Cenomanian/Turonian organic-rich sediments are widespread in the western Mediterranean and Atlantic and have been studied in outcrops, DSDP/ODP sites, and petroleum exploration wells. This study includes sections from Italy (Apennines, Southern Alps), Tunisia (Bahloul), Algeria, Morocco (Rif Mountains, Atlas Mountains, Tarfaya), Gibraltar Arch, Spain (Celtiberic Ranges, Betics, Bay of Biscay, Galicia Margin), Senegal (Cape Verde Basin, Casamance), and Nigeria (Benue, Calabar Flank).

Detailed data from thick pelagic sequences (e.g., Tarfaya basin) made it possible to construct a detailed stratigraphic framework using planktonic foraminiferal, radiolarian, and cephalopod zonations. We calculated sedimentation rates and organic-matter-accumulation rates based on this stratigraphic framework. The results are compared to paleogeography and paleobathymetry, in order to recognize variations of middle Cretaceous organic matter accumulation in time and space. The following general trends in organic-matter accumulation were observed. Cenomanian/Turonian organic-rich sediments occur in a wide range of bathymetric settings, except for coastal and shallow-marine shelf areas. Although total organic carbon content (TOC) can be high in the deep sea, highest accumulation rates of organic matter occur in outer-shelf environments of low-latitude areas (e.g., Tarfaya and Senegal shelf basins).

Pre-Cenomanian sediments of the Western Mediterranean and adjacent Atlantic margin are characterized by low TOC with an important terrestrial component. During the Cenomanian, TOC increased, and the marine component became dominant, culminating around the Cenomanian/Turonian boundary with TOC up to 40%. Since the Turonian, organic-rich sediments have progressively declined, being replaced by more oxygenated sediments.

Local subsidence history and tectonic stress led to different stages of maturation of the organic matter. In areas of thick accumulation (especially the shelf environment) and favorable thermal history, deposits around the Cenomanian/Turonian boundary can be of economic interest as petroleum source rocks.

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