Les terrains jurassiques du sondage de Couy (Cher, France). Leur analyse séquentielle détaillée à partir de la description des carottes et des courbes diagraphiques
Jurassic Formations in the Couy Borehole (Cher Department, France). Their Detailed Sequential Analysis from the Core Sample Descriptions and Well-Logging Curves
Gaz de France
2 Université Paris VI
Le forage de Couy-Sancerre (Programme Géologie profonde de la France) avait pour objectif d'atteindre l'Anomalie magnétique du Bassin Parisien. La série sédimentaire entièrement carottée, complétée par un jeu complet de diagraphies modernes (micro-résistivité, sonique, gamma-ray, densité, neutron, PEF, pendagemétrie), est le plus souvent compréhensive, mais présente parfois des condensations. L'analyse séquentielle du forage de Couy a permis de mettre en évidence 17 séquences dans le Jurassique inférieur et une partie du Jurassique moyen. Dans le Jurassique inférieur, les 11 séquences caractérisées (la dernière passant dans le Jurassique moyen) sont très semblables à celles proposées par Haq et ai. (1988) . Pour le Jurassique moyen, les 8 séquences reconnues sont assez délicates à corréler avec celles proposées par Haq et al. (1988) , aussi bien que par Rioult et al. (1991) en Normandie ou par Gonnin et al. (1993) en Poitou. Le forage de Couy, par ses données lithostratigraphiques et biostratigraphiques, ainsi que par ses enregistrements diagraphiques bien calés, constitue un sondage de référence pour la stratigraphie du Bassin Parisien.
The Couy-Sancerre borehole (Deep Geology Program of France) had the target of reaching the Paris Basin magnetic anomaly. It was located in the eastern part of the Biturige subblock, more precisely between the Armorican Block and the Burgondy Block, on the edge of the Loire Trough, on the site of the Mesozoic Marly Trough. It was located on a small intermediate horst (Fig. 1). The sedimentary series has been entirely cored, and this has been completed by a complete set of modern well logs (microresistivity, sonic, gamma ray, formation density, compensated neutron, photoelectric effect, dipmetering). This series is often comprehensive, but sometimes shows condensations. Petrographic and mineralogic studies (Delavenna et al. , 1989; Mélières, 1991) and the determining of numerous ammonites (Lorenz et al. , 1987) make this borehole a first-rate stratigraphic reference concerning the Lower and Middle Jurassic series in the southern part of the Paris Basin. The lithostratigraphic analysis has already been the subject of a publication (Lorenz et al. , 1992), and a description of the sequences has been sketched out (Gely and Lorenz, 1991). At the same time, the implementation of lithostratigraphic data and the description of the well logs provide greater accuracy in interpretation in terms of desposit sequences, while the stratigraphic calibration of the well-log signatures gives a reference on the scale of the Paris Basin. The sequences defined in the Couy borehole are compared to the ones already published elsewhere on a global scale (Haq et al. , 1988; Vail et al. , 1987) and on a regional scale (Rioult et al. , 1991; Gonnin et al. , 1992, 1993; Bessereau and Guillocheau, 1994, (Figs. 2 and 3). Discontinuities corresponding to a sedimentation gap are usually represented by traces of bioturbation or by perforations in the top surface of a bed or else by a surface of gullying. In other cases, the sequence boundaries do not seem to be so clearly expressed but correspond to highly bioturbated bands, a single limestone bed in the midst of marls or perhaps an abrupt lithological change. For a limestone bed situated at the upper boundary of a sequence, we can see that this latter is often perforated or bioturbated. This enables them to be distinguished from other limestone levels situated at the transgressive maximum, and which contain pelagic fossils or authigenic minerals. On well logs, these two types of beds often have a comparable signature. The sequence boundaries are clearly shown by the well logs, which show clearcut curve breaks and are often capable of orienting or confirming the choice of identification criteria of the boundaries in the core samples. However, it can be seen that there is no direct relationships between the visible size of the discontinuity and the existence of an appreciable biostratigraphic gap. The Domerian-Toarcian boundary does not show any biozone gap, but it is particulary well expressed by gullying. Whereas the very large gap in the Lower Bajocian is not marked by any appreciable discontinuity, but only by a perforated surface, the gap nonetheless corresponds to four biozones. From the absence of erosion traces of a subaerial nature, we can deduce that the drop in sealevel at the end of the sequences was less than subsidence (Type 2 discontinuity) in the area around the Couy borehole. Located between the Biturige subblock and the Loire Trough, the Couy borehole, as the result of the evolution of local tectonic subsidence, reveals either carbonate facies of the Berry platform (sequence of dominantly carbonate deposits) or thick clayey facies of the Marly Trough (sequence with dominantly clayey deposits). In these sequences, maximum transgression corresponds to the