keywords: Bima sandstone, grain-size analysis, palaeocurrent, palaeoenvironment, Yola arm
Field studies and laboratory analyses (granulometry and thin-section petrography of light/heavy minerals) were carried out on exposed sections of the Upper Bima Sandstone (B3) to determine their source area(s) and depositional environment. The field studies revealed that the sediments consist of ferruginous and non-ferruginous sandstones which are made up of fine to medium-grained sandy sediments with scattered pebbles and cobbles. The B3 is characterized by mainly planar, cross-bedded, and occasional herring-bone cross-beddings. The result of the grain-size analysis showed that the sediments are made up of medium to coarse-grained, poorly-sorted, fine-skewed and leptokurtic sandstone. The grain-shapes vary from angular to sub-angular suggesting a relatively short distance of transportation. The thin-section indicated that the sandstone averagely consists of 69.5% quartz (total quartz), 16.2% feldspar, 9.2% rock fragment and 5.1% cement. The heavy mineral separation analysis showed that the sandstone is composed of 6% rutile, 24.5% zircon, 4.3% tourmaline, 4.7% kyanite, 2.5% apatite, 1.9% sillimanite and 60.6% opaque minerals. Palaeocurrent analysis indicates the direction of ancient current to be in the SSE-NNW direction. The heavy mineral associations suggest that the ancient sediments of the B3 in the study area were derived from nearby acidic igneous and metamorphic rocks of the Adamawa-Sardauna and Hawal Massifs belonging to the Precambrian Basement Complex of Nigeria. Fluvial to beach environment is proposed for the B3sandstone on the basis of textural characteristics and sedimentary structures.