 5 occurs in the whole «Untere Sulfatzone» and «Obere Sulfatzone», in particular in the «Untere Anhydritschichten» and «Ober Anhydritschichten». Occasionally, some layers of subtidal sulfates are intercalated in the halite beds as well.
4.2.6 LFA 6—Halite deposited from perennial brine
The variability among the halite lithofacies indicates a highly dynamic depositional environment and cause various developments of halite beds and associated changes in lithofacies (Fig. 9). The halite of the Zeglingen Formation recrystallized, especially in areas where the halite beds served as detachment and became affected by dynamic recrystallization (Fig. 8B). Primary structures are, therefore, rare and the assignment of a specific lithofacies to a LFA and associated genetic processes is difficult. Consequently, only two LFA were specified differing by the time the sediment surface was covered by brine: LFA 6 comprises halite precipitated from perennial brine and LFA 7 is made-up by halite precipitated from ephemeral brine. The classification of dynamically recrystallized intervals may be questionable. However, this is limited to individual intervals and should have little effect on the overall interpretation.
Similar to the formation of subaqueous sulfates, two processes play a role in the subaqueous in situ formation of halite: formation of cumulates and bottom growth (bottom nucleation). The first indicates precipitation on the bottom of a stratified water body and thus, a comparatively great brine depth, whereas bottom-growth fabrics (chevron halite) are suggestive of rather shallow and unstratified water masses (Hovorka et al., 2007). Depth, however, has to be taken relative; due to high brine density and its damping effect on waves and vertical circulation as well, brine bodies may become stratified in depths of ten meters and less (Kendall, 2010). Due to the recrystallization of the halite beds, a distinction whether certain halite intervals were formed primarily as cumulates or as chevron halite is difficult. Halite laminites can be considered as cumulate formations and vertically elongated halites as chevron halites as described for the halite beds in northwestern Switzerland (Widmer, 1991). Laminites are rare in the halite beds of the Zeglingen Formation (Dronkert et al., 1990) but color banded halite occurs frequently.
If pure halite (LF 15) does not show evidence of desiccation (see Sect. 4.2.7), it is considered to record formation out of a perennial brine. Subordinate contamination, especially by clay-minerals, can be explained by aeolian input as well as by weak freshwater inflow carrying suspended load, floating as buoyant lense on denser brine. Besides clay-minerals, gypsum can precipitate in the upper part of the water body due to weak dilution and form cumulates, resulting in the formation of thin seams or interlayers (LF 16). Recrystallization and coarsening of the halite crystals in intervals without strain may lead to coarse-grained halite having impurities in the intercrystalline space (LF 18, Fig. 8C). If a freshwater body reaches down to the sediment surface, the uppermost sediment is dissolved and a sharp truncation surface is formed. This surface is planar and thus differs from an irregular dissolution surface with microkarst voids formed by desiccation.
Rather pure halite beds can also be formed by resedimentation. These are fine-grained or, if recrystallized, coarse-grained. Resedimentation may also form floatbreccias consisting of sulfate or carbonate clasts within a halite matrix (LF 17) if halite intercalated with other lithologies experienced slumping. Halite and gypsum breccias in Miocene salt beds in the Carpathian foreland have been interpreted as proximal mass flow deposits (Peryt & Kasprzyk, 1992; Peryt & Kovalevich, 1997; Bukowski et al., 2007). In general, floatbreccias with brittle-deformed clasts of LF 17 are interpreted as mass flow deposits if they lack characteristics of collapse breccias (LFA 7, see Sect. 4.2.7). LFA 6 occurs only in the halite beds of the «Untere Salzschichten» and «Obere Salzschichten».
4.2.7 LFA 7—Halite deposited from ephemeral brine
Halite layers of this LFA are characterized by desiccation features like salt cubes in mud (Fig. 9) and subaerial dissolution phenomena like irregular dissolution surfaces (Figs. 6D, 8D, 9). The latter is clearly distinguishable from sharp truncation surfaces formed by freshwater inflow (see above).
If a halite layer, however, is overlain by another lithology, overprinting of the halite sediment and the overlying layer depends on the degree of saturation of the upward-flowing fluid (evaporate pumping). If the pore fluid is saturated with respect to halite, for instance due to proximity to a perennial brine pan, it will result in the growth of displacive halite crystals in the overlying sediment. If the pore fluid is undersaturated with respect to halite, it causes dissolution of the halite and the formation of associated collapse structures. When halite is completely dissolved, only the dissolution structures remain, and the sediment is classified as LFA 9 or LFA 10. However, if dissolution is incomplete, the clasts of the collapse breccia may sink into a mixture of halite and dense brine forming a floatbreccia within halite matrix (LF 17, Fig. 6H). Thus, floatbreccias of LF 17 are assigned to LFA 7 (i) if they are overlain by collapsed layers, especially if these show signs of desiccation, and (ii) if the clasts are of the same lithology as overlying layers and show signs of vertical displacement, suggestive of desiccation with partial dissolution. Halite surrounding such sunken clasts in floatbreccias as well as halite under distinct desiccation surfaces often show a red to yellow color. The color can be attributed to small amounts of iron oxides in the salt (Fischbeck et al., 2003) and, consequently, indicates intermediate oxidizing conditions such as prevailed during desiccation.
Repeated desiccation, dissolution, and halite growth within the sediment results in blurring of the primary sedimentary structures. Collapse into dissolution cavities and displacive halite growth in the sediment can lead to complete destruction of the primary fabric. These processes are called haloturbation (Smith, 1971). Haloturbated sediments are classified as LF 19 (dominant halite) or LF 20 (dominant argillaceous and sulfatic sediments). LFA 7 occurs only in the halite beds of the «Untere Salzschichten» and «Obere Salzschichten».
4.2.8 LFA 8—(Distal) sheet flood deposits
Siliciclastics can be brought into evaporite basins by aeolian and aqueous transport (Dronkert et al., 1990; Kendall, 2010). Accordingly, clay-rich layers not associated with sulfate nodules are interpreted as being of aeolian origin or having settled from flowed-in suspension. Aqueous transport may occur in form of seawater washovers (storms) and surface runoffs (sheet floods). Aeolian transport may result in both, episodic or continuous input whereas sheet floods and storms represent episodic events and cause single layers. Such layers, whether formed by landward or seaward current, are addressed together in LFA 8 as sheet flood deposits. In general, decimeter-thick layers are seen as sheet flood rather than aeolian deposits.
