Trace fossils from the Nanka Formation, Southeastern Nigeria

Abstract

The Eocene Nanka Formation of the Anambra Basin of southeastern Nigeria is a thick, abundantly cross-stratified, flaser and lenticular bedded unfossiliferous unit; it is composed dominantly of medium to coarse grained quartz sand with shale and claystone at several horizons. Various kinds of burrows, confined to the sand beds, are the only evidence of past organic activity within the otherwise unfossiliferous formation. These burrows have been identified to be of the ichnogenera Ophiomorpha and Skolithos. The former occurs as both isolated and randomly oriented tubes and as dichotomously branching burrows possessing tuberculate ornamentation. The latter is seen as short and narrow vertical to steeply inclined straight to curvilinear burrows. A third unidentified burrrow type occurs as horizontally oriented, unlined tubes probably constructed by sediment ingesting worms. Trace fossils have proved to be reliable environmental indicators. Both Ophiomorpha and Skolithos have been known to occur mainly in a marginal marine environment. Their presence in the Nanka Formation is, therefore, indicative of the deposition of Nanka sands in an intertidal to shallow sublittoral zone. This interpretation, based on trace fossil study, finds support on other evidences independently obtained from the lithostratigraphic and sedimentologic features of the formation. It appears then that the presence of trace fossils in otherwise unfossilliferous strata could provide a reliable basis for environmental diagnosis.