LITHO-BIOFACIES CHARACTERISATION OF THE CENOMANIANTURONIAN SEDIMENTS IN THE SOUTHERN BENUE TROUGH, NIGERIA

Abstract

This research on the litho-biofacies characterisation of the Cenomanian-Turonian interval in the southern Benue Trough, is an encompassing approach towards facies relationships of the stages. The aim of this research is to establish the depositional environments and depositional system of the Cenomanian-Turonian successions through integrated lithofacies and micropaleontological analyses. Various attributes of rock units including texture, fossil content, sedimentary structures and architecture, complimented with petrographic analysis were used to identify lithofacies and biofacies. Sample preparation for foraminifera and ostracod as well as palynomorphs followed standard preparation procedures. The sandstones are classified as subarkose, sublitharenite, quartz arenite and lithic arkose. The carbonate microfacies include mixed siliciclastic-carbonate facies, bioclastic packstone, bioclastic oolitic packstone, planktic foraminifera wackestone and ostracod wackestone. The microfacies indicate deposition in low to moderate energy marine environments. A total of 30 foraminifera species identified comprised 10 species of agglutinated tests of Ammobaculites, Ammotium and Haplophragmoides, six (6) calcareous benthic species distributed among six (6) genera and five (5) families; and 14 planktic species, distributed among eight (8) genera and six (6) families. The calcareous benthic group include Cibicidoides, Valvulineria and Praebulimina. The planktic group, characterized by low diversities, are dominated by Heterohelicids and Hedbergella. The benthic assemblages are characterized by low diversity indices (Fisher α diversity range 1.0 -7.0; dominance, range 0.7-1.0), low to high planktic/benthic values ranging from 0.0 -100.0 %, as well as occurrence of only two agglutinated foraminiferal tests morphotypes. A total of nine (9) species of marine ostracods, distributed among six genera and five families were recorded namely, Bythocypris sp., Bairdia malzi, Bairdia illaroensis, Bairdia sp., Cytherella ix aegyptopuntata, Cytherella farafraensis, Xestoleberis sp., Paracosta parakefensis , and Basslerite sp, listed in other of decreasing abundance. Palynological analysis yielded low to moderate diversity of palynomorphs of terrestrial and marine origins. The recorded pollen and spores include: Longapertites marginatus, Longapertites sp., Cyathidites minor, Zlivisporites blanensis and Retidiporites sp. Based on assemblage distribution, environmental preferences of foraminifera and ostracod taxa, diversity indices, planktic diversity, P/B ratios and morphotypes, an inner neritic to upper bathyal depositional settings is deduced. Facies analysis on outcrops and subsurface drill cores allowed recognition of 17 sedimentary facies namely, matrix supported, massive conglomerate (Gmm), clast-supported massive conglomerate (Gcm), massive mudstone/shale or siltstone facies (Fm), laminated mudstone/shale or siltstone facies (Fl), massive sandstone facies (Sm), horizontally stratified sandstone facies (Sh), trough cross bedded sandstone facies (St) and normally graded sandstone facies (Sg). Others include herringbone cross stratified sandstone facies (S-hb), planar cross stratified sandstone facies (Sp), swaley cross stratified sandstone facies (Ssc), parallel to wavy laminated sandstone facies (Slw) and sandstone-shale heterolith facies (Hsf). Remaining ones are bioturbated sandstone facies (Sb), coal facies (C), massive limestone facies (Lm) and rippled cross bedded limestone facies (Lr(w).The facies were grouped into six facies associations (FAs). The FAs are tidally influenced channel deposit (FA1), tidally influenced foreshore to shoreface deposit (FA2), middle to lower shoreface deposit (FA3), offshore deposit (FA4), flood plain/marsh/lagoon deposit (FA5) and fluvial deposit (FA6) with implications of relative sea level changes and some stratal bounding surfaces. The study has led to the identification of two depositional sequences in the Cenomanian to Turonian strata of the southern Benue Trough.