The doubly vergent Alpine orogen (Fig. 1) is the consequence of the late Cretaceous to present continent–continent collision between the European and Adriatic plates (Schmid et al., 1996; Handy et al., 2010). It is comprised of a double vergent nappe stack wherein the material on the northern side was derived from the European and Adriatic plates, while the rocks on the southern side is only of Adriatic provenance. In the centre, the Alps expose a crystalline core of European origin referred to as the Lepontine dome and the external massifs (Fig. 1; e.g. Aar massif; Spicher, 1980). At deeper crustal levels, the Alpine orogen is underlain by a thick crustal root made up of a stack of lower crustal material derived from the European continental plate (Fry et al., 2010; Fig. 1b). Beneath the core of the orogen, the ca. 160 km long (Lippitsch et al., 2003) lithospheric mantle slab of the European continental plate bends and thus downwarps the foreland plate towards the SE (Fig. 1b). This bending was mainly driven by slab load due to the relatively large density of the subducted lithospheric mantle in comparison with the surrounding asthenosphere as seismo-tomography data reveal (Lippitsch et al., 2003). On the northern side of the Alps, the structurally highest unit is made up of Austroalpine nappes that structurally overlie the Penninicum, which in turn are underlain by the Helvetic thrust nappes (Fig. 1a). The front of the Helvetic and Penninicum units is referred to as the basal Alpine thrust (Fig. 1b). On the southern side, the Alps are made up of the south alpine thrust sheets that consist of crystalline basement rocks and sedimentary units of African origin. This fold-and-thrust belt is bordered to the south by the Po basin (Fig. 1a). The northern side of the Alps are separated from the southern side by the north-dipping Periadriatic line that accommodated most of the shortening during the Oligocene and the early Miocene by backthrusting and right-lateral slip (Schmid et al., 1996).

Recently, slab load has been considered the major driving force of the subduction history of the European plate and for the exhumation of crystalline rocks (Kissling and Schlunegger, 2018). This also concerns the exhumation of the Lepontine dome (Fig. 1), where normal faulting along the Simplon detachment fault resulted in rapid tectonic exhumation of the dome between late Oligocene and early Miocene times with a peak recorded by thermo-chronometric data at 20 Ma (Fig. 1b; Hurford, 1986; Mancktelow, 1985; Mancktelow and Grasemann, 1997; Schlunegger and Willett, 1999; Boston et al., 2017). This was also the time when the Aar massif, situated on the European continental plate (Fig. 1b), experienced a period of rapid vertical extrusion (Herwegh et al., 2017). Herwegh et al. (2017) and Kissling and Schlunegger (2018) proposed a mechanism referred to as rollback subduction to explain these observations. According to these authors, delamination of crustal material from the European mantle lithosphere along the Moho resulted in a stacking of buoyant lower crustal rocks beneath the Lepontine dome and the Aar massif, forming the crustal root (Fry et al., 2010; Fig. 1b). These processes are considered to have maintained isostatic equilibrium between the subducted lithospheric mantle and the crust and thus the elevated topography (Schlunegger and Kissling, 2015). Additionally, they most likely balanced, through the stacking of the crustal root (Fry et al., 2010, Fig. 1b), the rapid removal of upper crustal material in the Lepontine dome at ca. 20 Ma (Schlunegger and Willet, 1999; Boston et al., 2017). Delamination of crustal material has also been invoked to explain the contemporaneous rapid exhumation and northward thrusting of the Aar massif along steeply dipping thrusts (Herwegh et al., 2017). These processes were contemporaneous with (i) the reorganization of the drainage network of the Central Alps (Kuhlemann et al., 2001a; Schlunegger et al., 1998), (ii) the decrease in sediment flux to the basin, as revealed by sediment budgets (Kuhlemann, 2000; Kuhlemann et al., 2001a, b), and (iii) the Burdigalian transgression in the Swiss part of the Molasse basin. We will thus refer to these processes when discussing the controls on the Burdigalian transgression within a geodynamic framework.

The Molasse basin is approximately 700 km long and striking ENE–WSW from France to Austria, where it broadens from <30 km to a maximum width of ca. 150 km (Pfiffner, 1986; Fig. 1a). It is limited to the north by the Jura Mountains, the Black Forest, and Bohemian massifs, as well as to the south by the basal Alpine thrust (Homewood et al., 1986).

