Sedimentological Characterization of the Pre-Santonian Siliciclastic Deposits of Lower Benue Trough, Southeastern Nigeria

The study which involves forty-one (41) sandstone samples’ collected from the area delimited by longitudes 7°00'E and 8°30'E and latitudes 5°00'N and 6°30'N, entailed evaluating and characterizing these sediments. Detailed field studies to establish the lithostratigraphy was carried out and fresh samples were subjected to petrographic, granulometric and heavy mineral analyses. Field observations revealed that the sandstones of the Awi Formation are angular to subangular, feldspathic, and generally fines upward and Amaseri are calcareous, indurated and laterally extensive. Agbani Sandstone consists of thick vertical sequences of ferruginized and indurated sandstones. Granulometric study shows medium to coarse-grained (0.78-1.30?), poorly to moderately sorted (0.87-1.19?), coarse to strongly fine skewed (-0.17-0.57?) and very platykurtic to leptokurtic (0.66-1.31?). This indicates mostly high energy condition and this is characteristic of fluvial environments. Petrographic studies for Awi samples indicate that they are texturally immature to submature, subarkosic arenites from a continental block provenance, while Amaseri Sandstones are texturally submature sublitharenites from a recycled orogen provenance. Agbani Sandstones are texturally immature to mature subarkosic arenites, sublitharenites and quartz arenites. Heavy mineral studies revealed zircon, tourmaline, rutile and hornblende to be common to all formations, sillimanite, kyanite, garnet, and staurolite are mostly common to Awi and Amaseri Sandstones while Agbani Sandstone has a dominance apatite with spinel. This suite of minerals suggests an acid igneous rock to typical dynamothermal metamorphic rock sources. The ZTR indices (74.0-94.4%) indicate mineralogically mature sediments. The study established that the pre- Santonian siliciclastic sequences of the Lower Benue Trough are texturally immature to mature and compositionally submature to supermature products of a continental block and recycled orogenic provenance under humid climates and deposited in fluvial to tidally influenced fluvial environments with high to moderate energy conditions humid climates which are deposited under high to moderate energy conditions.