SASEG Annual Conven.on FIELD TRIP Sunday 23 June 2019 Field trip Leader: Prof. Dr. Andrea Moscariello Dr. Silvia Omodeo Salé Introduction The area studied here corresponds to the westernmost extension of the Alpine Molasse basin. We shall concentrate on the area referred to as “Greater Geneva area”, which covers the Canton Geneva and surrounding France. It is comprised between the Alpine front to the southeast (marked by the Prealps and Subalpine Massifs in figure 1) and by the Jura mountain range to the northwest. Within the Molasse basin of the Geneva area, the small SW-NE trending range of the Salève and its southwestern extension are incorporated within the Jura domain (Fig. 1). The Greater Geneva area is limited to the southwest by the NW-SE trending Vuache Mountain, also part of the Jura. Tectonic framework of the western part of the Swiss Molasse basin This field trip will present the general structural features and lineaments affecting the Geneva Basin, as well as its overall stratigraphy illustrated through Mesozoic carbonate and Cenozoic 1 Molasse outcrops. The theme of potential georesources in several stratigraphic intervals in the area will be addressed. The field trip will consist of 4 stops. Stop 1 at the top of the Mount Salève gives a general overview of the main structural elements constituting and delimiting the Geneva Basin (or “Geneva cuvette”). Stop 2 and 3 are located in France along the Vuache Fault Zone, an ancient geothermal conduit today exhumed; Stop 4 is located in the Canton of Geneva and will visit the locally bitumen-impregnated Cenozoic Molasse derived from Toarcian organic-rich shales. GEX-2 GEX-1 GEo-01 JURA DOMAIN Thônex-1 GEX-3 GEX-4 GEX-5 GEX-6 Humilly-2 Humilly-1 Salève-1 Musiège-1 Salève-2 Savoie-106 Savoie-109 Savoie-108 SUBALPINE Savoie-107 Savoie-105 DOMAIN Savoie-104 Simplified stratigraphy and geological map of the Geneva Basin based on outcrops and hydrocarbon wells (from Clerc et al, 2015; Brentini, 2018) Stop 1 OVERVIEW OF THE GENEVA BASIN This stop is located in France, at the top of the Salève Mountain, ca. 1300 m a.s.l (the southern part of Canton of Geneva). From this point of view the morphologic expression of the Geneva Basin (or Geneva Basin) bounded to the southeast by the Salève range, to the northwest by the folded Jura, and to the southwest by the Vuache Mountain is well visible. To the NE, the lake Geneva extends for ca 72 km separating the Prealps reliefs and the Swiss Plateau. To the west the Fort de l’Ecluse gorge separating the Vuache Mountain from the folded Jura, where the Rhône River exits the Geneva Basin it is visible. The backbone of these surrounding mountain ridges is made of outcropping Mesozoic carbonates. A geological cross-section of the Geneva Basin can be established from 2D seismic data. It extends from the Alpine front in the southeast up to the Vuache Mountain in the northwest. 2 Geological cross-section across the Greater Geneva area derived from 2D seismic lines. This section crosses the Alpine foreland basin from the Alpine front up to the Vuache Mountain/Jura. Geological cross-section of the Salève Mountain according to Joukowsky and Favre (1913) Stop 2: THE VUACHE FAULT WALL AT LA BALME DE SALLANGY In this stop we will visit a spectacular fault mirror, i.e. a fault surface characterized by clear kinematic indicators such as striations, friction grooves, calcite steps and sigmoidal/lithons structures. The Vuache fault bounds to the SW the anticline of Mandallaz which represents 3 the southern extension of the Salève, of the same axial direction, with respect to which it is simply offset. The former exploitation of the quarry of Petite Balme (illegal and unauthorized or at least uncontrolled and unregulated) has cleared in previous years this superb looking mirror. Thus, without the exploitation of this career, this site would still be hidden! The anticline of Mandallaz has a very open vaulted form, which is clearly visible thanks to the natural notch, almost orthogonal to its axis. This produces from its southern end (top of La Tête) the escarpments of La- Balme-de Sillingy, whose foot forms an almost continuous cliff. The observed geometry (see map) indicated that the fault mirror belongs to a conjugate fault plane connected with ca 30 ° angle to the main fault (see the map below). Despite the high angle, this secondary fault has been interpreted as a Riedel structure assoc iated with the major detachment. On July 15, 1996 at 02:13 am, an earthquake with an estimated magnitude of 5.2 on the Richter scale shacked the region. Thus was highlighted, at the foot of the mountain of Mandallaz, one of the largest fault mirrors, of almost 2,000 m², never seen in Europe. This is certainly the most spectacular fault mirror in France. Stop 3: THE VUACHE FAULT: AN EXUMED GEOTHERMAL SYSTEM