Stratigraphy and Paleoenvironment(s) of the Late Cretaceous deposits in Ohafia area, Afikpo Basin, Southeastern Nigeria

Abstract

Lithofacies and biofacies data extracted from three outcropping sections of Nkporo and Mamu Formations’ sediments in Ohafia area were used for paleoenvironmental and sequence stratigraphic interpretations. The five (5) lithofacies distinguished from the sections were Dark-Grey to Black Shale Facies (A), Oolitic Limestone Facies (B), Calcareous Siltstone Facies (C), Sandstone Facies (D) and Heterolithic Facies (E). The attributes of these lithofacies, including lithotype, rootlets, bioturbation, reducing conditions and microfossil contents, were used to discriminate the units into various environments of deposition and paleodepths. The oolithic limestone facies for instance was interpreted as deposits in shallow coastal areas with high turbulence, especially, in warm shallow marine environments. Paleobathymetry of the lithofacies ranged from non-marine through coastal deltaic, shallow inner neritic, inner neritic to middle neritic paleodepths. Micropalaeontological analysis of the sediments revealed a complete absence of planktonic foraminifera while agglutinated benthic assemblages (dominated by Haplophragmoides spp., Textularia spp. and Anomalinoides spp.), calcareous benthic foraminifera (Gavelinella guineana and Planulina nacatochensis), Gastropods, Ostracods, and Arenaceous indeterminate were recovered. The recognition of Bolivina afra-Haplophragmoides talokaense assemblage zone in the Mamu Formation conferred Maastrichtian age to the deposits. The presence of palynological assemblages like the Adallussiella polymorpha were used to assign Late Campanian age for the Nkporo Formation. Sequence stratigraphic analysis using the integration of foraminiferal, paleontological and lithofacies data identified two incomplete depositional sequences. The first sequence identified in the Nkporo Formation, comprised a Highstand systems tract (HST) bounded at the top by a sequence boundary (SB) dated 75 Ma. The SB also marked the beginning of the second depositional sequence that occurred in the successions of the Mamu Formation and comprised of Transgressive systems tract (TST), bounded at the top by a maximum flooding surface (MFS) dated 69.8 Ma. The results of this study have indicated the presence marker beds and constrained stratigraphic surfaces which can be used to map the identified play elements (source, reservoirs, traps and seals) in the study area. The surfaces are thus essential ingredients in basin-wide correlations for improved hydrocarbon exploration and development in the Afikpo and other adjoining inland basins of Nigeria.