Abstract
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.
Highlights
The Benue Trough is defined as an Intracontinental Cretaceous basin about 1000 km in length stretching in NE-SW direction and resting unconformably upon the Precambrian Basement (Benkhelil et al, 1989). Offodile, (1989) stated that geologically, the Benue Trough consists of a linear stretch of sedimentary basin running from about the present confluence of the River Niger and Benue to the northeast and bounded roughly by the Basement Complex areas north and south of the Benue River (Figure 1).www.ccsenet.org/esrEarth Science Research (a)Vol 2, No 1; 2013 ° N 14 W E S SB (b) CNB
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
High energy conditions are considered to have prevailed dominantly during the deposition of the basal sand units of the Awi Formation but abruptly dropped during the deposition of the upper sand units. This energy drop is visible in the log motif of the Awi Formation which shows a fining-upward sequence (Figure 4).The drop in the velocity of the transporting medium stabilized during the deposition of the Amaseri Sandstone and at the lower units of the Agbani Sandstone, further upwards on the stratigraphic sequence of the Agbani Sandstone, there are intermittent fluctuations between high and moderate energy conditions which is reflected by frequent increase and decrease in grain size as well as the degree of sorting
Summary
The Benue Trough is defined as an Intracontinental Cretaceous basin about 1000 km in length stretching in NE-SW direction and resting unconformably upon the Precambrian Basement (Benkhelil et al, 1989). Offodile, (1989) stated that geologically, the Benue Trough consists of a linear stretch of sedimentary basin running from about the present confluence of the River Niger and Benue to the northeast and bounded roughly by the Basement Complex areas north and south of the Benue River (Figure 1).
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