Stratigraphie et analyse sédimentologique du passage Oxfordien/Kimméridgien dans le Boulonnais

Abstract

Samples were selected in cored sections of two boreholes drilled through the Oxfordian/Kimmeridgian boundary close to Boulogne-sur-Mer (Boulonnais area, North of France). By the end of the Jurassic, this area was subjected to a mixed carbonate/siliciclastic sedimentation in an epicontinental environment close to emerged land (London-Brabant Massif) (Fig. 1). This study provides additional data on a series of formations not recognized in outcrop since the beginning of the 20th century (Pellat, 1867, 1878, 1880a, b; Rigaux, 1872, 1893, 1901; Pruvost, 1921; Dutertre, 1925). It completes the recent regional revision of the late Bajocian to early Oxfordian formations (Magniez et al., 1984; Vidier et al., 1995; Thierry et al., 1996) and the Kimmeridgian-Tithonian formations exposed in the cliffs between Boulogne-sur-Mer and Cape Gris-Nez (Deconinck et al., 1983; Wignall, 1991; El Albani et al., 1993; Geyssant et al., 1993; Proust et al., 1993, 1995; Wignall et al., 1996). Description of the formations (facies, microfacies) The studied section includes four distinct formations from the base up: a) The "Argiles du Mont des Boucards" Formation (15-30 m thick; Pellat, 1867). It consists of homogeneous claystone to marlstone with occasional mollusk shells. The formation is generally subdivided into two members: the "Argiles à pyrite" Member at the base and the "Argiles à Ostrea subdeltoidea" Member above. The second member commonly contains Ostrea beds. Coral boundstone may be observed locally between the two main members -this facies, the "Calcaire de Brucquedal" Member consists of marlstone with abundant echinoderm debris, brachiopods, shell fragments and calcareous build-ups dominated by microbial crusts (thrombolites). The poorly diversified coral fauna include Thamnasteria (dominant), Stylosmilia, Comoseris, Fungiastraea and Enallhelia genera. The limited palaeogeographic extension of the reefal facies can be mapped (Fig. 6). b) The "Grès de Brunembert" Formation (1-10 m thick; Pellat, 1878). This formation consists of decimetre- to metre-thick alternations of bioclast-rich sandy beds and silty to sandy marl. It is separated from the underlying formation by an erosional surface. c) The "Oolithe d'Hesdin-l'Abbé" Formation (about 10 m thick; Pellat, 1867). It comprises decimetre- to metre-thick beds of bioturbated limestone consisting of poorly-sorted ooliths and pisoliths in a marly matrix. The formation is fairly fossiliferous (pelecypods, brachiopods, echinoderms, bryozoans, foraminifera). Oncolites are also observed. d) The "Caillasses d'Hesdigneul" Formation (5 m thick; Pruvost, 1921). This formation is composed of micritic and compact limestone. Results and discussion The biostratigraphic framework used in the boreholes is based on both palynologic data (dinoflagellate cysts) and published data (ammonites). The ammonite zonation defined by the Groupe français d'étude du Jurassique (1997) for the subboreal realm in the West European basins is used here (Fig. 2). The lowermost part of both boreholes is comprised between the top of Plicatilis Zone and the base of the Cautisnigrae Zone (Glosense/Tenuiserratum ammonite zones in the Boreal ammonite scheme, middle/late Oxfordian). The top of the boreholes belongs to the Cymodoce zone (early Kimmeridgian) (Fig. 7). In addition to facies and microfacies analysis, a high-resolution determination of the clay mineral assemblages and a study of the organic content (Rock-Eval pyrolysis, palynofacies observations) of the well-preserved borehole (SCP8) have been carried out (Fig. 8). Grain-size data were obtained on the insoluble fraction, with the hypothesis that this fraction reflects variations of terrigenous input. The clay assemblages are composed of kaolinite, illite, mixed illite-smectite layers and chlorite. Variations of the kaolinite and illite content are similar and in direct contrast to variations of the smectitic mineral content. A smectite-rich interval occurs in the "Argiles à pyrite" Member and is correlated with a grain-size increase of the insoluble fraction, interpretated as a terrigenous input. Kerogen studies have been essentially limited to the marly or clay intervals. The organic content is generally low (TOC <1%) and dominated by a poorly preserved continental organic matter (Type III and IV). Marine organic matter (dinoflagellates, foraminifera) is more abundant in the "Argiles à Ostrea subdeltoidea" than in the "Argiles à pyrite". A depositional model (Fig. 9) and a sequence analysis of the deposits are proposed (Fig. 8). Comparison between the variation of the clay minerals and the sequence stratigraphy shows that smectite-rich intervals occurred during late highstands and lowstands, whereas illite and kaolinite were dominant during transgressions and early highstands. This unusual relationship, previously observed in the Kimmeridgian-Tithonian deposits of the Boulonnais area (Proust et al., 1995) (Fig. 11), seems to be characteristic of Late Jurassic deposits along the western edge of the so-called London-Brabant Massif. Variations of detrital sources associated with physiographic changes of the coastline and tectonic or climatic control of the sedimentation are discussed briefly to explain this signal.

Key words: Oxfordian, Kimmeridgian, Biostratigraphy, Clay minerals, Organic matter, Sequence stratigraphy, Pas-de-Calais (France), Boulonnais.