JURASSIC FORMATIONS IN THE COUY BOREHOLE (CHER DEPARTMENT, FRANCE) - THEIR DETAILED SEQUENTIAL-ANALYSIS FROM THE CORE SAMPLE DESCRIPTIONS AND WELL-LOGGING CURVES

The Couy-Sancerre borehole (Deep Geology Program of France) had the ta rget of reaching the Paris Basin magnetic anomaly. It was located in t he eastern part of the Biturige subblock, more precisely between the A rmorican Block and the Burgondy Block, on the edge of the Loire Trough , on the site of the Mesozoic Marry Trough. It was located on a small intermediate horst(fig. 1). The sedimentary series has been entirely c ored, and this has been completed by a complete set of modem well logs (microresistivity, sonic, gamma ray,formation density, compensated ne utron, photoelectric effect, dipmetering). This series is often compre hensive, but sometimes shows condensations. Petrographic and mineralog ic studies (Delavenna et al., 1989; Melieres, 1991) and the determinin g of numerous ammonites (Lorenz et al., 1987) make this borehole a fir st-rate stratigraphic reference concerning the Lower and Middle Jurass ic series in the southern part of the Paris Basin. The lithostratigrap hic 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 lithos tratigraphic 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 bo rehole are compared to the ones already published elsewhere on a globa l 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 Guilloc heau, 1994, (Figs. 2 and 3). Discontinuities corresponding to a sedime ntation gap are usually represented by traces of bioturbation or by pe rforations in the top surface of a bed or else by a surface of gullyin g. In other cases, the sequence boundaries do not seem to be so clearl y expressed but correspond to highly bioturbated bands, a single limes tone bed in the midst of marls or perhaps an abrupt lithological chang e. For a limestone bed situated at the upper boundary of a sequence, w e can see that this latter is often perforated or bioturbated. This en ables them to be distinguished from other limestone levels situated at the transgressive maximum, and which contain pelagic fossils or authi genic minerals. On well logs, these two types of beds often have a com parable signature. The sequence boundaries are clearly shown by the we ll logs, which show clearcut curve breaks and are often capable of ori enting or confirming the choice of identification criteria of the boun daries in the core samples. However, it can be seen that there is no d irect relationships between the visible size of the discontinuity and the existence of an appreciable biostratigraphic gap. The Domerian-Toa rcian boundary does not show any biozone gap, but it is particulary we ll expressed by gullying. Whereas the very large gap in the Lower Bajo cian is not marked by any appreciable discontinuity, but only by a per forated surface, the gap nonetheless corresponds to four biozones. Fro m the absence of erosion traces of a subaerial nature, we can deduce t hat the drop in sealevel at the end of the sequences was less than sub sidence (Type 2 discontinuity) in the area around the Couy borehole. L ocated between the Biturige subblock and the Loire Trough, the Couy bo rehole, as the result of the evolution of local tectonic subsidence, r eveals either carbonate facies of the Berry platform (sequence of domi nantly carbonate deposits) or thick clayey facies of the Marry Trough (sequence with dominantly clayey deposits). In these sequences, maximu m transgression corresponds to the minimum values of the gamma-ray log in the basin series and to maximum values on the platform domains (Lo renz and Gely, 1994). Hence, in the platform domain, the transgressive maximum may be shown by clayey limestones with rare ammonites, indica ting a maximum value of the gamma-ray log. However, in other places th is 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 maxi mum distancing of continental areas from a fixed point, thus having a good chance of involving a slowing down of sandy and clayey terrigenou s 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 oo liths (Niortense and Garantiana zones, and above in the region, the Ma crocephalus, Lamberti and Cordatum zones). The influence of regional s ubsidence is superimposed on the successions of deposits, linked to se alevel changes. These two factors control the accomodation and the hyd rodynamic conditions of the basin. Tectonic activity can obliterate th e role of eustatism to the extent of preventing any deposit, and entir e sedimentary sequences may then not be expressed locally, as is the c ase for the Humphriesianum zone in the Poitou and Berry regions. On th e other hand, a speeding up of the subsidence of the Marry Trough occu rred during the highstand interval in the Lotharingian (Sequence 3), c ausing the deposition of back basin clays, thus situating the maximum paleodepth exceptionally at the top of the sequence underneath a noner osive discontinuity. A contrast can also be seen between the marry ser ies of the Upper Pliensbachian, such as the Bathonian, and the condens ed limestone series of the Lower and Middle Bajocian. The installation of the carbonate platform in the Aalenian-Lower Bajocian (platform wi th an accumulation of bioclast sands of the ''keep-up carbonate platfo rm'' type, Sarg, 1989), with already a first glimpse of the upermost T oarcian (micritic limestone platform with gryphites, of the ''catch-up carbonate platform'' type, according to Sarg, 1989), was probably cau sed by a great slowing down of subsidence. The same pattern can be fou nd in the regional Callovian with periods of erosion or nondeposition. We are led to highlight three periods of tectonic activity in the Cou y area (Lotharingian, Aalenian-Bajocian, Callovian), which do not alwa ys correspond to the tendencies described in other paleogeographic dom ains, for example such as Champagne in the Middle Jurassic (