Abstract: Cylcic Sedimentation in Cretaceous Pelagic Sediments

Timothy D. Herbert, S.L. D'Hondt, J. Park

Rhythmic oscillation in sediment composition are found in many sections of Cretaceous age from different ocean basins. Carbonate cycles are the norm, but variations in redox state, biogenic silica, microplankton composition, clay mineralogy, and trace-fossil content often accompany the carbonate rhythms. By integrating results of ocean drilling and outcrop studies, it is now possible to draw some lessons on the influence of orbital forcing on Cretaceous sedimentation.

In nearly all cases where well-sampled records exist, the influence of the earth's precessional (circa 20 kyr) cycle runs through the fabric of sedimentation. Particularly long histories of precessional forcing come from the South Atlantic (20 m.y. of precessional cycles) and from the Barremian-Cenomanian of Italy (22 m.y. of precessional cyclicity). In some but not all cases, the earth's eccentricity signal is also recorded. Individual anoxic events of the Barremian through Albian interval seem paced by swings in precession, both in Italian pelagic sections, and in Atlantic cores recovered by DSDP. The evidence from geochemical flux estimates suggests that in most cases the black shale deposits originated in conditions of relatively low surface fertility. Carbonate rhythms in the Cam anian-Maastrichtian of the South Atlantic seem to reflect a combination of carbonate dissolution and terrigenous dilution. The cycles occur from sites ranging from 0.5 to 4 km paleodepth, indicating large vertical variations the carbon chemistry of the Late Cretaceous South Atlantic basin.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994