Description of geological units

Basement

The Variscan crystalline basement (basementsensu stricto) is buried under more than 3000 meters of sediments in the GGB, and has never been reached by the drill. Close to the GGB, three wells have reached this unit in the Jura Mountains: Bonlieu-1, Essavilly-101 and La Chandelière-1D (see Figure 4.1 for location). They show that the basement is mainly composed of biotite-rich gneiss, although green schist and porphyric granite have also been recognised.

The closest basement outcrops are located in the subalpine area (Haute-Savoie, France), where (Marro and Manigler, 1998) have described quartz- and plagioclase-rich micaschists, of probable sedimentary origin. Bordet (1961) and Ménot (1988) have studied the nearby, external, crystalline Belledone Massif, where they have recognised three main lithologies;

according to their description and nomenclature, the micashists described by Marro and Manigler (1998) might be part of the "série satinée". Granites and gneisses considered as basement rocks appear also further north in the Jura Mountains, in the Serre Massif.

Similarly to the Massif Central and Vosges Mountains, the latter forms a NE-SW oriented horst, which was emerged during most of the Mesozoic and played a key role in clastic material supply and facies distribution in this area (Coromina and Fabbri, 2004 ; Bichet and Campy, 2013).

Permo-Carboniferous

Permo-Carboniferous sediments infill half-graben troughs, where they onlap the crystalline basement (angular uncorfomity). Important lateral thickness variations are observed, de-pending on trough location and depth, but consistent lithologies are described both in wells and outcrops. In the Geneva Basin, the only well that reached the Permo-Carboniferous is Humilly-2. However, eleven other deep wells have penetrated this interval in the vicinity of the studied area (Chaleyriat, Charmont, Chatelblanc, Chatillon, , Faucigny, Chapery, La Tailla, Essavilly, and Treycovagnes, Figure 4.1). The lithology consists of tight brownish conglomerates and arkosic sandstones (Figure 4.3-1a), intercalated with silty, argillaceous and organic-rich layers (Figure 4.3-1c). Recent analyses of the organic matter have revealed concomitant continental and marine origins(Do Couto and Moscariello, 2016). Thus, a pos-sible prevailing deltaic depositional environment is suggested for this unit, by contrast with the strictly continental conditions previously interpreted according to the abundant coal chips and beds found in this unit (Figure 4.4). This unit has been dated with palynomorphs that systematically indicate a Stephanian to Autunian age. Marro and Manigler (1998) have studied this unit on nearby outcrops in the subalpine Alps and recognised lithologies ccompatible with those of the Carboniferous sediments in wells. Plant debris gave a Middle Stephanian age for the entire unit, whereas palynomorphs could not be used because of the high degree of metamorphism which affected the rocks. Therefore to date, in the area of study, none of these sediments has proved to be of Permian age, and this period of time has been represented by a hiatus.

4.3. Sedimentology and stratigraphy 91

Figure 4.4: Conceptual model of the depositional environment during the Carboniferous.

92 Chapter 4. Stratigraphy and sedimentology Triassic

Historically, Triassic units have been named according to the German nomenclature, which has been used in the present study. Triassic units seldom outrop in the GGB and surround-ings. Marro and Manigler (1998) have described Lower and Middle Triassic outcrops in the Arly valley (subalpine Alps). The lithologies observed (conglomerate, cornieules and dolomitic limestone) are linked to the Helvetic domain, which corresponds to the distal part of the European margin at the time of deposition. However, the GGB shows rather proxi-mal facies, which are associated with the Jura domain. Therefore, the study of Marro and Manigler (1998) has not been used for correlation purposes, and only well data and outcrops equivalent to the Jura domain have been considered. In the closest surroundings of the GGB, only the Keuper can be observed, whereas the Buntsandstein, Muschelkalk and Lettenkohle units do not outcrop (Figure 4.5).

Based on well observations, the Buntsandstein (also called "Grès bigarrés") unconformably overlies the Carboniferous or directly overlies crystalline basement (seen in La Chandelière-1D well, Figure 4.1). It is mainly composed of fine to coarse-grained quartz sandstone with argillaceous to dolomitic cement (Figure 4.3-1b). Few conglomeratic and argilaceous silty layers are intercalated in this Formation, which thickens northwards (10-30m in HU-2 and FAY-1 to 60-90m in BLU-1 and ESS-101, Figure 4.1). This Formation appears quite sand rich in the north (Jura, ESS-101 and BLU-1, Figure 4.1), whereas towards the south it con-tains argillaceous to evaporitic cements which plug the interparticle porosity. The transition Buntsandstein-Lower Muschelkalk is observed in Essavilly-101 and Humilly-2 wells, (Figure 4.1). It shows a higher clastic input in the Essavilly-1 to the North, where the nearby Serre Massif is a significant source of detrital material.