Sheet flood layers normally show sharp and plane bases and signs of current-driven transport like intraclasts. They are clearly distinguishable from aeolian deposits by these. Differentiation from channel sediments sometimes appears arbitrary. In general, clast-supported conglomerates (LF 11) were interpreted as channel fill whereas matrix-supported ones (LF 11 or LF 13) and pure clay layers (LF 22) were interpreted as sheet flood deposits.
Because the area was located about 100 km from the nearest potential source area of siliciclastics (Ziegler, 1990), no sand layers or other proximal sheet flood deposits could be found. Only distal deposits in the form of clay layers, partly with reworked coarser mud- or anhydrite-clasts, occur.
4.2.9 LFA 9—Dissolution remnants (probably eogenetic)
All dissolution phenomena are grouped in two LFAs: Early, eogenetic dissolution phenomena characterize LFA 9 whereas LFA 10 comprises late, meso- or telogenetic dissolution feature. All LF associated with dissolution phenomena occur in both LFAs. A distinction between the two LFAs is made by integrating the corresponding intervals into a stratigraphic and regional context (see Sect. 4.2.10). For example, residue intervals associated with significant increase in thickness of overlying strata indicate the formation of a dissolution depression that was later filled with sediment and are, therefore, interpreted as mesogenetic and are ascribed to LFA 10.
Primary dissolution phenomena are collapse breccias (LF 9, 10, 13) and associated LF. These are in particular ductile-deformed anhydrite and clay mixtures (LF 13, 14). Collapse breccias are one of the most reliable indicators of the former presence of evaporites (Warren, 2016). They ideally show typical LF successions: (i) undisturbed bedding, (ii) fractured and still well recognizable bedding (packbreccias, LF 9), (iii) mosaic floatbreccia with still recognizable former bedding (LF 10), (iv) rubble floatbreccia without signs of former bedding (LF 13). Similar features occur in the studied cores. The rubble floatbreccias generally form the main part of collapse breccias in particular and residue intervals in general. The clasts of these rubble floatbreccias sometimes show signs of dissolution and of ductile deformation (Fig. 5F). Ductile-deformed anhydrite-clay beds occur both at the base and partly in between (LF 14). Occasionally such ductile-deformed layers also constitute an entire dissolution interval and are then distinguishable from other ductile-deformed layers (LF 7) by vertically displaced components indicating substantial dissolution (Table 2).
LFA 9 is generally restricted to the “Untere Brekzien” and “Obere Brekzien”. The fact that these strata have stratigraphic character indicates widely similar environmental conditions that allowed deposition of halite but made its preservation impossible.
4.2.10 LFA 10—Dissolution remnants (probably meso- or telogenetic)
LFA 10 basically comprises the same LF as LFA 9. However, increased thickness of overlying strata as well as stratigraphically different position of the corresponding residue interval (not within the «Untere Brekzien» or «Obere Brekzien»; in Böttstein and Riniken at the base of the halite beds, indicating dissolution by ascending fluid, in Marthalen-1 replacing the entire halite beds) indicate dissolution postdating that typical of LFA 9. Sediments above a residue interval exhibiting clear evidence of extensional vertical deformation indicate that dissolution occurred after their deposition. Furthermore, the lithologic features of the corresponding LF differ from the counterparts in LFA 9. So, the residue intervals ascribed to LF 14 in the boreholes Böttstein and Marthalen-1 contrast with all other dissolution intervals grouped into in LFA 9 due to their very distinct ductile as well as brittle deformation structures and evident displacement features.
5 Sequence stratigraphy
In general, sequence stratigraphic studies are subject to some limitations (Warren, 2016): Sea-level fluctuations within hydrographically isolated evaporite basins do not necessarily correlate with those outside. Furthermore, deposition rates of evaporites may be an order of magnitude higher than those of marine carbonates. Correlations with global short-term sea-level fluctuations should therefore be taken with caution. Nevertheless, detailed sequence stratigraphic analyses can be carried out within evaporite basins in general (e.g., Tucker, 1991; Sarg, 2001; Husinec, 2016) and have been carried out in the Central European Middle Muschelkalk.
The Central European Triassic constitutes a 2nd order transgressive–regressive cycle (Aigner & Bachmann, 1992). The Muschelkalk is sequence-stratigraphically subdivided differently by different authors (Fig. 3). Aigner and Bachmann (1992) as well as Nitsch et al. (2020) subsume Middle and Upper Muschelkalk into a 3rd order transgressive–regressive cycle where the Middle Muschelkalk together with the lowermost part of the Upper Muschelkalk represents the transgressive part. Szulc (2000) subdivided the Middle and Upper Muschelkalk into two 3rd order cycles with a sequence boundary (SB) at a "middle carbonate horizon", which Szulc (2000) correlates with the «Mittlerer Dolomit» of Friedel and Schweizer (1989) that, according to log correlations, corresponds to the «Untere Anhydritschichten» in the study area (see Sect. 6.4). All mentioned authors consider the base of the Middle Muschelkalk as SB between the highstand system tract (HST) of the Lower Muschelkalk and the sediments of the lowstand system tract (LST) at the base of the Middle Muschelkalk even if both the exact SB position and the stratigraphic range of the LST differ in detail between studies (Fig. 3). The Middle Muschelkalk in northern Germany (Karlstadt, Heilbronn and Diemel formations) was subdivided into 9 parasequence cycles or 4th order cycles (Brückner-Röhling, 1999; Röhling, 2000). These parasequence cycles were combined by Nitsch et al. (2020) into 2 cycles, MM1 and MM2. MM1 comprises the parasequence cycles 1 to 4 of Röhling (2000), MM2 5 to 8. Röhling's parasequence cycle 9 is already seen as the first parasequence cycle of the Upper Muschelkalk by Nitsch et al. (2020). The boundary of Röhling’s parasequence cycles 1 and 2 corresponds to the SB between HST (Kaiseraugst Formation) and the LST (Heilbronn Formation). MM2 begins with the «Zwischendolomit» of southwestern Germany or the «Untere Anhydritschichten» in the study area (Fig. 3). Nitsch et al. (2020) note the parasequence to cycle ratio of 1:4 and discuss the possibility that they could correspond to 100 kyr and 400 kyr Milankovitch cycles. However, due to the strong influence of synsedimentary tectonics potentially overprinting climatic signals, Triassic cycles should be treated cautiously in terms of their formation mode (Szulc, 2000, 2008b). Nevertheless, although diverging in detail, the cycle subdivisions and sequence-stratigraphic correlations of earlier authors together already indicate a certain synchronicity of individual stratigraphic horizons.
