Facies and Granulometric Analysis as Proxies for the Paleodepositional Environment of the Imo Formation, Southeastern Nigeria

Abstract

Facies and granulometric analysis were carried out so as to decipher the paleodepositional environment of the Imo Formation in the study area. Results suggest the presence of eleven lithofacies, which are grouped into six lithofacies association, namely; swaly cross-bedded sandstone, laminated shale, sandstone / shale intercalations, bioturbated sandstone, fossiliferous shale and planar crossbedded sandstone facies associations. Results of pebble morphometric indices suggest a high energy (beach) environment for the sandstones of the Imo Formation. Pebble form indices for the formation have coefficient of flatness (F.R. %) =37.342 ± 8.916 (beach), elongation ratio (E.R.) = 0.743 ± 0.138 (torrents, brooks and rivulets), maximum projection sphericity index (M.P.S.I.) = 0.565 ± 0.090 (beach) and oblate - prolate index (OP index) = -2.313 ± 5.72 (beach). Scatter plots of coefficient of flatness versus sphericity and sphericity versus oblate-prolate index suggests that pebbles were formed in a shallow marine high energy (beach) environment. Bivariate plots of sand textural parameters such as graphic skewness versus graphic standard deviation and second moment skewness versus second moment standard deviation also confirm the high energy (beach) origin of sandstones. These results thus suggest a generalized spectrum of marine environments ranging from upper offshore through lower to upper shoreface to the foreshore for the Imo Formation

Keywords: Imo Formation, Lithofacies, Sphericity, Oblate – Prolate Index, Coefficient of Flatness