Depositional facies and ichnology of a tidally influenced coastal plain deposit: the Ogwashi Formation, Niger Delta Basin

Abstract

The Ogwashi Formation is re-interpreted as tidally influenced coastal plain deposit based on the detailed outcrop description of exposures and facies analysis. Facies associations comprise strata interpreted to be the deposits of the following environments: fluvio-estuarine channels, tidal channels, tidal flats, coastal plain channels and coastal floodplain/mire. The fluvio-estuarine channels deposits are characterised by high-energy conglomerate and coarse-grained sandstone that are strongly to weakly bioturbated with trace fossils belonging to the Skolithos and the depauperate Cruziana ichnofacies. The tidal channel deposit contains mud chips and mud lenses (at the base) and includes mud draped planar and trough cross-beds. The tidal flat deposit comprises sandy heterolithic sediments that are intensely burrowed. The coastal plain channel deposits are mud-filled channels, with sand sheets interpreted as splay deposits and variegated sandy claystone which has undergone pedogenic alteration. The floodplain and mire deposits are light grey mudstone and dark grey carbonaceous mudstone with plant remains and lignite. These depositional successions show a change from marine to non-marine conditions, interpreted to be a result of relative sea level changes. The architectural elements of the coastal plain succession are grouped into channelised, sandy cross-stratified and floodplain deposits. The channelised elements occur either as isolated or as multistorey channel deposits. Sequence stratigraphic interpretation reveals one depositional sequence with a sequence boundary and an erosive transgressive surface of erosion identified by the occurrence of the Glossifungites ichnofacies with sharp, erosive contacts.