Stratigraphic Architecture of the Oweli Sandstone, South-Eastern Nigeria

Abstract

Stratigraphic analysis and correlation of thirteen measured outcrop sections confirms that the Nkporo Group includes a major sandstone interval (the Oweli Sandstone) that is encased by two distinct mud rock intervals: a basal fossiliferous shale and limestone that correlates with the shallow shelf Nkporo Formation and a coal-bearing mud rock that correlates with the coastal plain Enugu Shale. Lithofacies and environmental analysis reveals that the Oweli Sandstone contains two depositional facies: a basal, fine-medium grained delta front facies and an overlying medium-coarse-grained, tidally-influenced fluvial channel facies. Down-dip sequence stratigraphic correlation of the stratigraphic units and depositional environments reveals a gradational transition from the underlying shallow shelf facies of the Nkporo Formation to the distal delta front facies of the Oweli Sandstone. The contact is defined by the 76Ma maximum flooding surface that records the onset of the Oweli clastic influx into the Campanian Sea. The Oweli delta front facies is truncated at the top by the 75 Ma unconformity surfaces over which the tidally-influenced fluvial channel facies of the sand body accumulated. The contact between the tidally-influenced fluvial facies and the overlying coal-bearing mud rocks of the Enugu Shale is sharp and represented by a transgressive surface of erosion (TSE). Analysis of trends in thickness and facies changes reveals that the Oweli Sandstone exhibits a general proximal-fluviatile and distal-marine character. These new findings should serve as useful information toward the better understanding of the stratigraphic architecture of the Nkporo Group in south-eastern Nigeria.