The Middle Muschelkalk of the study area is built up of mini-cycles with thicknesses ranging from a few dm to about 3 m. These mini-cycles mainly show shallowing-upward or brining-upward trends. The different recurrent mini-cycle types are shown in idealized form in Fig. 10 but are often not ideally developed. The different types of mini-cycles are named A to I (Fig. 10). Many of them can be auto-cyclic and might have been subjected to early diagenetic solution events. Consequently, in general, mini-cycles are not correlatable. However, they often form cycles of higher order, some of which are correlatable regionally and are mentioned below. Five cycle sets (CS) of higher order with thicknesses of about 5 to 15 m, composed of mini-cycles and cycles of different number, could be identified and correlated over the entire study area (Figs. 11, 12, 13). They are interpreted as 4th order shallowing-upward cycles.
5.1 Cycle set 1: «Untere Sulfatzone»
In the study area, the basal SB of the Middle Muschelkalk is marked by mostly arenitic anhydrite and dolomite layers, often showing an erosional base and signs of transport, overlying dolomites with anhydrite nodules and locally mudcracks. This SB generally coincides with the base of the Zeglingen Formation. Upward, anhydritic algal laminites, followed by sabkha sediments, and locally occurring syn- to early postsedimentary residual intervals show regionally widespread desiccation which forms the top of the first shallowing-upward cycle of the Middle Muschelkalk. In the western part of the study area, a haloturbated layer at the base of the «Untere Salzschichten» forms the top of this first shallowing-upward cycle. East of the well Stadel-3, this haloturbated layer does not occur. Instead, a few dm thick layer of subaqueous anhydrite overlies the top of the first shallowing-upward cycle followed by a second shallowing-upward cycle in the «Untere Sulfatzone» (Figs. 11, 13). Where present, this layer of subaqueous anhydrite is clearly developed as minimum in the GR log. It seems plausible to correlate the first shallowing-upward cycle of the «Untere Sulfatzone» across the whole study area and consequently correlate the second shallowing-upward cycle of the «Untere Sulfatzone» in the east with the lowermost shallowing-upward mini-cycles within the halite beds in the west. CS 1 is formed mainly from type A and B mini-cycles, although the latter are rarely completely developed.
5.2 Cycle set 2: «Untere Salzschichten»–«Untere Brekzien»
The onset of halite beds, mostly deposited from perennial brine, marks the base of CS 2. At some locations, the lowest strata within the halite beds show a short transgressive phase with a transition from halite deposited from ephemeral brine to halite deposited under perennial conditions. As discussed above, the base in the eastern part of the study area may already be located within the «Untere Sulfatzone» at a distinct layer of subaqueous anhydrite, intercalated in mostly supratidal sediments. The halite beds of CS 2 consist of a strongly varying number of shallowing-upward or brining-upward mini-cycles of type C, D and E. Many of the mini-cycles exhibit a dissolution horizon or signs of desiccation at the top. Where the «Untere Brekzien» occur, consisting of sabkha-sediments and residue intervals, they provide clear evidence of local desiccation at the CS top. The «Untere Brekzien» consist of mini-cycles of type B, in most cases without subaqueous anhydrite, and of type I.
5.3 Cycle set 3: «Untere Anhydritschichten»–«Obere Brekzien»
The base of CS 3 is marked by an abrupt transition from either halite or sabkha sediments of the «Untere Brekzien» towards the more or less subaqueous anhydrite of the «Untere Anhydritschichten». The transition from halite or sabkha sediments to subaqueous anhydrite implies increased inflow of less saline water, due to regional correlatability and the thickness of the anhydrite layers, more likely of marine origin than meteoric. At the same time, an increase in water depth can be observed. Both clearly indicate a transgression of less saline water, which may have affected the shallow topography of the top of CS 2 virtually isochronously. The sediments of CS 2 are either directly overlain by subaqueous anhydrite beds, indicating a local rapid character of this transgression, or by intercalations of microbial mats and subaqueous anhydrite, indicating a comparatively slow transgression. After the initial transgressive phase, the mini-cycles within the «Untere Anhydritschichten» can be grouped to two shallowing-upward cycles in most wells. Depending on the degree of desiccation, the top of the lower cycle is formed by microbial mats or sabkha sediments. The top of the second cycle is marked by either the first sabkha sediments and residue intervals belonging to the overlying «Obere Brekzien» or, where occurring, the onset of halite of the «Obere Salzschichten». Mini-cycles of type B and F form the «Untere Anhydritschichten». Mini-cycles of type B, usually starting with intertidal microbial mats, and of type I form the «Obere Brekzien». In most cases, the mini-cycles of the «Obere Brekzien» form three distinct shallowing-upward cycles of subaqueous anhydrite or microbial mats at the base and sabkha sediments and residue intervals at the top. In the study area, the «Obere Salzschichten» only occur at Stadel-2 and -3 and Bachs-1 (see Sect. 6.5) where they are formed by one mini-cycle of type C.
5.4 Cycle set 4: «Obere Anhydritschichten»–«Dolomit-dominierte Schichten»
On top of the supratidal sediments of the «Obere Brekzien», a transgression with a transition from sabkha sediments to microbial mats and finally subaqueous anhydrite is again evident and forms the base of CS 4. As already described for the «Untere Anhydritschichten», the transgression occurs either abruptly with subaqueous anhydrite directly overlying the «Obere Brekzien», or gradual with intercalations of microbial mats and subaqueous anhydrite. Subaqueous anhydrite and anhydritic microbial mats form the «Obere Anhydritschichten», which—like the «Untere Anhydritschichten»—occur in whole northern Switzerland. Towards the top of the «Obere Anhydritschichten», again microbial mats are intercalated indicating a shallowing-upward trend. Within the «Übergangsschichten» and the lower part of the «Dolomit-dominierte Schichten» (GR unit UES in Figs. 11, 12, 13, see Sect. 6), overlying the «Obere Anhydritschichten», dolomite and dolomitic to anhydritic marl layers increasingly occur, which in turn show signs of desiccation towards the top, such as anhydrite nodules, chicken wires and mudcracks. Intercalated microbial mats become increasingly dolomitic towards the top of this succession. CS 4 is formed by mini-cycles of type B and F in the lower part and mini-cycles of type B and G in the upper part.
