OSTRACODS AND FORAMINIFERA BIOSTRATIGRAPHY AND PALEOENVIRONMENTAL STUDIES OF FIKA SHALE, UPPER BENUE TROUGH, NIGERIA

Abstract

Biostratigraphy and paleoenvironmental studies of Fika Shale were carried out on outcrop samples of Upper Ridge and West Q sections at Ashaka, Upper Benue Trough using foraminifera and ostracods as proxies. Lithological studies showed that the sections consist of fissile grey shales interbedded with light grey limestone. Twenty (20) samples of shale and limestone were obtained from each of the two exposed outcrop sections and were subjected to conventional micropaleontological processing techniques. Five genera of foraminifera and four genera of ostracods were recovered and described. The foraminiferal species recovered include; Ammotium nkalagum, Ammotium nwalium, Ammotium bornum, Ammobaculites coprolithiformis, Ammobaculites spp., Fursenkoina wilcoxensis, Saccammina spp., Placentammina sp., Trochammina sp., Trochammina dutsuna, Ammodiscoides sp., Ammobaculites benuensis, and Heterohelix globulosa. The ostracods species recovered are Brachycythere sp., Bythocypriss sp., Cytherella sp., Ovocytheridea reniformis, Ovocytheridea symmetrica. The paleoenvironment of deposition of the investigated sections of the Fika Shale were interpreted based on the paleoecological parameters; salinity, oxygenation and bathymetry. The occurrence of a predominantly agglutinated foraminifera assemblage, Platycopids and Cytherocopids ostracods suggests hyposaline and dysoxic environment of deposition in the investigated sections. The Planktic/Benthic (P/B) ratios of the foraminiferal assemblages suggest paleowater depth not exceeding 40 metres which suggest inner to middle neritic shallow marine deposition.