A Tectonic Implication Of The Eruption Of Pyroclastics In Uturu, Southern Benue Trough, Southeast Nigeria

Abstract

The origin of the Benue Trough is linked to rifting associated with the separation of South America from Africa in the Cretaceous times. The vast igneous activities in the trough are thought to either precede the separation of the two continents or post-date Albian sediments deposited in the trough after the separation. Exposures of pyroclastics within flatland in Uturu, east of Okigwe, were studied with a view to determining the implication of the eruption that emplaced the pyroclastics on the tectonic evolution of the Lower Benue Trough. Field expressions show that the pyroclastics erupted parallel to the axial plane of the Abakiliki anticlinorium in NE-SW direction and are spatially associated with shales of the Asu River Group and Nkporo Shale. Mud fragments of diverse sizes (1cm-20cm) and shapes of the host Asu River Group are incorporated in the pyroclastics. The sizes of the fragments of the pyroclastics fall mainly within the range of tuff and lapilli composed of mainly pyroxene, plagioclase, melilite and opaque minerals. The Nkporo Shales form cover unit over the pyroclastics and do not show any effect of the volcanism on them. The stratigraphic relationship between the pyroclastics and the surrounding sedimentary units shows that the pyroclastics erupted after the deposition of Asu River Group shales (mid-Albian) but terminated before the deposition of the Nkporo Shales. This corresponds to period between Cenomanian and Santonian and suggests a link to the tectonic events that post-dated the separation of South America from Africa.

Keywords: Benue Trough, Tectonism, Abakiliki anticlinorium, pyroclastics, lapilli.

Global Journal of Geological Sciences Vol. 6 (2) 2008: pp. 123-128