Sedimentology of the Jura Molasse: Miocene tidal clastics and freshwater carbonates from the Tramelan-2 Borehole, NW Switzerland

Abstract

The Tramelan-2 borehole (Canton Bern, Switzerland) continuously cored a 275.60 m sequence of Palaeogene to Neogene sediments, providing a rare opportunity for sedimentological analysis of the Jura Molasse. Lithostratigraphy, sedimentary facies and heavy minerals allow correlation with the classical Swiss Molasse. Evidence for clastic input from Alpine and non-Alpine sources is consistent with deposition at the northern feather edge of the Molasse Basin. Grey sandstones at the base of the succession are tentatively interpreted as fluvial facies of the Lower Freshwater Molasse (USM). These are overlain by erosively-based conglomerates, interpreted as winnowed storm lags of locally-derived clasts and distantly-sourced Alpine material at the transgressive base of the Upper Marine Molasse (OMM). Above this, a range of plane-laminated and cross-bedded bioclastic sandstones are interpreted as the deposits of a meso- or macrotidal flat or estuarine complex. Facies evolution in the upper OMM records a series of stacked regressive cycles within an overall upward trend of reducing current energy, reflecting a transition from offshore to nearshore shallow marine environments. The unconformably overlying Upper Freshwater Molasse (OSM) passes upwards from micaceous siltstones and marls into a series of freshwater carbonates, dominated by lacustrine limestones which are arranged in repeated regressive cycles, each capped by organic-rich horizons. The middle part of the carbonate interval comprises palustrine limestones with pedogenetic fabrics recording periodic subaerial exposure. Stable isotope signatures from the palustrine facies reflect subaerial pedogenetic overprint, while the lacustrine carbonates record cyclical variations in δ¹³C which may reflect increases in organic material during progressive lake fill and/or an increased influence of aquifer waters during flooding events. The Jura Molasse onlapped a Mesozoic carbonate pediment hosting a regional karst system which remained active at least until the Burdigalian transgression on footwall highs bounding the Delémont Basin, and locally into the Langhian near Tramelan. This configuration controlled subsequent deposition of OSM carbonates which onlapped erosional and potentially early tectonic relief, and were deposited in groundwater-fed seasonal lacustrine/palustrine environments, where the karstic aquifer controls on seasonal hydrology appear to have been comparable to Caribbean freshwater carbonate wetland systems today.

1 Introduction

The Jura (Figs. 1 and 2) forms an arcuate range of folded and thrusted Mesozoic carbonates lying to the northwest of the Molasse Basin in Switzerland (Trümpy, 1980; Burkhard & Sommaruga, 1980), France and Germany. While the external sector of the Tabular Jura to the northwest is largely undeformed, the more internal Plateau Jura to the southeast is characterised by localised areas of intense deformation in the Faisceau Jura (Chauve et al., 1980; Madritsch et al., 2008; Ziegler & Fraefel, 2009). By contrast, the structure of the Folded Jura of NW Switzerland (Laubscher, 1965, 1977, 1981, 1987, 1992; Schori et al., 2021) is defined by NE-SW trending anticlinal limestone ridges and intervening synclinal valleys where Cenozoic continental and marine sediments rest directly on the karstified Mesozoic surface. This region lies at the northern, former feather edge of the Molasse Basin (Heim, 1919; Homewood et al., 1986; Matter & Weidmann, 1992).

Fig. 1

a Map showing location of Switzerland in central Europe. Yellow box shows area in b. b Geological map of the Swiss Alps and Jura, modified after Rime et al. (2019). MB: Molasse Basin. URG: Upper Rhine Graben; BG: Bresse Graben. Yellow box shows area in Fig. 2

Fig. 2

a Simplified geological map of NW Switzerland, simplified after Spicher (1980): Tektonische Karte der Schweiz (1: 500,000). Schweizerische Geologische Kommission. Yellow box shows area in b. Thick dashed grey line: national frontier. Thick dashed purple line: course of the A16 Trans-Jura motorway. b Geological map of the Tramelan syncline showing location of the Tramelan-1 and Tramelan-2 boreholes

Evidence of clastic input from non-Alpine sources to the north led several authors to discuss the possibility that the Molasse Basin was at least intermittently connected with the Upper Rhine Graben (URG) via a NNE-SSW sedimentary conduit through the Jura area termed the “Rauracian Depression” (Baumberger, 1927; Reichenbacher et al., 1996; Berger et al., 2005b; Mojon et al., 2018). A degree of structural linkage between the two areas is implied by the continuation of N-S to NNE-SSW trending regional faults bordering the URG into the Delémont area of the Jura as shown in Fig. 2 (see also Roussé, 2006; Pirkenseer et al., 2018).

The Lower Freshwater Molasse (USM), Upper Marine Molasse (OMM) and Upper Freshwater Molasse (OMM) of the Jura provide a record of Palaeogene to Neogene deposition in the area between these important depocentres. The Oligocene and Miocene strata preserved in Jura synclines today appear to record complex and potentially distinct sedimentary histories with laterally variable facies which are challenging to correlate (Berger et al., 2005a, 2005b, 2011).

The Palaeogene to Neogene succession of the Molasse Basin is dominated by clastic rocks (Matter et al., 1980; Homewood et al., 1986; Keller, 1989; Kempf et al., 1999; Platt & Keller, 1992; Schlunegger et al., 1993, 1997a, 1997b) and thins northwards as it progressively onlaps a karstified pediment of Mesozoic carbonates in the Jura (Keller, 1992). There are significant stratigraphic gaps, for example in the Aquitanian which is absent in many areas of the Jura.

Carbonate intervals also occur in the Molasse at the northern margin of the Molasse Basin and in the Jura. The basal USM Lower Limestones are developed between Yverdon and Neuchâtel (Platt, 1992; Becker et al., 2001a, 2001b) while the USM Delémont Limestones occur in the central Jura (Picot et al., 1999). A younger series of freshwater carbonates referred to as the ‘Oehningian’ also occurs in the OSM (Kälin, 1993; Clement & Berger, 1999; Becker, 2003).

The areal distribution of the key lithostratigraphic units is shown in the excellent regional correlations and palaeogeographic maps presented by Berger et al., (2005a, 2005b) as also updated recently by Mojon et al. (2018) and Pirkenseer et al. (2018).

The drilling of the Tramelan-2 borehole in one of the synclines in the central Jura Mountains to a Total Depth of 275.60 m (Bertrand, 1990) permitted detailed study of the Molasse succession at this location. This paper presents sedimentological descriptions and interpretations of the clastic and carbonate units, with additional heavy mineral data and stable isotope results also collated from the clastics and carbonate rocks respectively. Together, these allow an assessment of the likely sediment sources active during deposition and provide important insights into basin evolution and palaeohydrology during Palaeogene to Neogene deposition in this area.

2 Material and methods

Drilling of the two Tramelan boreholes in the late summer and autumn of 1988 formed part of a hydrological study of groundwaters within an actively circulating karst system developed at the top of the Jurassic succession (Bertrand, 1990). A first borehole, located at the eastern edge of Tramelan village (co-ordinates 2575.980/1230.395) reached Mesozoic strata at a depth of only 5.75 m, subsequently penetrating a Marine Jurassic succession of Tithonian to Kimmeridgian age and reaching a Total Depth of 164.50 m. A second borehole was located 2 km to the east of Tramelan and south of the River Trame near La Tuilerie, at co-ordinates 2577.410/1230.935. Drilling penetrated a thick sequence of Palaeogene-Neogene Molasse sediments before reaching Total Depth in sandstones of the Lower Freshwater Molasse at 275.60 m.

Both boreholes acquired continuous 10 cm diameter cores. The core from the Tramelan-2 borehole was transferred to the Institut für Geologie at the Universität Bern for sedimentological logging, sampling for heavy minerals, micropalaeontological determinations and stable isotope analysis. Facies descriptions and stratigraphic analysis were carried out on 1 m core sections. Core photographs are included as supporting information (Additional File 1). 50 thin sections were made for microfacies analysis. Heavy mineral samples were taken every 5–10 m; the depths given refer to the first and last sample in each case.