The Muschelkalk and Lettenkohle have been penetrated in Humilly-2 in the GGB and in several other wells in the Jura Mountains. Homogeneous facies are described in cores, show-ing packstone dolomite with anhydrite nodules alternatshow-ing with anhydrite and argillaceous laminations (Figure 4.3-2d). This interval thickens southwards, from 110-130m in the Jura domain to 240m in Faucigny-1 close to the Helvetic domain (Figure 4.1). Depositional envi-ronments are presented in Figure 4.6.

The Keuper is separated in two evaporitic units, visible in few wells. Only the upper part outcrops in the Champfromier area ("diapir of Champfromier", described by Meyer (2000b), Figure ??). The lower unit is characterized by massive halite with gypsum/anhydrite layers, and is overlain by the upper unit made of alternating dolomite, anhydrite and argillaceous horizons similar to the Muschelkalk unit. The ductile salt interval often acts as decollement horizon and generates thrusts and duplexes, which trigger large lateral variations in thickness (240 to 350m in wells). Close to the Alpine front, the massive halite unit may be absent and laterally grades to carbonates, as observed in Faucigny-1 (Rigassi, 1977 ; ESSO REP, 1970).

Transition to the Rhaetian is marked in outcrops by a marly dolomitic interval (about 5m thick). Above, the Argiles noires à estheries and Grès blonds units make up this interval but are not always distinguished in well descriptions. The former unit is a dark shale that contains Cyzicus (Euestheria) minuta (VON ZEITEN) (lateral equivalent to "Argiles de Levallois" (Caire, 1970 ; Kerrien and Landry, 1982)) indicating euryhaline conditions. The

4.3. Sedimentology and stratigraphy 93

Figure 4.5: Triassic units, outcropping in the Jura Mountains (La Sandézanne, Champfromier area (Figure 4.1)), correlated with the entire Triassic unit penetrated in the Humilly-2 well.

94 Chapter 4. Stratigraphy and sedimentology

Figure 4.6: Conceptual model of the depositional environment during the accumulation of the Muschelkalk.

4.3. Sedimentology and stratigraphy 95

Figure 4.7: Lateral thickness variation of Liassic units in a NW-SE transect across the GGB.

latter unit is composed of a quartz sandstone unit becoming rich in bioclasts upwards. Based on available information, a laterally coherent thickness of some 20m can be defined for this stratigraphic interval.

Lower Jurassic (Liassic)

The Triassic-Jurassic transition is progressive and is therefore challenging to distinguish (Meyer et al., 2000). However, good biostratigraphic and lithological markers in both Rhaetian and Hettangian surrounding units help to precise this boundary. Carbonate beds overlying theGrès blonds show a decreasing detrital content, whereas marine fauna dated as Hettangian (Mouterde and Corna, 1997) dominates gradually. The Triassic-Jurassic bound-ary is located in this transitional facies, which testifies to the Early Liassic transgression (Meyer et al., 2000).

The Liassic is subdivided into two parts consisting of two distinct subunits, the lower car-bonat and the upper marls, respectively. In outcrops, several units can be differentiated within these two subunits, mainly according to the faunal content (Loup, 1993 ; Meyer, 1995

; Vilpert, 1996 ; Donzeau et al., 1997 ; Meyer et al., 2000): (1) Calcaires gréseux à Chlamys

96 Chapter 4. Stratigraphy and sedimentology

Figure 4.8: Conceptual model of depositional environment during the early Upper Liassic.

4.3. Sedimentology and stratigraphy 97 (bivalves, Chlamys valoniensis), characterized by Planiinvoluta carinata, and supporting a transgressive trend (2) Calcaires à Gryphées (bivalves, Gryphaea arcuata) (3) Calcaires argileux à cassure conchoïdale, rich in ammonites (4)Marnes calcaires à belemnites (Figure 4.3-5d)(5) Marnes à amalthées (ammonite, Amaltheus margaritatus) (6) Marnes noires à nodules et Tisoa (bioturbation), poorly constrained by fauna, and only dated through sur-rounding units (7) Dalle échinodermique, showing a marked lithological contrast and well dated by ammonites (Pleuroceras spinatum) (8) Schistes cartons, platy marls with a poorly diversified fauna, but a characteristic lithology in wells (9) Alternances micacées à bancs durs, a transitional facies at the Toarcian-Aalenian boundary.

