Ocean Drilling Program, United States

Mid-Cretaceous organic-rich sediments of mainly Cenomanian and Turonian age were recovered during Ocean Drilling Program (ODP) Leg 207 along a presumed paleowater depth transect on Demerara Rise. The entire sequence shows decimeter- to meter-scale cyclic alternations between carbonate-rich and organic-rich sediments. An interpretation of the cyclic pattern is derived from time series analysis of sediment color data in selected intervals in combination with thin section analysis of representative lithologies. Results are compared to time-equivalent sediments in the Tarfaya Basin, Morocco. At Demerara Rise, there are two main types of carbonate-rich lithologies: pelagic carbonate beds and planktonic foraminiferal packstone layers. Pelagic carbonate-rich intervals are present throughout the two deepwater sites (ODP Sites 1257 and 1258) and in the upper part of the sequence at the shallower sites (Sites 1259-1261). Planktonic foraminiferal packstones are attributed to either current- or wave-induced winnowing in relatively shallow water. Packstones are the dominant carbonate-rich lithology in the lower part of the sequence at Sites 1259-1261 but decrease in frequency higher in the sequence because of continued subsidence; they persist longer at the two shallowest sites (Sites 1259 and 1261) than at the intermediate water depth site (Site 1260). Cyclic variation in lithology at Demerara Rise is inferred to represent eccentricity and precession cycles with, at most, a weak obliquity component. Large variations in the thickness of the inferred precession cycles are attributed to climate-dependent variation in sedimentation rates of carbonate and siliciclastic material in combination with variable degrees of compaction and, within packstone layers, variable rates of removal of sediment because of winnowing. Lithologic cycles within the latest Cenomanian oceanic anoxic event are correlative between the Tarfaya Basin and Demerara Rise. A weak obliquity signal at Demerara Rise, in contrast to the strong obliquity component inferred for the Tarfaya Basin, can be explained if the obliquity signal represents variation in subsurface ventilation. The sequence at Demerara Rise represents continuous anoxia, suggesting that any variation in ventilation would have little impact if all oxygen is consumed before it could reach the area. The strong eccentricity- precession bundles are inferred to represent an atmospheric and/or oceanic circulation signal, which controlled rainfall and siliciclastic sedimentation rates as well as upwelling intensity and surface water productivity. Winnowing of carbonate-rich layers at shallow water depths at Demerara Rise implies that general circulation, and thus any upwelling, was most vigorous during deposition of carbonate-rich levels, which ultimately implies that organic-rich sediments are primarily related to enhanced anoxia.