International Journal of Science and Technology

International Journal of Science and Technology>> Volume 7, Number 9, September 2017

International Journal of Science and Technology


Sedimentological and Depositional Enviroment of the Mid-Maastritchtian Ajali Sandstone, Anambra Basin, Southern Nigeria

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Author Odumoso, S.E . Oloto, I.N . Omoboriowo, A.O
On Pages 26-33
Volume No. 3
Issue No. 1
Issue Date January, 2013
Publishing Date January, 2013
Keywords Ajali, Sandstone, Anambra, Sedimentology, Enviroment


Abstract

Outcrops of Mid-Maastrichtian strata belonging to the Ajali Sandstone in the Anambra Basin, Southeastern and Southern Nigeria were mapped and logged. Samples were studied and used to determine textural and mineralogical characteristics, chemical composition, and paleoccurrent direction, provenance, palaeoenvironment, palaeogeography and diagenesis so as to develop a depositional history for the rocks in the study area. On the basis of the lithologic and sedimentary characteristics, the rocks have been grouped into three facies from bottom to top; shale unit; cross-bedded, burrowed sandstone unit; and ferruginized sandstone unit. The sandstone units exposed in most part of the study area.. on the basis of Grain size analysis, it reveals that the sandstones are fine-grained to coarse-grained poorly sorted, leptokurtic, positively skewed, burrowed, planar and trough cross-bedded, but exhibit a fining-upwards texture gradient. The characteristics ichnofossils associated with the sandstones include dwelling burrows of Ophiomorpha and Palaeophycus which belong to the Skolithos ichnofacies. The sandstone generally lack both microfauna and macrofossils. On the basis of framework composition, the sandstones are grouped as quartz arenites but few are sublithic arenites. Heavy mineral analysis revealed a provenance of metamorphic and igneous rocks from Nigerian Basement complex and Oban Massif. Palaeocurrent analysis of the cross beds of the sandstones shows bimodal patterns indicating mainly SSW-NNE and as minor E-W directions of flow current as being responsible for their transportation. A continental (fluvio-deltaic) environment is proposed for the sandstones on the basis of grain size analysis, ichnofacies, geochemical and palaeocurrent analysis. A depositional history proposed for the sandstone is that ancient sands were derived from both igneous and metamorphic rocks of the Nigerian Basement Complex by two ancient rivers flowing n both SSW-NNE and E-W directions.

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