160 samples of representative carbonate facies were selected for isotope analyses and taken from split core sections before being reacted in an on-line automated carbonate preparation system linked to a VG Prism II ratio mass spectrometer. Approximately 10 mg of powdered sample was reacted in “100%” H₃PO₄ for 10 min at 90 °C. Reproducibility of istotopic ratios of standard materials in the system was better than 0.1 ‰ for δ¹⁸O and 0.05 ‰ for δ¹³C.

3 Geological setting and lithostratigraphy

The Tramelan syncline (Fig. 2) trends ENE-WSW, with northern and southern limbs exposing Upper Jurassic Kimmeridgian and Tithonian marine carbonates. Molasse sediments of Oligocene and Miocene age are present in the axis of the syncline (Aufranc et al., 2016a).

The established lithostratigraphy of the Jura was based on the work of Rollier (1892, 1893), who also carried out the original mapping (Rollier, 1898, 1900). Later studies permitting refinements and modifications of the lithostratigraphy were carried out by Frei (1925), Liniger (1925), Baumberger (1927), Rothpletz (1933), Forkert (1933), Schlaich (1934), Ziegler (1956), Antenen (1973) and Becker (2003) as summarised by Aufranc et al., (2016a, 2016b; 2017). The stratigraphy of the Upper Rhine Graben to the north was reviewed by Sissingh (1998).

The chronostratigraphy of the Molasse Basin to the south of the study area was established by Schlunegger (1997a), while the sedimentary evolution of the Molasse Basin was evaluated in detail by Keller (1992) and Strunck and Matter (2002). Important regional correlation studies were presented by Berger et al., (2005a, 2005b) and Berger (2011), drawing on the work of Picot (2002) and Becker (2003). Further syntheses were presented in relation to the Palaeogene by Mojon et al. (2018), with significant modifications and correlations with the URG also proposed by Pirkenseer et al. (2018). New data referring to the Neogene (Burdigalian) section in the Jura were provided by Prieto et al. (2018).

Lithostratigraphic schemes in the area are complex and feature nomenclature in two languages. Table 1 therefore provides a glossary of English and equivalent German and French stratigraphic units in common use. Figure 3 presents a simplified lithostratigraphy for the area in English, while Fig. 4 places the local succession in the context of an updated and more detailed summary of Palaeogene-Neogene stratigraphy in the Jura compiled by Becker (2022 unpublished data).

Table 1 Summary chart of key stratigraphic units referred to in the text, together with their German and French language equivalents

Fig. 3

Generalised stratigraphy and facies of the Jura Molasse. Chronostratigraphic ages are from Cohen et al. (2022)

Fig. 4

Generalised stratigraphy and facies of the Jura Molasse. The blue dashed rectangle illustrates the succession penetrated in the Tramelan-2 borehole. Adapted from unpublished figure courtesy of and with kind acknowledgment to Dr. Damien Becker (2022 unpublished data). Molasse Basin lithostratigraphy adapted after Keller (1992) and Sommaruga (1996). See Pirkenseer et al. (2018) for alternative stratigraphic synthesis and correlations with the URG. Chronostratigraphic ages are from Cohen et al. (2022)

Throughout the Jura, a mineralised deposit known as the Siderolithic, including thin sandstones and gastropod limestones, rests on and is preserved in pockets within Mesozoic strata which form a regional karstified pediment (Baumberger, 1923; Rollier, 1910; Weidmann, 1984). The Siderolithic is generally assigned to the Eocene on the basis of mammal teeth finds and the Oligocene age of overlying deposits. However, Hofmann et al. (2017) postulated that the Siderolithic was a terra rossa deposit which continued to be formed until significantly later, with new dating from the northern and eastern Jura indicating that karstification and/or active karstic circulation continued or resumed during the Miocene up to c. 15 M yr BP.

The Siderolithic is unconformably overlain by the Palaeogene-Neogene succession, and appears to record a stratigraphic gap when Jurassic and Lower Cretaceous marine carbonates were subaerially exposed. The length of this stratigraphic gap is interpreted to increase northwards.

Thus on the northern margin of the Molasse Basin at the southern edge of the Jura, the Siderolithic is overlain by Oligocene fresh-water carbonates of the Lower Limestones. Further north (as in Tramelan-2), these strata may be missing and the karst surface appears to be overlain by sandstones of the Alsace Molasse, thinning to the northwest (Aufranc et al., 2016a) and marls and fresh-water carbonates of the Delémont Limestones. These units form the Lower Freshwater Molasse (USM), which is present in the Tramelan-2 borehole and at La Tuilerie (Fig. 2) in the SE of the Tramelan syncline, but absent in Tramelan-1 (Bertrand, 1990) and in western areas of Tramelan village 1.5 km and 3.5 km to the west respectively (Aufranc et al., 2016a).

The unconformably overlying Upper Marine Molasse (OMM) of the Jura has four units (Becker, 2003; Zulliger, 2008; Aufranc et al., 2016b; Garefalakis & Schlunegger, 2019). The basal Burdigalian comprises grey sandstones of the Grey Molasse and the calcareous Shelly Sandstones (together the Lucerne Formation). The upper Burdigalian comprises coarse clastics of the Polygenetic Conglomerates and Pebbly Sandstones (making up the St-Gallen Formation).

In the southern Jura, these OMM deposits rest on variably truncated sections of the USM. However, at Glovelier, only 10 km NE of Tramelan-2, the USM is missing and the OMM succession rests directly on the karstified top Mesozoic surface, which includes karstic pockets with infill of Burdigalian age (Hug et al., 1997; Becker, 2003; Prieto et al., 2018).

The OMM is overlain by brackish to freshwater strata of the Langhian-Serravallian Upper Freshwater Molasse (OSM), comprising grey and black carbonates of the Freshwater Limestones and Marls, and green or grey marls and sandstones of the Micaceous Sandstones.

4 Borehole sequence and lithostratigraphic correlation

A simplified borehole log is shown in Fig. 5, with the main units described in the text below.

Fig. 5

Borehole log showing lithostratigraphy and heavy mineral zones (see text discussion below)

4.1 Grey sandstones (275–252 m)

The basal part of the succession comprises a series of 1 m bedded, dull grey-greenish medium-fine grained micaceous sandstones, locally cross-bedded with rare 30 cm thick coarse- to medium-grained graded grey sandstone units and intercalated 20 cm conglomerate horizons.

4.1.1 Interpretation

Sedimentological analysis of this interval is challenging. Neither marine indicators nor shale interbeds are present and these strata are interpreted as high energy braided stream deposits.

4.1.2 Stratigraphic assignment

The colours, lithology and abundance of muscovite are similar to those of the Alsace Molasse (Rollier, 1893), suggesting attribution to the USM. Further north in the URG, Roussé (2006) describes a sandy Lower Alsace Molasse containing shallow marine sedimentary structures, overlain by a continental fluviatile Upper Alsace Molasse which is potentially similar to the section in Tramelan-2. However, Antenen (1973) described the Alsace Molasse of the Jura as a series of sandstones interbedded with mottled overbank marls and paleosols similar to those developed in the Molasse Basin USM (c.f. Platt & Keller, 1992) but which are absent here.

The Alsace Molasse is overlain by marls and freshwater carbonates of the Delémont Limestones across much of the central and northern Jura, which were described by Kälin (1993, 2001) and Becker (2003) and dated as MP29-30 (Upper Chattian to basal Aquitanian). Thicknesses of the Délemont Limestones are variable, with construction of the A16 Transjura motorway near Tavannes 10 km to the east (Zulliger, 2008) revealing a section c. 5 m thick, overlain by a red sandy interval dated as MN2 (Aquitanian). The absence in Tramelan-2 of in situ Délemont Limestones and of this overlying Aquitanian section is consistent with localised truncation of the USM succession at an erosional unconformity below the base OMM.

