Physical and Textural Characteristics of Eze-Aku Sandstone at Akpoha and Environs, Afikpo Basin, Nigeria
Lithofacies defined by physical and textural characteristics, lithological contacts and diagenetic features in outcrops were studied to determine the paleo-depositional processes and environments of deposition, and petrographic classification of Eze-Aku sandstone at Akpoha and Evirons. Sedimentary structures identified include hummocky and swaley cross stratifications, decalcification holes, concretions, planar cross beds, inclined parallel bedding in aggradation-progradational sandstone units, parallel laminations to wave ripple laminations and some weathering features. Six lithofacies were identified and they include laminated silty shales/shales, hummocky and swaley cross stratified fine-grained sandstones, inclined parallel bedded medium- to coarse-grained sandstones, calcareous sandstones, cross bedded medium grained sandstone and horizontal to wave-ripple laminated very fine- to fine-grained sandstone. The characteristics and associations of lithofacies show that the sandstones of Eze-Aku Formation in the study area were deposited in storm dominated shallow marine environment. The sandstones are variably indurated and thin sections analysis shows cementation and intense compaction; angular to subrounded, poor to moderately sorted and fine to coarse grains, as well as minerology of grains consisting of monocystalline quartz, feldspar, rock fragments and cement with average percentage compositions of 55-60.5, 23-26.5, 1-5, and 5-8, respectively. The sandstones were therefore classified as immature arkosic sandstones formed from sediments transported by low density fluid and sourced from plutonic rocks. The variable degrees of induration of samples from different outcrops can be attributed to variable level of compaction by overburden load or tectonic compression and possibly mild metamorphism caused by Santonian thermotectonic event.
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