ASTRONOMICAL TUNING OF CENOMANIAN SEDIMENTS IN THE ATLANTIC SECTION OF THE TETHYS

Oceanic Anoxic Events (OAE’s) represent major disruptions of the ocean system resulting in massive deposition of organic-rich layers in marine environments. Based on drilling results of Cretaceous marine sediments by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP), it is apparent that special recurrent conditions in the Cretaceous oceans were conducive to the accumulation of large amounts of organic-rich sediments.

A comprehensive review of the drilling data shows that most of the Cretaceous organic-rich sediments occurred in the Tethyan domain, and in the Pacific. Black shales alternate rhythmically with greenish-gray burrow-mottled mudstones in the more expanded sections of the deep bathyal zones of the Tethys. These cyclic patterns of events can also be traced into epicontinental seas where extensive black shales appear to be coeval with the deeper-water facies, as for instance in the North Sea, the northeastern Mexican margins and northern South America.

Several controlling factors have been debated to explain the origin or mechanism of deposition of organic-rich layers, e.g. stagnation of bottom water with expansion of the oxygen minimum zone, increased productivity and rapid sedimentation, tectonovolcanic events, among others. However, these lithologic sequences are usually characterized by distinct repetitive patterns of light and dark colored levels with frequencies comparable to similar well known recurrent variations in Pleistocene deposits that are climatically induced. For instance, proxy spectral analyses in the Cenomanian section at Leg 43, Site 386, located in the present western Atlantic on the Bermuda Rise, were carried out and show different frequencies of cycles indicative of astronomical modes, which overprint each other. The most obvious and commonly recorded tempo in the successions can be related to precession, as exemplified in 386-47-5. If the details of the astronomical incidence curve resemble the details of the proxy curve, the astronomical tuning of the succession can be almost certain as the controlling mechanism of lithofacies variations in Tethyan sediments.