Reconstructions of the evolution of the Molasse basin (Fig. 2a) have been the focus of many research articles over the past years (e.g. Matter et al., 1980; Homewood and Allen, 1981; P. A. Allen, 1984; Keller, 1989; Schlunegger et al., 1996; Sinclair, 1997; Kempf et al., 1999; Kuhlemann and Kempf, 2002; Ortner et al., 2011; Reichenbacher et al., 2013). This has resulted in the general notion that the large-scale subsidence history of the Molasse basin was closely linked with the geodynamic evolution of the Alps (Sinclair et al., 1991; Kuhlemann and Kempf, 2002; Pfiffner et al., 2002; Ortner et al., 2011; Schlunegger and Kissling, 2015). The development of this basin as a foreland trough has been considered to commence with the closure of the Alpine Tethys in late Cretaceous times (Lihou and Allen, 1996; Schmid et al., 1996). This was the time when subduction of the European oceanic lithosphere with a large density beneath the Adriatic continental plate started. Large slab load forces resulted in a downwarping of the European foreland plate and the formation of a deep marine trough (Schmid et al., 1996), where sedimentation occurred by turbidites (Sinclair, 1997) on submarine fans (Allen et al., 1991; Sinclair, 1997; Lu et al., 2018; Reichenwallner, 2019). This first period of basin evolution has been referred to as the flysch stage in the literature (Fig. 2b; Sinclair and Allen, 1992). The situation changed at 35–32 Ma when the buoyant continental lithosphere of the European plate started to enter the subduction channel. Strong tension forces started to operate at the stretched margin of the European continental crust, particularly beneath the Central Alps (Schmid et al., 1996), with the result that the subducted oceanic lithosphere of the European plate broke off (Davies and von Blanckenburg, 1995). The consequence was a rebound of the European plate, a rise of the Central Swiss Alps, and an increase in sediment flux to the Swiss Molasse basin (Sinclair, 1997; Kuhlemann et al., 2001a, b; Willett, 2010; Garefalakis and Schlunegger, 2018), which became overfilled at ca. 30 Ma (Sinclair and Allen, 1992; Sinclair, 1997). The subsequent post-30 Ma stage of basin evolution has been referred to as the Molasse stage (Sinclair and Allen, 1992).

Molasse sedimentation occurred from ca. 30 Ma onward and was recorded by two large-scale, transgressive–regressive mega-sequences (Fig. 2b; e.g. Sinclair, 1997; Kempf et al., 1997, 1999; Kuhlemann and Kempf, 2002; Cederbom et al., 2004, 2011). These two mega-sequences consist of four lithostratigraphic groups. The first mega-sequence comprises the Lower Marine Molasse (UMM; Diem, 1986) and the Lower Freshwater Molasse (USM; Platt and Keller, 1992), and the second mega-sequence consists of the Upper Marine Molasse (OMM; Homewood et al., 1986) and the Upper Freshwater Molasse (OSM; Matter et al., 1980). Sedimentation in the Molasse basin continued up to ca. 10–5 Ma, when a phase of uplift during Pliocene times resulted in erosion and recycling of the previously deposited Molasse units (Mazurek et al., 2006; Cederbom et al., 2004, 2011). This erosion reached deeper stratigraphic levels in the western part of the Molasse basin than in the eastern segment (Baran et al., 2014) with the consequence that the OMM deposits are only fragmentarily preserved in the west (Fig. 2a).

Sediment dispersal changed during the Molasse stage of basin evolution. Prior to 20 Ma, during USM times, measurements of sediment transport directions (Kempf, 1998; Kempf et al., 1999) and sediment provenance analysis (Füchtbauer, 1964) revealed that the sedimentary material was transported to the east by braided to meandering streams. At that time, a coastline was situated near Munich, separating a deep marine trough farther east from a terrestrial environment to the west of Munich (Kuhlemann and Kempf, 2002). During OMM times, heavy mineral assemblages reveal that the Swiss Molasse basin operated as a closed sedimentary trough, where all supplied material was locally stored (Allen et al., 1985). During OSM times, from ca. 16.5 to 5 Ma, heavy mineral data imply that material with sources in the Hercynian basement north of Munich or the Bohemian massif was supplied to the Swiss Molasse (“Graupensandrinne”; Allen et al., 1985; Berger, 1996), suggesting that material transport occurred towards the west (Kuhlemann and Kempf, 2002). The details of the reversal of the drainage direction have not yet been elaborated, and related scenarios lack a database with palaeo-flow information, particularly for the OMM. The establishment of such a database, particularly the assignment of a more precise age to the drainage reversal, will be part of the scope of this article.

