BIOSTRATIGRAPHY AND PALEOENVIRONMENTS OF CRETACEOUS DEPOSITS OFF THE COAST OF ABIDJAN (GUINEA GULF)

The D1-1X well, located 15 km SSW of Abidjan, gives geological informa tion 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, con sisting essentially of sandstones. The upper strata contain some claye y intercalations with planktonic foraminifera (ticinellids and hedberg ellids) of late Albian age. The upper sequence (late Albian-Maastricht ian) is 1020 m thick, sandy and clayey at the base, argillaceous at th e top, and contains many unconformities. Biostratigraphic analysis rev eals that some of these correspond to important depositional gaps (lat e Cenomanian, Santonian, Campanian-Maastrichtian boundary, earliest Pa leocene). Despite the scarcity and poor preservation of the foraminife ra, the study of the assemblages allows 4 units to be distinguished. T he lower unit, !late Albian-Cenomanian in age, is characterized by a p lanktonic foraminiferal assemblage (Favusella, Hedbergella, Praeglobot runcana and Rotalipora) and rare benthic foraminifera (Cassidella tegu lata, Lingulogavelinella modesta). The Turonian-Coniacian unit is esse ntially characterized by Whiteinella and a very poor assemblage of ben thic foraminifera. The overlying unit, with a benthic foraminiferal as semblage essentially composed of lituolids and buliminids, has been at tributed to the Campanian (?). he upper unit contains a foraminiferal assemblage composed of planktonic foraminifera of Maastrichtian age, a nd benthic foraminifera dominated by Orthokarstenia and Eponides pseud oelevatus. This argillaceous unit is below an earliest Paleocene (eugu bina Zone) depositional gap, and is overlain without any visible uncon formity by early Paleocene clays (pseudobulloides Zone). Micropaleonto logical and lithological study of the Cretaceous sequence of the D1-1X well reveals a gradual evolution from coarse to fine detrital deposit s, a deepening bathymetry, and a decrease in the dissolution of the fo raminiferal tests from the Albian to the Maastrichtian. This complex s edimentary evolution of the Cretaceous deposits reflects the progressi ve opening of the South Atlantic and the proximity of active transform continental margins. (C) 1997 Academic Press Limited.