Large thickness variations are highlighted in the carbonate compartment between outcrops (30m thick in average) (Donzeau et al., 1997) and Humilly-2 observations (180m thick) (S.N.P.A., 1969) (Figure 4.7). This difference has been also pointed out on seismic (Signer and Gorin, 1995). The Triassic and Lower Liassic units are thinner westwards of the paleo-Cruseille-Humilly fault zones, which might indicate a relative shallower depositional envi-ronment to the West (Figure 4.8). This tendency seems to be inverted during the Upper Liassic and Dogger (see later), and can be explained by the reactivation and inversion of NW-SE tectonic lineaments.

Middle Jurassic (Dogger)

As mentioned before, the Lias-Dogger boundary is transitional. Therefore, in the study area, the Aalenian has been attributed either to the Upper Liassic (ESSO REP, 1970 ; S.N.P.A., 1964) or to the Lower Dogger (Donzeau et al., 1997). Officially, the Aalenian stage is part of the Lower Dogger (Cohen et al., 2013), and has been considered as such in this study.

in the subsurface, the Dogger unit is only approximately subdivided in wells, because only few cores are available in this interval. On the contrary, outcrops provide better lithological constraints, and the literature review shows six different units. The Calcaires gréso-micés à Cancellophycus unit (bioturbation also named Zoophycos) shows a distinct microfaunal assemblage (abundantPlaniinvoluta carinata and Trochammina sp. and occurrence of Cor-nuspira orbicula (Clerc, 2005), which allows the interpretation of the depositional environ-ments but does not provide a clear time constraints. According to Metzger (1988), this unit is diachronous towards the Jura Mountains. It begins in the Early Aalenian in the northern Jura Mountains and Swiss tabular Jura, and extends till the Early Bajocian in the southern Jura. In the studied area, it shows a constant thickness and facies towards the southern Jura and commonly reaches a thickness of 30m (Contini, 1970 ; Donzeau et al., 1997) (Figure 4.9).

Facies in the Bajocian show large lateral variations between the southern Jura and Geneva area (Piuz, 2004 ; Clerc, 2005 ; Charollais et al., 2013b ; Donzeau et al., 1997, and references therein). These variations reflect the deepening of the platform towards the GGB, and the nomenclature used to date has to be adapted. The most recent and detailed studies on the regional Bajocian (Piuz, 2004 ; Clerc, 2005) allow the following stratigraphic subdivi-sion to be proposed. At the base, the Alternances inférieures de calcaires fins unit extends for about 25m, and is composed of siliceous limestone with frequent echinoid fragments,

98 Chapter 4. Stratigraphy and sedimentology

Figure 4.9: Lateral thickness variation of Dogger units in a NW-SE transect across the GGB.

4.3. Sedimentology and stratigraphy 99

Figure 4.10: Conceptual model of depositional environment during the Bajocian.

100 Chapter 4. Stratigraphy and sedimentology peloids, and poorly diversified foraminifera assemblage (mainly Planiinvolutina and Oph-talmidium). In Humilly-2 only, an ooid-rich interval is observed, which is time-equivalent to coral bioconstruction events ("Calcaires à polypiers", photozoan assemblage) found in more proximal environments (Burgundy area). The overlying Calcaires à entroques unit includes alternating crinoid-rich limestone beds (Figure 4.10) with fine siliceous and marly limestone (>50m). In the Haute-Chaîne, the outcrop described by Wernli (1970) and Piuz (2004) (torrent du puits d’Enfer), is marked at the base and top by well-defined crinoid-rich beds (10m and 7m thick respectively), whereas in Humilly-2, this lithology forms one single bed.

Transition from Lower to Upper Bajocian is marked by changes in the microfaunal content, which generally indicate a deepening of the depositional environment. The corresponding unit is namedAlternances supérieures de calcaires et marnes, and is composed of alternating limestones and marls (about 50m think), where the oyster Praexogyra acuminata typcally found in the Champfromier area is not observed in outcrops and wells closer to the Geneva area. On top of this unit, oolithic beds (15m), which might be correlated with the Cal-caires oolithique unit defined in more proximal settings westwards in the meridional Jura Mountains (Piuz, 2004), are observed in the Haute-Chaîne, but not in Humilly-2. Therefore we have not separated it from the underlying unit. The Bajocian ends up with a Fe-rich oolithic hard ground (Thiry-Bastien, 2002), which marks an abrupt transition to a deeper environment in the Bathonian.

