Terrestrial influence on the late Cretaceous Tethyan-Atlantic Epeiric Sea in the Yola Basin, Northern Benue Trough, Nigeria

Abstract

A multi-proxy investigation was carried out on three outcrop sections from the Upper 
Cretaceous Numanha Formation extended in the Yola Basin of the Northern Benue 
Trough, Nigeria. The Numanha Formation is a predominantly shale sequence that pre
serves the Turonian–Coniacian depositional history of the Atlantic and shallow southern 
Tethyan seaway. The investigation included palynofacies characterization of the sedimen
tary organic matter content of 61 samples collected from three sections, NL10, NL8, and 
NL4, in addition to analyses for the total organic carbon (TOC) and carbon:nitrogen (C/N) 
ratios in 44 samples from all of the three studied sections. Palynofacies data confirms 
an overall marginal marine (estuarine) depositional setting for the Numanha Formation 
which was strongly impacted by terrestrial and freshwater sources. This is based on the 
following observations: 1) near-constant presence of marine dinoflagellate cysts and/or 
benthic planispiral microforaminiferal inner linings, 2) strong terrestrial contribution of 
transported vascular plant fragments such as opaque, structured, and degraded phyto
clasts with fluxes of cuticles at certain intervals, 3) near-constant presence of the fresh
water alga Pediastrum which was also recorded in high proportions at certain intervals. 
TOC content for the majority of the analyzed samples was generally low (<1%) indicating 
poor organic matter preservation potential which was probably due to high energy and 
well-oxygenated conditions. C/N ratios were mostly below 20 for the analyzed samples 
from all sections confirming the inferred marine depositional settings. There were, how
ever, two exceptionally high C/N anomalies that were seen in all of the three studied 
sections, but were especially pronounced in the NL4 section, which were also coupled 
with elevated TOC content. These are herein interpreted to represent periods of increased 
terrestrial input, probably as a result of high continental runoff, associated with selective 
diagenetic consumption of nitrogen by microbial degradation.