Cyclicity of Cenomanian-Turonian organic-carbon-rich sediments in the Tarfaya Atlantic Coastal Basin (Morocco)

Abstract

Density logs from six completely cored and logged exploration wells across the Cenomanian-Turonian boundary in the Tarfaya Basin (Morocco) reveal cyclic sedimentation patterns of different frequencies. The cyclicity is mainly expressed as fluctuations in organic carbon and pelagic carbonate content. Two large scale cycles are observed, one from the top of theRotalipora cushmaniplanktic foraminiferal biozone to the upper part of theWhiteinella archaeocretaceaZone and one from the top of theW. archaeocretaceaZone to the upper part of theHelvetoglobotruncana helveticaZone. The large scale trends are related to major changes in organic carbon burial and probably reflect 3rd order sea-level fluctuations. Maximum organic carbon burial corresponds to benthos-free, laminated sediments, indicating bottom water anoxia. These periods coincide with sea-level highstands of the Vail-Juignet sea-level chart. Milankovitch-scale cyclicity is present, superimposed on these large scale cycles. The obliquity (39ky in the Cretaceous) signal is the most pronounced cyclicity and allows the duration of theW. archaeocretaceaZone to be estimated as 720ky. Individual obliquity cycles have a pronounced signature in the well logs and can be used as correlation horizons within the entire basin.