The Upper Marine Molasse (Burdigalian, Ottnangian) in Southwest Germany facies interpretation and a new lithostratigraphic terminology

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1 Z. dt. Ges. Geowiss., 161/3, p , 7 figs., 1 tab. Article Stuttgart, September 2010 The Upper Marine Molasse (Burdigalian, Ottnangian) in Southwest Germany facies interpretation and a new lithostratigraphic terminology Nicola Heckeberg, Martina Pippèrr, Benjamin Läuchli, Florian U. M. Heimann & Bettina Reichenbacher* Heckeberg, N., Pippèrr, M., Läuchli, B., Heimann, F.U.M. & Reichenbacher, B. (2010): The Upper Marine Molasse (Burdigalian, Ottnangian) in Southwest Germany facies interpretation and a new lithostratigraphic terminology. [Die Obere Meeresmolasse (Burdigal-, Ottnang-Stufe) in Südwestdeutschland Fazies-Interpretation und eine neue lithostratigrafische Gliederung.] Z. dt. Ges. Geowiss., 161: , Stuttgart. Abstract: The geology of the lower Miocene Upper Marine Molasse (OMM) of the North Alpine Foreland Basin (Molasse Basin) has been studied intensively. However, one problem that remains concerns the different informal names that have been assigned to the OMM units by local workers. Moreover, there is still no general consensus with regard to the precise lithostratigraphic content of these units. In this study, three outcrops in the Lake Constance area (SW Germany) displaying OMM sediments are investigated with regard to lithofacies, sedimentology and microfossils (benthic foraminiferal assemblages). The Kalkofen Formation is introduced as a formal lithostratigraphic name for the so-called Sandschiefer, which represents a mud-dominated succession of an inner to middle neritic environment. The overlying newly defined Baltringen Formation includes the former Bodmansande or Baltringer Schichten, which are sand-dominated deposits indicating a subtidal environment with strong tidal currents. The lower boundary of the Baltringen Fm is marked by a basal erosion surface and a pebbly basal layer (Baltringer Horizont). The Steinhöfe Formation is introduced as a formal lithostratigraphic term for the so-called Deckschichten or Feinsandserie. It overlies the Baltringen Fm and is marked by an unconformity at base. The Steinhöfe Fm comprises finer-grained sand/silt alternations indicative of a less high-energetic but still tidally influenced marginal marine setting. The abrupt shift between the mud-dominated Kalkofen Fm and sand-dominated Baltringen Fm is a result of the wellknown shallowing-upward event that separates the two classical OMM sedimentation cycles (sensu Lemcke et al. 1953). The unconformity at base of the Steinhöfe Fm adds support to the hypothesis of a third sedimentation cycle. Kurzfassung: Die Geologie der untermiozänen Oberen Meeresmolasse (OMM) im nordalpinen Vorlandbecken (Molassebecken) ist grundsätzlich gut bekannt. Allerdings gibt es für den süddeutschen Anteil des Molassebeckens noch keine lithostratigrafische Gliederung der OMM. Stattdessen existieren eine Reihe von informellen Schichtnamen für lokale oder regionale Faziesentwicklungen, deren stratigrafischer Inhalt nicht immer einheitlich gehandhabt wird. In der hier vorliegenden Arbeit werden drei OMM-Aufschlüsse im Gebiet des Bodensees (SW-Deutschland) hinsichtlich ihrer Lithofazies, Sedimentologie und Mikrofossilien (benthonische Foraminiferen) vorgestellt. Die Kalkofen-Formation wird als lithostratigrafischer Name für die bisher als Sandschiefer bezeichneten Schichten eingeführt; sie repräsentiert eine überwiegend tonigsiltige Abfolge des inneren bis mittleren Neritikums. Die darüber folgende neu definierte Baltringen-Formation umfasst die früher als Bodmansande oder Baltringer Schichten bezeichneten Sedimente; es handelt sich um Sand-dominierte Ablagerungen des subtidalen Bereichs mit starken Gezeitenströmungen. Die Untergrenze der Baltringen-Formation ist durch eine Erosionsdiskordanz und eine basale Gerölllage (Baltringer Horizont) charakterisiert. Für die über der Baltringen-Formation folgenden feinkörnigeren Sedimente, die bisher als Deckschichten oder Feinsandserie bezeichnet wurden, wird der Name Steinhöfe-Formation eingeführt. Die Steinhöfe-Formation zeigt an ihrer Basis ebenfalls eine Diskordanz. Sie besteht überwiegend aus Feinsand/Silt-Wechsellagerungen, die in einem weniger hochenergetischen, aber immer noch Gezeiten-beeinflussten Milieu abgelagert wurden. Der sehr deutliche Wechsel zwischen der Ton-Silt-dominierten Kalkofen-Formation und der Sand-dominierten Baltringen-Formation belegt die schon von Lemcke et al. (1953) festgestellte Verflachung des Beckens, die zur Untergliederung der OMM in zwei Sedimentationszyklen geführt hat. Die Diskordanz an der Basis der Steinhöfe-Formation unterstützt die Annahme eines dritten OMM-Sedimentationszyklus. Keywords: Molasse Basin, Southwest Germany, Lake Constance, Upper Marine Molasse (OMM), Ottnangian, tidal sediments, benthic foraminifera, Graupensandrinne Schlüsselwörter: Molasse-Becken, Südwestdeutschland, Bodensee, Obere Meeresmolasse (OMM), Ottnang-Stufe, Gezeitensedimente, benthonische Foraminiferen, Graupensandrinne *Addresses of the authors: B.Sc. Nicola Heckeberg, Dipl.-Geol. Martina Pippèrr, M.Sc. Benjamin Läuchli, Prof. Dr. Bettina Reichenbacher (corresponding author; b.reichenbacher@lrz.uni-muenchen.de), Department of Earth and Environmental Sciences, Palaeontology & Geobiology, Ludwig-Maximilians-Universität München, Richard-Wagner-Str. 10, München, Germany; M.Sc. Florian Heimann, WSL Swiss Federal Research Institute for Forest, Snow and Landscape Research, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland E. Schweizerbart sche Verlagsbuchhandlung, Stuttgart, Germany DOI: / /2010/ / $ 8.10