5.5 Cycle set 5: «Dolomit-dominierte Schichten»–«Dolomitzone»
The base of CS 5 is characterized by a dm-thick subaqueous anhydrite bed marking the transgression on the sabkha sediments and microbial mats of the uppermost part of CS 4. This anhydrite bed forms a distinct marker bed in GR logs. Only in the well Riniken, first signs of the corresponding transgression are already found ca. 1 m below the marker bed (Fig. 12). Upwards, the transition to the dolomitic layers of the «Dolomitzone» consists mainly of dolomitic microbial mats. The general shallowing-upward trend is interrupted by episodic intercalations of dolomitic mudstones, marls, dolomitic arenites and oolites. The top of the cycle set is marked by the diachronous transgressive surface of the Upper Muschelkalk and the general change to fully marine conditions. CS 5 mainly consist of mini-cycles of type G and H.
6 2D regional correlation
All GR units defined in this study as well as most GR subunits could be correlated well between the investigated boreholes. Thus, the Middle Muschelkalk sediments in the study area can be correlated stratigraphically at a scale of a few meters (GR subunits). The GR units and subunits of all studied boreholes were correlated in three 2D sections A, B, and C (Figs. 11, 12, 13). These sections are flattened to the top of the uppermost GR unit MM.9. Mini-cycles can be recognized within each borehole but are only rarely correlatable between different wells. Mini-cycles of the boreholes Stadel-3, Bözberg-1 and Riniken are displayed exemplarily (Figs. 11, 13). The cycle sets correlate well with individual GR units (see Sect. 6). Thus, they are not shown for each borehole individually in the sections.
6.1 GR units MU.GB (Geislingen Bed) and MU.OO («Obere Orbicularismergel»)
In the study area, the first evaporitic interval of the Muschelkalk is the Geislingen Bed, which corresponds to the GR unit MU.GB and represents a prominent marker horizon (Figs. 11, 12, 13; Becker et al., 1997). The Geislingen Bed as well as the overlying «Obere Orbicularismergel» were not sedimentologically analyzed in this study but integrated into regional correlations. Whereas the Geislingen Bed is evaporitic in whole northern Switzerland, a facies transition to dolomite and limestone occurs north of the study area in Baden-Württemberg (Hagdorn & Simon, 2005; Geyer et al., 2011). GR units MU.GB and MU.OO are well correlatable between all studied boreholes. The top of GR unit MU.OO largely correlates with the base of the Zeglingen Formation. The thickness of the interval is rather constant in the study area with an increasing trend toward the west (Fig. 14 A).
6.2 GR unit MM.1 («Untere Sulfatzone»)
GR unit MM.1 is well correlatable between all studied boreholes. The base of the Zeglingen Formation largely corresponds to the base of the GR unit MM.1 and is, therefore, interpreted as almost isochronous in terms of stratigraphic resolution. GR unit MM.1 contains three subunits of which only the lowest one (MM.1 a) occurs in all boreholes. The two upper ones (MM.1 b and c) occur only in the eastern part of the study area (Fig. 14 B). GR subunits MM.1 b and c practically disappear over a short distance between boreholes Bülach-1 and Stadel-3 and only the base of subunit MM.1 b occurs west of Bülach-1 (Fig. 13). Where MM.1 b occurs entirely, its base coincides with a layer of subaqueous anhydrite intercalated in mostly supratidal sediments dividing the GR unit MM.1 into two shallowing-upward cycles. GR correlations suggest that the time equivalent of MM.1 b and c, where they are absent, is located already in the basal halite layers (Fig. 13). Thus, the basal subaqueous anhydrite layer of MM.1 b is correlated with the onset of the halite beds in the western part of the study area, also for sequence stratigraphic reasons (see Sect. 5.2). Apart from the differences in thickness between wells where MM.1 b and c occur and the rest, GR units MM.1 shows a distinct decrease in thickness northwards (Figs. 11, 12, 14B).
6.3 GR units MM.2 («Untere Salzschichten») and MM.3 («Untere Brekzien»)
Where present, both GR units, MM.2 and MM.3, are well correlatable between the studied boreholes. Except for the boreholes Weiach, Marthalen-1 and Trüllikon-1, the GR unit MM.2 occurs in all studied boreholes. The GR unit MM.3 occurs only in the western part of the study area and in Weiach. Where present, GR unit MM.3 always rests on top of the GR unit USS in the study area and in northwestern Switzerland (Widmer, 1991). The regional correlations imply that where present, the «Untere Brekzien» correspond to the upper part of the «Untere Salzschichten» elsewhere. In Profile A peaks of GR unit MM.3 can be correlated over almost 90 km, but GR unit MM.2 in Stetten-M correlates to only the lowest part of GR unit MM.3 in Siblingen and Weiach (Fig. 11). The halite beds in Stetten-M, hence, correlate with the «Untere Salzschichten» in northern Switzerland and the «Unterer Tonanhydrit» in Ühlingen-2 and Stetten-M correlates with the GR unit MM.3 and the «Untere Brekzien». GR unit MM.2 shows the highest thickness variations of all GR units in the study area (Fig. 14C). The halite beds in the regions ‘Jura Ost’ and 'Nördlich Lägern' show distinct signs of tectonic influence such as stretching lineation of halite crystals (Fig. 8 B) and a corresponding mylonitization of some halite intervals. However, the distinct differences in thickness of the halite beds, especially in ‘Nördlich Lägern’, are interpreted to be mainly depositional. On the one hand, deformation of the halite beds is mainly parallel to the bedding and on the other hand the halite beds in 'Nördlich Lägern' can be correlated very well (Fig. 13), which clearly would not be the case if deformation would have been significant.
