Field and geochemical evidence of cretaceous anoxic events in parts of the Northern Benue Trough, Nigeria

Abstract

Field and geochemical evidences presented in this study report the occurrence of 
Cretaceous anoxic events in the Northern Benue Trough, Nigeria. Five Cretaceous forma
tions (Bima, Yolde, Pindiga, Dukul, and Fika) within the Northern Benue Trough were inves
tigated, and the field evidences reveal the occurrence of dark-colored, finely laminated 
shale units lacking bioturbation, which are mostly associated with coarsening upward 
sequences. This is typical of sediments deposited under transgressive waters with prevail
ing anoxic conditions which were ubiquitous during the Cretaceous. Furthermore, total 
organic carbon values obtained from the geochemical analyses of the dark shales were 
considerably high, ranging from 1.35% as the lowest value to 3.67% as the highest value, 
consistent with concentrations of organic matter preserved due to the anoxic condition 
of deposition, while the sulfur content of the studied samples were found to fall within 
ranges below 1% (the highest being just about 0.6%). Rock-eval pyrolysis data obtained for 
the studied samples suggest kerogen types III and IV, implying organic materials derived from either continental or mixed continental and marine depositional environments. Tmax
 data obtained shows that most of the studied shales are thermally immature, implying a 
negligible to poor hydrocarbon generation potential.