2 286 Nicola Heckeberg et al. 1. Introduction The mainly siliciclastic sediments of the Upper Marine Molasse (= OMM 1 ) represent an important segment of the basin fill of the North Alpine Foreland Basin and Molasse Basin, respectively. The OMM sediments in Switzerland and Southwest Germany vary in thickness between 200 and 800 m (Strunck 2001, Frieling et al. 2009c), but may reach more than 1000 m thickness in southern Bavaria and Austria (Müller 1978, Lemcke 1988, Rupp & Husen 2007). Characteristic assemblages of benthic foraminifera indicate that the OMM sediments were deposited during the early Miocene, i.e. the Burdigalian stage of the Mediterranean, or Eggenburgian and Ottnangian stages of the Paratethys (Hagn 1961, Wenger 1987a, b, Pippèrr & Reichenbacher 2009, 2010; Fig. 1A). Previous investigations have addressed a diversity of topics concerning the geology of the OMM, including sedimentary facies, cyclicity, biostratigraphy and palaeoclimate (e.g. Lemcke et al. 1953, Berger 1985, Faupl & Roetzel 1987, Keller 1989, Wenger 1987a, b, Scholz 1989, Zweigel et al. 1998, Janz & Vennemann 2005, Pippèrr et al. 2007, Bieg et al. 2008, Kocsis et al. 2009, Frieling et al. 2009a, b, c, Heimann et al. 2009, Pippèrr & Reichenbacher 2009, 2010). A formal lithostratigraphic terminology has been established for parts of the OMM in Austria (Piller et al. 2004) and Switzerland (Keller 1989, Strunck 2001), but is lacking for southern Germany (STD 2002, Doppler et al. 2005). In addition, the individual OMM deposits in southern Germany have been given a number of different local (informal) names (Fig. 1B), and there is still no consensus regarding the precise stratigraphic extent of several of these informal OMM units (see below). This study contributes to the establishment of a formal terminology for the OMM in the Southwest German part of the Molasse Basin, to facilitate basin-wide correlations and advance the understanding of the interplay between tectonic events, eustatic sea level rise and fall, and global climate change during the Burdigalian. 2. Geological overview The Molasse Basin is a typical foreland basin that extends from western Switzerland (Lake Geneva) to Austria, where it passes to the Vienna Basin and the Carpathian Foredeep. The Molasse Basin of Switzerland and Southwest Germany belongs to the Western Paratethys, while the Molasse Basin of Southeast Germany and Austria represents the western part of the Central Paratethys. During the late Eocene and early Oligocene, accumulation of marine marls and sand- 1 We abbreviate the Upper Marine Molasse as OMM after the German term Obere Meeresmolasse here to avoid misunderstanding because UMM means in Germany Untere Meeresmolasse (= Lower Marine Molasse). stones was prevalent throughout the Molasse Basin (Bachmann & Müller 1991). At the base of the late Oligocene (early Chattian), however, a regression in Switzerland and Southwest Germany resulted in the deposition of fluvial and brackish sediments in these areas (Lemcke 1988, Reichenbacher et al. 2004, Kempf & Pross 2005; Fig. 1A). Transgression of the Tethys and Paratethys seas during the early Burdigalian then, again, produced a marine, mainly siliciclastic succession, i.e. the Upper Marine Molasse (OMM; Lemcke 1988, Berger 1985, Rögl 1998, Kuhlemann & Kempf 2002). From the late Burdigalian onwards, another regression led to fluvial sedimentation in the western Molasse Basin (Lemcke 1988, Doppler et al. 2005, Abdul Aziz et al. in press), while marine conditions prevailed to the East (Piller et al. 2004). Accumulation of Molasse sediments ended in the late Miocene; since then, erosion has removed and is still removing parts of the Upper Freshwater Molasse (Lemcke 1988). This paper focuses on the Ottnangian part of the OMM in the Southwest German part of the Molasse Basin. In this area, the OMM succession is underlain by fluvial and lacustrine sands and marls of the Lower Freshwater Molasse and overlain by the Upper Brackish Molasse (Fig. 1A). The Ottnangian OMM sediments can be subdivided into at least two fining-upward sedimentation cycles (Lemcke et al. 1953, Lemcke 1988; Fig. 1B), which represent base level cycles according to Frieling et al. (2009c). The first cycle comprises the Heidenlöcher Schichten and Sandschiefer, while the second, according to Lemcke et al. (1953) and most subsequent authors, comprises the Bodmansande and Deckschichten (= Baltringer Schichten + Feinsandserie). In addition, a third sedimentation cycle started in the Deckschichten or its equivalents (Gall 1975, Pippèrr et al. 2007). Deposition of the Upper Brackish Molasse (OBM) is characterised by a complex palaeogeography with various environmental settings (Lemcke et al. 1953, Doppler & Schwerd 1996). The most characteristic event was the incision of a large river, i.e. the Graupensand river (Moos 1925, Kiderlen 1931; see Tipper et al for an alternative explanation). This river extended along the northwestern margin of the South German Molasse Basin to the Southwest and eventually flew into the still existing OMM sea in the Swiss Molasse Basin (Kiderlen 1931, Reichenbacher et al. 1998; Fig. 2). The Graupensand river initially produced a valley that was km wide and up to 80 m deep, the socalled Graupensandrinne, but then changed from erosion to accumulation and deposited the gravels and sands of the Grimmelfinger Schichten. After this, a short-term transgression from the OMM sea in the Swiss Molasse Basin into the river valley occurred, depositing fossiliferous brackish silts and marls, i.e. the Kirchberg Formation. In the Lake Constance area, the southern margin of the Graupensandrinne widens toward the S and SW, and forms the so-called erweiterte (extended) Graupensandrinne (Haus 1951). In this area, a conglomerate layer occurs at the base of the Kirchberg Fm, and the Kirchberg Fm itself overlies OMM sediments (Grimmelfinger Schichten are lacking; Erb et al. 1961, Schreiner 1989). This conglomerate layer has been

3 The Upper Marine Molasse (Burdigalian, Ottnangian) in Southwest Germany 287 Fig. 1: (A) Age in million years and stratigraphy of the Molasse succession in the South German part of the North Alpine Foreland Basin (Lemcke 1988, Doppler et al. 2005); age correlation of the Paratethys stages principally follows Piller et al. (2007), but correlation of the Ottnangian/Eggenburgian boundary is tentative. (B) Conventional lithostratigraphy of the Ottnangian Upper Marine Molasse and Upper Brackish Molasse in the Southwest German Molasse Basin (Schreiner 1976, 2008). termed Mischhorizont or Mischgeröllhorizont in previous studies, and is composed of pebbles indicative of a source in the Alps to the SW, i.e. the Hörnli and the Napf fans (Haus 1951, Erb et al. 1961, Schreiner 1976, 1989, 2008, Werner 1994). Contemporaneous deposits of the Kirchberg Fm outside the area of the Graupensandrinne include the calcretes and freshwater limestones of the so-called Albstein (Rutte 1955, Zöbelein 1985), as well as marls and sands containing a brackish microfauna (so-called Süßbrackwassermolasse; Lemcke et al. 1953, Reichenbacher 1993; Fig. 1B).