Only the Lower and Middle Bathonian are found in the GGB area and seem lateraly constant in thickness. They are maded of the Calcaires terreux unit, which consists of fine, marly, bioclastic limestones with more marly intercalations. This interval is well dated by the rich ammonite content (Mangold, 1970). It is overlain by the Calcaire d’Arnans (oolithe ferrugineuse) unit, dated as Middle Callovian and forming the last unit of the Dogger. It consists of a thin condensed bed in the GGB (about 5m thick), with abondant Fe-rich ooids, Fe-crusts, phosphate and glauconite (Charollais et al., 2013b).

Mangold (1984) noticed that overall the Aalenian to Lower Bathonian units are thinning westwards (Ile Crémieux, Vignoble and Revermont), whereas this trend is reversed in Upper Bathonian and Callovian deposits going eastwards (GGB). This author suggests that topo-graphic highs are responsible for this observed differential sedimentation, originally present in the Burgundy area, and later moving towards the Haute-Chaîne at the end of the Dogger.

These highs are underlined by numerous sedimentary hiatus during Upper Bathonian and Callovian times. Signer and Gorin (1995) indicate that on seismic data, prograding sedi-mentary bodies can be recognised in Middle Jurassic crinoidal limestones, extending their palaeogeographical realm southwards.

Upper Jurassic (Malm)

The Dogger-Malm boundary is recognised by the presence of 10 cm thick breccia beds (Pseudo-brèche (ferrugineuse) de Saint-Claude) in the Haute-Chaîne (Vilpert, 1996 ; Nuss-baumer, 1995), dated as late Callovian(?)-early Oxfordian (Ecoffey, 1994).

The Malm has been penetrated in both Humilly-2 and Thônex-1 wells in the Geneva Basin s.s. (Figure 4.11). Several other wells in the Rumilly Basin provide additional information

4.3. Sedimentology and stratigraphy 101

Age

Marbre bâtard Chambotte Fm

Couches de la

Corraterie Vions Fm

Calcaires de Thoiry Pierre-Châtel Fm

Purbéckien Goldberg Fm

Portlandien Tithonien Tidalites de Vouglans

Kimmeridgien auct. Kimmeridgien auct.

Sequanien Calcaires

pseudolithographiques

Rauracien Calcaires lités

Geissberg Effingen Birsmendorf Domérien Pliensbachien sup

Carixien Pliensbachien inf Lower-Jurassic

Former names Current names

Cretaceous

Néocomien ou

Infra-valanginien Berriasien

Upper-Jurassic

Oxfordien Argovien

Table 4.1: Comparative table of former stratigraphic nomenclature and the one proposed in this work in the GGB for the Lower Jurassic, Upper Jurassic and Cretaceous.

on facies and lateral development throughout the basin. This unit is also well observed in outcrops, because it is mainly composed of competent lithologies. It forms a substantial part of the surrounding anticlines and crest of the Jura Haute-Chaîne. The Malm can be subdivided lithologically into two main zones. The lower one is marly (commonly called the "combe oxfordienne") and forms topographical depressions. The upper one is made of competent beds and shows the transition from marly limestone to fully calcareous lithologies at the top. Over the years, authors have subdivided the Malm into numerous units according to facies criteria: Argovien, Rauracien, Sequanien, Kimmeridgien auct., Portlandien. This nomenclature corresponds to former definitions of age intervals and is now obsolete. The link with the present-day nomenclature is not straightforward, and the most reliable correlations to date have been included in the interpretation proposed in this work (table 4.1).

The Oxfordian starts with the Couches de Birmensdorf unit, dated as Middle Oxfordian (Enay, 1966) and directly overlying the Callovian. TheMarnes à Creniceras rangeri (Lower Oxfordian) described westward in the southern Jura are missing in the studied area (Mas-trangelo, 1973 ; Blondel, 1990 ; Graezer, 1995 ; Nussbaumer, 1995 ; Vilpert, 1996 ; Donzeau et al., 1997 ; Charollais et al., 2013b). TheCouches de Birmensdorf are composed of greyish, fine, nodular limestone with frequent sponge debris and ammonites. This unit presents a con-stant facies and a thickness of less than 10 m. In geological maps, this unit is usually grouped with the underlying Callovian Calcaires d’Arnans. The Couches d’Effingen and Couche du Geissberg are grouped in one single unit in the area (Couches d’Effingen-Geissberg) because their facies is very similar, whereas it becomes distinct westwards. This unit is composed of marly limestones, rich in ammonites providing coherent datations (Figure 4.3-5c and 5d).

102 Chapter 4. Stratigraphy and sedimentology

Figure 4.11: Lateral thickness variation of Malm units across a broad NW-SE transect in the GGB.

