P. C. de Graciansky, Ecole Nationale Supérieure des Mines de Paris and

• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: North Atlantic
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.544.1411; http://www.deepseadrilling.org/80/volume/dsdp80pt2_38.pdf

Four holes at Sites 549 and 550 located along a transect across the northwestern European continental margin recov-ered calcareous carbonaceous sediments of mid-Cretaceous age. They are part of the more widespread occurrence of North Atlantic black shales. At both sites, alternating dark laminated calcareous siltstone and burrowed light-colored calcareous siltstone were recovered. At Site 549 the sequence is lower Albian to lower middle Albian and is 181 m thick. Terrigenous influx to the site was apparently relatively important because of its shallow paleodepth (500 m) and the proximity of an emerged land mass. At Site 550, which was the deepest site (2000 m paleodepth), biogenic calcareous sediments dominate the 91-m thick sequence of upper Albian to middle Cenomanian strata directly overlying oceanic basalt. A problem is raised by the cyclic character of the deposits. The cycles may have been climatically induced by consequent variations in turnover of bottom waters. Bottom currents seem also to have played a role in their formation. In spite of the similar aspect ç>f the sediments at both sites, the organic geochemistry data document differences be-tween their environments of deposition. At Site 549 the low organic carbon content (< 1%) and the absence of marine organic matter in the dark facies both suggest that the bottom waters were moderately to well oxygenated. In contrast, at Site 550 the preservation of marine organic matter within the dark lithology implies a temporary restriction and oxy-gen depletion in the bottom waters during Cenomanian time. The coeval Cenomanian chalks deposited at the shallower Site 549 were well-oxygenated, pure white sediments. The narrow and complex geometry of the depression located in the oceanic trough, which opened during the Cenomanian, was probably favorable to restricting circulation of deep waters and also to the preservation of middle Cenomanian sediments, which have not been found very frequently in the deep North Atlantic.