4 288 Nicola Heckeberg et al. Fig. 2: Palaeogeography of the Eggenburgian and Ottnangian (after Haus 1951, Lemcke 1988, Doppler & Schwerd 1996). The Graupensandrinne formed during the late middle Ottnangian and eroded the OMM sediments in the area of its valley. Asterisks indicate location of the studied outcrops and type sections. 3. Material and methods Three outcrops with exceptionally well-preserved OMM sediments were studied in the area to the NE of Lake Constance (Figs. 2, 3): (1) The Eichhalde outcrop (R H , m NN; Fig. 3A), 1 km south of the village of Kalkofen; sediments are exposed from SW to NE along a small forest path, (2) the Maria im Stein outcrop (R H , m NN; Fig. 3B), 4 km NE of Owingen in the valley of the river Aach, in the vicinity of the farm Steinhöfe; the pilgrimage church Maria im Stein is located within the outcrop area, (3) the Burghöfe outcrop (R , H , m NN; Fig. 3B), 500 m to the W of the Maria im Stein outcrop, located at a steep slope along the southern margin of the river Aach; the name refers to the nearby farm Burghöfe. Lithofacies and sedimentology of the three outcrops were examined in the field. Fourteen samples were taken for micropalaeontological analysis and processed in the laboratory by soaking in hydrogen peroxide, washing under running water through 1000 μm, 200 μm and 63 μm mesh sieves. Microfossils were handpicked under a stereomicroscope from representative splits of the dried residues. Benthic foraminifera were identified to species-level, largely following Wenger (1987a), Cicha et al. (1998) and Rupp & Haunold-Jenke (2003). The widely used Fisher s α diversity index was calculated (see Hammer & Harper 2006). The Fisher s α diversity index remains constant regardless of the number of individuals included; high values indicate high species diversity. Planktonic foraminifera were specified as one group and counted in order to obtain the percentage of planktonic versus benthonic foraminifera (P/B ratio). Generally the P/B ratios grow with increasing water depth and distance to the shore; ratios of <20 % are suggestive of inner shelf (inner neritic) environments, % of middle shelf (middle neritic), and % of outer shelf (outer neritic) environments (Murray 1991). Sedimentological and micropalaeontological data served as a basis for the interpretation of the palaeoenvironment according to the actualistic approach. Literature used for comparison includes Visser (1980), Clifton (1981, 1983), De Mowbray & Visser (1984), Reading (1996), Schäfer (2005) for sedimentology, and Sen-Gupta (1999) and Murray (2006) for benthic foraminifera. 4. Results 4.1 Eichhalde outcrop At the Eichhalde outcrop, the Molasse sediments consist of (in ascending order) the uppermost beds of the Lower Freshwater Molasse, ~30 m OMM sediments (Winder 1983) and a few metres of Upper Brackish Molasse (Haus 1951, Erb et al. 1961). During our field work (April 2009), OMM sediments were best exposed in the northeastern part of the outcrop; however, the transition into the Upper Brackish Molasse was not accessible. We recorded and sampled the upper part (about 11 m) of the OMM sediments from four sections (Fig. 4).

5 The Upper Marine Molasse (Burdigalian, Ottnangian) in Southwest Germany 289 Fig. 3: Geographic settings of the outcrops Eichhalde (A), Maria im Stein and Burghöfe (B) and Baltringen (C). Lithofacies and sedimentological features The OMM sediments include a marly and an overlying sandy segment (see Fig. 4A). The marly segment is exposed in section A (0 6.9 m) and at the bottom of section C; the sandy segment in sections A to D (thickest in D, with 6 m). Section A displays blue-grey marls at base (0 0.6 m), overlain by a series of alternating beige and light brown marls with glauconitic sand lenses. From 2.8 m upward in the section, the succession of beige and light brown marls continues, but sand lenses are absent. From 3.7 m upward, the marls again contain glauconitic sand lenses, while sand lenses once more are absent between m. The transition from the marly to the sandy segment is marked by a basal erosion surface (Figs. 4A, D). The bottom of the sandy segment is exposed in sections A and C. It is marked by a thin (~0.1 m) sandy-gravelly layer with large clasts (up to 20 cm) consisting of quartzites, sandstones, Jurassic limestones (some with bivalve boreholes) and granites; oyster fragments and shark teeth may sporadically occur. Upward in sections A, C and D follow fine to medium-grained, glauconitic, grey sands with a few coarse-grained sand layers; bioclasts are frequent (sec-

6 290 Nicola Heckeberg et al. Fig. 4: Eichhalde outcrop. (A) Schematic logs of the Sandschiefer (Kalkofen Fm) and the Bodmansande (Baltringen Fm). (B), (C) Tidal bundles (B) and sand/silt alternations with flaser bedding and superimposed ripples (C) in the Baltringen Fm. (D) Basal erosion surface between the Kalkofen Fm and the Baltringen Fm. Position of (B) (D) and micropalaeontological samples is marked in (A). tion A) or rare (section D). The sands are non-bedded in sections A ( m) and D ( m). Upward in sections B, C and D, intercalations of sand/silt-alternations with flaser bedding or superimposed ripples (Fig. 4C), submarine dunes with internal cross-bedding, and mud drapes on individual cross-bedded layers are recognizable. A distinct planar cross-stratification is present in section C and the upper part of section D ( m). Alternating beds of sand are separated by thin mud drapes (Fig. 4B). Measurements from cross-bed sets indicate palaeocurrents toward the W and NW ( azimuth). Micropalaeontology Six samples (E-A, E-1 to E-5) were taken from the marly segment and three (E-6 