Les dépôts à phosphorites du Crétacé moyen de la plate-forme helvétique: Rôle de l'eustatisme et des criculations océaniques

Abstract

During the middle Cretaceous (late Aptian — middle Cenomanian) silicoclastic shelf deposits overlained the shallow-water Urgonian limestones of the Helvetic domain. In the southern part of this area, both biostratigraphic analysis and sequence stratigraphy give evidence for four depositional sequences within the mid-Cretaceous deposits: S1 (Upper Aptian) starts with transgressive bioclastic deposits containing phosphatic allochems. These are overlained by argillaceous siltstones which pass upwards to calcarenaceous sands. This vertical evolution represents the build-up of a bioclastic and silicoclastic shelf with a shallowing upward trend. The sequence S1 is interrupted by an important unconformity which erodes locally down to the Urgonian substratum. S2 (late Upper Aptian — Middle Albian) and S3(Upper Albian pro parte) arerepresented by thin bioturbated phosphate-rich greensands. During the formation of these two sequences, sedimentation was partly controlled by the morphology of the basal unconformity. For instance, the sequence S2 begins with an allochem-rich sandstone which was deposited only in erosional scours. S4 (late Upper Albian — middle Cenomanian) iscomposed of highly condensed hemipelagic glauconiterich biomicrites which are usually capped by ministromatolites. This last mid-Cretaceous sequence is covered by the pelagic limestones of the Seewen formation (late Cenomanian — Santonian). Except for the sequence S1, all the mid-Cretaceous sequences show low sedimentation rates (5 cm.Y −1 to 1 m.Y −1) which cannot be explained only by eustatic control. The regional distribution of mid-Cretaceous condensed deposits compared to published palaeoceanographic reconstructions suggests that the mid-Cretaceous sedimentation on the North Tethyan Margin may have been controlled by a subsurface oceanic current.