In Marthalen-1, a distinct residual interval could be distinguished at the stratigraphic position of the missing «Untere Salzschichten». It consists of ductile deformed anhydrite-clay mélange and a rubble floatbreccia with internally brecciated clasts, indicating multiphase dissolution processes (LFA 10). For Trüllikon-1 no cores of the corresponding interval were recovered during drilling, but FMI log (Formation Micro-Imager) shows similar deformed sediments as the residue horizon in Marthalen-1 (Additional file 1: Figs. S1, S2). In Böttstein, the «Untere Salzschichten» and GR unit MM.2 are underlain by an interval of ductile-folded, steeply dipping anhydrite and clay, interpreted as residue interval caused by ascending fluids (LFA 10).
6.4 GR unit MM.4 («Untere Anhydritschichten»)
Except for Bözberg-1, where the GR units MM.4 to MM.6 occur in strongly reduced thickness and cannot be delineated from each other, GR unit MM.4 is well correlatable between all studied boreholes. Although the Middle Muschelkalk in Bözberg-1 is tectonically overprinted, the reduced thickness of the GR units MM.4 to MM.6 is considered primary. Like the ductile overprinting of the halite beds, the brittle deformation of the «Obere Sulfatzone» in form of faults is bedding parallel and therefore cannot be responsible for significant offsets and associated thickness variations. The base of GR unit MM.4 correlates with the base of CS 3. The thickness of the GR unit MM.4 increases in general to northwest and shows in Schlattingen-1, in a comparatively marginal basin position, the lowest thickness of the examined boreholes (Fig. 14D). A local minimum in thickness of GR unit MM.4 occurs at Stadel-2 where equivalents of the upper part of GR unit MM.4 form the GR unit MM.5 (compare Sect. 6.5; Fig. 13). Despite this anomaly, an increase in thickness towards northwest is seen (Fig. 14D). According to the GR log correlations, MM.4 and the corresponding «Untere Anhydritschichten» correlate to the «Zwischendolomit» and the «Zwischenanhydrit» at Ühlingen-2 and Stetten-M (Figs. 11, 12).
6.5 GR unit MM.5 («Obere Salzschichten»)
In the study area, the GR unit MM.5 and the corresponding «Obere Salzschichten» occur only at Bachs-1, Stadel-2, and Stadel-3. GR log correlations between these and adjacent boreholes indicate that the deposition of the «Obere Salzschichten» started in Bachs-1 and Stadel-2 while in other places the deposition of the anhydrites of GR unit MM.4 were still ongoing. In Stadel-3, halite deposition began later, simultaneously with the base of GR unit MM.6. Deposition of the «Obere Salzschichten» ended at all three locations within the lowermost GR subunit of MM.6 (Fig. 13).
6.6 GR unit MM.6 («Obere Brekzien»)
In well Bözberg-1, the MM.4–MM.6 sequence occurs in strongly reduced thickness (see above). In Ühlingen-2, the MM.6–MM.8 succession is not segregable and shows a strongly reduced thickness (Fig. 12) due to subrecent dissolution processes (Sawatzki et al., 2005). The GR unit MM.6 is well correlatable across all other wells. The characteristic tripartition is traceable in all boreholes and can be correlated from the study area across the well Stetten-M, 70 km to the north (Fig. 11), to at least the well Urach-3, 110 km to the northwest (Martin & Zedler, 2014). In Stetten-M and Urach-3, the GR unit MM.6 largely corresponds to the «Oberer Tonanhydrit». A significant difference in thickness of the interval MM.6–MM.7 is evident between Benken and Marthalen-1 where the interval is around 5 m thicker than in Benken. This is accompanied by a facies change within the «Untere Anhydritschichten», the «Untere Brekzien» and the «Obere Anhydritschichten». All of them show a higher proportion of subaqueous anhydrite (LFA 5) in Marthalen-1 than in Benken, where LFA 1 and 2 dominate in the corresponding interval. Subaqueous conditions and a comparatively large number of slumps and brecciated layers at Marthalen indicate a steeper relief than in Benken during deposition of GR units MM.6 and MM.7. Together with a distinct residue interval in Marthalen-1 at the level of the «Untere Salzschichten», these findings indicate at least partial dissolution of the halite beds at the time of deposition of the «Obere Sulfatzone».
6.7 GR unit MM.7 («Obere Anhydritschichten»)
With the exception of the borehole Ühlingen-2 (see above), the GR unit MM.7 is well correlatable between all studied boreholes. Its subunits could be correlated over the entire study area, the lowest subunit also to Stetten-M, where the GR unit MM.7 corresponds to the «Oberer Anhydrit» (Fig. 11). The base of GR unit MM.7 correlates with the base of CS 4.
6.8 GR unit MM.8 («Übergangsschichten»/«Dolomit-dominierte Schichten»)
With the exception of the borehole Ühlingen-2 (see above), the GR unit MM.8 is well correlatable between all studied boreholes. In Trüllikon-1, GR unit MM.8 shows a local thickness minimum (Fig. 14G). Overall, an increase in thickness towards the west can be detected. In Stetten-M, the GR unit MM.8 largely corresponds to the «Obere Dolomit-Formation» of Rogowski and Simon (2005), which is now defined as Diemel Formation (Simon et al., 2020b). In Ühlingen-2, the lower part of GR unit MM.8 shows reduced thickness due to dissolution processes (see above), but the upper part corresponds to the lower part of the «Obere Dolomite 1» which forms the part of the Diemel Formation underlying the so-called «Hornsteinbank».
6.9 GR unit MM.9 («Dolomitzone»)
The GR unit is correlatable between all boreholes. The lowest GR subunit MM.9 a could be recognized in all boreholes, the others in most of the studied boreholes in the study area. MM.9 a corresponds to a distinct anhydrite bed marking the base of CS 5. In the south of the study area, the top of GR subunit MM.9 e corresponds largely to the base of the Upper Muschelkalk Schinznach Formation. Towards north, the formation boundary rises to the top of GR subunit MM.9 g. This transition is accompanied by a decrease in thickness of GR unit MM.9 (Fig. 14). Thus, the northern part of the study area may have been more elevated due to lowered subsidence and therefore was not initially affected by the transgression. Further north in the Stetten-M borehole, the base of the Upper Muschelkalk declines even further to the level of GR subunit MM.9 b (Fig. 11).
