PALEO-ENVIRONMENTAL RECONSTRUCTION OF ROCK TYPES FOUND IN OZALLA AND ENVIRONS, NKANU WEST LOCAL GOVERNMENT AREA, ENUGU STATE, SOUTHEASTERN NIGERIA
Keywords:
Agbani Sandstone, Enugu Shale, Mamu Formation, Anambra Basin, Grain Size Distribution Analysis, Depositional Environment, Fluvial, Shallow Marine.Abstract
The study area, Ozalla and environs, is located in Nkanu West Local Government Area of Enugu State, Southeastern Nigeria. It lies between latitudes 06°16ꞌ0ꞌꞌN and 06°21ꞌ0ꞌꞌN and longitudes 07°27ꞌ0ꞌꞌE and 07°32ꞌ0ꞌꞌE, covering an area of approximately 85∙6km2. This study aimed to determine the geology and depositional environments of the units through field relationships and grain size distribution analysis. The field data show that the area is underlain by two lithological units: the shale unit and the sandstone unit. The shale unit comprises dark gray shale that is fissile, laminated, and bedded. The shale consists of ironstone beds separating the shale units. There are fractures and faults seen within this unit, which covers about 70% of the area. The Consolidated Sandstone unit ranges from fine to coarse grain, and in some locations, it shows a coarsening-up sequence. The color ranges from white, purple, reddish, yellowish, and brown, and there are presence of quartz pebbles that are angular-rounded. It covers approximately 30% of the study area. The average dip of the area is 4° with SW as the dip direction. Structures such as bedding, cross-bedding, lamination, fissility, color branding, fractures, faults (normal fault), joints, biogenic structures, and ripple marks were observed in the study area. The anoxic nature of the shale unit shows that it is deposited in a marine setting, probably deep marine setting. Sieve analysis of the sandstone unit shows fine– medium grain sandstone, moderately well sorted, negatively skewed, and platykurtic. In addition, the mode of deposition is a fluvial environment due to the fining down sequence and lack of carbonate cement. The bivariate result reveals a fluvial/continental environment. It also shows beach sand; thus, a shallow marine environment is suggested. Economic materials that can be exploited in the area include: sandstone, ironstone, shale, and clay.
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