Following the scope of the paper, we established the facies relationships and sediment transport patterns during the transgressive phase of the OMM and thus mainly focused on the OMM-I. We proceeded through sedimentological investigations of key sites (Fig. 2a), which expose the related succession in proximal (Entlen section east of the Napf units; Sense section west of the Napf units), central (St. Magdalena site, Gurten drill core), and distal positions (Lake Neuchâtel and Wohlen areas). The lithofacies in the Entlen and Sense sections was investigated in the field at the scale of 1:50. At each outcrop along these sections, data were collected as notes in the field book and hand drawings on digital photos (available from the senior author upon request). The results are then presented as logs in Fig. 4 and in Tables 1 and 2. The St. Magdalena site and the Lake Neuchâtel and Wohlen areas only display outcrops rather than sections. Therefore, the sediments at these locations have been sketched in the field and on digital photos, thereby paying special attention to collecting information about the orientation and thickness of cross-beds. The sedimentary material of the ca. 200 m deep Gurten drill core is not available. However, the sediments were photographed at high resolution at the University of Bern in 1989 (see Fig. S3 in the Supplement). We used these photos to extract information on the lithofacies association encountered in the drilling.

The lithofacies have been identified (e.g. Schaad et al., 1992; Keller, 1989) based on the assemblage of sedimentary characteristics, including grain size, thickness, lateral extent if applicable, sedimentary structures, basal contact, colour, and fossil content (Tables 1 to 5). The lithofacies types correspond to individual bedforms (see Tables 1 to 5 for references), which bear information on flow strengths, flow directions, sediment supply, and water depths (e.g. Keller, 1989). The combination of these parameters, usually recorded by distinct assemblages of lithofacies types, can be used to identify distinct sedimentary settings. Related concepts of facies analysis have been documented for fluvial deposits (Miall, 1978, 1985, 1996; Platt and Keller, 1992) but are less standardized for shallow marine deposits. Here, we followed Keller (1989) and Schaad et al. (1992), who developed a concept for shallow marine deposits whereby lithofacies types are grouped into facies assemblages in a hierarchic order, based on which distinct shallow marine settings can be interpreted. We followed these authors and assembled the various lithofacies types into five depositional settings, which are from land to sea: terrestrial, backshore, foreshore, nearshore, and offshore. We then mapped the depositional settings at the scale of 1 : 25 000 at various sites across the Swiss Molasse basin where suitable outcrops were present (see Fig. 2a for visited sites).

Sediment transport directions were determined from orientations of clast imbrications, gutter casts, and dip directions of cross-beds. In addition, the orientation of the coastline can be inferred from sediment transport within the surf-and-swash zone at the wet beach where rolling grains carve millimetre-thin rills in the beach deposits, which are oriented perpendicular to the coast. These rills are recorded by linear grooves, or parting lineations, on the surface of sandstones (J. R. L. Allen, 1982; Hammer, 1984). We thus measured the orientation of these features where visible. We also determined the strike direction of oscillation ripple marks to infer the orientation of waves and thus of the coastline.

We analysed the sediments in the key sections according to their palaeo-water depths. For oscillation ripple marks, the ripple metrics (spacing between ripple crests and ripple heights) together with the grain size can be used to infer water depths at the time the oscillation ripples were formed (Diem, 1985; Allen et al., 1985; see the Supplement). We thus measured the ripple metrics with a metre stick together with grain sizes in the field and calculated the water depths following Allen (1997). Likewise, minimum water depths can be inferred from the heights of cross-beds as examples from modern streams have shown (Bridge and Tye, 2000; Leclair and Bridge, 2001). Please refer to the Supplement for the deviation of the related equations. Published information from deep drillings (Boswil 1; Hünenberg 1; Schlunegger et al., 1997a) and seismostratigraphic data (line 8307, Schlunegger et al., 1997a; line BEAGBE.N780025; Fig. S2 in the Supplement) completed the available database.

The sedimentary suite of the ca. 370 metre thick OMM-Ia at Entlen (Figs. 4a and S4a in the Supplement) records a large diversity of lithofacies types (Table 1). Parallel-laminated (Sp), fine- to medium-grained sandstone packages are centimetres to decimetres thick and normally graded. These deposits alternate with decimetre-thick low-angle cross-bedded units with tangential lower boundaries (Sc, Scta) and layers with a massive structure (Sm). Gravel and pebble layers (Sg) and shell fragments (Shf) are visible where sandstone units have erosive bases. Current (Scr) and oscillation ripple marks (Sos), locally with branching crests (Sbr), as well as flame fabrics or sand volcanoes (Sv) are present only in some places. Fine-grained lithofacies include millimetre- to centimetre-thick parallel-laminated to massive mudstone layers (Mp, Mm). Siltstone climbing ripples (Mcl) are subordinate in the OMM-Ia suite. Mudstone drapes (Md), a few millimetres thick, mostly occur on top of current ripple marks (Scr). In places, root casts are associated with yellow to ochre mottled colours.