The newly established lithofacies associations as well as detailed facies and GR log correlations form the base to refine the temporal and spatial facies evolution of Middle Muschelkalk sediments in northern Switzerland and to embed them in a sequence stratigraphic framework. Furthermore, previously only locally defined lithostratigraphic units of the Middle Muschelkalk can now be correlated across northern Switzerland and also into rather interior parts of the CEB. The cyclic lithostratigraphic units introduced by Widmer (1991) for northwestern Switzerland can be distinguished in entire northern Switzerland and the majority of their boundaries form isochronous horizons in terms of stratigraphic resolution.
7.1 Facies evolution
The Geislingen Bed forms the first evaporitic interval of the Muschelkalk in the study area, representing a prominent marker horizon (Becker et al., 1997). The facies transition from carbonates in the north of Baden-Württemberg to evaporites in the study area indicates a salinity increase towards south. During deposition of the «Obere Orbicularismergel» above, salinity decreased and under oxygen-deficient conditions partially bituminous marls were deposited Widmer, 1991; (Simon et al., 2020a, 2020b). Similar deposits are common at the base of thick evaporite successions (e.g., Richter-Bernburg, 1955; Brongersma-Sanders, 1972; Legler et al., 2005; Matano, 2007; Scotchman et al., 2010; Tzevahirtzian et al., 2022). The first evaporites of the Zeglingen Formation occur in form of dm-thick anhydrite layers, often having an erosive base, intercalated in the dolomites above the «Obere Orbicularismergel». The number of dolomite layers interbedded in the anhydrite decreases towards the top, indicating the increasing salinity for the «Untere Sulfatzone».
The sediments of the «Untere Sulfatzone» show a distinct shallowing-upward trend with desiccation features in the study area and the development of sabkha facies. A transgression of hypersaline waters led to the deposition of the basal halite layers in the west of the study area. Since initial precipitation of gypsum or anhydrite would be expected in case of a sea-level rise, this transgression may be interpreted as the result of a local subsidence pulse that allowed hypersaline water already saturated in other parts of the basin to ingress into the study area. In the east, this transgression seems to have led only to the deposition of a decimetric anhydrite layer that was covered by sabkha sediments (Fig. 15A). The sudden facies shift between the boreholes Stadel-3 and Bülach-1 and the associated substantial westward increase in thickness of the «Untere Salzschichten» that significantly exceeds the thickness of GR unit MM.1 b can be related to WNW–ESE striking faults that were active during Middle Muschelkalk times (Pietsch, 2023). Synsedimentary differential subsidence also led to strong variations in thickness of the halite beds in other areas in northern Switzerland (Hauber, 1993; Pietsch, 2023). While halite deposition continued in the east of the study area with recurrent short dissolution and desiccation phases, the hypersaline conditions in the west commenced by soon permanent desiccation, at Weiach and probably also at Siblingen even immediately after deposition of the «Untere Sulfatzone», and a sabkha plain formed (Fig. 15B). Thereafter, halite was accumulated only episodically and was rapidly redissolved, forming typical dissolution breccias. The halite layers of the study area exhibit several desiccation and dissolution horizons. A prominent desiccation event between the lower so-called «Zwickelsalz» or «Unteres Salz» and the upper so-called «Bändersalz» is described from SW Germany (Balzer, 2003). Whether any of the desiccation events encountered in the study area correspond to the boundary between «Zwickelsalz» and «Bändersalz» in SW Germany remains unclear for the moment. The «Untere Salzschichten» can be traced over whole northern Switzerland to the Upper Rhine Graben (Widmer, 1991; Häring, 2002). The «Untere Brekzien» are absent in the area of the Riburg saltworks (unpublished data Schweizer Salinen AG) but they occur again in the Basel region (Widmer, 1991) indicating widespread extent of sabkha plains bordered by salinas and ephemeral ponds.
The change in brine chemistry causing the lithology transition from the «Untere Salzschichten» to the «Untere Anhydritschichten» is interpreted to have been caused by an inflow of less saline water affecting whole northern Switzerland. According to log correlations, the «Untere Anhydritschichten» partly correspond to the «Mittlerer Dolomit» in SW Germany (Figs. 11, 12) and may record a transgressive phase represented by the MM2 cycle (Nitsch et al., 2020). As already described for the Geislingen Bed, the facies transition from dolomite and anhydrite in SW Germany to pure anhydrite in the study area indicates a salinity increase towards south also at the time when the «Untere Anhydritschichten» formed. Both the good correlatability in the GR log and the lithofacies evidence in the underlying strata for an extent flat topography of a sabkha plain suggest an isochronous onset of the «Untere Anhydritschichten» in terms of stratigraphic resolution.
While sedimentation of the «Untere Anhydritschichten» continued in the rest of the study area, halite of the «Obere Salzschichten» was deposited in the area of the wells Bachs-1 and Stadel-2 (Fig. 15C). Halite deposition continued in the area of the wells Bachs-1, Stadel-2 and -3 but in the meantime large parts of the study area desiccated with the onset of the «Obere Brekzien». In NW Switzerland, the «Obere Salzschichten» were described by Widmer (1991) for a salt exploration well. In the current brine fields of the Riburg saltworks the «Obere Salzschichten» occur areawide (Hauber, 1980, 1993; unpublished data Schweizer Salinen AG). The local occurrence or absence of the «Obere Salzschichten» can be attributed to differential subsidence at the time of the Middle Muschelkalk (Pietsch, 2023).
In Marthalen, partial dissolution of the «Untere Salzschichten» led to a steeper relief and sedimentation of subaqueous anhydrite (Fig. 15 D). At the time of sedimentation of GR subunits MM.6 b and c, sabkha facies prevailed throughout the study area and large parts of the South German Basin («Oberer Tonanhydrit» in SW Germany). A further transgressive phase with inflow of less saline waters affected whole northern Switzerland and led to the sedimentation of the «Obere Anhydritschichten» (Fig. 15E). As already described for the «Untere Anhydritschichten», both the good correlatability in the GR log and the lithofacies evidence in the underlying «Obere Brekzien» for an extent flat topography of a sabkha plain suggest an isochronous onset of the «Obere Anhydritschichten» in terms of stratigraphic resolution.