An alternative interpretation could perhaps see the lowermost interval in Tramelan-2 as laterally equivalent to a significantly younger sandy Upper Marine Molasse sequence which overlies the Delémont Limestones in the adjacent Tavannes syncline and is assigned to mammal zone MN 2 (Late Aquitanian) by Zulliger (2008). In their Figure 3–13, Becker (2003) placed this series in Early Burdigalian zone MN3a, a unit of the Grey Molasse which forms part of the OMM and more typically displays marine sedimentary structures.

4.2 Green and grey sandstones and conglomerates (252–190 m)

A 35 cm thick conglomerate horizon at 249.25 m contains polymict clasts 2–5 cm across. This conglomerate is overlain by a thick alternation of polymict conglomerates and rippled or cross-bedded pale green or pale grey coarse sandstones. The sandstones have erosive bases, rare rippled layers and intercalated mudstone horizons. Interbedded coarser-grained layers up to 10 cm in thickness contain abundant 1–5 mm white bivalve fragments. Towards the top of the unit, the sandstones include reworked cm-scale sandstone chips. The presence of rip-up clasts, cross-bedding and ripple structures indicates a relatively high energy and shallow water environment.

The conglomerates contain clasts of quartzite, gneiss and limestone up to 10 cm in size. The conglomerate horizon at 235 m is underlain by a 1 m interval of white ostracodal limestone, which in turn rests on a 2 m thick grey-green mottled marl.

4.2.1 Interpretation

The varied clast sizes of the conglomerates and their sharply erosive bases are consistent with deposition in high energy events which reworked the underlying substrate while supplying pebbles from erosion in the catchment area.

Many of the pebbles are of Mesozoic carbonate and are probably locally derived, while the gneiss and quartzite pebbles were likely transported from more distant sources prior to deposition as winnowed storm lags. As described above, the context of the 1 m limestone interval at 236 m potentially suggests deposition as a large clast of reworked Delémont Limestone.

The poor sorting of the coarser sandstones and their sedimentary structures indicate deposition by high energy currents as may occur in tidal channels and bars (de Raaf & Boersma, 2007). Rapid migration of tidal channels is likely to have favoured preservation of the channel facies while the sediments of topographically higher bars and sand flats were preferentially eroded.

The medium- to coarser grained units show low angle cross-bedding and numerous reworked sandstone and/or mudstone clasts. Their poor sorting suggests variable current energy, and low angle cross-bedding is consistent with lateral migration of large scale sedimentary structures; similar deposits were interpreted by Keller (1990) as point bar deposits of large tidal channels.

4.2.2 Stratigraphic assignment

Garefalakis and Schlunegger (2019) inferred northward onlap of a basal OMM1 section of lower Burdigalian MN3a age in northern areas of the Molasse Basin bordering the Jura to the SW of Tramelan near Lake Neuchâtel (see Fig. 2). This interpretation is consistent with an MN3a age determination for the basal OMM in the Tavannes syncline obtained by Beaumont et al. (1984).

In the northern axis of the Molasse Basin, Garefalakis and Schlunegger (2019) assigned an MN3b age to the Shelly Sandstones. This unit contains abundant molluscan shell fragments (Allen et al., 1985) and in many areas east of the Napf Fan also contains debris of the echinid Scutella sp., while further west comprising sandy pelecypod-bearing packstones with barnacle fragments.

The interbedded and overlying polymict conglomerates are typically assigned to the Polygenetic Conglomerates. Elsewhere in the Jura, as here, this unit commonly includes reworked clasts of Delémont Limestone as well as of Shelly Sandstone (Antenen, 1973).

According to Antenen (1973), the Shelly Sandstones and Polygenetic Conglomerates are rarely found together. Conglomeratic intervals have historically been attributed to the “Helvetian”, an informal local stratigraphic name used for conglomerate-bearing facies (see discussion in Becker, 2003), with a later Burdigalian age inferred on the basis that the conglomerates must be somewhat younger than the sandstone units from which some of their clasts were derived.

In the Tramelan-2 borehole, conglomerate and shelly sandstone facies appear interbedded. In the absence of biostratigraphic control, an alternative interpretation could thus be considered that sedimentation reflected successive erosion and reworking during the Burdigalian transgression. In this interpretation, the Shelly Sandstones and Polygenetic Conglomerates would represent regionally persistent and diachronous sedimentary facies laid down during episodic sea level rise (Figs. 2 and 3; discussion in Naef et al., 1985). Conglomerates would then represent the initial transgressive deposits in each case, overlain by Shelly Sandstones, before the deposition of further conglomerate horizons containing reworked Shelly Sandstone clasts.

A similar contact between the USM and the overlying OMM Lucerne Formation was exposed by the A16 motorway in the Tavannes syncline 5 km to the east (Mojon et al., 2018). A 2 m thick basal lag of sandy conglomerate at that locality featured pebbles of probable Alpine origin, as well as shark teeth, bivalves, bryozoans and well-rounded mammalian bones and reworked caliche nodules. This initial transgressive lag was then overlain by bioclastic sandstones assigned to the Shelly Sandstones by Zulliger (2008).

Similarly, Aufranc et al. (2016b) describe a polymict conglomerate of quartzite, granitic and limestone clasts resting on the Delémont Limestone at La Tuilerie near Tramelan. Although no detailed analysis was reported, this assemblage may reflect mixed source areas, with quartzite likely to have been derived from the Napf Fan to the south while porphyrite and Triassic Buntsandstein clasts seen further north in the Delémont Basin point to source from the Vosges.

4.3 Pale beige and grey sandstones (190–95 m)

The grey and green Shelly Sandstones and Polygenetic Conglomerates are overlain by a thick series of sandstones containing rare carbonate pebbles. Pale beige deposits dominate from 190 to 169 m and from 142 to 127 m with mid-grey rocks between and above.

The lower part of this interval comprises fine-laminated muddy sandstone–siltstone–mudstone alternations with a wide variety of sedimentary structures including abundant current ripples, small-scale load structures and grading as well as lenticular, flaser and convolute bedding. Molluscan shell fragments are common in the coarse sandstone units of flaser sets.

These facies are overlain by a series of more massive sandstones above. Two thin limestone conglomerate horizons at 154.8 m and at 142 m contain sub-rounded beige, grey and brown carbonate clasts from 2 to 10 cm in size. The lower of the two horizons is 80 cm thick and rests on a heavily mottled grey marl horizon, while the upper one is 20 cm thick.

4.3.1 Interpretation

The range of sedimentary structures in coarser units indicates deposition by unidirectional currents of moderate velocity (> 0.45 ms⁻¹), while sandstone–siltstone beds are interpreted as suspension- and bed-load deposits (Reineck & Singh, 1980). The preservation of intercalated mudstone layers records deposition at intervening slack water phases. Variable current régimes are characteristic of tidal settings (Klein, 1975a; 1975b; Longhitano et al., 2012) with relatively low current velocities being typical of the upper zones of muddy tidal flats and salt marshes.

Rapid sedimentation leads to instability of saturated sediments and the formation of convolute and deformed bedding via dewatering. A rotated block of laminated fine sandstone at 105 m is interpreted as tidal flat material reworked via the erosional collapse of a tidal channel margin.

The green-grey fine muddy sandstones and marls which follow contain burrows and cm-scale brown or grey-green mottling but display few primary sedimentary structures. Their massive and homogeneous character is thought to reflect destratification through extensive bioturbation and root action from a well-developed terrestrial vegetation cover during emergence. These deposits are interpreted as being deposited in the marshy, supralittoral zones of a coastal tidal flat.

4.3.2 Stratigraphic assignment

The series of beige and grey sandstones in Tramelan-2 is assigned to the Pebbly Sandstones, part of the OMM. Comparison with Antenen’s (1973) lithostratigraphy suggests that intercalated thin conglomerate horizons at 154.8 m and 142 m correspond to the Vêlé Limestone Conglomerate.