4.3. Sedimentology and stratigraphy 103

Figure 4.12: Conceptual model of the depositional environment during the Upper Kim-meridgian, at the early stage of the reef sequence.

104 Chapter 4. Stratigraphy and sedimentology The overlyingCalcaires lités unit shows a similar facies (Figure 4.3-5a), but is individualized according to ammonite zones (Enay, 1966). The "combe oxfordienne" unit is overlain by the Calcaires pseudolithographiques unit. This first competent lithology is described as dm-thick micritic limestone beds with rare marly intercalations. This unit extends until the end of the Oxfordian, whereas, an additional unit (the Calcaires d’Arans) is present west of the Champfromier area.

The Couches à céphalopodes unit marks the base of the Kimmeridgian. It is composed of soft lithologies dominated by marly limestones rich in ammonites, which provide good age determinations. The facies is similar to that of the Couches d’Effingen-Geissberg, and only biostratigraphic data help to differentiate them (Nussbaumer, 1995). The transition to the overlyingCalcaires de Tabalcon unit, also calledFormation du Coin at Mount Salève, is pro-gressive to reach pure carbonate beds. The increase in bioclast content and the occurence of Tubiphytes morroniensis are typical of this unit (Figure 4.3-4a). The so calledReef complex, currently named Etiollets Formation (Fm) varies rapidely laterally and vertically (Bernier and Enay, 1972 ; Donzeau et al., 1997 ; Meyer, 2000a ; Widmer, 2001 ; Charollais et al., 2013b) (Figure 4.12). Three units are differentiated in the area according to their facies: (1) the Calcaires récifaux, (2) the Calcaires plaquetés and (3) the Calcaires de Landaize. The first one comprises patch reefs (Figure 4.3-3bis.b and 3bis.c), mud mounds (Figure 4.3-3bis.d) and surrounding coarse inter/peri-reefal deposits (Figure 4.3-3bis.a). These bodies are lim-ited in space, and show brittle deformation. The second one is composed of platty micritic limestones, whose faunal content can be restricted and where dolomite occurence is common, as well as that of organic matter, which can form even bitumen laminites in a few places (Orbagnoux, France (Davaud et al., 2014)). This unit is mechanically more easily deformed and kinks are observed close to major fault zones (Donzeau et al., 1998 ; Widmer, 2001 ; Charollais et al., 2013a). The third unit is made of coarse bioclastic limestones, whose fau-nal content corresponds to the patch reef composition. Both the Calcaires de Tabalcon unit and Etiollet Fm are diachronous across the GGB based on ammonites chronostratigraphy (Bernier, 1984 ; Deville, 1990 ; Enay and Boullier, 2000).

The Couches du Chailley unit, which overlies the Etiollet Fm, is mainly developed west-wards in the southern Jura, and seems to pinch out in the studied area. Whereas this unit shows great thickness variations, its facies is relatively constant and shows highly bio-turbated (Thalassinoides) biomicritic limestone deposited in a calm, subtidal and lagoonal environment (Guyonnet, 1988 ; Nussbaumer, 1995). In the GGB, this unit is reduced or absent, and is grouped with the overlyingTidalites de Vouglans unit. In the new lithostrati-graphic framework, they are regrouped under the name ofTwannbach Fm. TheTidalites de Vouglans (previously called "Portlandien flammé") is the last unit recognised in the Malm and is composed of micritic limestone showing a tidal influence, e.g., algal mats, dessication features, fenestrae and gypsum pseudomorphosis (Figure 4.3-2a, 2c, 3d). The proxymity of the continent is also recorded in the fossil content through the occurence of charophyte algae. No significant biostratigraphic marker is found in this unit, but the age of the latter is constrained by surrounding formations to the Tithonian and base of the Berriasian.

Overall, the first part of the Malm is characterized by deep depositional environments, which

4.3. Sedimentology and stratigraphy 105 progressively grade to platform settings during the second part of the Kimmeridgian. At that time, the palaeotopography was shaped by active extension tectonics. It controlled the facies distribution and explains the important lateral variations in the Etiollet Fm.

Patch reefs developed on highs, isolating quieter, muddier, more or less restricted, lagoonal environments (Meyer, 2000a). Progressively, reefs prograded southeastwards and large tidal flats extended over the entire area at the end of the Jurassic. The paleotopography marked

Patch reefs developed on highs, isolating quieter, muddier, more or less restricted, lagoonal environments (Meyer, 2000a). Progressively, reefs prograded southeastwards and large tidal flats extended over the entire area at the end of the Jurassic. The paleotopography marked