to E-8) from the overlying sandy segment (see Fig. 4 for sample position). Sample E-A (not indicated in figure 4) comes from a small section located to the West of the Eichhalde outcrop, a few metres above the boundary of the Lower Freshwater Molasse. The samples from the marly segment contain a marine microfauna composed of echinoid spines, ostracods, benthic and planktonic foraminifera (see Appendix). Benthic foraminifera are relatively frequent; the assemblage is composed of 41 taxa (Appendix). The tests are usually small and well or moderately well preserved; a few specimens are poorly preserved. Sample E-A from the bottom deposits of the OMM produced the most diverse benthic foraminiferal assemblage. Twenty-six species were identified (Fisher s α = 8.4); the dominant genus is Cibicidoides, other common genera include Elphidium, Elphidiella and Ammonia (Tab. 1). In contrast, benthic foraminiferal assemblages from samples E-1 to E-5 are clearly less diverse (Fisher s α = ). The predominant taxa are Elphidiella minuta (E-1), Cibicidoides lopjanicus / C. tenellus (E-2, E-3, E-5) and Ammonia

7 The Upper Marine Molasse (Burdigalian, Ottnangian) in Southwest Germany 291 Tab. 1: Number of individuals, diversity and relative abundance (%) of the most abundant benthic foraminifera in the samples from the Eichhalde outcrop. Bold-faced are the two most frequent taxa of every sample. Samples Eichhalde E-A E-1 E-2 E-3 E-4 E-5 E-6 E-7 E-8 Number of benthic foraminifera Fisher s α diversity index Elphidium spp. keeled Elphidium spp. altogether Elphidiella minuta Nonion commune Ammonia beccarii s.l Cibicides lobatulus Cibicidoides lopjanicus / C. tenellus Heterolepa dutemplei Hanzawaia boueana beccarii s.l. (E-4); other common species include Elphidium spp., Nonion commune and Hanzawaia boueana; in E-1 also Heterolepa dutemplei (Tab. 1). Planktonic foraminifera and ostracods are abundant only in sample E-5. The samples from the sandy segment (E-6 to E-8) contain foraminifera, ostracods, balanids, bryozoans, echinoid spines and fish teeth. The benthic foraminiferal assemblage (~17 taxa) is less diverse than that of the underlying marly segment (Fisher s α = ). However, many specimens are comparatively large and quite well preserved. The most abundant taxa are Elphidium spp., Elphidiella minuta, Ammonia beccarii s.l., Cibicidoides lopjanicus / C. tenellus and Hanzawaia boueana (Tab. 1, Appendix). Planktonic foraminifera are generally rare in the sandy segment. 4.2 Maria im Stein outcrop The Maria im Stein outcrop extends for about 120 m from SE to NW; four sections (A D) were recorded in detail (Fig. 5). The sections expose about 14 m of sandy sediments. Lithofacies and sedimentological features The lowermost portion of the OMM sediments at Maria im Stein is exposed in section A (Fig. 5A), and consists of structureless, mainly medium-grained, poorly sorted glauconitic sands (~100 cm). Above follow three coarse- to mediumgrained sandy bodies of laterally varying thickness and extension (Figs. 5A, E). These bodies show cross-bedding, truncated cross-bedding and bidirectional cross-bedding (herringbone structures); cm-sized gravels occur in some places. Each of the sand bodies is overlain by a marl layer, which is thin on the two lower bodies and 33 cm thick on the sand body showing the herringbone structure (section A: m). This thick marl layer can be traced for 25 m to the NW where it is thinning down to 1 cm. In section A two additional cross-bedded sand bodies with laterally varying thickness and extension (up to 36 m, up to 3 m) follow above the marl and are again overlain by thin marl layers. A structureless sand body, which is laterally discontinuous and extends for ~28 m, appears in the uppermost part of section A ( m). The dip orientation of the various cross strata in section A suggests bimodal palaeocurrents toward the SE and NW. The marl layer at base of section B corresponds to the marl lense at m in section A (indicated with dotted line in Fig. 5A). Fine- to coarse-grained structureless sands and a laterally thinning marl bed with cm-sized ripples follow section-upward. They are overlain by structureless sands with prominent slumping structures (at 4.1 m in section B; Figs. 5A, D). A cross-bedded, medium-grained sand body with scattered gravel-sized clasts and up to 25 cm long and 2 cm high wave ripples follows (up to 4.6 m; Figs. 5A, C). Sands with sigmoidally bedded lenses, internal planar bedding and megaripples, each up to 200 cm long and 25 cm high, form the upper part of section B ( m; arrow in Fig. 5C). This segment is overlain by a sand/marl alternation ( m) with planar and cross-bedding in the lower portion and mainly planar bedding in the upper part. Sections C and D continue the OMM succession upwards and show that the uppermost sand/marl alternation of section B grades into a silt/marl alternation ( m in section C). Then follows a medium-grained sand body with internal cross- and planar bedding, trough bedding and megaripples up to 377 cm long and 29 cm high ( m; Fig. 5B, just below the arrows). This sand body is cut by an unconformity (Figs. 5A, B [arrows]). Above follows a ~200 cm thick succession of regularly plane-parallel bedded silts and fine- to medium-grained sands, in places with superimposed ripples. Sedimentary structures include internal cross- and planar bedding, bidirectional cross-bedding (herringbones) and small-scale grading. The top deposits in sections C and D consist of a thick, fine-grained sand body ( m) with internal cross- and planar bedding and a glauconitic, medium-grained sand layer ( m); the latter is traceable

8 292 Nicola Heckeberg et al.