In the Molasse Basin, Garefalakis and Schlunegger (2019) described offshore deposits of the Shelly Sandstones as overlain by an upper OMM2 (St.-Gallen Formation) unit of likely MN4 age (Jost et al., 2016) comprising nearshore deposits of the Coarse Sandstone (Grobsandstein).

4.4 Siltstones, limestones and Marls (95–5 m)

A thin coal at 95 m is overlain by a series of green and grey-brown marls, locally containing reworked 1–10 mm angular clasts of limestone and organic-rich mudstone, with abundant gastropods and intercalated 1–2 m pale beige limestone horizons. Marls dominate up to 65 m, with limestones increasing above this and forming the entire sequence from 55 to 34 m.

The overlying interval from 34 to 5 m comprises grey-brown marls with abundant gastropods and interbedded 1–2 cm pale beige limestone horizons. Palaeontological determinations on samples from 32.4 to 10 m are presented in Table 2.

Table 2 Palaeontological determinations from the Upper Freshwater Molasse (kind courtesy of the late Prof. J.–P. Berger)

The intervals from 65 to 55 m and 45–34 m are arranged in decimetre- to metre-scale asymmetric cycles, commencing with sharp-based pale beige, laminated ostracod packstones and passing gradationally up into mid- and darker grey intraclastic or charophyte wackestone capped by cm-scale dark grey to black organic-rich or coaly horizons. This succession is interrupted from 55 to 45 m by a series of white brecciated limestones with pedogenetic fabrics.

Similar cycles are found at outcrop nearby (Fig. 6; see also Aufranc et al., 2016b). The cycles begin with pale grey carbonate, darkening upward towards dark grey, organic-rich limestones. Rarer cm-scale sapropelic coal horizons are preserved beneath sharp and well-defined cycle tops.

Fig. 6

Outcrops near the Tramelan-2 borehole site, showing sedimentary cycles from light to dark lacustrine carbonates. a Vertical section, younging to top. b Detail showing weakly laminated limestone units capped by cm-scale sapropelic horizons (stratigraphic younging to the left)

Thin sections from the Tramelan-2 borehole reveal a variety of microfacies (Fig. 7):

• 1) Pale to mid-grey micritic limestones, with abundant gastropod and ostracod fragments (Fig. 7A), charophyte stems and gyrogonites and micritic limestone intraclasts (Fig. 7B) These limestones locally contain abundant spar-filled tubes 0.1 mm in diameter and up to 0.5 mm long (Fig. 7B and C).

  Interpretation

  The open, clotted fabric of this facies (Fig. 7B and C) is consistent with algal carbonate production while the sparry tubes appear similar to green algae described in Miocene lacustrine bioherms from the margins of the Ries Crater in Germany by Riding (1979). The absence of lamination suggests deposition in shallow, unstratified carbonate lakes.

• 2) White to pale grey strongly brecciated limestones, with angular 2–10 cm intraclasts of green marl and grey limestone, glaebules and circumgranular cracks (Fig. 7D).

  Interpretation

  Fabrics typical of pedogenetic brecciation and desiccation (Alonso Zarza, 2003) favour an interpretation as palustrine carbonates modified during seasonal subaerial exposure.

• 3) Darker grey carbonates with abundant gastropods and a higher content of organic matter (Fig. 7E). Detrital quartz grains are locally abundant. Some limestones (e.g. at 45.4 m) show abundant angular cm-scale intraclasts.

  Interpretation

  The presence of intraclasts and detrital quartz indicates clastic influx and reworking. These carbonates are interpreted as organic-rich limestones (“gyttjas”; Stankevica et al., 2016) deposited in shallower water at a vegetated lake margin.

• 4) Homogeneous green marls with gastropod shells show only local weak lamination.

  Interpretation

  Lower carbonate contents suggests that these rocks were laid down during the early stages of freshwater deposition when clastic influx initially diluted carbonate production.

• 5) Light brown to beige laminated gastropod biomicrites. This rare facies shows weak mm-scale lamination and abundant gastropod shell fragments (Fig. 7F).

  Interpretation

  The preservation of weak lamination suggests limited bioturbation, consistent with deposition below a dysoxic water column, possibly in slightly deeper water.

Fig. 7

Thin section micrographs from carbonates in the Upper Freshwater Molasse. a Ostracod wackestone, b Clotted algal micrite with intraclast (i; yellow dashed outline), charophyte stem (arrowed) and area of detail shown in c. c Detail of image b, showing green algal filaments in longitudinal and cross sections. d Peloidal wackestone with circumgranular cracking, e weakly laminated organic rich limestone with shell fragments, f micritic mudstone overlain sharply by darker shelly wackestone with abundant gastropod shell fragments, grading up into organic rich mudstone

4.4.1 Discussion

The varied facies of this limestone succession are interpreted as recording deposition in freshwater carbonate wetland environments characterised by limited fine clastic input, the accumulation of organic matter and colonisation by terrestrial snails.

A sharp-based basal limestone conglomerate horizon at 87 m may record initial flooding. The overlying strata from 86 to 65 m comprising green and brown marls, laminated siltstones and intercalated 50 cm thick limestone beds are interpreted as early phase ephemeral pond deposits.

These marls pass up into a series of stacked regressive cycles, each commencing with green lacustrine marls, overlain by open lacustrine ostracod packstones and algal wackestones and organic-rich marly-peaty horizons with abundant terrestrial snails. As noted above, these darker units are typically described as gyttjas (Stankevica et al., 2016), humic deposits which are rich in detrital organic matter and capped by cm-scale sapropelic coals (see Fig. 6). Similar facies are found in the Middle Miocene of the Steinheim Basin in SW Germany (Tütken et al., 2006).

Analogous facies occur in many modern temperate marl lakes of North America, with examples including Green Lake, New York (Dean & Fouch, 1983), and Lake Littlefield and Sucker Lake, Michigan (Murphy & Wilkinson, 1980; Treese & Wilkinson, 1982; Dustin et al., 1986). In these lakes, cyclical carbonates recorded successive periods of increased organic productivity and progressive shallowing resulting from sedimentary infilling (Platt & Wright, 1991). An ancient, large-scale analogue may be provided by the Lower Cretaceous Peterson Limestone of Wyoming and Idaho, which covers 20,000 km² and is up to 60 m thick (Glass & Wilkinson, 1980).

The brecciated palustrine limestones from 55 to 45 m clearly record periodic subaerial exposure, pointing to deposition in freshwater algal marsh environments subject to seasonal or longer periods of emergence (Platt, 1989; Alonso Zarza, 2003) and similar to those found in modern Caribbean coastal wetlands such as the Florida Everglades (Platt & Wright, 1992).

The influence of groundwater recharge on shallow palustrine-lacustrine systems was postulated by Wright and Platt (1995). Carbonate wetlands both in Florida and the Yucatán Peninsula of Mexico (Platt & Wright, 2019, 2023) overlie extensive regional carbonate pediments and are fed by underlying karst aquifer systems which control seasonal fluctuation of water levels. Regional karst systems also underlie extensive palustrine and lacustrine carbonates in the Early Cretaceous and the Palaeogene of Iberia and NW Europe (Platt, 1989; Armenteros et al., 1997; Mas et al., 2019) which may rest either directly on a karstified carbonate pediment (Azêredo et al., 2015) or upon clastic successions which infill tectonically-created relief above it (Platt, 1995; Platt & Wright, 2023).

A range of continental facies and environments were recognised in the OSM by Kuhlemann and Kempf (2002), passing from alluvial fans in the south to more distal fluvial and lacustrine environments in northern areas of the Molasse Basin and the Jura. Becker (2003) proposed palaeogeographic models showing onlap of OSM deposits onto the marine Jurassic. We now interpret these OSM strata as recording seasonal subaerial exposure and pedogenetic modification of freshwater carbonates in response to variations in aquifer level in the underlying karstified Mesozoic carbonate pediment and its overlying clastic cover.

