Abstract:
Striking cyclic interbeds of laminated dark-olive to black marlstone and bioturbated white to light-gray limestone of Neocomian age have been recovered at Deep Sea Drilling Project (DSDP) and Ocean Drilling Project (ODP) sites in the North Atlantic. These Neocomian sequences are equivalent to the Maiolica Formation that outcrops in the Tethyan regions of the Mediterranean and to thick limestone sequences of the Vocontian Trough of France. This lithologic unit marks the widespread deposition of biogenic carbonate over much of the North Atlantic and Tethyan seafloor during a time of overall low sealevel and a deep carbonate compensation depth. The dark clay-rich interbeds typically are rich in organic carbon (OC) with up to 5.5% OC in sequences in the eastern North Atlantic. Sequences in the eastern North Atlantic, particularly off northwest Africa, contain more abundant and better preserved hydrogen-rich, algal organic matter (type II kerogen) relative to the western North Atlantic, probably in response to coastal upwelling induced by an eastern boundary current in the young North Atlantic Ocean. The more abundant algal organic matter in sequences in the eastern North Atlantic is also expressed in the isotopic composition of the carbon in that organic matter. In contrast, organic matter in Neocomian sequences in the western North Atlantic along the continental margin of North America has geochemical and optical characteristics of herbaceous, woody, hydrogen-poor, humic, type III kerogen. The inorganic geochemical characteristics of the dark clay-rich interbeds in Neocomian sequences of North America suggest that most of the material was derived from the North American continent during relatively wet intervals. Inorganic geochemical characteristics of the clastic material in the white, carbonate-rich interbeds off North America suggest that they contain a mixture of clastic material from North American and less-weathered African sources, perhaps by wind-transported material during dry intervals.