Sedimentology and Calcareous Nannofossil Biostratigraphy of Alaka Well, Niger Delta, Nigeria

Abstract

Calcareous nannofossil biostratigraphy of Alaka well was carried out to identify the biohorizons that exist, ultimately for biozonation, chronostratigraphy and geologic age calibration of the studied interval. The study involved sedimentology analysis, gamma ray description and preparation of two hundred and twelve (212) ditch cuttings, sampled at 9.144m intervals for the study of calcareous nannofossils on smear slides. Sedimentology and gamma ray analyse showed deposition within shallow marine paleoenvironments with alternating minor transgressions in a generally progradational delta setting, accounting for the low calcareous nannofossil occurrences. The calcareous nannofossil assemblages recovered from the slide preparations consist of fourteen (14) species, comprising nine (9) long ranged, non-stratigraphic significant forms and five (5) short ranged stratigraphic significant forms. The biohorizons identified included Base (B) occurrence of Catinaster coalitus at 3578m; Discoaster hamatus at 2874m; Top (T) occurrence of Discoaster hamatus at 2518m and Base absence (Ba) of Reticulofenestra pseudoumbilicus at 2280m. Other biohorizons included Top occurrence (T) of Discoaster bollii at 2435m and Discoaster deflandrei at 3578m respectively. The biohorizons defined boundaries between biozones and showed that the studied interval penetrated four biozones from CNM11 to CNM14 of Backman et al. (2012), NN7 to NN10 of Martini (1971) and CN5b to CN8a of Okada and Bukry (1980). The geologic age of the studied interval ranged between about 11.60 Ma within CNM11/NN7/CN5b zone to about 8.80 Ma within CNM14/NN10/CN8a zone, the standard chronostratigraphy lies within Tortonian stage. Two chronostratigraphic surfaces were mapped in the studied section and comprised one global maximum transgression at 3514m corresponding to 10.5 Ma maximum flooding event, and one localized maximum transgression at 2847m corresponding to 9.5 Ma maximum flooding surface, separated by sea-level lowstand mapped at 3331m and corresponding to 10.35 Ma.