9 The Upper Marine Molasse (Burdigalian, Ottnangian) in Southwest Germany 293 as a marker horizon in the Maria im Stein outcrop (upper boundary indicated by dotted line in figure 5A). Micropalaeontology Three samples (M-1 to M-3) were taken from the sands of each of the sections (see figure 5A for sample position). Sample M-1 contains fragments of balanids, bryozoans and a few fish teeth, while sample M-2 lacks microfossils. Sample M-3 produced a few, mainly well preserved benthic foraminifera; most abundant are Elphidium spp. and Ammonia beccarii s.l. (Appendix). 4.3 Burghöfe outcrop The Burghöfe outcrop (Fig. 6) exposes the uppermost part (~100 cm) of the OMM sediments as preserved in the Maria im Stein outcrop, and several metres of sands that probably represent a sandy facies of the Kirchberg Formation (Upper Brackish Molasse; see chapter 4.5). Lithofacies and sedimentological features The OMM sediments consist of medium- to coarse-grained cross- and planar bedded glauconitic sands with intercalated thin mud layers (Fig. 6A), which may represent the remnants of former megaripple bodies. The sediments are overlain by a coarse-grained layer with up to 14 cm thick pebbles composed of limestones, quartz, quartzites and granites (Fig. 6A C [arrows]). Above follow ~3 m of medium- to coarse grained sands showing cross-bedding, truncated cross-bedding, planar bedding and thin layers of marl intercalations (Fig. 6D). Micropalaeontology One sample (M-4) was taken from the marine OMM sediments. It yielded rare but relatively well preserved benthic foraminifera, including Elphidium subtypicum and Hanzawaia boueana. The overlying medium- to coarse grained sands were sampled in another outcrop located 1.5 km to the N (R H , 540 m NN). This sample (M-5) contained a single test of Lenticulina, several bryozoan and balanid fragments and a few echinoid spines. 4.4 Biostratigraphy Characteristic assemblages of benthic foraminifera indicate an early Miocene age for the OMM deposits in the North Alpine Foreland Basin (Burdigalian stage of the Mediterranean, Eggenburgian and Ottnangian stage of the Paratethys; Fig. 1A). The biostratigraphy of the OMM in the Lake Con- stance area is also based on assemblages of benthic foraminifera (Hagn 1961, Wenger 1987b). According to Wenger (1987b), the OMM in this area is Ottnangian in age. However, Hagn (1961) suggested that the base of the OMM, i.e. the Heidenlöcher Schichten (Fig. 1B), may be late Eggenburgian in age. For a further subdivision, Hagn (1961) used typical assemblages of benthic foraminifera, as well as the abundant occurrence of particular species. Based on these observations, Hagn (1961) correlated the Sandschiefer (Fig. 1B) with the early Ottnangian, and the Bodmansande and Deckschichten with the middle Ottnangian. Ottnangian sediments can be positively identified based on the presence of the index species Amphicoryna ottnangensis (e.g. Wenger 1987a, Cicha et al. 1998, Rupp & Haunold-Jenke 2003). As A. ottnangensis occurs in the Eichhalde sections, both in the marly (E-4) and the sandy segment (E-8; Appendix), the OMM exposed at the Eichhalde outcrop can be assigned to the Ottnangian with confidence. Four of the six samples studied from the marly segment (which corresponds to the Sandschiefer, see below) contain S. pectinata (Appendix), which is a typical species in lower Ottnangian benthic foraminiferal assemblages (Neuhofener Schichten; Wenger 1987a). As a result, the samples described in this study generally support the age proposed by Hagn (1961). 4.5 Lithostratigraphy In the Eichhalde outcrop (Fig. 4), lithofacies and microfossils permit the correlation of the marly segment with the informal unit Sandschiefer (Fig. 1B). The conglomeratic layer (at 6.9 m) corresponds to the Baltringer Horizont, and the fine to medium-grained unconsolidated sands ( m) represent the Baltringer Schichten (or Bodmansande). This parallelisation is consistent with previous studies at this outcrop (Haus 1951, Erb et al. 1961, Hagn 1961, Winder 1983). In the Maria im Stein outcrop (Fig. 5), lithofacies and microfossils indicate that the lower part of the succession, up to the unconformity at 8.8 m, can be parallelised with the Baltringer Schichten (or Bodmansande), while the finergrained segment above correlates with the Feinsandserie (or Deckschichten). The Maria im Stein outcrop is not mentioned in the literature, but the parallelisation proposed in this paper is consistent with the general stratigraphic succession in the area north of Lake Constance (Haus 1951, Erb et al. 1961). The Burghöfe outcrop displays the upper part of the Feinsandserie (or Deckschichten; Fig. 6A, lower part). The overlying coarse layer containing cm-size clasts (Figs. 6A C: arrows) most probably correlates with the conglomerate at Fig. 5: Maria im Stein outcrop. (A) Schematic logs of the Bodmansande (Baltringen Fm) and Deckschichten (Steinhöfe Fm). (B) Unconformity at the lower boundary of the Steinhöfe Fm (indicated with arrows), and megaripples in the uppermost beds of the Baltringen Fm (just below the unconformity). (C), (D) Close-up of section B with megaripples (C, indicated with arrow) and wave ripples, and with slumping structures (D). (E) Dune with fine-draped foresets (?tidal bundles) in the lowermost Baltringen Fm of section A. Position of (B) (D) and micropalaeontological samples is marked in (A).

10 294 Nicola Heckeberg et al. Fig. 6: Burghöfe outcrop. (A) (C): Uppermost part of the Deckschichten (Steinhöfe Fm) and overlying conglomerate (indicated with arrows). The conglomerate can probably be attributed to the basal conglomerate of the Kirchberg Formation (see text). (D) Facies of the sands (?Kirchberg Formation) above the conglomerate layer. base of the Kirchberg Formation in this area (Mischhorizont or Mischgeröllhorizont; Haus 1951, Erb et al. 1961, Schreiner 1992). If this correlation is accurate, then the overlying cross-bedded sands (Fig. 6D) represent a rather coarsegrained facies of the Kirchberg Fm. However, the Kirchberg Fm in the area north of Lake Constance usually consists of horizontally bedded, well sorted silts or fine sands (Erb et al. 1961, Schreiner 1992); coarse-grained sands and cross-bedding, as observed in the Burghöfe outcrop, have only rarely been reported (Haus 1951: 52, 56). 5. New lithostratigraphic terminology The Sandschiefer, Baltringer Schichten (or Bodmansande) and Feinsandserie (or Deckschichten) each are characterised by a distinct lithofacies that makes their identification easy and discriminate them from one another in outcrops and boreholes. Therefore, formal lithostratigraphic names according to the international stratigraphic guidelines (see Salvador 1994, Steininger & Piller 1999) can (and should) be introduced for these units (Fig. 7). 5.1 Kalkofen Formation Name and derivation of name The name refers to the village of Kalkofen, 8 km NE of Stockach. Kalkofen is the closest village in the vicinity of the Eichhalde outcrop, which is selected as the type section. Synonyms Sandschiefer (e.g. Erb et al. 1961, Schreiner 1966, 1989, 1992, 2008, Erb 1989, Werner 1994, Szenkler 1995, Szenkler & Ellwanger 2001); Sandmergelserie (Lemcke et al. 1953, Schwerd et al. 1996, Doppler et al. 2005).

