Sedimentological charateristics of the Ajali and Benin formations around Arochukwu–Odoro Ikpe Axis southeast Nigeria

Abstract

The crop-outs of Ajali and Benin Formations within the Arochukwu – Odoro Ikpe Axis Southeast Nigeria, was studied for sedimentological characteristics. Standard methods applied in fieldwork and sedimentological studies were used for this study. The results of the particle size distribution of the Ajali Formation is as follows grain size (0.73- 2.23, mean 1.38); sorting (0.9 - 2.7, mean 1.29); Skewnes (-0.06 - 0.67, mean 0.03); kurtosis (1.19 - 1.39, mean 1.27); grain size distribution – Unimodal to Polymodal. Benin Formation, grain size (-1.92 to 1.16, mean -0.35); sorting (0.60 to 2.24, mean 1.45); Skewnes (-0.22 to 0.76, mean 0.19); kurtosis (0.60 to 1.80) and grain size distribution is Unimodal to Polymodal. Both formations are coarse to very fine and pebbly (mean median grain size), poorly sorted, negatively skewed to strongly finely skewed (mean nearly symmetrical) and leptokurtic Also both formations are very poorly to poorly sorted, have similar mode of transportation which generated these sediments at the time of deposition, which seems to be traction and rolling – typical of fluvial setting. Petrographic analysis of the Ajali and Benin Formations show that quartz is the dominant mineral in both formations (75% - 90%), followed by rock fragments (5% - 25%) and feldspars (0% - 5%). The sediments are interpreted as quartz arenites; this agrees with earlier work on both formations. The dominant sedimentary structures within the Ajali Formation are herringbone cross-stratification, crossbedding, heteroliths and skolithos; bed boundaries are sharp to erosional; with pebble lags. Whereas the dominant sedimentary structures in the Benin Formation are crossbedding and large-scale forest beds; bedding contacts are gradational to erosional with pebble lags. The similarity in sedimentological characteristics of both formations is suggestive of similar origins – the Oban Massif and Obudu Plateau and conditions at the time of deposition. Both formations have similar depositional environments interpreted as fluvial – stream deposits except the upper part of the Ajali Formation which has marine character – tidal.

Keywords: Ajali Formation, Benin Formation, Quartz arenites, sedimentological characteristics