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Hydrodynamics and Depositional Implications of Rhythmic Thickness Variations in the Nanka Formation at Umunnya, South Eastern Nigeria

ABSTRACT

The rhythmic thickness variations in the Nanka Formation exposed in a roadcut at Umunnya were measured and analyzed to confirm (i) that the rocks were deposited under strong tidal influence, and (ii) to determine the specific tidal system that prevailed at the time of sedimentation. Facies analysis reveals three lithologic intervals: sandy, claystone and heterolithic intervals. The basal part of the section consists of well sorted and cross bedded sandstone characterized by thick-thin laminae alternations associated with double mud drapes, reactivation surfaces, Ophiomorpha, Chondrites, and Paleophycus. Paleocurrent analysis reveals a consistently bipolar-bimodal pattern with a dominant northeasterly flow direction. The claystone interval includes two discontinuous, lens-shaped, ferruginized, fine to coarse grained, poorly sorted, unidirectionally cross bedded sandstone beds containing extra-formational clasts. The heterolithic interval consists of rapid alternation of thin beds of grey claystone, siltstone and very fine to fine grained ferruginized sandstone. The gross sedimentary characteristics and facies relationships of the Umunnya section suggest prograding tidal flat sedimentation- the basal sandstone facies accumulated in a tidally influenced channel, the claystone facies association accumulated in a lagoon, while the overlying, sandy heteroliths accumulated in sand flats. Plots of bed thickness against foreset numbers demonstrate the presence of diurnal inequality of the tide. The trends of thick-thin alternations are superimposed upon lower frequency neap/spring cycles, thus indicating that the beds formed under the influence of a semi-diurnal tidal regime.

Keywords: Nanka Formation, Umunnya, rhythmites, tidal sedimentation, diurnal inequality