Palaeogeographical and palaeoenvironmental settings 1

The sedimentary succession in the Geneva Basin overlies a crystalline basement which formed during the Variscan (Hercynian) orogeny (basement sensu stricto). This oldest unit results from the collision between the Gondwana (South) and Laurasia (North) continents (Debrand-Passard et al., 1984 ; Ziegler, 1990 ; Dercourt and Vrielynck, 1993). From the end of the Carboniferous until the Triassic, the development of the North Atlantic and Tethys ocean rift systems caused crustal thinning and extensional features in the basement of Western and Central Europe (Van Wees et al., 2000 ; Ziegler, 2001 ; Wilson et al., 2004 ; Ziegler and Dèzes, 2006). In the studied area, it created SW-NE trending, low angle normal faults and half grabens in the basement (McCann et al., 2006). In the GGB, these depressions were filled by continental siliciclastic material eroded from the surrounding topographic highs (Massif Central, Vosges, Serre, Morvan, Bohemian Massif). Coal seams intercalated in these deposits testify to a vegetation thriving in a humid environment during Carboniferous times. The crystalline basement and these mainly continental deposits form the basement sensu lato, and the contact between this unit and the overlying Triassic sediments is marked by an angular unconformity (Sommaruga, 1999 ; Signer and Gorin, 1995 ; Guellec et al., 1990, and references therein).

At the beginning of the Mesozoic, a large part of the Pangea megacontinent started to break up (Golonka, 2007). In the Lower Triassic (unit and age called "Buntsandstein" ac-cording to German nomenclature, Figure 1.2a), continental sedimentation was progressively replaced by carbonates and evaporites deposited in a shallow epicontinental sea coming from the North and the East (marine transgression until the Middle Triassic Muschelkalk), whose depocenter was located in the Northern Germany domain (Sommaruga, 1997)(Figure 1.2b).

Sedimentation along the northwestern Tethys margin was partially isolated from the open sea by the Alemanic-Vindelician High acting as a barrier. Consequently, a thick anhydrite

1Co-authored with Maud Brentini

22 Chapter 1. Introduction

Granite and gneiss, external crystalline massifs Fault

Wrench fault in the GGB, interpreted from seismic data Thrust

Figure 1.1: The Greater Geneva Basin, A) stratigraphic units, B) situation, geological units and tectonic features (structural interpretation in the GGB from Clerc, (in prep.), and simplified geological map (1:1000000) after Chantraine et al. (1996)). The studied area is outlined by the black dashed line.

1.3. Geological setting 23 and dolomite unit developed from the Northern Jura domain towards the Geneva area (Pao-lacci, 2013). Similar palaeoenvironmental conditions lasted until the Late Triassic (Keuper) and then evolved into more restricted environments (anhydrite, salt and shale deposits).

Another phase of transgression occurred during the Rhaetian and Lower Jurassic (Lias-sic) times on the NW European platform (Figure 1.2c). At this time, the Alemanic High (individualized from the Vindelician High) was still active and partially disconnected the epicontinental platform from the Tethyan ocean realm. Although the Alemanic High was flooded during the Middle Jurassic (Dogger), it still formed as a "submarine swell" (McCann, 2008) influencing and compartmentalizing the marine sedimentation. The part of the plat-form corresponding to the present-day Jura relief was then progressively tilted towards the SE (Rigassi, 1962 ; Mangold, 1984). This movement continued throughout the Late Juras-sic (Malm), during which the Alemanic High subsided accordingly. The boundary between the shallow platform and external and deeper environments following a general NE-SW axis Geneva-Bern was therefore shifted to the SE (Sommaruga, 1997). During the Upper Malm (Kimmeridgian-Tithonian, Figure 1.2d), this shift is indicated by the southward and south-eastward progradation of patch reefs and oolithic bars developing on inherited topographic highs in the carbonate platform of the Jura domain (Meyer, 2000a). Overall, the Jurassic period is characterized by two transgressive-regressive cycles, ending in the Lower Berriasian (locally known as "Purbeckian"). The top of the latter sequence is marked by typical emer-sion features well documented in the studied area (Deconinck and Strasser, 1987 ; Strasser and Davaud, 1982, 1983 ; Müller, 1986).