11 The Upper Marine Molasse (Burdigalian, Ottnangian) in Southwest Germany 295 Fig. 7: Correlation of the new OMM formations with previous informal units in the Molasse Basin of Southern Germany. Type section and description The Eichhalde outcrop (Fig. 4) represents the type section; the thickness of the Kalkofen Fm is about 26 m (Erb et al. 1961). Interbedding of marls with glauconitic sands and silts appear in the lower part (Haus 1951, Winder 1983), while a mud-dominated alternation of blue-grey and brown marls with less sand content prevails in the upper part (this study). Bivalves and gastropods occur at places, but are generally rare. The microfauna is marine and dominated by relatively diverse assemblages of benthic foraminifera. The environment was a marine inner to middle neritic setting (see 6.1 for details). Geological age Early Ottnangian. Regional aspects The Kalkofen Fm is well known from geological mapping and drillings in the area NW of Lake Constance (Erb et al. 1961, Schreiner 1966, 1989, 1992, 2008, Erb 1989, Werner 1994). It grades into the Sandmergelserie in the central part of the South German Molasse Basin and represents an equivalent of the Neuhofener Schichten in the eastern part (Fig. 7). The Kalkofen Fm generally reaches a thickness of about 60 m, but may vary at places, i.e. from 20 to 70 m. It may overly the Lower Freshwater Molasse (type section), the Heidenlöcher Schichten or the OMM Basisschichten. Locally, a fossiliferous sandstone, up to 2 m thick, containing a rich fauna of bivalves, gastropods, bryozoans, shark teeth and marine mammals, occurs at base of the Kalkofen Fm (Erb et al. 1961, Erb 1989). To the Northwest of the Lake Constance area, the coarsegrained sandstones and sands of the Grobsandzug represent a time-equivalent of the Kalkofen Fm (Fig. 7). These m thick sediments were deposited in a shallow marine coastal setting with strong tidal currents (Erb et al. 1961, Schreiner 1966, 1989, Werner 1966, Bieg et al. 2007). Another coastal equivalent of the Kalkofen Fm is a fossiliferous limestone, i.e. the Randengrobkalk. Both Grobsandzug and Randengrobkalk strike SW NE, along the former coastline, and are characterised by distinctive tidal sedimentary structures (Werner 1966, Bieg et al. 2007). As they display a specific lithofacies, they may be defined as separate lithostratigraphic formations in a future study. 5.2 Baltringen Formation Name and derivation of name The name refers to the village of Baltringen (30 km S of Ulm) where the type section is located (Figs. 2, 3C). Synonyms Bodmansande (sensu Erb et al. 1961, Schreiner 1966, Erb 1989, Werner 1994); Baltringer Schichten (sensu Doppler 1989, Schreiner 1992, 2008, Doppler & Schwerd 1996, Schwerd et al. 1996, Doppler et al. 2000, Szenkler 1995, Szenkler & Ellwanger 2001, Heimann et al. 2009); Baltringer Horizont (sensu Lemcke et al. 1953, Werner 1994). Note that the Baltringer Schichten according to Erb et al. (1961), Schreiner (1966, 1989), Lemcke (1988) and Erb (1989) include the Baltringen Fm AND the Steinhöfe Fm. Type section and description The type section is described in detail in Heimann et al. (2009). It exposes an 8 m thick succession of the Baltringen Fm and also records the boundary to the overlying Steinhöfe Fm (see below). The lithofacies is characterised by an alternation of dm-thick, medium- to coarse-grained sands with cm-thick fine sand and silt layers. The deposits show crossbedding, planar bedding, sand waves, ripples and tidal structures such as herringbones and fine draped reactivation sur-

12 296 Nicola Heckeberg et al. faces (Heimann et al. 2009). Palaeocurrents are bidirectional and show a dominant SE-ward current. Macrofossils are generally rare in the type section, but are known from the pebbly basal layer (Baltringer Horizont) in former quarries in the vicinity of Baltringen (see Heimann et al for further details). A marine microfauna is present. The Baltringen Fm was deposited in a subtidal, high-energy environment (see chapter 6.2 for details). Geological age Middle Ottnangian. Regional aspects The Baltringen Fm is known from geological mapping and drillings in the Molasse Basin of Southwest and South Germany. In nearly all outcrops and drillings, the base of the Baltringen Fm is marked by a conglomeratic, pebbly layer (Baltringer Horizont), which may reach up to 2.5 m thickness and displays a high content of fossils and especially fossil debris (see Heimann et al for further references). The Baltringen Fm corresponds to the Achen Fm in southern Bavaria (Pippèrr et al. 2007) and represents an equivalent of the Glaukonitsande + Blättermergel in southeastern Bavaria (Fig. 7). In the Lake Constance area, the thickness of the Baltringen Fm generally ranges from a few metres to 10 m (Erb et al. 1961, Schreiner 1966, 1989, 1992, Erb 1989, Werner 1994, Szenkler 1995, Szenkler & Ellwanger 2001), while it measures up to 50 m in the central part of the South German Molasse Basin (Lemcke et al. 1953, Doppler 1989, Doppler & Schwerd 1996, Doppler et al. 2000). The Baltringen Fm is often absent in the area of the Graupensandrinne valley due to the erosive incision of the Graupensand river (Schreiner 1989, Werner 1994). However, in the area of the extended Graupensandrinne (Haus 1951), where the Eichhalde outcrop and probably also the Maria im Stein outcrop are located, the Baltringen Fm may be present, but then is most often reduced in thickness (e.g. Eichhalde). Reference sections The Eichhalde and Maria im Stein outcrops are selected as reference sections because they show the typical lithofacies as it can be seen in the type section Baltringen and additionally expose the lower boundary (Eichhalde) or the upper boundary (Maria im Stein) of the Baltringen Fm. In the Eichhalde outcrop, the lower boundary of the Baltringen Fm forms a basal erosion surface (Figs. 4A, D) and is marked by a conglomeratic layer, which corresponds to the Baltringer Horizont mentioned in previous studies (Lemcke et al. 1953, Werner 1994). In the Maria im Stein outcrop, sections A C display a 9 m thick segment of the Baltringen Fm that show the typical facies, i.e. cross-bedded, planar-bedded and structureless glauconitic sands; sand/silt/marl alternations occur as well (Fig. 5A). Sections C and D of the Maria im Stein outcrop show the upper boundary of the Baltringen Fm, which is marked by an unconformity (Figs. 5A, B). 