Compound Dunes in a Tide-dominated Estuary Succession: Example from the Daban Fulani Member of Pindiga Formation, Gongola Sub-basin, Northern Benue Trough, NE Nigeria

Abstract

This work documents criteria for identification of compound dunes from the tidalites of the Daban Fulani Member of Pindiga Formation, Gongola Sub-basin, Northern Benue Trough, Nigeria. It is based on the study of well exposed outcrop analogue along recently constructed Gombe-Kumo road. This definition is important because tidal compound dunes and tidal bars are similar in scale and in 2D cross-section even though they originate by forward-accretion and lateral accretion processes respectively.Field measurements reveal the following characteristics for the compound dune: 1) medium to very coarse grained moderate to well-sorted cross bedded sandstones arranged in clinoforms between which thin mud-draped intervals occur; 2) the clinoforms display tangential oblique geometry with each consisting of fining-up grain size profile, large-scale planar cross bedding at its lower part and small-scale planar and trough cross bedding at its upper part with some of the small-scale cross bedded parts, displaying herringbone cross-bedding; 3) the clinoforms display sharp lower and upper contacts and range in thickness from 10 cm to 40 cm; 4) overall, coarsening and thickening-up of the clinoform sets with corresponding decrease in thickness of mud drapes is characteristic, as a result the clinoforms become amalgamated up-section; 5) the sandstones are generally not bioturbated but the mud intervals display mottled texture with up-section decrease in intensity. Compound dunes and tidal bars are deposited adjacent to each other in tide dominated systems and their peculiarities significantly impact on the prediction of fluid flow properties as well as estimation of hydrocarbon reserve especially in frontier basins.