CRETACEOUS BIVALVES AND PALAEOENVIRONMENTS OF THE CALABAR FLANK, SE NIGERIA.

Abstract

Bivalves of Albain, Cenomanian, Turonian and Campanian age are documented from the Calabar Flank, SE Nigeria. Benthic megafossils are associated mainly with limestone and marls; thin beds of mudstone, siltstone and shelly layers intercalated within a thick sequence of shales. The latter contain abundant ammonites and planktic foraminifera, while a few of the benthic foraminifera are represented mainly by arenaceous forms. Although the state of preservation of the bivalves is poor and has hindered identification of some specimens to generic and specific levels, it is obvious that diversity is low. Three dimunitive forms are however abundant. Integration of the bivalves data with other fossils as well as ichnological evidence has given a better palaeoenvironmental picture. It is shown that the Calabar Flank was subjected to frequent oceanographic fluctuations during the late lower and the Upper Cretaceous. The Middle-Upper Abian times experienced shallow tropical marine conditions with well aerated bottom while the Cenomanian had long periods of marine oxygen-deficient conditions frequently int`errupted by shorter spells of marginal marine oxic bottomed environments. The Turonian witnessed a more uniform shallow sea with oxygenated bottoms while the Campanian rocks show an inner neritic marine setting with occasional arid brackish water environments.


KEY WORDS: Albian-Campanian bivalves, Palaeoenvironments, Calabar Flank, Nigeria


Global Jnl Geological Sciences. Vol.2(1) 2004: 15-36