Figure 8 provides an integrated facies model for the OSM carbonates in Tramelan-2, drawing on similarities with the marl-peat cycles of the Michigan lakes described by Murphy and Wilkinson (1980) and Treese and Wilkinson (1982). Although modest cycle thicknesses of only 0.5 to 1 m may partly reflect compaction, they are consistent with formation in very shallow water bodies. The occurrence of organic-rich and rare laminated horizons is interpreted as reflecting episodic overproductivity resulting in intervals of dysoxia at the lake floor.

Fig. 8

Facies model for Upper Freshwater Molasse carbonates in the Tramelan-2 borehole

4.4.2 Stratigraphic assignment

A dark grey to black sapropelic coal at 95 m is interpreted as the base of the OSM, resting on the OMM below. This is overlain by a sandstone conglomerate and then by medium- to fine-grained sandstones. A limestone conglomerate at 87 m marks the sharp base of a carbonate succession in Tramelan-2 which appears to show a facies evolution similar to ‘Oehningian’ carbonates described from near La Chaux-de-Fonds and Le Locle, some 30 km to the southwest (Becker, 2003; Eichenberger et al., 2020). Mojon et al. (2018) recognised three lithological units in that area:

• A lower and marly Unit 1, transitional with the marine Burdigalian, locally displays a red colour ascribed to reworking of the Siderolithic (Kübler, 1962) and dated as MN4;

• The overlying ‘Lower Oehningian’ Unit 2 consists of fine grained, highly fossiliferous freshwater carbonates and marls and is dated as spanning zones MN5, 6 and 7–8;

• Finally an ‘Upper Oehningian’ Unit 3 contains dark organic-rich horizons near the top.

Kälin and Kempf (2009) recognised thirteen characteristic local mammal assemblages within the OSM distribution across the North Alpine Foreland Basin from SW France, through Switzerland and into Southern Germany. These assemblages span the European Neogene mammal (MN) units from MN4 to the base of MN9 (ca. 17.5–11 Ma), noting that further east in Southern Germany, Reichenbacher et al. (2013) recognised a transitional ‘Upper Brackish Molasse’ (OBM) of MN4a age between the OMM and OSM, with the base of the OSM placed at 16.6 Ma BP in that area and somewhat later at 16.0 Ma BP in the Swiss Molasse Basin, and noting also that the basal contact of the OSM may be erosive in both areas.

Biostratigraphic data from the Le Locle area to the SW of Tramelan-2 point to OSM carbonate sedimentation within a well-constrained interval in the Langhian–Serravallian from 14.0 to 13.5 Ma BP (Mojon et al., 2018). However, the basal unit (MN4) at La Chaux-de-Fonds/Le Locle and Tramelan-2 appears to be absent in the Jura synclines of La Sagne, Les Ponts-de-Martel, Vermes and Golat (see Fig. 2) where the oldest Oehningian strata are dated as MN5 (Kälin, 1993; Kälin & Kempf, 2009). This mild diachroneity was interpreted as a reflection of early Jura tectonics by Kälin et al. (2001), while Mojon et al. (2018) interpreted units 2–3 as recording the rich ecosystem of a large carbonate lake formed in a developing syncline surrounded by karstic topography. Freshwater carbonate deposition may thus have initiated in small and potentially isolated basins before expanding via onlap and overstep across a wider area.

The OSM succession at Tramelan-2 appears to show a broadly similar evolution of facies to that around Le Locle, in commencing with marls and passing through palustrine and lacustrine carbonates into cyclic alternations of organic-rich carbonate facies above, within which a grey-black marl with gastropod fragments at 21.3 m was dated from MN7 + 8 (Kälin, 1993). These correlations suggest that OSM environments and facies evolved in parallel across the area, either via direct connection, or via a common groundwater level which served to control water bodies which were intermittently isolated in structural compartments within the Jura at this time.

5 Stable isotope analyses

5.1 Isotopic data

Carbon and oxygen stable isotope data for 160 samples are presented on a cross-plot in Fig. 9A and versus depth in the core in Fig. 9B.

Fig. 9

Stable isotope data from the Upper Freshwater Molasse: a all data, plotting lacustrine/palustrine and pedogenetic carbonates. Data from underlying Tithonian Twannbach Formation marine carbonates from Rameil (2005). b Vertical profiles, showing correlation with sedimentary cycles

The pedogenetically modified carbonates from 57 to 45 m show only modest vertical variation in isotopic composition, displaying a narrow range in δ¹³C from − 6 to – 4 ‰ PDB and in δ¹⁸O from – 4 to – 1 ‰ PDB, while over much of this interval δ¹³C and δ¹⁸O remain fairly constant at – 5 and – 2 ‰ respectively. A subset of samples with the lowest δ¹³C values appears to show a degree of co-variance between δ¹³C and δ¹⁸O as shown by the trend line in Fig. 9A.

The lacustrine sediments show a wider range in stable isotope ratios, with cyclical variations of up to 4 to 5 ‰ in δ¹⁸O and up to 6 ‰ in δ¹³C carbon across intervals 50 to 100 cm thick. Carbon and oxygen isotopic ratios track broadly in parallel within several individual cycles (e.g. between 36–35 m and 44–42 m; Figs. 9B and 10), with more negative (lighter) isotopic ratios in darker facies of cycle tops and with abrupt positive shifts occurring at cycle boundaries.

Fig. 10

Stable isotope data showing typical carbonate sedimentary cycles in lacustrine carbonates from the Upper Freshwater Molasse between 45.0 and 41.0 m. Cycle boundaries shown by red dotted lines, with δ¹³C trends shown by red arrows: sharp cycle bases at flooding events with onset of open lacustrine conditions; gradual depletion in δ¹³C towards the top of sedimentary cycles associated with increased concentration of ¹³C-depleted organic matter

5.2 Discussion

The stable isotope compositions of non-marine carbonates have been discussed in several major review papers including those by Kelts and Talbot (1990) and de Boever et al. (2017).

The isotopically-light carbon and isotope oxygen compositions and suggestions of partial co-linearity shown by the pedogenetic facies suggest deposition in closed hydrological settings with long residence times (Talbot, 1990). Similar stable isotopic values and trends are reported from palustrine and pedogenetic successions in the Upper Jurassic of the western USA (Dunagan & Turner, 2004), the Berriasian of Northern Spain (Platt, 1989), the Eocene of the Swiss Molasse Basin (Platt, 1992) and the Miocene of central Spain (Bustillo & Alonso Zarza, 2007) and are consistent with the incorporation of atmospheric CO₂ within pedogenetically overprinted carbonate modified during subaerial exposure (Cerling, 1984, 1991; Cerling et al., 1989).

Within the current dataset, the subset of samples from the palustrine facies appearing to show colinearity in Fig. 9 may reflect greater desiccation during subaerial exposure and pedogenesis, with colinearity reflecting evaporative effects on δ¹³C and δ¹⁸O. The remaining palustrine samples with δ¹³C and δ¹⁸O values which are more similar to the lacustrine samples may reflect lesser subaerial exposure and/or bioturbational mixing in more hydrologically open conditions.

More negative δ¹³C values in the palustrine carbonates may also record increased oxidation of organic material in seasonally flooded and subaerially exposed settings. Pederson et al. (2019) linked repeated wetting and drying with the delivery of isotopically light ¹³C to the dissolved inorganic carbonate (DIC) pool from which the freshwater carbonates of the Florida Everglades were precipitated. Similar stable isotope compositions within parts of the Clino core drilled on the Great Bahama Bank were interpreted by Oehlert and Swart (2014) as reflecting subaerial exposure during Quaternary low sea-level stands.

Early stage meteoric dissolution and reprecipitation of carbonate may be common in many Holocene and ancient marine and brackish carbonate environments (Mazzullo & Bischoff, 1992; Munnecke et al., 2023; Fetrow et al., 2022; Pederson et al., 2023) and this process can also potentially play a significant role in creating light δ¹³C and δ¹⁸O values.