The increasing dolomite content of the «Übergangsschichten» and «Dolomit-dominierte Schichten» and the pure dolomites of the «Dolomitzone» indicate the decreasing salinity in the upper part of the Middle Muschelkalk and the gradual transition to the normal marine conditions of the Upper Muschelkalk. Chicken-wires, mud cracks, and microbial mats indicate predominantly inter- to supratidal conditions during deposition of the upper part of the Middle Muschelkalk. At the time of sedimentation of GR unit MM.9, inter—to supratidal conditions prevailed in the study area while interior parts of the CEB already were affected by the transgression of the Upper Muschelkalk. The prolonged sedimentation of the intertidal «Dolomitzone» indicates the existence of elevated regions in the northeastern part of the study area at the end of the Middle Muschelkalk (Fig. 15F).
7.2 Controls of Middle Muschelkalk cyclicity
The lowest order cyclicity found in the Middle Muschelkalk of the study area corresponds to the mini-cycles (see Sect. 5, Fig. 10). With few exceptions, these mini-cycles cannot be correlated between different boreholes. Likewise, occurrence and quantity of solution or desiccation horizons cannot always be correlated between different wells. Both imply auto-cyclic and rather local control of most mini-cycles. Marked variations in thickness of the «Untere Salzschichten» and the «Obere Salzschichten», as well as selective occurrence of the «Untere Brekzien» and the «Obere Salzschichten» suggest considerable tectonic control of the deposition of these units. Also, the diachronous onset of the deposition of the halite of the «Untere Salzschichten» and the limestones of the Upper Muschelkalk, respectively, indicates subsidence-controlled facies changes.
However, the isochronous onset of the «Untere Sulfatzone», the «Untere Anhydritschichten», the «Obere Anhydritschichten» and their temporal equivalents as well as their widespread occurrence suggest a synchronous control on the transgression of CS 1, 3, and 4 outside the study area as well as surrounding regions. Tectonic control of marine gateways to the Tethyan realm, climatic changes, or eustatic sea-level fluctuations could have caused the transgressions. CS 1 and CS 3 represent supraregional sequence stratigraphic marker beds. The base of CS 1 corresponds to the basal sequence boundary of the Middle Muschelkalk according to Aigner and Bachmann (1992) and Nitsch et al. (2020). The base of CS 3 correlates with the base of the MM2 cycle of Nitsch et al. (2020).
Because of the different number of cycles, it remains unclear for the moment whether the CS 1 and 3 and their counterparts correspond to parasequences of Brückner-Röhling (1999) and Röhling (2000) as suggested by Nitsch et al. (2020). In the study area, no cyclicity could be detected that consistently fit to the 100 kyr or 400 kyr Milankovitch cycles. The majority of the shallowing-upward minicycles are considered to be the result of the high sedimentation rates of the evaporites, which are expected to significantly exceed the enhanced subsidence during the Middle Muschelkalk.
7.3 Implications on the configuration of marine connections to the Tethyan realm
Salinity increases to the south are evident for the Geislingen Bed and the «Untere Anhydritschichten», two supraregional marker beds. This, as well as the later onset of the Upper Muschelkalk transgression in the study area, suggests inflows of normally marine water from the north throughout the time of the Middle Muschelkalk. Thus, marine connections to the Tethyan realm appear to have existed only through the East Carpathian and the Silesian-Moravian gateways during Middle Muschelkalk times.
7.4 Quantification and timing of dissolution
To estimate the original thickness of the «Untere Salzschichten» at Marthalen-1, Trüllikon-1, and Böttstein, different scenarios can be calculated using the above-mentioned salt purity values. For the estimation of the minimum original thickness of the «Untere Salzschichten», halite beds having 70% purity are assumed, for the best guess 80% purity are assumed, and for the estimation of the maximum original thickness, very pure halite beds with 85% purity are assumed (Table 3). The thickness of the residue intervals is assumed to record the original amount of insolubles. The residue interval in Marthalen-1 consists of three parts. The uppermost and lowermost are interpreted as meso- or telogenetic (LFA 10). If only these two parts classified as LFA 10 are considered in the calculation, a low estimate of the possible original salt thickness is obtained; if the entire interval is considered in the calculation, a higher estimate is obtained (Table 3).
The estimated values of dissolved halite at Marthalen-1 are similar to those obtained by simply subtracting the thickness of the residue interval in Marthalen-1 from the thickness of the «Untere Salzschichten» in Benken (12.94 m «Untere Salzschichten» at Benken; 10.07–11.16 m dissolved halite at Marthalen-1). Consequently, an amount of 7.0–11.5 m of dissolved halite is considered the best guess. As discussed above, a significant difference in thickness of the interval MM.6–MM.7 between Benken and Marthalen-1 of ca. 5 m accompanied by a higher proportion of subaqueous anhydrite in Marthalen-1 than in Benken indicate at least partial dissolution of the halite beds at the time of deposition of the «Obere Sulfatzone». The estimated thickness of dissolved halite exceeds the difference in thickness of the interval MM.6–MM.7 of around 5 m between Marthalen-1 and Benken. Consequently, it cannot be excluded that part of the halite could also have been dissolved after the deposition of the Zeglingen Formation. Since permeable faults are required for dissolution, it can be assumed that dissolution occurred primarily during tectonically active periods. Without repeated displacement along the faults, healing and thus sealing of the faults takes place rapidly in evaporites. Thus, potential times for further solution of the halite in Marthalen-1 could be following times of enhanced tectonic subsidence, that are the Anisian to Ladinian Upper Muschelkalk, the Aalenian Opalinuston, and the Oxfordian (Wetzel et al., 2003). As there is no evidence of gypsification of the «Obere Sulfatzone» in the mentioned wells, recent sub-surface erosion can be ruled out.
No cores of the Middle Muschelkalk were recovered in Trüllikon-1 and the identification of the residue interval is based only on the absence of the «Untere Salschichten» and facies similarities of the corresponding stratigraphic interval between the FMI in Trüllikon-1 and the cores of the residue interval in Marthalen-1. In the Middle Muschelkalk, no thickness anomalies were detected in the layers overlying the residue interval. Consequently, dissolution of the «Untere Salzschichten» is most likely to have taken place later, probably at least partly during the deposition of the Upper Muschelkalk, as a positive thickness anomaly can be detected in these sediments.
