Petrography and depositional environments of the sandstones in areas around Abuul, in Ushongo, southern Benue trough, Nigeria

Abstract

The study focuses on the petrography and depositional setting of the sandstones in areas around Abuul, Ushongo; lower Benue Trough. The integration of outcrop,  textural and petrographic studies aided the inference of the depositional framework of the sandstones in the area. Indurated sandstone samples were cut into thin  sections for petrographic analysis while a total of 100 pebbles from OAM/1/Abuul were subjected to morphometric analysis. The thin section studies involved the determination of the texture and mineral/framework element compositions of the sandstones through point counting of about 100 constituent grains. The geologic mapping indicates that the sandstones are deposited proximal to the Basement Complex rocks. Thin section results show that the sandstones are coarse-grained and poorly sorted lithic arkoses, predominantly comprising of angular framework elements, with low textural and chemical maturity, possibly deposited in a fluvial setting. The result from pebble morphometry analysis also confirmed that the sands are fluvial. The bivariate plots of the pebble studies indicate a dominating population of fluvial-influenced pebbles in association with a subordinate population of beach influenced pebbles. However, minor grain compression is evident through the  observed sediment compaction, stretched grains and presence of quartz  overgrowths in some of the sandstones. This study has shown that the sandstones in the area are deposited by fluvial dominated processes, with the interaction of beach processes, though to a lesser degree.

Keywords: textural analysis, sandstone petrography, depositional setting