5.3 Steinhöfe Formation Name and derivation of name The name refers to the farm Steinhöfe, which is located close to the Maria im Stein outcrop; sections C and D represent a composite type section (Fig. 5). Synonyms Deckschichten (sensu Erb et al. 1961, Schreiner 1966, 1989, 1992, 2008, Erb 1989, Werner 1994, Szenkler 1995, Szenkler & Ellwanger 2001); Feinsandserie (sensu Lemcke et al. 1953, Doppler 1989, Doppler & Schwerd 1996, Schwerd et al. 1996, Doppler et al. 2000, Heimann et al. 2009). Type section and description Sections C and D of the Maria im Stein outcrop represent the type section. They preserve an about 6 m thick succession of the Steinhöfe Fm and also record the boundary to the underlying Baltringen Fm, which is marked by an unconformity (Figs. 5A, B). The Steinhöfe Fm is dominated by fine-grained sediments and thin-bedded sand/silt alternations. A marine microfauna is present but scarce. The lithofacies, together with the benthic foraminifera, suggest a tidal influence in marginal marine settings with less high-energy influence (see chapter 6.3 for details). Geological age Middle Ottnangian. Regional aspects The Steinhöfe Fm is known from geological mapping and drillings in the Southwest and South German part of the Molasse Basin. In the Lake Constance area, it reaches a thickness of between a few to 15 m (Erb et al. 1961, Schreiner 1966, 1992, Szenkler 1995), while it is up to 50 m thick in the central part of the South German Molasse Basin (Doppler 1989, Doppler & Schwerd 1996). The Steinhöfe Fm can be correlated with the Glaukonitsande + Blättermergel in southeastern Bavaria (Schwerd et al. 1996) and probably corresponds to the Ulperting Fm in southern Bavaria (Pippèrr et al. 2007). The Steinhöfe Fm is often missing in the former area of the Graupensandrinne valley due to the erosive incision of the Graupensand river, but may be present in the area of the extended Graupensandrinne (Schreiner 1989, Werner 1994). Reference sections The Burghöfe outcrop (Fig. 6) is selected as a reference section because it shows the typical lithofacies as it can be seen in the type section and additionally exposes the upper boundary of the Steinhöfe Fm. This upper boundary is marked by a conglomeratic layer (Fig. 6A), which probably corresponds to the base of the Kirchberg Fm (see chapter 4.5).

13 The Upper Marine Molasse (Burdigalian, Ottnangian) in Southwest Germany Environmental setting 6.1 Palaeoenvironment of the Kalkofen Formation At the type section Eichhalde, the Kalkofen Fm is dominated by marls that may contain a high proportion of glauconitic sand lenses. Marls containing sand lenses are interpreted as reflecting periods with some wave (or current) energy in the basin, whereas marls devoid of sand lenses probably were deposited during periods with a very low wave (or current) energy. Benthic foraminifera are most diverse in the lowermost beds of the Kalkofen Fm (E-A; α = 8.4), but diversity decreases upward in the section (E-1 to E-5; α = 4 6.4; Appendix). According to the diversity of benthic foraminifera in comparable environments (see Murray 2006), α values of 4 are indicative of a marine-euhaline shelf habitat. Decreasing diversity and predominance of small-sized tests in the upper parts of the Kalkofen Fm (referred to as Sandschiefer) have also been reported from other exposures in the Lake Constance area (Hagn 1961). This suggests that the upper part of the Kalkofen Fm formed in a marine setting with environmental stress for benthic foraminifera, for instance salinity fluctuation or oxygen depletion of bottom water. Due to the fact that several benthic foraminiferal taxa display characteristic depth ranges (e.g. Murray 1991, 2006, Leckie & Olson 2003), the benthic foraminifera from the Kalkofen Fm can be used to estimate water depths. The abundance of shallow water foraminifera such as Elphidium, Ammonia beccarii s.l. and Hanzawaia boueana (see Tab. 1) point to an inner to middle neritic environment with a water depth of predominantly less than 50 m. The rarity of taxa that prefer an outer neritic to bathyal environment such as Pullenia, Melonis pompilioides and Uvigerina provide additional support for this assumption. In all, the bathymetric estimate is also supported by the proportion of planktonic foraminifera (P/B-ratio). In most samples, the P/B-ratio is <10 % (Appendix), which suggests an inner neritic environment (Murray 1991). However, the uppermost part of the Kalkofen Fm (sample E-5; Fig. 4A) displays a P/B ratio of 39 %, which may point to a middle neritic environment. It would be interesting to see whether foraminifera obtained through a highresolution sampling reflect the bathymetrical differences through time that could explain the changes of energy regimes indicated by varying contents of sand lenses in the marls. 6.2 Palaeoenvironment of the Baltringen Formation The Baltringen Fm at the type section near Baltringen (Heimann et al. 2009), as well as at the reference sections Eichhalde and Maria im Stein, preserve sedimentological structures that suggest a tide-influenced subtidal environment. These include (1) rhythmic bedding of cross-bedded sand couplets (tidal bundles; Fig. 4B; Reineck & Singh 1980, Reading 1996); (2) herringbones and bimodal patterns of palaeocurrents (NW + SE); (3) sand/silt alternations with superimposed ripples (Fig. 4C); (4) fine-grained drapes on individual cross-bedded sand layers (fine-grained drapes or layers indicate slack water periods, while the cross-bedded sands represent the ebb and the flood currents; Visser 1980); (5) mud lenses cutting cross-bedded sand bodies (Fig. 5E) (mud lenses presumably represent deposits of slack water periods, while the cross-bedded sands may be exceptionally thick tidal channel foresets; Heimann 2009). In addition, a high-energy subtidal environment can be assumed based on the occurrence of medium- to large-scale cross-bedding with directionally uniform main cross-bedding directions, medium- to large-scale channels, lentiform sand-bodies and megaripples (Figs. 4 5B; see also Clifton 1983). The benthic foraminifera recovered from these deposits provide additional information about the palaeoenvironment (Heimann et al. 