P. C. de Graciansky

Deep Drilling Results of Leg 47b (Galicia Bank Area) in the Framework of the Early Evolution of the North Atlantic Ocean

Lithologic and stratigraphic evidence from D.S.D.P. Site 398 (3910 m water depth, 1740m total penetration) and regional seismic reflexion data are placed in the context of the early tectonic evolution of the North Atlantic ocean. The morphology of the western Iberian continental margin is the result of two main tensional episodes dated Permo— Lias and Upper Jurassic - Lower Cretaceous, during which the initial basins between Grand Banks and Iberia were created by subsidence and tilting of continental blocks. A limited oceanic opening had probably occurred in Jurassic time between these two tensional episodes. There was no relative motion during Lower Cretaceous between North America and Iberia. One of the main results is that the 398 drillhole penetrated into the basement structure of a tilted block of the continental margin. Borehole data indicate an Uppermost Aptian age for the end of the Upper Jurassic - Lower Cretaceous tensional episode at the level of the site. The subsequent beginning of sea floor spreading in the Uppermost Aptian is associated with a change of sedimentary facies from graded sequences interbedded with slump beds or debris flows to dark, detritic shales. The continental margin had subsided on a regional scale since this time.

The Cretaceous oceanic events (anoxia and hiatus) within a sequence stratigraphic framework

The authors revision of the stratigraphy and organic geochemistry of the North and South Atlantic DSDP boreholes has demonstrated the large range of occurrences and settings of the Cretaceous black shales during the so-called Oceanic Anoxic Events. No simple model has yet been found to explain the exact timing of the anoxic layers and stratigraphic gaps associated with these three particular events. Sequence stratigraphic analyses of outcrop sections in the Western Interior basin (Black Hills area), in the Paris basin, and in the Southeast basin of France provide data to precisely date the timing of the anoxia and oceanic gaps and to show their relationship to long and short term changes of relative sea level. The Oceanic Anoxic Events correspond to periods of severe starvation events where several depositional sequences, and more particularly their maximum flooding surfaces, are merged. Three strongly condensed stratigraphic periods are recognized in the deep Atlantic ocean: event E1 a during the early Aptian, event E1b during the late Aptian-early Albian, and event E2 from the late Cenomanian to the middle Turonian. They coincide on the adjacent landmass with the overall backstepping of the depositional environments due to the overall rapid rising of relative sea level.