Unitary Associations of the Aptian-Albian calcareous nannofossils of the Essaouira-Agadir Basin (NW Africa)

Abstract

Although the Essaouira-Agadir Basin (EAB) in Morocco is one of the most complete successions of the Lower Cretaceous in Northern Africa, recent investigations have indicated major basinal-scale hiatuses, which may hinder interregional correlation. To test the quality of the existing stratigraphic scheme and provide a robust biozonation, nannofossil occurrence data from four sections were processed using the Unitary Association (UA) method. Based on critical evaluation of the singletons, which are all of a very important stratigraphic value, they were included in the analyses. The UA range chart of the studied time interval indicates six distinct segments, which are all comparable to the Standard Tethys Zones. The Aptian/Albian contact is also well defined. The results demonstrated that empirical index nannofossil zones have different quality depending mainly on their vulnerability to dissolution. In addition, some secondary marker species (i.e., their appearance/disappearance is located within a specific nannofossil zone), which exclusively occur in only one UA, can substitute the zonal index species. Although marker nannofossil species are different from section to section due to poor preservation, the UA method still provides a robust stratigraphic scheme at considerable resolution. The Aptian/Albian boundary was marked by a dramatic increase in the extinction rates and decrease in the species richness, whereas the Early Albian interval was characterized by relatively low diversity and high turnover rates.