Biostratigraphie et paléoenvironnements des dépôts crétacés au large d'Abidjan (Golfe de Guinée)

Abstract

The D1-1X well, located 15 km SSW of Abidjan, gives geological information on the offshore Cretaceous sedimentary series of the Ivory Coast Basin. The basal sequence is very thick (over 2000 m of sediments have been drilled without reaching the basement) and coarsely clastic, consisting essentially of sandstones. The upper strata contain some clayey intercalations with planktonic foraminifera (ticinellids and hedbergellids) of late Albian age. The upper sequence (?late Albian-Maastrichtian) is 1020 m thick, sandy and clayey at the base, argillaceous at the top, and contains many unconformities. Biostratigraphic analysis reveals that some of these correspond to important depositional gaps (late Cenomanian, Santonian, Campanian-Maastrichtian boundary, earliest Paleocene). Despite the scarcity and poor preservation of the foraminifera, the study of the assemblages allows 4 units to be distinguished. The lower unit, ?late Albian-Cenomanian in age, is characterized by a planktonic foraminiferal assemblage (Favusella,Hedbergella,PraeglobotruncanaandRotalipora) and rare benthic foraminifera (Cassidella tegulata,Lingulogavelinella modesta). The Turonian-Coniacian unit is essentially characterized byWhiteinellaand a very poor assemblage of benthic foraminifera. The overlying unit, with a benthic foraminiferal assemblage essentially composed of lituolids and buliminids, has been attributed to the Campanian (?). The upper unit contains a foraminiferal assemblage composed of planktonic foraminifera of Maastrichtian age, and benthic foraminifera dominated byOrthokarsteniaandEponides pseudoelevatus. This argillaceous unit is below an earliest Paleocene (eugubinaZone) depositional gap, and is overlain without any visible unconformity by early Paleocene clays (pseudobulloidesZone). Micropaleontological and lithological study of the Cretaceous sequence of the D1-1X well reveals a gradual evolution from the coarse to fine detrital deposits, a deepening bathymetry, and a decrease in the dissolution of the foraminiferal tests from the Albian to the Maastrichtian. This complex sedimentary evolution of the Cretaceous deposits reflects the progressive opening of the South Atlantic and the proximity of active transform continental margins.