Cenomanian-Turonian Organic Facies in the Western Mediterranean and Along the Adjacent Atlantic Margin

Wolfgang Kuhnt, J. P. Herbin, J. Thurow, J. Wiedmann

Pre-Cenomanian sediments of the western Mediterranean and adjacent Atlantic margin are characterized by low total organic content (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%. After the Turonian, organic-rich sediments progressively disappeared and were replaced by more oxygenated sediments.

Study methods include considering data from outcrops, DSDP/ODP sites, or petroleum wells. Detailed data from onshore locations allowed the development of high-resolution stratigraphy, analysis of depositional environment, and calculation of sedimentation rates. Analysis of these data indicates Cenomanian-Turonian organic-rich sediments can be observed in a wide range of bathymetric settings. They are widespread in the western Mediterranean and Atlantic and have been especially studied in Italy (Apennines, southern Alps), Tunisia (Bahloul), Algeria, Morocco (Rif Mountains, Atlas Mountains, Tarfaya), Gibraltar arch, Spain (Betics, Bay of Biscay, Galicia margin), Senegal (Cape Verde basin, Casamance), and Nigeria (Benue, Calabar flank).

Four environments have been discriminated: (1) coastal area with dolomites, tempestites, and coquinites; (2) shelf basin with calcareous black shales; (3) continental margin basin with black shales characterized by calcareous background sedimentation; and (4) deep-water basin with black shales characterized by noncalcareous background sedimentation and organic-rich turbidites. Except coastal areas, the organic-rich sediments can be observed in all environments. 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.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.