North Atlantic clay sedimentation and paleoenvironment since the late Jurassic

Abstract

The large abundance of clay particles in ancient marine sediments raises the question of their origin. Detailed mineralogical, chemical and lithological investigations on numerous DSDP Mesozoic and Cenozoic sediments of the North Atlantic margins point to insignificant contribution of au~ tochthonous sources compared to continental detrital supply. Thus the clay sediments strongly reflect the continental environmental variations in the geological past, rather than the present marine environment. Selected examples show how mineralogical changes may contribute to explaining major geographical events: 1) Climate: Cretaceous and Paleogene smectite chiefly inherited from hydromorphic soils of low-relief areas under hot temperature and contrasting humidity; irregular increase of primary minerals in Cenozoic rocks due to a world-wide cooling associated with more regular rainfall. 2) Tectonics: strong supply and rapid accumulation of a mixture of primary and pedogenic minerals in the NW Atlantic during the middle-late Jurassic, related to ocean initiation and continental drift. 3) Physiography: detrital attapulgite (palygorskite) in Albian time, and attapulgite and sepiolite in Early Eocene time, indicators of closed or semi-closed marginal basins, in which fibrous minerals formed under warm and confined conditions. 4) Currents and global tectonics: strong increase of primary minerals in the upper Cretaceous, probably induced by the onset of longitudinal deep-water circulation at the time of the NV! Atlantic widening. A tentative chronological and geographical interpretation of clay mineral variations in· marine deposits is proposed which takes into account the highly variable train of environmental factors.