The overlying ca. 430 m thick OMM-Ib (Figs. 4a and S4a in the Supplement) comprises fine- to medium-grained sandstone packages with mudstone interbeds (Table 1). Low-angle trough (Sctr) or tabular (Scta) cross-bedded sandstone beds are several decimetres thick. The Sctr sandstones contain current ripple marks (Scr) at their base, whereas laminae sets of Scta sandstones are interbedded with current ripples (Scr) recording an opposing sediment transport direction. At one site, decimetre-thick sandstone beds display a planar base and a wavy top with a wavelength of several metres (Spw; Fig. 4a). Parallel-laminated (Sp) and massive-bedded (Sm) sandstone beds are decimetres thick and mainly found at the top of the OMM-Ib unit. Mudstones mostly occur as mudstone drapes (Md) on top of current ripple marks (Scr). Lenticular and flaser interbeds (Mle, Mfl) are decimetres thick and characterized by current ripple marks with truncated crests. The OMM-Ib then ends with a metre-thick mudstone displaying yellow to reddish mottling, root casts, and caliche nodules.

Estimates of palaeo-water depths (see the Supplement) reveal that the OMM-Ia sedimentary rocks were deposited in shallow conditions <5 m deep (Fig. 4a and Table S2). At the base of the OMM-Ib palaeo-water depths were >15 m and thus deeper compared to the OMM-Ia unit (see the Supplement). The OMM-Ib then shallows towards the top.

Measurements of the bedform orientations of the OMM-Ia deposits reveal sediment transport directions between 315∘ NW and 60∘ NE, with a dominant NE-directed transport (Fig. 4a). During OMM-Ib times, transport directions were bidirectional, and measurements reveal the full range between 260∘ W and 70∘ E (Fig. 4a). Dominant transport directions of the OMM-Ib sediments change towards the N and to the W up-section.

The Napf units, which are a terrestrial interval of the OMM and the OSM (Fig. 3; Schlunegger et al., 1996), are ca. 1550 m thick and include a succession of conglomerates, sandstones, and mudstone interbeds (Matter, 1964), which we categorize into five lithofacies types (Table 2, Fig. S4d in the Supplement). Individual conglomerate beds are up to 10 m thick and display stacks of 2–3 m thick beds with massive- (Gm) to cross-bedded (Gc) geometries. The sandstone beds occur as massive-bedded (Sm) and cross-bedded (Sc) units. Interbedded mudstones are horizontally bedded (Mp) and have a yellowish–reddish mottling, caliche nodules, and root casts. Palaeo-flow measurements imply a change from a NE-directed transport during USM times to a NW-directed sediment transport between OMM-I and OSM times.

The OMM at Sense (Figs. 4b and S4b) starts with a ca. 200 m thick succession of predominantly sandstones with some mudstone interbeds. Medium- to coarse-grained sandstone beds, up to 2–3 m thick, are massive-bedded (Sm), parallel-laminated (Sp), and trough cross-bedded (Sctr) (Table 3). They also occur as metre-scale tabular cross-beds (Scta) forming several metre-thick sigmoidal foresets (Sc) with top and bottom sets and pebbly lags (Sg). These packages are well exposed along a nearby road-cut (Heitenried, Fig. 2a; 46∘49′27′′ N/7∘18′42′′ E; Fig. S4c in the Supplement). Some of these Sctr cross-beds contain current ripple marks (Scr), which are draped with a muddy layer (Md). Ripple marks also build up tabular sandstone bodies. They are either asymmetric (Scr) or symmetric (Sos) and may display branching crests (Sbr). In places, the sandstone bodies are highly bioturbated (Sf). Mudstone interbeds are 10–20 cm thick, massive- (Mm) to parallel-laminated (Mp), and strongly bioturbated (Mf).

The first occurrence of 5–10 m thick sandstone beds at the 200 m stratigraphic level (Fig. 4b) marks a distinct shift in the stratigraphic record where several metre-thick cross-bedded sandstone beds dominate the sedimentary succession. At this level, (i) 5–10 m thick normally graded sandstone beds overlie an erosive base and display epsilon cross-beds (Sce); (ii) cross-bedded sandstones (Scta) are several metres thick and tens of metres wide, and individual laminae sets are superimposed by current ripples (Scr) with an opposite flow direction than the cross-beds themselves; nearly all laminae sets of cross-beds (Sc) are superimposed by mudstone drapes (Md); and (iii) medium-grained sandstones display ridge-and-swale bedform geometries (Spw) with a small amplitude of a few decimetres and a large wavelength of several metres. Some of these Spw facies are occasionally covered with oscillation ripple marks (Sos). The Sense section ends with an alternation of decimetre-thick mudstone beds (Mm) and metre-thick massive- to cross-bedded conglomerates (Gm, Gc). These conglomerates then evolve towards an amalgamation of several metre-thick, massive-, and cross-bedded packages characterizing the uppermost ca. 50 m suite of the Sense section (Fig. 4b).