This stop offers the opportunity to examine a complex fault system known as the Vuache Fault, a NW-SE trending left-lateral transpressive regional fault separating the Geneva Basin to the NE from the Rumilly Basin to the SW. This fault, linked most likely to basement fault (Charollais et al., 1983) active since Mesozoic time, is considered active as demonstrated by the earthquake of M1: 5.3 intensity occurred in 1996, whose epicentre was located ca. 12 km to the SE. The kinematic analysis of the earthquakes suggest a detachment surface located at the base of the Mesozoic sequence, corresponding with Triassic evaporites (Baize et al., 2011). From this stop the SW-NE trending thrust faults forming the northern flank of the Salève and Montagne de Mandallaz mountains are also visible. 4 Continental Molasse Chattian Marine Molasse 3 Burdigalian Urgoni an Lmst Cross section of the Vuache mountain throughout history (A: Schardt, 1891 B: Arikan, 1964; C: Charollais et al., 2013) and geological map of the Vuache Mountain, central part (Charollais et al., 2013). Seismic line across the Vuache Fault, southeast of the Vuache Mountain At this stop the Mesozoic stratigraphy of the Vuache Mountain which well-represents the subsurface geology of the Geneva Basin (Charollais et al., 2013; Rusillon, 2018) will be presented. The Chaumont and Malpas, sitting on Lower Cretaceous shallow marine platform carbonates (“Urgonian” facies). Looking at the Vuache Mountain to the north and the 5 Musiège Moutain to the south, the opposite dips of Mesozoic strata in both mountains, which are separated by the Vuache Fault can be observed. A) Synthetic stratigraphic log of the Geneva Basin with indication of possible reservoirs. B) Typology of the thermal anomalies identified in the Geneva Basin. Approximate depth is shown for reference but may strongly vary from one place to another. From Chelle-Michou et al., 2017) STOP 4: THE PETROLEUM SEEPAGES IN THE MOLASSE OF DARDAGNY Walk downstream of the Roulave Stream near Dardagny (Canton of Geneva) to see outcrops of Lower Freshwater Molasse (USM; Oligocene). The USM consists of sandstones and marls deposited in a low sinuosity river and flood plain environment. These sandstone are impregnated by bitumen. Geochemical analysis have been performed on these seepages, but unfortunately few information could be extracted, as the bitumen is strongly biodegraded. However, it seems clear that they have been generated by a marine source rock, which in the area is most likely the Toarcian Posidonia shales source rock. Several galleries and small pits have been dug since the 19th century to recover the bitumen. These bituminous sandstones are already mentioned in 1770 by H.B. de Saussure. The first attempts to exploit them in the area date back to 1826; the digging of galleries in the Roulave Valley began around 1845, and phases of production and abandonment went on until 1894. During the two world wars, the exploitation was reinitiated with the digging of wells and deep galleries. Probably a total of several tens of tons of viscous bitumen was extracted. It was used to asphalt some streets in the City of Geneva, lubricate carriage wheels, caulk boats or for medical purposes (Wildi et al., 2014). Bitumen derived from Toarcian Posidonia shales have also been encountered in cores taken in the karstified upper part of the Urgonian limestones. 6 (a) Chromatogram of the oil seepages in the Roulava stream (a1), indicating high biodegradation; (b) and (c) microphotograph in fluorescencee light of the Toarcian Posidonia shales in the Geneva area. Abundant algae of different size are dispersed in the shale matrix, typical of a Type II kerogen. Geneva Basin stratigraphy illustrated by the well Humilly-2 and calibration of seismic reflectors (after Paolacci 2012). On the right hand-side, a summary of the petroleum system in the Greater Geneva area. 7 References: Baize, S., Cushing, M., Lemeille, F., Gelis, C., Texier, D., Nicoud, G. and Schwenninger, J.L. (2011). Contribution to the seismic hazard assessment of a slow active fault, the Vuache fault in the southern Molasse basin (France). Bull. Soc. géol. France, 182/4, 347-365. Brentini M. 2018 Impact d’une donnée géologique hétérogène dans la gestion des géo-ressources: Analyse intégrée et valorisation de la stratigraphie à travers le bassin genevois (Suisse, France). PhD thesis. University of Geneva. Terre et Environnement, 140, 216 p. Charollais et al. (1988). 1:50'000 Geological Map of France (BRGM), sheet Annecy-Bonneville. Charollais, J., Wernli, R., Mastrangelo, B., Metzger, J., Busnardo, R., Clavel, B., Conrad, M., Davaud, E., Granier, B., Saint Martin M. and Weidmann, M. (2013). Présentation d’une nouvelle carte géologique du Vuache et du Mont de Musièges (Haute-Savoie, France) : stratigraphie et tectonique.