Characterization And Depositional Environments Of Upper Cretaceous Sediments Exposed At Ikpankwu Quarry And Km 75, Port Hacourt- Enugu Road, Okigwe

ABSTRACT

The study is on the characterization and depositional environment of upper Cretaceous sediments exposed at the Ihube-Ikpankwu Axis, Okigwe. The results of field and laboratory studies on selected samples of the study area show that the sediments in the study area are fine to medium grained, with an average mean size of 1.808Φ. The standard deviation (dispersion sorting) values range from 1.38 – 2.48 i.e. poorly to moderately sorted with an average value of 1.805Φ. The studied samples range from Mesokurtic to Leptokurtic, although locally Platykurtic, negatively skewed to near symmetrical probably due to turbulence within a predominantly unidirectional flow. The histogram plots of the cumulative weight percent against the phi scale show that the sandstones in the study area are mostly unimodal, The probability curves for the samples of the studied sediments show that the sandstone units were deposited majorly by suspension, saltation, and traction, which is indicative of marine setting with fluvial incursions at certain periods of the depositional history. The results of bivariate plots for sediments in the study area indicate beach and fluvial deposits, while multivariate results indicate mainly shallow marine and fluvial deposits. A total of nine lithofacies were distinguished from the studied sections. These include: Crossbedded sandstone facies (A), Massive sandstone facies (B), Mudstone facies (C), Bioturbated sandstone facies (D), Trough cross-bedded sandstone facies (E), Laminated shale facies (F), Shale and siltstone facies (G), Shale and mudstone facies (H), Very fine laminated sandstone and siltstone facies (I). Deductions from field relationship and associated sedimentary structures suggest the sediments were deposited in fluvial to shallow marine environment. Biostratigraphic data of studied shale samples from the study area reveal dominance of foraminifera assemblages that belong to Ammobaculites coprolithiformis, Haplophragmoides sahariense, Ammobaculites amabensis, Haplophragmoides talokaense and Ammobaculites texanus. This assemblage was used to assign a Maastrichtian age to the shale unit of Mamu. Palynomorphs study of samples in the study area showed that samples from Mamu Formation at km 75 Enugu-Portharcourt road are characterized by forms such as Leiotriletes adrennis, Retitricolporites annulatus, Lycopodiumsporites sp, Ephedripites sp, Noothofaqidites sp and Hexaporotricolpites. Some organic walled microplanktons such as Dinogymnoids indet was also found. These microspore assemblage gave an age range of Campanian-Maastrichtian for the sediments exposed at Km 75. At the Ikpankwu section, the upper section suspected to be Nsukka Formation comprise of entirely different assemblages such as ongapertites sp, Trifossapollenites sp and Apectodinium sp. The palynomorphs recovered here are mostly pollen. However, Dinogymnoids indet is the only organic walled microplankton recovered. The assemblage confirms a late Maastrichtian-Paleocene age for this formation.