minimum values of the gamma-ray log in the basin series and to maximum values on the platform domains (Lorenz and Gely, 1994). Hence, in the platform domain, the transgressive maximum may be shown by clayey limestones with rare ammonites, indicating a maximum value of the gamma-ray log. However, in other places this must not be confused with a comparable well-log signature provided on the platform by lagoonal marls situated at the top of the sequences. In the basin domain, the transgressive maximum is linked to the maximum distancing of continental areas from a fixed point, thus having a good chance of involving a slowing down of sandy and clayey terrigenous influxes. For the other components of the sediment, the slowing down of clayey sedimentation causes a decrease in dilution. Enrichments in phosphate appear (Bifrons and Bomfordi subzones, Opalinum, Niortense and Garantiana zones), in glauconite (Hodsoni zone), in ferruginous ooliths (Niortense and Garantiana zones, and above in the region, the Macrocephalus, Lamberti and Cordaturn zones). The influence of regional subsidence is superimposed on the successions of deposits, linked to sealevel changes. These two factors control the accomodation and the hydrodynamic conditions of the basin. Tectonic activity can obliterate the role of eustatism to the extent of preventing any deposit, and entire sedimentary sequences may then not be expressed locally, as is the case for the Humphriesianum zone in the Poitou and Berry regions. On the other hand, a speeding up of the subsidence of the Marty Trough occurred during the highstand interval in the Lotharingian (Sequence 3), causing the deposition of back basin clays, thus situating the maximum paleodepth exceptionally at the top of the sequence underneath a nonerosive discontinuity. A contrast can also be seen between the marly series of the Upper Pliensbachian, such as the Bathonian, and the condensed limestone series of the Lower and Middle Bajocian. The installation of the carbonate platform in the Aalenian-Lower Bajocian (platform with an accumulation of bioclast sands of the keep-up carbonate platformtype, Sarg, 1989), with already a first glimpse of the upermost Toarcian (micritic limestone platform with gryphites, of the catch-up carbonate platformtype, according to Sarg, 1989), was probably caused by a great slowing down of subsidence. The same pattern can be found in the regional Callovian with periods of erosion or nondeposition. We are led to highlight three periods of tectonic activity in the Couy area (Lotharingian, Aalenian-Bajocian, Callovian), which do not always correspond to the tendencies described in other paleogeographic domains, for example such as Champagne in the Middle Jurassic (Guillocheau, 1991). A sequential analysis of the Couy borehole has revealed 18 sequences in the Lower Jurassic and part of the Middle Jurassic. In the Lower Jurassic, 11 characterized sequences (the last one transiting into the MiddleJurassic) are similar to the ones proposed by Haq et al. (1988) , which for Western Europe must stem from the relative uniformity of Lower Jurassic deposits, often with clearcut boundaries, with generally abundant ammonites, and with a biostratigraphic scale that has been widely accepted by researchers. For the Middle Jurassic, the 8 recognized sequences are rather difficult to correlate with the ones proposed by Haq et al. (1988) as well as by Rioult et al. (1991) in Normandy, or by Gonnin et al. (1993) in Poitou. In the Middle Jurassic, the shallow carbonate platform facies, with numerous more or less prolonged sedimentation gaps, generally have fewer ammonite fauna than the Lower Jurassic deposits, and their lithological units are less clearcut and vary laterally very quickly. To complicate the analysis, at least in France, this was a period of slight tectonic instability linked to both the opening up of the Atlantic and the rifting period of the future Alpine domain. In distensional conditions this resulted in tiltings of blocks causing local variations in facies and thickness, and even emersions (Lorenz and Pomerol, 1985; Gabilly et al. , 1985). These relatively slight movements disturbthe recording of eustatic variations. The biostratigraphic dating of these in the Middle Jurassic in the Tethysian province and in the Boreal province. The Paris Basin is situated at the fluctuating boundary between these two provinces. Likewise, these two scales have not been definitively accepted by researchers, which sometimes makes the correlations of their results more delicate. In the Middle Jurassic, only a few major breaks reflecting the eustatism can be considered. It is to be feared that the others, even though biostratigraphically well dated, risk, for some time to come, representing local phenomena rather than general event. Because of its lithostratigraphic and biostratigraphic data, as well as its well-calibrated well-log recordings, the Couy borehole is a reference borehole for the stratigraphy of the Paris Basin. It has been possible to recognise third-other deposit sequences described on a global scale and to characterize them by a specific well-logging signature (curve break, ramps and major peaks) used for correlations on a basin scale.
© IFP, 1996