Late Miocene–Early Pliocene planktic foraminiferal biostratigraphy of deep offshore southwestern Niger Delta

摘要

The planktic foraminiferal biostratigraphy of the deep offshore southwestern Niger Delta has been studied from threewells (DPW-1, DPW-2 and DPW-3). A total of 418 ditch cutting samples (104 samples from DPW-1, 165 from DPW-2 and149 from DPW-3) taken at 18-m intervals were subjected to standard micropaleontological processing techniques. Intotal, 42, 52 and 40 planktic foraminiferal species were identified from DPW-1, DPW-2 and DPW-3, respectively. Theidentified planktic species allow the delineation of the studied section into four biozones-Globorotalia acostaensis(N16), Globigerinoides obliquus extremus/Globorotalia merotumida (N17), Globorotalia margaritae primitiva (N18) andGloborotalia margaritae evoluta/Globigerina nepenthes (N19) zones, based on the first appearance datums (FADs), lastappearance datums (LADs) of the nominate taxa and associated assemblages. The well sections dated Late Miocene toEarly Pliocene based on the occurrence of certain diagnostic planktic foraminiferal datums such as FADs Globorotaliaacostaensis, Globigerinoides obliquus extremus, Globorotalia merotumida/plesiotumida, Globorotalia margaritae primitivaand the LADs Globigerina nepenthes, Globorotalia margaritae primitiva and Globorotalia margaritae evoluta. The Miocene/Pliocene boundary was placed at the FAD Globorotalia margaritae primitiva. In the Niger Delta, this boundary is commonlyplaced at the FADs Globorotalia margaritae margaritae and Globorotalia crassaformis probably due to nonrecovery ofGloborotalia margaritae primitiva. However, in this studied section of the deep offshore southwestern Niger Delta, FADGloborotalia margaritae primitiva occurs consistently in the three study wells and is therefore considered an appropriatedatum for the placement of the Miocene/Pliocene boundary.