Estimates of palaeo-water depths range between 5 and 10 m (Fig. 4b and Table S4 in the Supplement) during the deposition of the lowermost 200 m. Conditions were deepest at the 200 m stratigraphic level, reaching water depths in the range of up to 30 m (see the Supplement). Measurements of sediment transport directions cover the range between ca. 0∘ N and 90∘ E (Fig. 4b) at the base of the Sense section, which then changed to an axial, bipolar SW–NE-directed transport and to a W-directed transport towards the end of the section (Fig. 4b).

The sandstones within a cave system near Fribourg (St. Magdalena; Figs. 2a and S4c in the Supplement) are medium- to coarse-grained and display an amalgamation of up to 1–3 m wide cross-bedded troughs (Sctr) with current ripple marks (Scr) at their base. The cross-bedded troughs and the ripple marks are both covered by mudstone drapes (Md). The amplitude of the troughs is in the range of several decimetres, whereas the cross-sectional widths span several decimetres to metres. The sandstones also occur as massive-bedded units (Sm). They are occasionally interbedded with current ripple marks (Scr) draped with mudstone layers (Md). Basal contacts are erosive. Measurements of morphometric properties (St. Magdalena; Fig. 2a) allow for an estimation of water depth, which is in the range of ca. 3 and 5 m (Table S1 in the Supplement). Sediment transport directions measured at the St. Magdalena site reveal a WSW–ESE-dominated sediment transport.

In the nearby ca. 260 m deep Gurten (Fig. 2a) drill core (Fig. S3 in the Supplement), OMM-Ia deposits occur as cross-bedded sandstones (Sc) topped with mudstone drapes (Md). These lithofacies associations (Table 4) are most abundant within the drill core and make up ca. 200 m of the log. However, because drill cores offer limited information about the dimensions of the encountered sediments, we were not able to determine if cross-beds can be assigned to tabular beds (Scta) or to troughs (Sctr).

Calcareous, shelly sandstones (Scc) occur in the basin axis and are an assemblage of various lithofacies. This Scc facies association is made up of 5–10 m thick, coarse-grained sandstone beds with low-angle cross-beds (Sc) that contain coquinas, shell fragments (Shf), and pebbles (Sg) in places. Interbedded fine-grained sandstones contain current ripple marks (Scr) recording an opposite flow direction relative to the cross-beds (Sc).

In the west (sites at Lake Neuchâtel area; Figs. 2a and S4c in the Supplement), mapping shows that Scc Muschelsandstein deposits are ca. 5 m thick and record NNE- to NE-directed sediment transport. Foreset thicknesses of these deposits thin to <1 m towards the front of the Napf megafan, where herringbone cross-beds imply SW- and NE-directed bimodal sediment transport. At the NE margin of the Napf, these Scc Muschelsandstein deposits grade into Slc Grobsandstein units, which show metre-thick tabular cross-beds (Scta) or decimetre-thick trough cross-beds (Sctr) wherein individual troughs have metre-wide diameters. Measurements of palaeo-flow directions reveal a SW- and SE-directed transport. These deposits are either time-equivalent sediments of the Scc Muschelsandstein, and are thus contemporaneous with the OMM-Ib succession, or they mark the base of the OMM-II succession (Jost et al., 2016). Farther east near the Wohlen area (Figs. 2a and S4d in the Supplement), foresets of Muschelsandstein cross-beds are 6 to 8 m thick and in some locations up to 10 m thick as reported by Allen et al. (1985). Sediment transport directions were oriented towards the SSW, covering the range between 230∘ SSW and 250∘ WSW and striking parallel to the topographic axis. Estimates of the water depths of the Scc Muschelsandstein reveal palaeo-water depths (Table S1 in the Supplement) between 60 and 100 m.

The OMM-Ia sedimentary rocks of the Entlen section are assigned to a backshore to upper nearshore realm within a wave-dominated environment (Fig. 4a, and please see Table 1 for references). Records of waves are inferred from (i) tabular, parallel-laminated and normally graded (Sp) sandstones, which are interpreted to represent sediments of the surf-and-swash zone near the wet beach where sedimentation occurs in the upper flow regime; (ii) low-angle cross-beds (Sc) with pebbly lags and shell fragments (Shf), which could reflect sand reefs (or shoals), rip channel fills, or storm layers; and (iii) oscillation (Sos) as well as branching ripple marks (Sbr) pointing to wave activity (Fig. 4a). Gravels and pebbly lags (Sg) are either evidence of high-energy storm events or river inflow from the backshore. Finer-grained lithofacies, which are either indicative of rapid sedimentation (Sv, Mcl; Table 1) or incipient pedogenesis (Mp, Mm; Table 1), are consistent with a shallow marine, wave-dominated environment.