The wider range of δ¹³C values shown by the lacustrine facies bears comparison with non-marine carbonates from a range of different settings including the Great Salt Lake, USA (Lindsay et al., 2017) and stream carbonates from the Denizli Graben in SW Turkey (Lopez et al., 2016; de Boever et al., 2017). Study of sediments from Lake Van in SE Turkey also shows how the contribution of different bioclastic and authigenic carbonate phases with diverse stable isotopic signatures can produce a relatively wide scatter of compositions (McCormack & Kwiecien, 2021).

The lacustrine carbonate facies show consistent vertical variations (Fig. 10). Light-coloured carbonate-rich biomicrites at the base of sedimentary cycles, e.g. at 44.7 m, 43.8 m, 41.4 m as shown in Fig. 10, generally show the most positive ¹³C values, passing upwards into darker, more organic-rich limestones displaying more negative δ¹⁸O and markedly more negative δ¹³C values.

These vertical variations in δ¹³C may reflect cyclical changes in organic productivity. While the lighter-coloured layers may thus record periods of authigenic carbonate sedimentation and extensive CO₂ degassing within a hydrologically open system where there was limited burial preservation or post-depositional oxidation of organic material, the dark layers may then reflect final lake filling stages when reduced CO₂ degassing and increased deposition of δ¹³C-depleted organic material led to a progressive depletion in δ¹³C towards the tops of each regressive cycle.

The occasional preservation of lamination could also indicate periodic deposition under a stratified water column which served to limit bioturbational mixing and oxidation of organic material while favouring the maintenance of an isotopically light DIC pool. The lighter sediment layers would then indicate flushing of the system associated with increased groundwater input.

An alternative interpretation could link the observed cyclical variation in δ¹³C to increased aquifer discharge during flooding events. While δ¹⁸O values are likely to reflect relatively rapid exchange between lake waters and the atmosphere such that aquifer signal may not be preserved, carbon isotopes may record groundwater influence. Mixing of contrasting dissolved inorganic carbon (DIC) sources from isotopically light surface waters (in equilibrium with terrestrial C₃ and aquatic plants) and isotopically heavier groundwaters hosted in underlying marine Jurassic carbonates could thus provide an explanation for the wide range of δ¹³C compositions observed. It is interesting that the highest δ¹³C values recorded here are similar to those obtained by Rameil (2005) from Twannbach Formation limestones of Tithonian age (Fig. 9) forming the immediate carbonate subcrop in the Tramelan syncline. Nevertheless, there is considerable variability in the stable isotopic composition of Jurassic marine carbonates in the area, with the deeper Reuchenette Formation (Kimmeridgian) showing significantly more negative δ¹³C values from + 2 to − 3 ‰ (Colombie et al., 2011).

6 Heavy minerals

6.1 Introduction

Early work on the heavy mineral assemblages of the Molasse Basin by Füchtbauer (1964) was followed by detailed outcrop sampling in the eastern Bernese Jura by Hofmann (1969) and Antenen (1973). Later results from Kälin (1993, 1997) were incorporated in a detailed study of heavy minerals from the Palaeogene of the central Jura by Becker (2003). Each of these earlier works demonstrated good correlation between heavy mineral zones and the lithostratigraphy while greatly assisting the understanding of clastic provenance in areas north of the Alps.

Strunck and Matter (2002) provided an overview of heavy mineral distributions in the USM of the western Molasse Basin, while Allen et al., (1985; their Fig. 3) recognised several characteristic heavy mineral assemblages in the OMM, each derived from distinct source areas in the south:

A southwestern zone in the western Molasse Basin delivered epidote-apatite-hornblende, with minor clinopyroxenes and serpentine and with pumpellyite decreasing eastwards. This assemblage derives from the western Alps via the Lake Geneva fan (Maurer, 1983);

A southern zone in the central Molasse Basin, corresponding to the Napf Fan between Thun and Lucerne, delivered assemblages dominated by epidote; while

Assemblages derived from an eastern zone were characterised by apatite and epidote with ultrastable minerals such as zircon and tourmaline, together with staurolite, chloritoid and the characteristic index minerals blue amphibole and lawsonite, and displaying higher garnet contents than in areas to the west. Allen et al. (1985) observed that epidote increases westwards while apatite and ultrastable minerals are more abundant to the east. These diverse assemblages are thought to reflect multiple sediment sources, with the presence of andalusite and topaz at certain horizons suggesting periodic clastic influx from the Bohemian Massif to the east.

6.2 Results

The 34 heavy mineral samples from Tramelan-2 show relatively uniform grain sizes throughout (except for the youngest samples and those at 266.3 and 266.0 m). Zircon and tourmaline grain morphologies appear to indicate first cycle derivation, while few of the apatite grains recovered show rounding. Changes in the proportions and the occurrence of key index minerals are therefore likely to reflect provenance rather than environmental effects. Heavy mineral assemblages show distinctive characteristics which differ from the Alpine Molasse as follows:

• Garnet is present in only moderate amounts. In the Alpine Molasse of western Switzerland (Strunck, 2001) garnet generally dominates heavy mineral assemblages in the Chattian, while being progressively replaced from the Aquitanian onwards by assemblages containing both apatite and garnet. This observation suggests that the Tramelan-2 succession is likely to be latest Chattian or younger in age;

• Only a single grain of chrome spinel was found, despite being generally abundant both in the UMM and as a key mineral in the Chattian USM of eastern and central Switzerland (Gasser, 1968; Schlunegger, et al., 1993, 1997a; Kempf et al., 1999; Strunck, 2001; Schlanke, 2015). Again this is consistent with absence of the lower USM at Tramelan-2.

• Blue amphibole is rare, and the associated mineral lawsonite is absent in Tramelan-2, contrasting with their characteristic occurrence in heavy mineral suites of Alpine derivation (c.f. Mange-Rajetzky & Oberhänsli, 1982).

• Pumpellyite is common in Tramelan-2, as it is from the Burdigalian onwards in the western Molasse Basin, where it likely derives from ophiolites in the Prealps and the schistes lustrés of the Western Alps (Mange-Rajetzky & Oberhänsli, 1986). Pumpellyite is typically much less common than serpentinite elsewhere in the Molasse (Strunck, 2001).

• Epidote-clinozoisite is present as very fresh, large crystals in Tramelan-2, in contrast with the smaller and commonly aggregate epidotes found in the Alpine Molasse.

• Allanite- or piemontite-cored epidotes were also recovered from Tramelan-2, and these types are also not generally found in the Alpine Molasse.

These observations point to a distinctive sediment provenance history in this part of the Jura. As discussed by Becker (2003) and Berger et al. (2005b), the presence of certain index minerals such as blue hornblende, pumpellyite and serpentine is diagnostic of sediment input from the western Alps delivered by an axial Lake Geneva drainage system (Genferseeschüttung), while other more northerly source areas may also have contributed.

6.3 Vertical evolution

Four discrete heavy mineral intervals are identified, with the thickest of these divided into two.

6.4 Unit 1

274.0–242.0 m. This unit broadly corresponds to the Grey Sandstones described from 275 to 252 m and is provisionally correlated with the fluviatile Upper Alsace Molasse of Roussé (2006). Epidote-clinozoisite is present in moderate amounts, with aggregate forms fairly common. Amphiboles are also important, with blue amphibole in the deepest samples. Apatite occurs in varying quantities, while zircon is sparse and only small amounts of tourmaline are present. Serpentine is rare, except in one sample, and pumpellyite occurs mainly in trace amounts.

6.4.1 Interpretation

This heavy mineral assemblage appears similar to Strunck’s (2001) heavy mineral unit 2 from the Molasse Basin, where the first appearance of hornblende occurs in the USM just below the Chattian/Aquitanian boundary. An alternative assignment to the basal OMM remains feasible.

6.5 Unit 2

239.3–162.5 m. This interval can be subdivided into two, both of which show limited numbers of tourmaline, a virtual absence of zircon, and varying, but relatively low quantities of apatite:

• a) 239.3–193.3 m. This unit corresponds broadly to the grey sandstones and conglomerates described above from 252 to 190 m. Epidote-clinozoisite is more abundant than in the underlying interval, but many fewer amphiboles are present. Serpentine and pumpellyite are relatively common.

