ASPECT OF SEQUENCE STRATIGRAPHY OF EXPOSED SEDIMENTARY ROCKS IN OKPEKPE AREA, WESTERN FLANK OF ANAMBRA BASIN, NIGERIA

Abstract

This study presents the mineralogical, textural characteristics, microfossil content and sequence stratigraphic information of the Ajali and Maastrichtian Mamu Formations, situated at Okpekpe, western flank of Anambra Basin, Southern Nigeria. The intent is to establish their environment and age of deposition. The investigative approach involved field studies and collection of samples from three (3) different outcrop locations, followed by laboratory studies which include; thin section petrography grain-size analysis (GSA), X-Ray Diffraction analysis and plynological analysis. Detail property examination showed that Mamu Formation is characterized by siltstone, sandstone (fine - coarse), clay, shale and heterolithic bed, while the Ajali Sandstone consists mainly of sandstone. Texturally, Ajali Formation is a medium to coarse grained, poorly to moderately sorted, and varied in colour from white to pinkish-brownish, negatively skewed, leptokurtic to platykurtic. Field observation, textural plot and bivariate parameters, skewness vs standard deviation, indicate fluvial environment of deposition. X-Ray Diffraction analysis of the clay mineral content of the shale samples from Mamu Formation, suggests continental through deltaic environment evidenced from the dominance of kaolinite, moderate illite and mix-layer clay minerals. A brackish to fresh water environment is suggested for Mamu Formation, based on the important environmentally significant species of microfossil encountered. Using palynological evidence, Late Campanian-Middle Maastrichtian age was assigned to Mamu Formation on the basis of some age-diagnostic palynomorph such as Longapertites marginatus, Cingulatisporites ornatus, Buttinia andreevi and Retidiporites magdalenensis, Rugulatisporites caperatus and Gleicheniidites senonicus. Sequence stratigraphic elements identified in the outcrop include Lowstand and Transgressivestand and Highstand Systems Tracts