The basal part of the OMM-Ib suite is assigned to a foreshore to lower nearshore setting shaped by the combined effect of wave and tidal activity (Fig. 4a). This is inferred by the observation that current ripple marks (Scr), which are situated on top of lamina sets of tabular (Scta) and trough cross-beds (Sctr), point towards an opposite flow direction than the cross-beds themselves (Fig. 4a). Mudstone drapes (Md) on top of ripple marks, together with lenticular and flaser interbeds (Mle, Mfl), are supportive evidence for a tidal environment (references in Table 1). The occurrence of waves, however, is inferred from parallel-laminated sandstones (Sp) with parting lineations and ridge-and-swale (Spw) structures at the base of the OMM-Ib suite. At the top of the OMM-Ib, massive sandstones (Sm) and mudstones with mottled colours, root casts, and caliche nodules mark the presence of a backshore, possibly terrestrial setting.

The change from a nearshore, wave-dominated environment (OMM-Ia) to an environment with tidal records (OMM-Ib) was additionally associated with a deepening from <5 to >15 m at the base of the OMM-Ib, followed by a regressive sequence. We thus consider the base of this unit to be the maximum flooding surface (MFS), separating the OMM-I into a transgressive OMM-Ia unit and a regressive OMM-Ib succession (Figs. 4a and 5a).

We interpret the association of massive- (Gm) to cross-bedded (Gc) conglomerates and massive- to cross-bedded sandstones (Sm, Sc) as deposits within a braided river system (Table 2; please see references there). In such an environment, conglomerates are common records of active channels. Massive- (Sm) to cross-bedded (Sc) sandstones alternating with mottled mudstones were most likely formed on the floodplains bordering the network of braided channels when bursts resulted in the accumulation of crevasse splay deposits (Platt and Keller, 1992). Mudstone interbeds (Mp) with evidence of palaeosol genesis formed when channel belts shifted away from the axis of the section (Platt and Keller, 1992). This facies association was mapped over tens of kilometres, both across and along strike of the basin orientation. It is thus assigned to an alluvial megafan (Schlunegger and Kissling, 2015), which was deposited by braided streams.

We assign the OMM of the Sense section to a tidal-dominated environment in which deltaic estuaries dominated the sedimentary facies (Fig. 4b, and please see Table 3 for references). This is inferred from (i) sigmoidal cross-bedded sandstones (Sc) with distinct top sets, foresets, bottom sets, and pebbly lags (Sg) that are indicative of a delta and (ii) trough cross-bedded (Sctr) and massive-bedded (Sm) sandstones that could represent mouth bar deposits where estuaries (or tidal inlets) end. In such environments, current ripple marks (Scr) with mudstone drapes (Md) at the base of tabular cross-beds (Scta) point to rhythmic changes in tidal current slack-water stages within a subtidal setting, whereas ripple marks (Sos) with branching crests (Sbr) and parallel-laminated sandstones (Sp) were most likely formed under the influence of waves close to the beach. Massive-bedded (Mm), parallel-laminated (Mp), and strongly bioturbated (Mf) mudstone interbeds are assigned to a tidal flat that established on the landside margin of the delta. Towards the top of the section, the facies successively evolves into a fan delta setting. This is inferred from the observation that the sedimentary suite thickens and coarsens upwards and ends with massive- to cross-bedded conglomerates (Gm, Gc), suggesting the progradation of a delta (e.g. Schaad et al., 1992).

The first occurrence of 5–10 m thick sandstone beds at the 200 m stratigraphic level (Fig. 4b) records a remarkable increase in the water depth when 5–10 m deep tidal channels (Sce sandstone beds with epsilon cross-beds) grade into several metre-thick subtidal sand waves (Scta) and nearshore tempestites (metre-scale Spw sandstones with ridge-and-swale geometries; see Table 3 for lithofacies and references). These two latter lithofacies (Scta, Spw) are interpreted to record the deepest palaeo-water depth in the Sense section, when water depths were in the range of up to 30 m. We consider this stratigraphic level to record the maximum flooding surface (MFS) within the Sense section (Fig. 4b), and we will use it for correlation purposes with the OMM succession at Entlen (see the Discussion section).

