Lithostratigraphy of the Maastrictian Nsukka Formation in the Anambra Basin, S.E Nigeria.

Abstract

A detailed lithostratigraphic study of the Campanian-Maastrichtian formations (Early basal Campanian to Maastrichtian) from a borehole and wells in the Anambra Basin has been undertaken. The lithofacies of the Nsukka Formation comprises shales, sandy clay and sands. The formation was encountered at several wells in the Anambra Basin. The Elf Nigeria Ltd well numbers 447/1/AAR-1, 7/4/AO-1 and 447/6/NM-1 penetrated the entire Campanian-Maastrichtian sequences of the formations. A lectostratotype has been proposed from the lithological differences between the Mamu and Nsukka Formations for the proper definition of the stratigraphic unit. This is to satisfy the code of accepted stratigraphic practice which states that stratotype should be proposed for established formations. It will also help to reduce the general assumption on similarities in the stratigraphic nomenclature of some lithofacies in the Mamu and Nsukka Formations. I. Introduction The Anambra Basin became the site of major deposition following the Santonian folding in the southeastern Nigeria (Fig. 1). Compressional uplift of the Lower Benue Trough succession (Albian to Coniacian) along a NE - SW axis was accompanied by tectonic inversion and downwarping of the Anambra platform. Estimates of total sediments thickness in the Anambra Basin from gravity measurements range from 1000 - 4500 m (Ladipo et al., 1992), out of which between 3000 - 3500 m were deposited during the late Cretaceous (Upper Campanian to Maastrichtian). The aims of this study are to provide a detailed description of the Nsukka Formation and to propose a type section for its elucidation. No type section was designated when the unit was established rather been generalized as similar to the Mamu Formation. A type section is the best means of defining a stratigraphic unit (Hedberg, 1976).