Geochemistry, Textural, and Mineralogical Maturity Indices of Gulma sandstone Member, type section of Gwandu Formation, Kebbi State, Sokoto Basin, Northwestern Nigeria

Abstract

Understanding the sedimentologi-cal processes, compositional evolution, and depositional environment of the Gulma Sandstone is essential for reconstructing its provenance, maturity, and paleoenvir-onmental history. Field mapping and laboratory analyses were conducted on Gulma Sandstone exposed in the Gulma area to elucidate its lithostratigraphy, geochemistry, petrography, mineralogical maturity, and paleoenvironment of deposition. Detailed field investigations involved systematic outcrop description based on colour, grain size, sorting, angularity, sedimentary structures, fossil content, composition, and diagenetic features. Laboratory analyses included Atomic Absorption Spectrometry (AAS) for major and trace element determination, heavy mineral analysis, petrographic examination of thin sections, and sandstone classification using modal point counts of quartz, feldspar, and rock fragments. The lithostratigraphic succession comprises whitish to purplish sandstone units exhibiting well-developed Liesegang ring structures, interpreted as products of chemical precipitation of authigenic minerals under fluctuating pore-fluid concentrations. These sandstones are overlain by claystone beds and capped by ferruginized granustone facies. Geochemical results from six representative sandstone samples show SiO₂ contents ranging from 54.60 to 57.29 wt% and Al₂O₃ from 29.20 to 31.25 wt%, while concentrations of heavy metals, trace elements, and rare earth elements (REEs) are generally low. Fe₂O₃ values are relatively elevated (4.60–6.50 wt%), indicating significant ferromagnesian mineral contributions. CaO concentrations range from 1.60 to 1.75 wt%, suggesting marine depositional influence associated with diagenetic calcite cementation. Other major oxides exhibit inverse relationships with SiO₂, reflecting progressive mineral dissolution with increasing transport distance. Sandstone maturity indices show SiO₂/Al₂O₃ ratios between 1.75 and 1.92, indicating appreciable clay content and limited chemical maturity. Alkali contents (Na₂O + K₂O = 1.60–2.15 wt%, average 1.91 wt%) imply moderate feldspar abundance and a felsic source rock provenance. Elevated Fe₂O₃/K₂O ratios (5.11–6.57) indicate ferruginized and mineralogically unstable sediments. Modal Mineralogical Index (MMI) values (1.11–2.35) place the sandstones within the Q = 50–75% and (F + RF) = 25–50% fields, suggesting texturally immature sediments. However, Log(Fe₂O₃/K₂O) versus Log(SiO₂/Al₂O₃) discrimination plots classify the sandstones predominantly as quartz arenites with subrounded to rounded grains, reflecting textural maturity. ZTR index values ranging from 54.05 to 83.63% further indicate mineralogically submature to mature sediments.