Etude sédimentologique et stratigraphique du bassin genevois

Les résultats proposés, à l’échelle nationale, par les deux projets HARMOS et RGF n’ont pas pu répondre directement aux besoins du canton de Genève. Un travail important a donc été accompli au cours de cette thèse pour bien comprendre leurs limites dans l’application régionale de leur méthode afin d’adapter la réflexion et de répondre, de manière plus adéquate, au besoin d’harmonisation de la stratigraphie genevoise. C’est dans le cadre de cette recherche que le besoin d’une étude sédimentologique et stratigraphique complète du bassin genevois s’est révélé nécessaire.

Si l’étude détaillée du bassin genevois présentée immédiatement ci-après, a permis de rassembler les données de surfaces et de sous-sol utiles à la compréhension globale de la géologie locale, elle s’est surtout avérée précieuse dans l’analyse de l’ensemble de ces données pour en garantir une sélection et une harmonisation cohérentes et adaptées dans la future base de données du canton de Genève. En d’autres termes, alors que le chapitre précédent a démontré la nécessité de disposer d’une liste déroulante des unités géologiques présentes dans les données géologiques lors de l’intégration de ces dernières dans une base de données, cette étude sédimentologique et stratigraphique du bassin genevois va permettre d’alimenter ladite liste déroulante de manière adaptée à la problématique spécifique genevoise.

Cette partie est co-écrite avec Elme Rusillon, qui vient de terminer une thèse de doctorat, également dans le cadre du programme GEothermie2020. Financée par le Service Industriel de Genève, Elme Rusillon a commencé sa thèse en juin 2013 et l’a soutenue en décembre 2017. Les objectifs de son projet ont été de constituer le socle de données géologiques et pétrophysiques du sous-sol genevois pour en caractériser les réservoirs potentiels afin de soutenir et d'orienter l’exploration géothermique de moyenne à grande profondeur. Elme Rusillon a ainsi travaillé sur l’ensemble de forages profonds présents dans la région du Grand bassin de Genève (« Greater Geneva Basin). Il a été choisi de rédiger ce chapitre ensemble car l’objectif était d’assembler les données du sous-sol et de surface (diplômes, thèse, publications) dans une seule et même étude sédimentologique et stratigraphique du bassin de Genève. Malgré des objectifs de thèse différents, la complémentarité de nos analyses a permis au travers de cette étude de répondre à chacune de nos problématiques. D’un côté, Elme Rusillon a pu constituer un socle stratigraphique solide pour caractériser ces données et de l’autre côté, ce travail commun m’a offert l’opportunité de proposer une liste exhaustive des unités géologiques à intégrer à la future base de données du canton de Genève.

La rédaction de cette partie consacrée à l’étude sédimentologique et stratigraphique du bassin genevois qui figure également dans le manuscrit de thèse de Elme Rusillon et en constitue le chapitre 4 : Stratigraphy and sedimentology, figure en anglais dans ce manuscrit car elle est intégrée à une publication future destinée à avoir une visibilité au-delà de la francophonie. De plus pour répondre aux besoins des acteurs locaux francophones, ces résultats fondamentaux sont traduits et transposés dans le catalogue stratigraphique présenté en détail dans le chapitre suivant (chapitre 9). La publication anglophone inclura, quant à elle, les données stratigraphiques, lithologiques et sédimentologiques présentées ci-dessous ainsi que les faciès sismiques et la signature structurale étudiés par Nicolas Clerc. Ce travail aidera également à détailler les variations latérales des faciès à travers le sous-sol du bassin de Genève. Cette étude commune regroupant ces trois travaux fondamentaux sur la géologie régionale sera plus précisément intitulée :

Integrated sedimentological and stratigraphical synthesis of the Greater Geneva Basin (rédigé avec E. Rusillon)

E. Rusillon, M. Brentini, N. Clerc and A. Moscariello

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Introduction Aims

The aims of this Chapter are to provide constraints on the distribution and geometry of potential reservoir bodies and to integrate the latter in a homogeneous and coherent stratigraphic framework at the basin scale. For this purpose, facies and lateral variations have been identified on new outcrops and subsurface well data have been integrated to previous regional studies, leading to coherent models of depositional environment developed during selected Mesozoic time. Structural aspects derived from 2D seismic data have also provided key constraints for facies distribution and unit thickness variations across the basin (Clerc, in prep.). These integrated sedimentological observations have allowed the definition of a synthetic stratigraphic cross-section for the GGB, updated according to currently used regional nomenclature and age boundaries. In suitable units, correlations with terminology recently harmonized at national scale by the Swiss geological survey (swisstopo) (HARMOS, (Morard, 2014)) have also been attempted.

