Sedimentary Structures and Petrography of Rocks within Akpoha Area of the Afikpo Basin Southeastern Nigeria; Constraints on Provenance

Abstract

Sedimentary structures and petrographic studies of Akpoha area in Afikpo/Abakiliki basin  was undertaken in an attempt to understudy and determining constraints  in depositional processes in the area . The major outcrops in the area are alternating succession of sandstones and shales striking in a NE-SW direction primarily as a result of folding due to past orogeny in the area. The sandstone outcrops at Akpoha are highly indurated due to age, compaction and tectonism and occur in ridges within the shale unit. The studied shale outcrops were poorly laminated and low in fossil content.Sedimentary structures identified include;Wavy/ripple lamination, Mud cracks, Parallel lamination and Fissility.The results of the petrographic modal analysis from thin section studies show that  the sandstone consists predominantly of quartz (55 - 64%); feldspar (22 - 30%) and rock fragment (1 – 9%). The sandstones were immature, poorly to moderately sorted with angular to sub-angular grain shapes which gives clear indication of high energy of sediment deposition and short transport history from provenance.The abundance of well preserved sedimentary structures  is an indication of prolong calmness of tectonic/orogenic activities in the area. The fissility observed in shale outcrops and the high induration of the Akpoha sandstones was as a result of compaction from overburden pressure before uplift and exposure.