The several metre-thick outcrops near Fribourg are interpreted as subtidal shoal deposits, which accumulated within a tidal-dominated environment (Table 4; please see references there). This is inferred from metre-scale cross-bedded troughs (Sctr) with current ripple marks (Scr) at their base (see also Homewood and Allen, 1981, for a similar interpretation). Alternatively, sandstone troughs (Sctr) could be assigned to a mouth bar environment in which massive-bedded sandstones (Sm) would represent records of rapid sedimentation. In contrast, mudstone drapes (Md) on top of the ripple marks (Scr) and cross-beds (Sc) are formed during low-energy tides or possibly during slack stages. Similar deposits (Sctr, or possibly Scta) within the Gurten drill core could also be interpreted as sediments of subtidal shoals; however, due to limited exposure, interpretations are non-conclusive. Shallow palaeo-water depths are also inferred from estimates of water depths ranging between 3 and 5 m.

We interpret the Scc Muschelsandstein (Table 5) sediments (containing coquinas, shell fragments (Shf), and pebbles – Sg) to have been deposited within the topographic axis of the Burdigalian seaway (see also Allen et al., 1985, and Jost et al., 2016, for a similar interpretation). Metre-scale cross-beds (Scta), locally superimposed by current ripple marks (Scr), have been interpreted to reveal deposition under strong tidal currents (Allen et al., 1985). These deposits are thus assigned to offshore, tidal-dominated sand waves for which sediment transport was NNE- (Lake Neuchâtel area; Fig. 2a) or SSW-directed (Wohlen area, Fig. 2a). In places, pebbly lags (Sg) are interpreted as flood-related splays of gravels into the offshore setting, derived from the neighbouring Napf megafan. In contrast, the coarse-grained sandstones (Slc Grobsandstein sediments; Table 5) reveal similarities to the subtidal shoal deposits encountered at the St. Magdalena site where trough cross-bedded sandstones (Sctr) and tabular cross-beds (Scta) dominate the facies assemblages. However, the deposits in the Wohlen area are coarser-grained, and cross-beds (Sc) have larger diameters but similar thicknesses. We relate the coarse-grained nature of these deposits to the proximity of the Napf megafan in the SW. The cross-beds with larger wavelengths and similar amplitudes possibly imply stronger currents compared to the subtidal shoal deposits near St. Magdalena.

The chronostratigraphic framework together with the sedimentological data and palaeo-flow directions are used to propose a scenario of how the basin evolved through time. During USM times (Fig. 6a), prior to the Burdigalian transgression, the basin was occupied by alluvial megafans at the proximal basin border, which gave way to an axially directed channel-belt system in the distal basin (Fig. 6a; Kuhlemann and Kempf, 2002). Analysis of heavy mineral assemblages by Füchtbauer (1964) and measurements of palaeo-flow directions in our study area (sole casts and cross-beds; this paper) and in eastern Switzerland (Kempf et al., 1999) revealed a NE-directed material transport towards the Munich region (Fig. 6a), which is consistent with the results of previous syntheses (e.g. Pfiffner et al., 2002; Kuhlemann and Kempf, 2002). In this area, the Molasse streams ended in a peripheral sea where neritic to open marine conditions prevailed (Kuhlemann and Kempf, 2002). Within the basin, a possible divide for sediment transport was situated somewhere SW of Geneva. We infer such a separation of sediment dispersal based on published sediment transport directions. In particular, south of Geneva, tidal cross-beds imply a sediment dispersal towards the south and thus towards the Tethys (Allen et al., 1991; Allen and Baas, 1993). To the northeast of Geneva, however, our own measurements and data from Kempf et al. (1999) reveal a NE-directed sediment transport to the Paratethys.

