Geological and Geotechnical Study of the Area North West of Afikpo, South- East, Nigeria

摘要

The study area is part of the Cretaceous sedimentary basins of South Eastern Nigeria as it lies on the south-eastern limb of the Ishiagu-Abakiliki anticlinorium and the Afikpo synclinal. The basin is a precursor to the oil producing Niger Delta basin. It consists of three geologic formations-the first and oldest being the Santonian Uplift. The intrusion resulted in the formation of ridges and sills. Subsequently the intruded formations, especially the shales were baked to form slates in the process loosing most of their natural shale properties. The second formation is Transitional consisting of unconformable shale and sandstone rock units tending to replace the Ndeabor formation, which is missing in the Afikpo area. The third formation in the study area is the Campanian-Maastrichtian Nkporo formation overlying the Santonian uplift sediments.lt has both shale and sandstone members but only one member-The Macgregor Sandstone unit, is found in the study area. Sedimentary features observed in the study area include desiccation cracks, worm burrows, solution cavities, cross lamination, ripple marks and joints. Amaseri sandstone was found a predominance of clay over silt to a ratio of 3% to 2% (the remaining 95% was fine-medium sands), while Junction shale had a predominance of silt over clay to a ratio of 30% to 20%. Based on these, siting of the engineering structures in these areas has to be carefully considered considering its heterolithic geology.