At Böttstein, the upper part of the «Obere Anhydritschichten» as well as the «Übergangsschichten» and «Dolomit-dominierte Schichten» contain sub-horizontal fibrous gypsum veins indicating vertical extension of these sediments. Since such an accumulation of sub-horizontal gypsum veins does not occur in any other of the studied boreholes classifying them as local phenomenon, they are interpreted to be genetically related to the residue interval at the base of the halite beds. The dissolution of the base of the halite beds must therefore have occurred after deposition of the «Obere Sulfatzone». Since the encountered thickness of the Upper Muschelkalk at Böttstein is a few m higher compared to surrounding wells, the dissolution of the basal «Untere Salzschichten» at Böttstein could have occurred at least partly during the deposition of the Upper Muschelkalk.
Based on a detailed facies analysis of Middle Muschelkalk evaporites in northern Switzerland, 22 lithofacies (LF) were distinguished and grouped into 10 lithofacies associations (LFA). High-resolution regional correlation of gamma-ray (GR) logs as well as sequence stratigraphic considerations provide a detailed stratigraphic framework of the Middle Muschelkalk sediments in the study area. This allows the reconstruction of the depositional history and forms the base to correlate and to compare Middle Muschelkalk sediments of northern Switzerland with equivalents in more interior parts of the Central European Basin (CEB).
In northern Switzerland, the Middle Muschelkalk is constituted by five 4th order shallowing-upward cycles and a varying number of lower-order cycles. Lithofacies pattern and distribution of sediment thickness are indicative of a tectonically active depositional setting during the Middle Triassic. That in turn explains that most lower-order cycles cannot be correlated across the study area and are, therefore, interpreted as auto-cyclic. Furthermore, short-term local increase in accommodation space was rapidly compensated by high rates of sedimentation. Most lithofacies encountered appear to have nearly flat bounding surfaces. Marine transgressions of the basal «Untere Sulfatzone», the «Untere Anhydritschichten» and the «Obere Anhydritschichten», thus, flooded wide areas in rather short time nearly simultaneously. Consequently, the corresponding deposits are considered isochronous over wide parts of the basin and serve as reference levels to integrate the peripheral facies into the supra-regional context.
Halite was deposited during two phases. In the first phase, halite accumulated in basins covering large areas, separated only locally by elevated regions on which sabkha plains formed. Eventually, these salinas desiccated and then sabkha plains covered large areas of northern Switzerland. The second phase, previously only identified in NW Switzerland, could now also be identified in central northern Switzerland. In contrast to the halite deposits of the first phase, the deposits of the second phase occur only very locally, bounded by widespread sabkha sediments. These sabkha plains appear to have been repeatedly flooded by brine, so that thin halite layers could be deposited there as well. However, these deposits were dissolved immediately after deposition but they can be recognized today as characteristic dissolution breccias. In addition, local eo- to mesogenetic dissolution was recognized in drill cores; it occurred during deposition of the Middle and Upper Muschelkalk in depths of tens of meters. The thickness of these dissolved Middle Triassic evaporites was quantified to have been smaller than 20 m. The Middle Triassic eo- to mesogenetic halite dissolution points towards subsurface fluid flow, likely associated with deeper seated faults.
Availability of data and materials
All relevant data that support the findings of this study are either included in this article and its supplementary material or can be requested from the authors.
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The authors thank Nagra for financing this project and permitting to publish and Schweizer Salinen AG for providing several unpublished studies and reports. Further, we thank P. Jordan and T. Voigt for their thoughtful reviews, which greatly helped to further improve the manuscript.
Core logging, facies analysis and regional correlation on which the study is based were funded by Nagra.
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Residue intervals of Trüllikon-1 and Marthalen-1. Fig. S1 Profile of the residue interval in Marthalen-1. Core scan, logged LF and LFA are provided in core depth, gamma-ray log in log depth. For the shown interval the shift between core depth and log depth is around 40 cm; this was corrected by shifting the gamma-ray log upwards now fitting to core depth. The residue interval consists of three parts from top to bottom. (1) 981.88 m–982.76 m: Ductile-deformed anhydrite of LF 14 (A). (2) 982.76 m–983.78 m: Rubble floatbreccia of anhydrite and marl clasts in clay matrix (LF 13) (B) with a layer of nodular anhydrite in clay matrix (LF 6) at 983.38 m–983.60 m (C). This middle part differs from the overlying and underlying parts and resembles in its appearance the breccias of the «Untere Anhydritschichten» or «Obere Anhydritschichten», respectively. Therefore, the floatbreccias were placed in LFA 9 whereas the nodular anhydrite was interpreted as secondary evaporite within a sabkha environment (LFA 1). (3) 983.78 m–984.75 m: In the upper part brecciated anhydrite with clay, clearly showing extensional brittle deformation with vertical extension and white anhydrite matrix (LF 10). A 10 cm-large clast at the base of this upper part is internally brecciated, possibly indicating multiphase extension. The lowermost part of the residue interval consists of a ductile anhydrite-clay mélange with clearly visible vertical displacements and an internally brecciated clast sunken into it (D). At the base, the residue interval has a sharp contact to underlying enterolithic folded anhydrite layers in anhydritic marl (LF 8). They clearly indicates sabkha environment (LFA 1) below the residue interval. Fig. S2 Residue intervals of Trüllikon-1 and Marthalen-1. Left: FMI of Trüllikon-1, showing ductile deformed sediments, interpreted as residue interval comparable to those in Marthalen-1. Right: 360° core scan and LFA of Marthalen-1, showing the residue interval shown in Figure S1. Green line: Base of «Untere Anhydritschichten», blue line: top of «Untere Sulfatzone»
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Pietsch, J.S., Wetzel, A., Deplazes, G. et al. Lithofacies, stratigraphy and depositional history of Middle Muschelkalk evaporites (Zeglingen Formation) in northern Switzerland. Swiss J Geosci 116, 16 (2023). https://doi.org/10.1186/s00015-023-00441-4