Lithofacies, architectural elements and tectonic provenance of Mio-Pliocene Dupitila Formation in the Cachar Thrust Fold Belt, Northeast India

Abstract

The post-orogenic terrestrial accumulations of Dupitila Formation (Mio-Pliocene) have been studied for the first time in the Cachar Thrust Fold Belt of Northeast India employing facies and architectural element analysis techniques. Six lithocolumns measuring 85 m in thickness were analyzed at the outcrop level with minimum resolutions of 20-30 cm. Seven lithofacies, namely clast-supported horizontally stratified gravel (Gh), matrix-supported planar cross-bedded gravel (Gp), trough cross-bedded sandstone (St), planar cross-bedded sandstone (Sp), horizontally bedded sandstone (Sh), laminated sand-silt-mud (Fl) and massive mud (Fm), were identified. The two lithofacies assemblages, i.e., interbedded gravel-sand and sand-mud associations, characterize the lower and upper divisions of Dupitila Formation, respectively. Identification of five architectural elements, i.e., channel fill complex (CHm), single channel (CHs), gravel bar and bedform (GB), sand bed form (SB) and floodplain fines (FF), was accomplished through lithofacies distribution and arrangement of bounding surfaces. Alternating gravelly and sandy facies succeeded by floodplain fines present an overall thinning and fining upward cycle resembling the gravelly and sandy braided fluvial stream deposits. A semi-humid climatic condition has been envisaged for the formation of lithic to sub-lithic (Q 66 F 9 R 25) Dupitila sandstone. These deposits have suffered early as well as late-stage diagenetic changes. The diversified detritus delivery to the fluviatile Dupitila basin was largely made by a recycled orogen comprising of Shillong massif, eastern Himalaya and the Indo-Burman Range.