BIOSTRATIGRAPHY AND DEPOSITIONAL ENVIRONMENT OF THE OIL SHALE DEPOSIT IN THE ABAKALIKI FOLD BELT, SOUTHEASTERN NIGERIA

Abstract

The oil shale deposit in the Abakaliki anticlinorium has not been demarcated. The present study focused on the age, correlation and depositional environment of the oil shale using abundance, planktonic/benthonic ratio and species diversity of foraminiferal and ostracod assemblages. The result shows that three prominent peaks or biozones namely 1, 2 and 3 were recognized. The 1 biozone showed that Praeglobotrucana and Guembelitria are the dominant species and ranged from Albian to Mid-Cenomanian (96–108 my). The 2 biozone indicated that Hedbergella and Heterohelix are the dominant species, and ranged from Upper Cenomanian to Early Turonian (92–95 my). The 3 biozone showed that Heterohelicids are the dominant species and ranged from Middle Turonian to Coniacian (82–91 my). The 1, 2, and 3 biozones correspond to the deposition of Asu River Group (Abakaliki Shale), Eze-Aku Formation and Awgu Formation respectively. The 2 peak is bimodal and has the highest frequency and this supports the maximum transgression occurring at the Cenomanian-Turonian boundary event (ocean anoxic event). The oil shale is deposited in outer shelf to bathyal environment and ranged from Upper Cenomanian to Early Turonian age and belongs to the Eze-Aku Formation.