Evaluation of Gombe Formation in the area around Kumo part of Gongola arm was carried out with a view to determine its depositional sub environment in the area through detailed geologic mapping, litholog construction and petrographic as well as sieve analyses of the sandstone samples. Detailed geologic mapping in the study was carried out hence the production of the geologic map of the area. Other pertinent geologic information such as color, texture, sedimentary structures, thicknesses of different beds and their vertical and lateral changes were determined. Four samples each were collected and subjected to laboratory analyses (i.e sieve analysis and petrography). Three stream sections were identified and logged in the field and plotted using sed log computer program. The sections include Kandahar stream section where a sequence of sandstones, mudstone and ironstone of Gombe Formation along a stream channel with a total thickness of 5.2 m is recognized; the GRA stream section where strata of Gombe Formation with a thickness of 2.06 m comprising sandstone, mudstone and ferriginized sandstone caping were logged and the Jauro Bose stream section where strata with a total thickness of 2.0 m comprising of sandstones and ferriginized reddish brown sandstone and mudstone with ferriginized sandstone interbeds. The Gombe Formation in the study area is characterized by yellow to brown sandstones with interbedded mudstones, ferriginized sandstones and ironstones. Grain size analysis indicate that the sandstones of Gombe Formation in the study area is fine to very fine grained, moderately well sorted, strongly coarse skewed and leptokurtic to very leptokurtic. Petrographic results indicate that all the analyzed samples contain large percentages of quartz in excess of 65 71 % with greater than 10% matrix and are characterized by angular to subrounded grains of quartz, feldspar and rock fragment and fall within the domain of feldspathic wacke and lithic wacke in the QFL diagram for sandstone classification scheme. Field evidences like interbeds of mudstones within the sandstone beds, flaser beddings, parallel laminations and bioturbations suggest that the sandstones were deposited in a transitional environment influenced by tides. Additional evidence is observed from laboratory analyses results especially sieve analysis that indicate fine to very fine grained sands and moderately well sorting, coupled with the characteristic shapes and the level of maturity obtained from the petrographic results like the large percentage of quartz and the subangular to subrounded grains. Integrating the field evidences and the laboratory results, the depositional sub environment for the Gombe Formation in the study area is interpreted to be intertidal environment of tidal flat of an estuary.
Key words: Gombe Sandstone, Sandstones, Gongola Basin, Northern Benue Trough, Depositional Environment, Estuary
|
