Petrographic Study of the Sandstones of Eze-Aku Formation (Turonian) in Abaomege, Southern Benue Trough, Nigeria.

Abstract

The Turonian Eze-Aku Formation in the study area uncomformably overlies the Albian Asu-River Group and unconformably underlies the Nkporo Group (Campano-Maastrichtian) in Afikpo Synclinorium. The petrographic study of the sandstones in Abaomege was carried out to determine the provenance, paleoclimate and source area tectonics for the sandstone facies. The lithofacies encountered in the area consist of fine to medium grained calcareous sandstones which correlate the sediments of Turonian Amasiri Sandstone and alternating sequence of light grey to dark grey shales, and siltstone. Petrographic analysis of the sandstone suggests that they are mineralogically mature and are essentially quartz arenites. Analysis and interpretation of sedimentary structures, mostly the cross beds, and also the structures of quartz grains indicates that the sandstones were derived primarily from granitic Basement Complex rocks from the Oban Massif, under humid climate. The paleocurrent plots for the sandstones show bimodal distribution pattern, suggesting a shore environment of deposition. The sandstones in Abaomege area are part of the sediments of the Late Cenomanian-Turonian Eze-Aku Formation of the Middle Cretaceous which outcrops at eastern flank of the Abakaliki Anticlinorium, Southern Benue Trough Nigeria. There has not been enough work done to establish the provenance regions, paleoclimatic setting and source area tectonics for the sandstones of Eze-Aku Formation in Abaomege area, southern Benue Nigeria. This study seeks to carry the petrography study and paleocurrent analysis of the sandstone and its implications on the provenance, sediment direction flow and paleoclimatic setting. (1) established an unconformity between the Turonian Eze-Aku Formation and the Campanian - Maastrichtian Nkporo Formation in the Afikpo Synclinorium. (2) reviewed the stratigraphy of the Eastern Nigeria, although some of the original names he adopted have been changed by ((3); (4)) in their later work. According to their report on tectonic activities, which occurred during the Cretaceous period, resulted in a series of folds in the sequence of the Asu River Group, the Eze - Aku Formation and created the Abakaliki Anticlinorium. Regional study of Pre - Santonian sandstones including the sandstones of the study carried out by ((5); (6)); they suggested that the sandstones are petrologically arkosic quartz arenites derived from the granitic/metamorphic basement rocks. An interest in paleocurrent analysis can be noted in the work of (7) which suggested that since cross bedding of any scale has paleocurrent value, all that is required is that its orientation be systematically measured and mapped. Paleocurrent indicators are orientated sedimentary structures interpreted to have been deposited by ancient flows. Cross beds slip face, pebble imbrications, parting lineation, tool marks and groove casts, and ripple crest orientation are all examples of possible paleocurrent indicators (8). Some paleocurrent indicators are unidirectional - that is, their shape provides unique information about the direction of the ancient paleoflow. (9) studied the Amasiri Sandstones in the Abaomege, Akpoha and Afikpo areas and noted that the sediment were derived from east of the study area and were deposited as tidal sand ridges in shallow marine environments. The Eze-Aku Formation outcrops out on both sides of the Asu River Group (Fig. 1). It is composed of calcareous shale, siltstone and thin sandy or shelly limestone as well as calcareous fine to medium grained sandstones (3). The Eze-Aku Formation has been dated to be generally of Lower Turonian age on the basis of diagnostic ammonites and microfauna (3). Cenomanian forams have also been recovered from the formation. It has an estimated thicknesss of 2100m (10). Table 1 shows the stratigraphic position of the Eze-Aku Formation in the southeastern Nigeria. Based on the study, the sandstones were suggested to be quartz arenites and the sediments were derived from the Basement Complex rocks of Oban Massif in a humid climate.