Microfacies and Biostratigraphy of the Limestone-Shale Sequences of Gboko Formation, Middle Benue Trough, Nigeria

Abstract

The drive to ascertain the paleodepositional environments of the inland sedimentary basins of Nigeria, necessitates a detailed biostratigraphy studies of limestone-shale sequences of Gboko Formation; Middle Benue Trough, Nigeria. A total of 26 representative samples were collected and properly packaged from different locations in Tse-kucha, Ukogh and Alonso-biam. Petrographic study, which is a routine carbonate mineralogical observation, textural description, and modal analysis were carried out on the rock slabs and standard thin section of eighteen (18) Limestone samples, under a petrographic microscope.  Biostratigraphic study, which is the study of rock strata using the fossils assemblages contained within them, was carried out on twenty-six (26) samples. The lithological assemblages show sedimentary sequences of shale and limestone, that represents transgressive-regressive cycle with many short lived environments. Petrographic studies which entails routine carbonate mineralogical observation, textural description and modal analysis under petrographic microscope, shows that the limestone as a whole have both mud-supported and grain supported textures with micrite forming over 75% of the bulk. The microfacies recognized include mudstone, bioclastic packstone, intrapelsparite packstone-grainstone, bioclastic grainstone, bioclastic wackestone-packstone, oncolitic grainstone-packstone, intrabiomicrite wackestone-packstone, stomatolitic boundstone and bioclastic wackestone microfacies. Foraminifera, algae, ostracods, gastropods, pelecypods and skeletal grains, constitute the major bioclasts, while pellets; oncoids and lithoclasts are the major non-bioclastic components of the microfacies. The systematic biostratigraphic analysis, which entails the identification, classification and description of the microfossils using the binocular microscope, yielded few stratigraphically significant planktonic and benthonic species of foraminifera. Some of the foraminifera species recovered from the sediments include Ammobaculites coprolithiforms, Reophax guineana and Gvellinella species. Ostracods, few echinoids remains and gastropods are the other microfuna recorded. The Integration of interpretations from the lithologic and biostatigraphic data suggests that the sedimentary sequence was deposited in a shallow marine, shelf lagoonal environment.