2009; Tab. 1, Appendix 1). The here studied specimens from the Eichhalde sections are relatively large, and the Fisher s α diversity ranges between 3.2 and 5.1. In modern marginal marine environments and brackish shelf seas, the majority of assemblages of benthic foraminifera have values of Fisher s α diversity <4 (Murray 2006). Accordingly, the here observed diversities indicate a marginal marine to inner neritic environment. This is also supported by the relative abundances of Elphidium species and Ammonia beccarii s.l. (Tab. 1). Several Elphidium species (nonkeeled species) are well known for their tolerance to salinity fluctuation, and Ammonia is a widespread euryhaline taxon that occurs in almost all marginal marine assemblages (Sen Gupta 1999, Murray 2006). However, a more or less normal marine salinity is indicated by the relatively high proportion of keeled Elphidium specimens (Tab. 1), together with the occurrence of other marine taxa (e.g. Cibicidoides, Nonion). In conclusion, sedimentological features and benthic foraminifera suggest an inner neritic subtidal environment for the Baltringen Fm that occasionally may have been affected by salinity fluctuations. 6.3 Palaeoenvironment of the Steinhöfe Formation The finer-grained sediments of the Steinhöfe Fm and their thinner bedding suggest a less high-energy environment than that of the Baltringen Fm. However, rhythmic pattern of sand/silt alternations suggest that some tidal influences still existed. Foraminifera are generally scarce in the Steinhöfe Fm (Hagn 1961, referred to as Deckschichten). Samples M-3 and M-4 contained a few specimens, predominantly of euryhaline taxa (i.e. Elphidium, Ammonia). The sedimentological features and benthic foraminifera generally are suggestive of a shallow, marginal marine, tidally influenced environment that may have been affected by intermittent fluctuations in salinity.

14 298 Nicola Heckeberg et al. 7. Regional geology The Burghöfe outcrop (Figs. 6A C) displays a conglomerate layer and series of cross-bedded medium- to coarse-grained sands above the Steinhöfe Fm (Fig. 6D). If the correlation of these sediments with the Kirchberg Fm is correct (see chapter 4.5), the region of the Aach River valley, where the Burghöfe outcrop is located, would belong to the extended Graupensandrinne. However, this contrasts the geological situation recorded on the Geological Map 8121 Heiligenberg (2001) and also described by Haus (1951), Erb et al. (1961) and Schreiner (1989). According to these authors, the southern margin of the Graupensandrinne occurs some 2.5 km W and NW of the Aach River valley, and the OMM succession in this area, N and NW of Owingen, is overlain by the Albstein (see also Erb et al. 1961: Fig. 2). As the Albstein records on the Geological Map 8121 Heiligenberg are apparently based on four drillings (no. 50, 95, 119, 347; Szenkler 1995), it may be possible that the so-called Albstein in these drillings represents samples of the basal limestones that are known from the base of the Upper Freshwater Molasse in this area (Schreiner 1976, Reichenbacher 1993). However, further studies are needed to determine whether the southern margin of the Graupensandrinne in the Lake Constance area is located approximately 3 km further to the South than suggested previously. 8. Summary and conclusion The term Kalkofen Formation is introduced for the informal OMM units Sandschiefer and Sandmergelserie (Fig. 7). The Kalkofen Fm represents a mud-dominated system; benthic foraminiferal assemblages suggest an inner to middle neritic environment. Varying contents of glauconitic sand layers and lenses point to changes in energy regime, which suggest bathymetrical differences through time and changes in the position of the shoreline. Possible reasons for this may include differences in basin subsidence, eustatic sea level rise and fall and climatic cycles. The term Baltringen Formation is presented as a formal name for the informal OMM unit Baltringer Schichten or Bodmansande (Fig. 7). The lower boundary of this Fm is marked by a basal erosion surface, which may be accompanied by a pebbly layer of high fossil content, i.e. Baltringer Horizont of previous studies (e.g. Heimann et al. 2009). The sediments of the Baltringen Fm demonstrate a sand-dominated system that was deposited in a shallow inner neritic basin. Sedimentary features indicate a subtidal environment with strong tidal currents and fairly long-period surface waves. The abrupt shift between the mud-dominated Kalkofen Fm and the sand-dominated Baltringen Fm represents the well known shallowing-upward event between the first and second sedimentation cycles of the Ottnangian part of the OMM (Lemcke et al. 1953, Frieling et al. 2009c; Fig. 1B). The Steinhöfe Formation corresponds to the informal OMM unit Feinsandserie or Deckschichten (Fig. 7). The lower boundary is characterised by an unconformity. The generally fine-grained sediments of the Steinhöfe Fm include sand/silt alternations, which are suggestive of less high-energetic conditions in a tidally influenced marginal marine setting. The unconformity that occurs at the base of the Steinhöfe Fm supports the hypothesis of a third OMM sedimentation cycle as suggested by Gall (1975) and Pippèrr et al. (2007). 9. Acknowledgements We are grateful to the LMU Master students Sabine Beierl, Marlies Schnell, Kristian Karl, Eckhard Schmidt and Kathrin Glas for their contributions to the lithological logs at the Eichhalde and Maria im Stein outcrops during our field trip in April Special thanks to M. Schnell for providing most of the photos of the figures 4 and 5. Many thanks go to Andreas Schäfer (Bonn) for helping with the interpretation of the sedimentary structures, to Gerhard Doppler (München) for critical discussion of the geological context, to James Nebelsick (Tübingen) and an anonymous reviewer for their constructive comments, and to Michael Krings (München) for improving our English style. 10. References Abdul Aziz, H., Böhme, M., Rocholl, A., Prieto, J., Wijbrans, J.R., Bachtadse, V. & Ulbig, A. (in press): Integrated stratigraphy and 40Ar/39Ar chronology of the early to middle Miocene Upper Freshwater Molasse in western Bavaria (Germany). Int. J. Earth Sci., DOI /s , Berlin. 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