An Integrative Laser Diffraction and Sieving Approach to Characterise Clastic Sediments of River Ero, SW Nigeria

摘要

Field studies and grain size analyses of the River Ero sediments illustrate the transportation, depositional processes, and depositional environment. The river channel sediments are sandy (devoid of fines), while the riverbank deposits are sandstones (thickly-thinly laminated) and mudrocks. Graphic litho-logs of the exposed riverbanks display sequences of sedimentary rocks deposited in two rhythmic cycles (I and II) of upward fining characteristics. The river channel sediments graphic mean size (M-z) ranges from coarse to medium-grained, inclusive graphic standard deviation (sigma(i)) of moderately well sorted to poorly sorted, inclusive graphic skewness (Sk(i)) of strongly coarse skewed to strongly fine skewed, and graphic kurtosis (K-G) that ranges from very platykurtic to very leptokurtic. The Laser granulometry results of the riverbank deposits showed M-z range from very fine sand to coarse silt), sigma(i) range of moderately sorted to poorly sorted, Sk(i) is symmetrical to fine-skewed; and K-G is platykurtic to leptokurtic. The bivariate plots of grain size statistics suggest river processes and deposition under fluvial conditions. The bivariate plots also indicated tractive mode transportation/deposition processes: rolling, graded suspension, and a blend of these for the river channel sediments; and uniform suspension for the riverbank deposits. The sandy and muddy lithofacies analysed are mainly products of suspension tractive mode, in a fluvial environment as flood-basin or riverbank deposits.