Geographical setting

Data have been acquired over the entire GGB. However, in this study, we concentrate on the basin surrounding the Geneva region, i.e., the Geneva Basin s.s., which is bounded to the southeast by the Mount Salève and to the southwest by the Mount Vuache (Figure 82).

Material and methods

This study includes two main study areas, i.e., the outcrops for surface data and the wells for the subsurface domain. The methods applied have encompassed (1) an outcrop review, based first on the extensive literature review of the regional geology, involving the valorisation of numerous unpublished MSc theses carried out in the past at the University of Geneva (Figure 82). The outcrop dataset has also been completed by an overall recognition of different units in the field, and sampling for microfacies analysis and cross-validation. (2) With respect to subsurface data, 45 wells reaching at least the Mesozoic strata have been selected. Reports and logs have been collected from the French and Swiss geological surveys (BRGM and Swisstopo).

The available core material has been described, and core samples have been retrieved for microfacies analysis on thin sections.

Sedimentology and stratigraphy

This study presents a synthesis of the main geological units, combining our studies of wells and outcrops with published and unpublished studies on sedimentological aspects of the GGB. The section below is structured as follows: for each main time unit, the transition with underlying deposits is described. Main facies characteristics and lateral variations are presented, in line with the regional structural framework. Geological units proposed here are not necessarily associated with a specific location and have therefore not always been related to formation names (according to the definition of "formation" of Foucault and Raoult (2010). Figure 83 summarizes the extension in space and time of such geological units within the Geneva Basin s.s.

(from the Jura Haute-Chaine to Mount Salève, passing through Humilly-2 well). It includes correlations with the national stratigraphic nomenclature (HARMOS, see details above), and with seismic horizons defined in the frame of GeoMol-CH project (Clerc 2016b). The Geneva Basin evolution has been considered according to the sea level curve (Haq et al. 1987), variations of depositional environments and 2nd order sequence stratigraphic framework (Rusillon, 2018). Information on potential reservoirs (for water, oil and gas ) has been collected from the regional well dataset and in the literature (Signorelli et al., 2004)

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Depositional environments proposed in Figure 83 have been interpreted according to microfacies analysis on outcrops and well samples (thin sections). Each of them has been characterized by various microfacies, which are illustrated in Figure 84.

Figure 82 : Situation map: the Greater Geneva Basin, wells studied and M.Sc. studies included. Structural interpretation in the GGB after Clerc, (in prep.), and digitized geological map from Chantraine et al. (1996).

150 Figure 83: (a) Schematic chronostratigraphical section of the Geneva Basin sedimentary succession (version 1.0). The horizontal axis is 25km, from the Jura Haute-Chaîne to Mount Salève,

passing through Humilly-2 well. (b) Correlation between common names used (inserted on the cross-section) and HARMOS nomenclature. (c) Seismic horizons defined in the frame of GeoMol-CH (Clerc, 2016). (d and e) Stratigraphical sequences defined at the 2nd order, and comparison with sea-level curve (Haq et al. 1987). (f) legend for the chronostratigraphical

section, including ranges of layer thickness measured in the GGB.

151 Figure 84 : Depositional environments observed along the sedimentary succession of the GGB, from Carboniferous to Lower Cretaceous units. (1a) Carboniferous unit, Humilly-2 well. (1b) Buntsandstein unit, Serre Massif. (1c) Carboniferous unit, Essavilly-101 well. (1d) Goldberg Fm, Savoie-104 well. (2a) Tidalites de Vouglans unit, Savoie-104 well. (2b) Goldberg Fm, Chaux-des-Crotenay, Jura Mountains. (2c) Tidalites de Vouglans unit, Savoie-109 well. (2d) Muschelkalk unit, Humilly-2 well. (3a) Vallorbe Fm, Savoie-109 well. (3b) Vuache Fm, Mount Vuache.

(3c) Goldberg Fm, Chaux-des-Crotenay, Jura Mountains. (3d) Tidalites de Vouglans unit, Savoie-104 well. (3bis-a) Etiollets Fm, Humilly-2 well. (3bis-d) Etiollets Fm, Valfin-lès-St-Claude, Jura Mountains. (3bis-c) Etiollets Fm, Reculet Nord, Jura Mountains. (3bis-d) Etiollets Fm, Humilly-2 well. (4a) Calcaires de Tabalcon unit, Savoie-109 well. (4b) Grand Essert Fm, Mount Vuache. (4c) Calcaires de Tabalcon unit, Etiollets path, Mount Salève. (4d) Grand Essert Fm, Gex-1 well. (5a) Calcaires lités unit, Savoie-107 well. (5b) Marnes calcaires à bélemnites unit, La Sandézanne stream, Jura Mountains. (5c) Couches d'Effingen-Geissberg unit, Humilly-2 well. (5d) Couches d'Effingen-Geissberg unit, Humilly-2 well.

