Cenomanian–Turonian organic sedimentation in North-West Africa: A comparison between the Tarfaya (Morocco) and Senegal Basins

Abstract

The Cenomanian–Turonian Oceanic Anoxic Event was recognised in North Western Africa in various depositional settings from abyssal areas to continental shelves. To derive information on environmental conditions in these different settings and define a depositional model, a petrographical and geochemical study of the organic matter was performed on sediments from the Tarfaya (Morocco) and Senegal Basins. The results obtained for these two locations were compared to those of previous studies, namely from DSDP wells. Petrographic and geochemical data allow the differentiation of two main organofacies: a shallow depositional facies (continental shelf) is characterised by low total organic carbon (TOC) contents (< 4%). As attested by low hydrogen index (HI) values (100 to 400 mg HC/g TOC), the organic matter (OM) is moderately preserved. Petrographically, this facies is composed of mixed OM with high proportions of reworked vitrinite indicating detrital material influence. The depositional environment is typical of dysoxic conditions (S/C < 0.36) exposed to high mineral inputs and oxygenated water currents. The second organofacies deposited in the deep marine environment (slope and abyssal) shows a high TOC content (> 7%). The predominance of fluorescing amorphous OM combined with high HI values suggests good preservation conditions. The S/C ratio (> 0.36) and abundance of organic-sulphur compounds support this interpretation and indicate a development of anoxic conditions. To explain the organic contrast between both environments a depositional model has been developed which is based on limited water exchange between both depositional settings. The main factor which has determined black shale sedimentation is the restricted water circulation related to the presence of isolated depositional environment during Atlantic Ocean opening. Compared to present upwelling zones, the palaeoproductivity in the studied area was relatively moderate during Cenomanian–Turonian and seems not to be the only determining factor of organic matter accumulation.