Mechanisms and biogeochemical implications of Cenomanian/Turonian black shale formation in North Africa:An integrated geochemical, millennial-scale study from theTarfaya-LaAyoune Basin in SW Morocco

Abstract

Cenomanian/Turonian (C/T; ~94 Ma ago) black shale successions from various N African basins, in particular from the Tarfaya-LaAyoune Basin (SW Morocco), have been studied in great detail using data from the field (including gamma-ray resistivity logging), sedimentology and advanced geochemical trace metal, biomarker and stable isotope methods. Deposition of these black shale units in most of the region was restricted to a short time envelope termed the C/T oceanic anoxic event (OAE2). During this short period, a favourable combination of factors existed which led to the development of exceptionally strong oxygen-deficiency in the N African Tethys and in particular in the southern proto-North Atlantic oceans. The C/T black shales in N Africa are laterally discontinuous and their distribution and thickness were controlled by the palaeorelief. The thickest and regionally most extensive C/T organic-rich shales in N Africa occur in Morocco namely in the Atlantic Tarfaya-LaAyoune Basin. The laminated biogenic sediments from this NW African shallow marine basin were deposited with very high sedimentation rates (av. 5-10 cm/ka) enabling the investigation of mid-Cretaceous paleoceanographic events at high temporal resolution with respect to rapid climate change and associated hydrocarbon source-rock formation. The low level of thermal maturity and the high degree of sulphurisation of the organic matter (kerogen) makes these black shale successions suitable for advanced inorganic and organic geochemical investigations.