Mechanical analysis of the Lauchapara Formation from the Susang Hills, Sherpur, Bangladesh: Implication for sedimentary environment

Abstract

 Mechanical analysis of the Lauchapara Formation following ASTM (D422-63) and BS 410 standard methods was carried out to depict the mode of sediment transportation as well as depositional process of the formation. This coarse-grained rock unit is outcropped along the lower course of Dumurtala Jhora, Karna Jhora and Jhulgaon Jhora streams in the south-western portion of the Susang Hills. Through detailed field survey, total five lithofacies were identified from two major outcrops of the Lauchapara Formation, namely: Massive sandstone facies (Sm), Parallel laminated sandstone facies (Sh), Trough cross-bedded gravel facies (Gt), Trough crossbedded sandstone facies (St) and Massive mud facies (Fm). Representative samples were also collected and analyzed from these facies. The Sm facies are moderately sorted, fine to strongly fine skewed, platykurtic to leptokurtic, strongly fine to fine sand; St facies are moderately sorted, fine skewed, leptokurtic, fine sand; Sh facies are moderately sorted, strongly fine skewed, leptokurtic to very leptokurtic, fine sand; and Gt facies are moderately sorted, fine skewed, very leptokurtic, coarse sand. Bivariate scatter plots suggest that this formation is composed of moderately sorted, strongly fine skewed, fine to coarse sandstone with occasional sparsely distributed gravels. In some outcrops, laterally extended thick gravel bed was also observed. CM pattern reveals that the Lauchapara Formation might be deposited through rolling to minor suspension, under high energy current of a braided stream. Inferring from the facies association of the Lauchapara Formation and tectonic position of the Susang Hills, it may be suggested that the formation was likely deposited under alluvial fan environment.