The Early Cretaceous is characterized by an intense tectonic and volcanic activity linked to the break-up of Pangea, which affected the climate, sea level and sedimentary dynam-ics (Föllmi, 2012). At this stage, despite another marine incursion flooded the platform of the Jura domain, near-emersion conditions prevailed until the end of the Barremian (Fig-ure 1.2e). The Aptian to Albian period is characterized by intensified greenhouse climate conditions (Föllmi, 2012). Several successive emersion and drowning episodes together with coeval tectonic structuration resulted in an intricate topography. Topographic depressions were filled with lacustrine to marine sediments rich in siliciclastic material and marked by numerous hiatuses and condensed beds (Pictet et al., 2016). Finally, pelagic chalk and lime-stones associated with a wide transgressive phase deposited over the entire area. The latter sediments were later eroded, and are today only found in karst pockets, which formed during the subsequent subaerial exposure of the Cretaceous sequence (see later). In the same area, Aptian-Albian deposits are well preserved in inherited topographic depressions, but seem to disappear eastwards of Bienne (Sommaruga, 1997 ; Pictet, 2016)(Figure 1.2f).

In the Early Cenozoic, convergence of African and Eurasian plates, together with base-ment uplift linked to the Vosges-Black Forest mantle dome, exhumed the Mesozoic strata (Trümpy, 1980 ; Karner and Watts, 1983). The sub-equatorial climate accelerated the ero-sion of the Cretaceous units and created a major unconformity at the top of the Mesozoic sequence. Lateritic sediments of Eocene age (Hooker and Weidmann, 2007 ; Becker et al., 2013) with reworked Aptian-Albian material filled karsts and fractures.

The overlying Oligocene Molasse (Figure 1.2g), covering the entire Swiss Plateau from

24 Chapter 1. Introduction

1. Bundsandstein

3. Rhaetien-Hettangien

4. Kimmeridgian-Oxfordian

5.Berriasien-Valanginien

6. Aptian-Albian

7. Oligocene 2. Carnien-Norien

Sand and conglomerate Sand

Sand and shale Carbonate and sand Carbonate Carbonate and shales Shale, some carbonate Shale

Organic shale Halite Sulphate Volcanic local Continental, lacustrine

Deltaic, coastal and shallow-marine clastics Shallow-marine, mainly shales Deeper-marine, mainly shales Deeper-marine clastics Carbonates, mainly shallow-marine Evaporite

Uninterpreted area Cratonic, mainly low relief

Depositional environments Lithological symbols

Areas of non-deposition

Tectonic symbols Others symbols

Inactive fold belts, moderate to high relief Active fold belts, high relief

Normal fault Transcurrentfault Active deformation

front of bold belt 0 Km 100

Direction of clastic influx Direction of marine incursion Direction of inta-basinal clastic

Figure 1.2: Palaeogeographical maps, modified after Ziegler (1990)

1.3. Geological setting 25 Geneva to Lake Constance and continuing to the east until the Pannonian region, in the GGB is divided in three palaeogeographical domains: the Bornes Plateau, the Geneva Basin and the Rumilly Basin (Berger, 1996 ; Morend, 2000 ; Charollais et al., 2007). The Bornes Plateau contains the Lower Marine Molasse (UMM according to the German nomenclature, see Strunck and Matter (2002, and references therein)) and Lower Freshwater Molasse (USM (Strunck and Matter, 2002)) units which were thrusted and folded during the Alpine orogeny to form the Subalpine Molasse. In the Geneva and Rumilly basins, the USM directly onlaps the eroded Cretaceous units or the Eocene lateritic sediments where present. In the Rumilly basin, the USM is overlain by the Upper Marine Molasse (OMM, (Strunck and Matter, 2002)) which transgressed from the SW and is to date present only in this domain. According to Schegg et al. (1997) more than 2000 m of USM and OMM are missing in the Geneva Basin and Bornes Plateau because of erosion.

The Quaternary period is characterized by several glaciations, and the resulting sediments record different progradation and retreat episodes of the Rhône Glacier and its related glacial, glacio-lacustrine and lacustrine environments (Moscariello et al., 1998 ; Fiore et al., 2011 ; Wildi et al., 2014). During the last glacial maximum, the Geneva Basin was covered by about 1000 m of ice (Rhône and Arve Glacier) as indicated by erratic blocs found on the Jura Mountains and near the top of Mount Salève (Moscariello, 2018).