Facies analysis and ichnology of a prograding river-dominated and wave- influenced deltaic deposit: The Nkporo Formation in the Itigidi-Ediba region of the Afikpo Sub-basin, south-eastern Nigeria

摘要

Integration of the sedimentological and ichnological characteristics of the Campanian-Maastrichtian succession of the Nkporo Formation, Afikpo Sub-basin, particularly in the Itigidi-Ediba area reveals a prograding deposit that is considered to record a river-dominated, wave-influenced and tide affected (Rwt) delta. Facies analysis of the succession indicates six facies associations (FA 1 to 6) suggesting deposition in the shelf, prodelta, distal delta front, proximal delta front, distributary mouth bar and bay-fill sub-environments. These deposits are characterized by abundant carbonaceous detritus; current-, storm-, wave-, and tide-generated sedimentary structures; low to moderate diversity, sporadically distributed trace fossil assemblages dominated by depauperate Cruziana and Skolithos ichnofacies that suggest relatively stressed environmental conditions; coarsening upward of most individual facies and the overall succession, and the presence of soft-sediment deformation structures. Zoophycus ichnofacies occurs as opportunistic burrows in fully marine deposits. The frequent occurrence of Diplocraterion sp. indicates a high rate of sedimentation. These ichnofossils are dominantly dwelling and feeding structures with limited grazing behaviour constructed by suspension and deposit feeders.