The palaeogeographic reconstruction based on our and published data for the period between ca. 20 and 19 Ma is shown in Fig. 6b. It illustrates that the central part of the Molasse basin changed to a shallow marine sea, which was ca. 40 km wide at that time. Our estimates of palaeo-bathymetric conditions and sedimentological data (Fig. 4a and b) reveal that the water depths corresponded to a subtidal and nearshore setting. Nearshore to possibly offshore conditions (30–50 m) are recorded by subtidal mega-sand waves (Allen and Bass, 1993) south of Geneva and by the predominance of sandstone–mudstone alternations within the Boswil and Hünenberg drill cores in the NE (Wohlen area, Fig. 2a; Schlunegger et al., 1997a). Subtidal shoals, in up to 5 m deep water, occupied the western part of the central Swiss Molasse (Fig. 6b). This was already proposed by Homewood and Allen (1981), and it is confirmed here by our sedimentological data and estimates of palaeo-water depths (see the Supplement). Measurements of sediment transport directions from the shoal deposits reveal bimodal, SW–NE-directed transport with a dominant NE orientation. This is particularly the case at the proximal basin border near the Sense section (Fig. 2a) where deltaic foresets accumulated within an estuarine setting (Fig. 4b). Mapping of depositional settings allowed us to trace the shoal deposits towards the northern tip of the Napf megafan, from which the shoals narrow from ca. 20 to ca. 10 km over a 70 km long distance along strike. It thus appears that the shoals were deflected towards the topographic axis through a dominant NE-directed material transport (Fig. 6b). This interpretation is additionally supported by measurements of the transport directions of the Napf megafan (i.e. clast imbrications) and the coastal deposits at the Entlen section (i.e. parting lineation, cross-beds) pointing material transport towards the NE (Figs. 4a and 5a). At the distal margin of the basin, field inspections show that beach sandstones gave way to subtidal shoal deposits up-section. It thus appears that the Molasse basin between the Lake Neuchâtel and Wohlen areas (Fig. 2a) was a region of sediment export to the NE and to the SW. Material transport was most likely accomplished through strong tidal currents that entered the Swiss Molasse as two major tidal waves from the Tethys in the south and the Paratethys in the northeast (Bieg et al., 2008).

The situation during ca. 19–18 Ma (Fig. 6c) started with the time when the maximum flooding surface was formed in the depositional record (MFS; Fig. 5a and b). The sedimentological data reveal that this time was characterized by a widening of the basin to widths up to 80 km, and it was dominated by offshore conditions in the topographic axis with water depths >50 m, as the Muschelsandstein deposits imply (Fig. 6c). There, cross-bed orientations (our measurements and data by Allen et al., 1985) and heavy mineral assemblages (Allen et al., 1985) reveal that sediment transport in the eastern basin axis (Wohlen area) occurred towards the SW, whereas sediment dispersal in the western basin axis (Lake Neuchâtel area) was directed towards the NE. We use this information to propose that a sedimentary depocentre established at the northern tip of the Napf megafan. In addition, this megafan is interpreted to have experienced a backstepping (see Sect. 6.3.3 for explanation). We infer such a scenario from the first appearance of a bimodal E–W orientation of material transport in the Entlen section (Fig. 5a). Because an E–W-oriented sediment transport requires a free passage for tidal currents along the southern basin margin, the seaside margin of the Napf megafan had to step back to allow such a passage to form (Fig. 6b and c).

The palaeogeographic situation shown in Fig. 6d comprises the time span between ca. 18 and ca. 14 Ma and displays the evolution from the OMM-II to the OSM. The OMM-II period followed a phase of non-sedimentation and possibly erosion across the entire Swiss Molasse basin, as our reassessment of the chronological framework of the OMM reveals (Fig. 5a and b). In addition, measurements of sediment transport directions reveal a SW-oriented sediment transport at proximal positions (Fig. 5a), which is consistent with the results of previous syntheses (e.g. Pfiffner et al., 2002; Kuhlemann and Kempf, 2002). This also implies that a possible E–W divide for sediment transport shifted towards the region near Munich or even farther east. Similar to Kuhlemann and Kempf (2002), we infer such a scenario from the supply of material with sources in the Hercynian basement north of Munich (Fig. 1a) or the Bohemian massif (Graupensandrinne, Fig. 6d; Allen et al., 1985; Berger, 1996; see also Sect. 2), which implies a westward tilt of the basin axis. This period ended with the progradation of the alluvial megafans during the time of the OSM.

In conclusion, this study confirms the results and syntheses of previous authors on the general sedimentation and material transport pattern during the deposition of the OMM. Nevertheless, our refinement of the chronological framework in combination with additional sediment transport data allow us to specify some further details on the development of the transgression of the OMM. These include (i) the establishment that the Burdigalian seaway was accompanied by both a deepening and widening of the basin. These mechanisms occurred contemporaneously and were associated with a northward shift of the topographic axis to the distal basin margin, where offshore and thus the deepest marine conditions established at 19 Ma. (ii) The reversal of the sediment transport direction from an originally NE-oriented sediment dispersal to a SW-oriented sediment transport started sometime after 20 Ma and was completed at 18 Ma at the latest. (iii) A wave-dominated coastline (with some tidal records) established on the eastern side of the Napf megafan (Fig. 4a), whereas a tidal-dominated estuarine environment characterized the proximal coastal margin on the western side of the Napf (Fig. 4b). These variations in sedimentation pattern appear to explain why the lithostratigraphic framework differs between the two regions (see Fig. 3a).