Granulometric, Heavy Mineral and Petrography Investigation of The Textural Properties, Paleoenvironment, and Classification of Wurno Sandstone, Sokoto Basin, Northwestern Nigeria

Abstract

Field outcrop description, granulometry, heavy-mineral, and petrographic investigations were carried out on Wurno Formation sediments to determine the textural characteristics, textural maturity, paleoenvironment of deposition, and transportation history of the sandstone samples. Sieving method in which various sieve sizes were placed on a sieve shaker and agitated for 15 minutes; retained particles were measured and entered into software to generate textural statistical measures. Heavy minerals were separated by the use of bromoform, while the petrographic analysis carried out was determined by the modal counts of quartz, feldspar, and rock fragments to determine the angularity of the grains. Lithostratigraphic succession of Runjin Beau outcrops of the Wurno Formation is characterised by bioturbated, herringbone structure unconsolidated medium sandstone, overlain by coarser light brownish sandstone, and clayey sandstone successively. The clayey sandstone is overlain by an intercalation of variegated friable sandstone and brownish laminated sandstone of different water current regimes. Gradational contact separates the laminated brownish sandstone from the overlying lignitic sandstone of a lagoonal deposit. This is overlain by orange sandstone with clay flasers, angular unconformably overlain by cross-laminated lignitic fine sandstone, capped by orange sandstone and folded ferruginized ironstone. Grain size of the samples varies from 1.37-2.13 (av.1.85), classified into the medium grain size class, except sample SRU2-T defined as fine-grained sediment. Sorting values vary from 0.55-1.035 (av. 0.72) of moderately well-sorted and moderately sorted classes. The grain size and sorting values indicate that the sediments were transported far away from the source and subjected to winnowing under multiple current effects. The skewness types are nearly symmetrical (0.04 to -0.07), negatively skewed (-0.179 to 0.22), and positively skewed (0.14). The Kurtosis are leptokurtic (1.18-1.23) and very leptokurtic (1.52-1.6) of an unimodal source. Bivariate plots indicate beach deposits except for sample SRU2 M2. The multivariate indices of -8.76 to -7.08 are indicative of an intertidal to tidal shelf depositional environment. ZTR indices vary from 47.3-56.4 %, indicative of mineralogically submature sediments. Modal count of quartz, feldspar, and rock fragments indicated two sediment types; quartz arenite, texturally mature and sublitharenite, which is texturally submature.