Depositional Environments and Petrographic Characteristics of Bida Formation around Share-Pategi, Northern Bida Basin, Nigeria

The Campanian Bida Formation is the oldest stratigraphic unit in the northern Bida Basin, central Nigeria. Facies analysis of the investigated sections indicates three main facies which include conglomerate, sandstone and claystone. The poorly sorted, matrix and grain supported conglomeratic facies which overly unconformably the weathered schists and granites is interpreted as gravity induced debris flow deposit. The massive to crossbedded sandstone facies with unidirectional paleo-flow dominantly to the NNE were deposited in braided fluvial channels. The sequence grades into claystone facies which probably was formed in localized non-marine floodplains.

Results of grain size analysis; standard deviation and skewness which range from 0.65 to 1.69 and -0.02 to 0.96 (average values of 1.29 and 0.11f) respectively suggest that the sandstones are predominantly texturally immature and, thus, imply fluvial origin. Average mean sizes of 0.79f suggest predominance of coarse sands and this suggest that saltation is the most prevalent mode of transportation. Pebble morphology study also indicates mainly fluvial processes for the clasts from the conglomerates and conglomeratic sandstones. Few individual clasts with sphericity and Oblate – Prolate (OP) index values less than 0.66 and -1.5 respectively, however, provide evidence of recycling of some of the clasts from older sediments.

Petrographic study reveals that quartz is the dominant framework grain ranging from 66.3 to 88.5%. Feldspar dominated by plagioclase type constitutes between 10 to 27.7%. The plotted QFR diagrams suggest subarkosic sandstones derived from rifted and uplifted continental basement rocks in a warm humid paleoclimatic setting.