• b) 183.6 m–162.5 m. This unit corresponds to the lower section of the Pebbly Sandstones displaying well-developed sedimentary structures. This interval is rich in epidote-clinozoisite and has fairly high proportions of epidote aggregates and very few amphiboles.

6.5.1 Interpretation

Although the Burdigalian Upper Marine Molasse strata in this interval may rest on a significant discordance representing an appreciable time gap, its association of a high epidote content and the key minerals hornblende and pumpellyite continues to be characteristic of the westerly-derived Lake Geneva system (Becker, 2003).

6.6 Unit 3

153.1–112.95 m. This interval corresponds with the upper, more massive section of the Pebbly Sandstones. These strata contain moderate amounts of apatite. Epidote-clinozoisite are abundant but epidote aggregates are rare. Staurolite and tourmaline occur in minor amounts, whilst zircon is virtually absent and amphiboles are rare. Traces of serpentine were found.

The base of mineral suite interval 3 is closely coincident with the occurrence of two key Vêlé Limestone Conglomerate horizons at 154.8 m and at 142 m, while the immediately overlying section between 112.95 m and 104.7 m, approximately 10 m below the interpreted top of the OMM at 95.0 m, shows a decrease in garnet and an increase in epidote and staurolite.

6.6.1 Interpretation

A switch to epidote-dominated assemblages in the Molasse Basin in the upper part of the OMM was interpreted by Allen et al. (1985) as recording a dilution of westerly-derived index minerals, associated with greater input from the Napf Fan, with transport over a significant distance from the Alps reflecting the assistance of strong tidal currents within a restricted seaway.

6.7 Unit 4

104.7–57.1 m. Samples from the marly basal section of the OSM between 57.10 and 91.0 m are highly micaceous with limited quantities of very fine-grained heavy minerals. These strata show the lowest epidote-clinozoisite contents encountered in the Tramelan-2 borehole, with rare epidote aggregates and the highest proportions of apatite. Tourmaline, zircon and staurolite are common. Several amphibole species are present in varying amounts, while serpentine is absent.

6.7.1 Interpretation

The abundance of mica in this interval and the occurrence of the common ultrastable heavy minerals tourmaline, zircon and staurolite are characteristic of the easterly-derived Micaceous Sandstone drainage system (the Glimmersandschüttung of Lemcke et al., 1953; Hofmann, 1969).

7 Sedimentological evolution

The basal deposits in Tramelan-2 (275.6–252 m) comprise sandy continental facies similar to those described from the fluviatile Upper Alsace Molasse (USM) of the URG by Roussé (2006), while the overlying OMM succession (252 m–95 m) bears comparison with the OMM of the Molasse Basin to the south (Homewood, 1981; Homewood & Allen, 1981; Allen, 1984; Allen & Homewood, 1984; Keller, 1989, 1990) where many of the same characteristic facies are developed.

The strata from 252 to 190 m appear typical of cyclical shallow water tidal deposits. Conglomeratic lags at cycle bases are interpreted as high energy storm beds recording erosional reworking of the underlying deposits and coarse fluvial input from the hinterland. The bioturbated sands seen above this from 190 to 95 m could be found in a range of shallow marine environments, but in this case are interpreted as subtidal shallow offshore deposits.

The overlying rippled and trough cross-bedded facies record moderate current action, most likely in tidal channels or on the flanks of sand bars. Laminated mudstone-siltstone-sandstone alternations are typical of the upper zones of muddy tidal flats and record deposition from slow-moving currents during slack water phases (Longhitano et al., 2012). Finally, the presence at cycle tops of mottled horizons suggests pedogenetic modification during subaerial exposure, as is commonly seen on the inland margins of tidal flats and salt marshes.

Following early descriptions by Klein (1963) and others, ancient tidal deposits have been described from all around the world (Coughenour et al., 2009). An excellent modern analogue comes from tidal flats inshore of the East Friesian Islands in Germany (Wunderlich, 1969; Reineck & Singh, 1980; Reineck & Wunderlich, 1988). Erosion through tidal channel migration leads to the removal of many of the aeolian and wave-influenced facies of the islands themselves, which show a relatively low preservation potential in comparison with tidal channel facies.

Tidal processes have long been documented from the OMM of the Molasse Basin (Allen & Homewood, 1984) and our work suggests that similar processes acted in the Jura. The OMM in Tramelan-2 is distinctive in showing few wave-generated structures. This is consistent with active tidal channel migration in a meso- or macrotidal, laterally restricted and structurally controlled seaway between the Molasse Basin to the south and the Rhine Graben to the north.

Jost et al. (2016) described wave-influenced facies as limited to the south of the Molasse Basin but not reaching the Jura, consistent with progressive onlap of the OMM to the north. Similarly, Garefalakis and Schlunegger (2019) inferred a wave-dominated coastline with limited tidal influence east of the Napf fan, while a tidal-dominated estuarine environment was developed on the proximal coastal margin to the west, potentially analogous to the setting of Tramelan-2.

High amplitude tides are characteristic of large estuarine systems displaying active tidal channels, sand shoals and broad mud flats. Modern analogues include the Elbe estuary of Germany as above and the Severn estuary of southern England (Allen & Duffy, 1998). Selli et al. (1978) describe how tidal amplification effects in the structurally controlled Messina Strait in southern Italy serve to produce macrotidal conditions within the microtidal Mediterranean.

Continental facies above 95 m are arranged in shallowing upwards sequences recording repeated flooding and infilling episodes. Interbedded marls and limestones from 84 to 66 m reflect ephemeral pond development before an overlying limestone package from 58 to 34 m was laid down in a longer period of lacustrine sedimentation. Seasonal subaerial exposure is inferred by the presence from 55 m to 45 m of palustrine carbonates displaying brecciated and pedogenetic fabrics.

Finally, the dominance of marls with abundant gastropods and peat horizons above 25 m points to the establishment of a freshwater swamp environment in the Tramelan-2 area in association with renewed clastic sedimentation, now including input from basement massifs in the east.

8 Stratigraphic development and regional context

8.1 Regional onlap

In Cenozoic times, the depocentre and distal pinch-out of each unit in the Molasse was displaced successively northwards ahead of the advancing thrust front and the isostatic load of sediments shed in advance of it (Pfiffner, 1986; Schlunegger et al., 1997c; Pfiffner et al., 2002). Keller (1990, 2012) elegantly interpreted regional stratigraphic variation at the northern margin of the Molasse Basin as reflecting progressive northward onlap and overstep onto the karstified Mesozoic of the European foreland, which itself rests on a top Palaeozoic surface forming a southward dipping regional slope (Rime et al., 2019). This model explains both the northward pinch-out of the USM across the central and eastern Jura (Becker, 2003; Naef et al., 1985: their Fig. 8) and the lateral variation observed in the OMM succession from south to north as discussed above.

At Glovelier, 10 km NE of Tramelan-2 (Fig. 11), Burdigalian strata rest on the marine Jurassic while also thinning northwards as a result of progressive basal onlap. Karst pockets here contain a Burdigalian fauna of MN4 age (Bolliger, 1997; Becker, 2003; Prieto et al., 2018), indicating that the Mesozoic pediment remained exposed into the Miocene or was unroofed by erosion at that time.

Fig. 11

Highly schematic section (not to scale) illustrating ENE-ward onlap of the Molasse succession onto the faulted and karstified foreland of the Jura. The dashed section passes over the Glovelier High with additional topographic relief (shown as a dashed line) which was exposed prior to OMM deposition. Intervals of karstic discharge are seen as favouring the deposition of freshwater carbonates at times when topographic relief was infilled and clastic supply was low

Figure 11 presents a regional onlap model, while Figure 12A provides a simplified facies model showing the main environments represented in the OMM at the onset of the Burdigalian transgression, illustrating progressive northward onlap in the Jura and towards basement massifs in the north.

