Comportement d'une bordure de massifs anciens et cortèges de minéraux argileux: l'exemple de la bordure occidentale du Bassin Anglo-Parisien au Callovo-Oxfordien

Abstract

Detailed sedimentological studies of Callovian to Oxfordian outcrops along the western border of the Anglo-Paris Basin (Maine, Normandy, S.W. England), carried out on a fine biostratigraphical background, showed that this sedimentary succession was controlled by two main factors on the margin of the Armorican and Cornubian massifs: (1) a swift levelling of the border area followed by terrigenous sedimentation (from uppermost Bathonian to Lower Oxfordian); (2) the initiation of a carbonate platform with a transition from terrigenous to carbonate sedimentation at the Lower/Middle Oxfordian boundary; (3) the rapid build-up of the carbonate platform (middle Oxfordian); (4) the burial of corals and the interruption of the platform development, favoured by a sudden change from carbonate to siliciclastic sedimentation occurring in early Upper Oxfordian. The sedimentation record was controlled by two main epirogenic structures: the “ Eperon du Perche” with a positive trend between southern Normandy and northern Maine provinces, and the “ Sillon marneux péri-armoricain”, a narrow area with subsident trend beneath the English Channel floor and between the Armorican and Ardenne blocks. After the study of clay mineral distribution from the base of the Callovian up to the Upper Oxfordian along this border area of ancient massifs, Armorican supplies are distinguished from external ones. Clay mineral supplies from Armorican lands strongly reflected epirogenic changes, physiographic variations, and regional current modifications affecting the future eastern border of the North Atlantic. These clay minerals are illite, kaolinite, more scarcely chlorite and mixed layers. “Primary” materials, such as illite and chlorite, occurred as well as smectites and kaolinite. There is not any appreciable evidence of diagenesis with depth of burial. The clay minerals being dominantly of detrital origin, their diversity was conditioned in part by the evolution of continental pedogenesis. The origin of smectites is more difficult to account for. No sign of volcanic material can be ascertained within the successions under study on the western border, so the most likely origin of these smectites was a terrigenous supply. In Middle Oxfordian times, three clay-mineral assemblages were deposited on the carbonate platform. Dominant smectites, accompanied by kaolinite and illite were deposited near the “Eperon du Perche”. A strong increase of smectites without kaolinite occurred in South Dorset, Pays d'Auge and Belinois (southern Maine). On the remainder of the western border, the composite clay-mineral assemblages contained kaolinite, illite and smectites in subequal concentrations. Similar associations also characterize the three other sedimentary episodes. But in South Dorset and Pays d'Auge, the smectites appeared in upper Lower Oxfordian, only during a remobilization of the Armorican basement. Clay minerals proceeded from weak soil erosion. Their supplies mainly depended on the physiographic evolution of the Armorican massif. The “Eperon du Perche”, favored transport of pedogenetic smectites. These supplies were also controlled by regional marine current along the coast. Differential settling processes occur frequently during off-shore migration of sediments. On the other borders of the Anglo-Paris Basin, detailed studies of mineralogical assemblages are still too scarce. Kaolinite was absent from Berry to Burgundy until the Lower Oxfordian, but was present in the Boulogne and Le Havre regions (until Middle Oxfordian). It disappeared from the Lower Oxfordian in South Dorset. One of the main external supplies may have come from the northeast or the north. Smectites were always present in the Middle and Upper Jurassic clay mineral assemblages of the Anglo-Paris Basin, except on the Pays d'Auge and South-Dorset. They were also wide distributed in Upper Jurassic deposits on the future margins of the North Atlantic. Western supplies of smectites were carried eastwards through the depression called “Sillon marneux péri-armoricain” and deposited in the Anglo-Paris Basin.