Tide-generated Sedimentary Structures, Lithofacies and Particle Size Distribution: Proxies to the depositional setting of the Ajali Sandstone in the Anambra Basin, Southeastern Nigeria.

摘要

Herringbone cross stratification, mud draped foresets, reactivation surfaces, tidal bundles, flaser beddings, ripple and horizontal laminations were identified on the Ajali Sandstone studied. The biogenic structures occur as dominantly vertical to sub-vertical and u -tubed burrows of the Skolithos ichnofacies commonly Skolithos verticalis and Ophiomorpha nodosa. Occasional Thalassinoides and Teichichnus isp burrows of the Cruziana Ichnofacies also occur. Three lithofacies were identified and these include; the medium to coarse grained cross bedded sandstone lithofacies, Fine grained cross bedded sandstone lithofacies and the mudstone lithofacies. The paleocurrent pattern from cross bed azimuths shows a bimodal – bipolar pattern indicative of tidal process. The grain size frequency histograms indicate both unimodal and bimodal asymmetrical patterns with shifting modes. The cumulative frequency curves suggest traction, saltation and suspension as the mechanisms of the sediment transport. The bivariate plots of the discriminant functions (Y1, Y2 and Y3) for the sands suggest deposition in a shallow agitated marine environment. Intertidal to subtidal shallow marine depositional setting is proposed for the Ajali Sandstone.