Fig. 12

Sedimentary facies models. a Basal OMM (Upper Marine Molasse). b OSM (Upper Freshwater Molasse)

During the Oligocene, freshwater carbonates were deposited in response to karstic discharge in an arc 30–80 km southwest of Tramelan between Neuchâtel and Lausanne (Platt, 1992), where the USM onlapped the Marine Jurassic and Lower Cretaceous of the southern Jura (Weidmann, 1984). These Chattian Lower Limestones also form a localised veneer on the Mesozoic karst surface in the Jura (Reichenbacher et al., 1996; Becker, 2003; Weidmann et al., 2008).

A similar configuration (Fig. 12B) is thought later to have facilitated renewed deposition of freshwater carbonates in the OSM, through regional onlap onto and karstic discharge from underlying marine Jurassic carbonates in a belt now located further north in the central Jura.

The geological map of Aufranc et al. (2016a) assigns the thin basal Molasse in Tramelan-1 to the USM, as at Tramelan-2, but progressive northward onlap sees the USM absent at Glovelier 10 km to the north, while 3.5 km to WSW of Tramelan-2 in western and southwestern areas of Tramelan village, the Jurassic pediment is overlain directly by younger OSM limestone conglomerates and limestones resting on a basal red marl horizon (Aufranc et al., 2016a; see also Forkert, 1933).

Aufranc et al. (2017) infer from these relationships that the OSM was deposited onto surface relief. This interpretation explains a degree of diachroneity observed within the basal OSM, with a thicker and slightly older section of transitional siltstones and marls resting on the OMM in topographic lows (as at Tramelan-2 and Le Locle), passing upwards into freshwater carbonates which overstep and rest on the karstified Jurassic and a basal conglomerate lag on highs. This onlap geometry would have allowed continued karstic discharge from the Jurassic pediment on highs while the aquifer rose through USM and OMM clastics draping the Mesozoic down slope.

A question then arises whether relief at the base OSM was exclusively erosional, recording the persistence of emergent highs after the OMM transgression, or whether it was partly structurally controlled while reflecting early deformation in the Jura as inferred by Mojon et al. (2018).

8.2 Structural controls on Palaeogene–Neogene sedimentation

The structure of the Jura (see Fig. 2) displays a series of en-échelon folds offset by NNE-SSW transverse faults and lateral ramps (Rime et al., 2019). Although the Jura is widely regarded as a predominantly thin-skinned fold-and-thrust belt (Laubscher, 1965, 1987; Sommaruga et al., 2017) compartmentalisation along strike appears to reflect significant pre-existing Variscan structure (Madritsch et al., 2008) which also defines the distribution of underlying Permo-Carboniferous troughs (Diebold & Naef, 1990; Diebold & Noack, 1997).

The same NNE-SSW fault trend defines both the eastern and western margins of the Delémont Basin, with the Glovelier High forming a relatively uplifted footwall block directly to the west. NNE-SSW structures were extensionally reactivated during formation of the URG to the north (Dèzes et al., 2004; Ustaszewski & Schmid, 2006), defining a series of ESE-dipping fault blocks on the northern flanks of the Jura which were passively onlapped by Palaeogene sediments (Hinsken et al., 2007; Ustaszewski et al., 2005) with clastic influx of Chattian Alsace Molasse from the south finally serving to infill this syn-rift relief (Roussé, 2006). With sedimentary gradients then equalised and clastic supply minimised, the Delémont Limestones were laid down as karstic discharge once again flowed onto the surface.

Aquitanian strata seen on the A16 at Tavannes (Zulliger, 2008) are absent at Tramelan-2, where only 1 m of vestigial or reworked Delémont Limestones is preserved. Lateral offset of the Tavannes and Tramelan anticlines suggests that a NNE-SSW transverse fault between them controlled reduced sedimentation or deeper erosion to the west. Karstification of the Glovelier High continued or resumed into the Burdigalian when regional transgression saw OMM deposits onlap pre-existing relief and overstep onto the remaining Jura highs and northern massifs. Formation of a narrow, strongly tidal seaway enabled clastic input from distant source areas in the Alps and from the Vosges and Black Forest. Roussé (2006) and Pirkenseer et al. (2018) envisaged structurally controlled sediment input via the Delémont Basin area (Fig. 13).

Fig. 13

a Palaeogeographic map of Western Europe in the Upper Rupelian (after Roussé, 2006). The area in b is shown in the orange inset box. b Schematic palaeogeography of the Molasse Basin and the URG, Early Chattian (Upper Alsace Molasse). Adapted after Pirkenseer et al. (2018)

Isotopic age dating by Looser et al. (2022) suggested an onset of tectonic activity in the Jura related to Alpine contraction commencing around 14.3 My BP, earlier than determined by biostratigraphy and coinciding with a Middle Miocene (Langhian) age at the latest. This suggests that tectonic enhancement of relief may already have begun by the time of OSM deposition.

Following Langhian sea-level fall, post-rift differential subsidence relief was infilled with basal OSM clastics and marls during renewed transgression, setting the scene for deposition of the overlying Serravallian carbonates as karstic aquifer discharge onto the sediment surface again saw the formation of a groundwater-fed lacustrine-palustrine system. Freshwater carbonate facies development is highly sensitive to tectonically-controlled differential subsidence (Platt & Wright, 1991, 2023), such that a series of small basins with distinct depositional histories and only partial connection may initially have formed along strike (Suter, 1978). However, parallel carbonate facies evolution across a wider area supports the establishment of a common aquifer level within a larger freshwater carbonate system extending SW-NE across the central Jura while connecting at least periodically with depocentres in the URG to the north.

Finally, renewed erosional unroofing of the flanking basement massifs saw clastic sedimentation resume with influx of the Micaceous Sandstones, dated at Tramelan-2 as basal MN7-8 (Kälin, 2001) and derived from the Vosges, Black Forest and Bohemian Massif to the north and east.

9 Conclusions

The Tramelan-2 borehole penetrated a 275.60 m thick sequence of sediments from the Lower Freshwater Molasse (USM), Upper Marine Molasse (OMM) and Upper Freshwater Molasse (OSM), presenting a rare continuously cored section through the Cenozoic of the Jura and permitting good correlation with the local and regional lithostratigraphy.

USM continental clastics are sharply overlain by shallow marine clastics of the OMM, passing upwards into non-marine clastics and carbonates in the OSM above. The succession displays distinctive heavy mineral assemblages reflecting derivation from a range of sources in the western Alps, from the Napf Fan, as well as from northerly non-Alpine source terranes.

Sedimentological facies analysis of the OMM in the Tramelan-2 borehole reveals a series of stacked shallow marine regressive cycles, each lying on a sharply-defined transgressive base.

Indications of strong current action and limited evidence for wave influence point to sedimentation in a protected meso- or macrotidal flat or estuarine complex within a structurally controlled narrow seaway between the Molasse Basin in the south and the URG in the north.

Later regression and erosion is thought to have resulted in rejuvenation of a regional karst system developed in the Mesozoic carbonate pediment. Carbonates and marls in the overlying Upper Freshwater Molasse onlapped and overstepped onto erosional and potentially early tectonic relief, while reflecting deposition in freshwater carbonate swamps, marshes and lakes. Repeated shallowing upward cycles capped by organic-rich calcareous peat record successive phases of lake infilling. The middle part of the succession comprises palustrine carbonates, which were periodicially modified by pedogenesis when lower aquifer levels resulted in seasonal or longer intervals of subaerial exposure.

Stable isotopic compositions of the freshwater carbonates reflect the influence of pedogenesis during subaerial exposure of palustrine facies. Cyclical variations in δ¹³C may reflect changes in lake productivity associated with the progressively increasing incorporation of ¹³C-depleted organic matter towards the top of lake-filling cycles. An alternative interpretation could nevertheless correlate positive δ¹³C excursions during episodic flooding events with increased aquifer discharge of groundwaters in equilibrium with underlying marine Jurassic carbonates.