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Description of geological units The Basement

The Variscan crystalline basement (basement sensu 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 82 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 (Bichet and Campy 2013; Coromina and Fabbri 2004)

Permo-Carboniferous

Permo-Carboniferous sediments infill half-graben troughs, where they onlap the crystalline basement (angular uncorfomity). Important lateral thickness variations are observed, depending 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 82). The lithology consists of tight brownish conglomerates and arkosic sandstones (Figure 84-1a), intercalated with silty, argillaceous and organic-rich layers (Figure 84-1c). Recent analyses of the organic matter have revealed concomitant continental and marine origins(Do Couto and Moscariello, 2016). Thus, a possible 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 85). 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 compatible 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.

153 Figure 85: Conceptual model of depositional environment during the Carboniferous.

Triassic

Historically, Triassic units have been named according to the German nomenclature, which has been used in the present study. Triassic units seldom outcrop in the GGB and surroundings.

(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 proximal 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 86).

154 Figure 86 : Triassic units, outcropping in the Jura Mountains (La Sandézanne, Champfromier area (Figure 82)),

correlated with the entire Triassic unit penetrated in the Humilly-2 well.

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 82). It is mainly composed of fine to coarse-grained quartz sandstone with argillaceous to dolomitic cement (Figure 84-1b). 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 82). This Formation appears quite sand rich in the north (Jura, ESS-101 and BLU-1, Figure 82), whereas towards the south it contains argillaceous to evaporitic cements which plug the interparticle porosity. The transition Buntsandstein-Lower Muschelkalk is observed in Essavilly-101 and Humilly-2 wells, Figure 82). 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, showing packstone dolomite with anhydrite nodules alternating with anhydrite and argillaceous laminations (Figure 84-2d). This interval thickens southwards, from 110-130m in the Jura domain to 240m in Faucigny-1 close to the Helvetic domain (Figure 82). Depositional environments are presented in Figure 87.

155 Figure 87 : Conceptual model of depositional environment during the accumulation of Muschelkalk.

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 82). 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 (ESSO REP, 1970; Rigassi, 1977).

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 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.

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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 the Grès blonds show a decreasing detrital content, whereas marine fauna dated as Hettangian (Mouterde and Corna, 1997)dominates gradually. The Triassic-Jurassic boundary 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 carbonate and the upper marls, respectively. In outcrops, several units can be differentiated within these two subunits, mainly according to the faunal content (Donzeau et al., 1997; Loup, 1993; Meyer, 1995; Meyer et al., 2000; Vilpert, 1996): (1) Calcaires gréseux à Chlamys (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 84-5d) (5) Marnes à amalthées (ammonite, Amaltheus margaritatus) (6) Marnes noires à nodules et Tisoa (bioturbation), poorly constrained by fauna, and only dated through surrounding 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 88). 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 environment to the West (Figure 89). 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.

Figure 88 : Lateral thickness variation of Liassic units in a NW-SE transect across the GGB.

157 Figure 89 : Conceptual model of depositional environment during the early Upper Liassic.

158

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-micacés à Cancellophycus unit (bioturbation also named Zoophycos) shows a distinct microfaunal assemblage (abundant Planiinvoluta carinata and Trochammina sp. and occurrence of Cornuspira orbicula (Clerc 2005), which allows the interpretation of the depositional environments 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 90).

Figure 90 : Lateral thickness variation of Dogger units in a NW-SE transect across the GGB.

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Facies in the Bajocian show large lateral variations between the southern Jura and Geneva area (Charollais et al. 2013; Clerc 2005; Donzeau et al. 1997; Piuz 2004 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 (Clerc 2005; Piuz 2004) allow the following stratigraphic subdivision to be proposed. At the base, the Alternances inférieures de calcaires fins tends for about 25m, and is composed of siliceous limestone with frequent echinoid fragments, peloids, and poorly diversified foraminifera assemblage (mainly Planiinvolutina and Ophtalmidium). 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 91) 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 named Alternances 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 Calcaires 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 laterally constant in thickness. They are made 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

Mangold (1984) noticed that overall the Aalenian to Lower Bathonian units are thinning