Fluvial sedimentation of the Proterozoic Alwar Group in the Lalgarh graben, northwestern India

Abstract

The Lalgarh graben, one of the several tectonic elements of the Proterozoic Delhi basin, was bounded on the east and southwest by southerly converging sub-parallel faults, and had a steep, rugged southern terminus with centripetal transport pattern. The depositional slope decreased northwestwards, where the graben opened towards a wide shallow sea. The Dausa uplift and Rajputana craton located to the east and south, respectively, were the main source areas. The graben is characterised by a sedimentary sequence more than 3000 m thick belonging to the Alwar Group, which rests unconformably on the basic volcanics of the Raialo Group. The Alwar sequence includes three WNW-ESE trending and northerly dipping sedimentary facies, showing progressive grain-size reduction upwards and northwards; these are: (1) matrix- and clast-supported boulder conglomerates which were deposited as debris flows and boulder sheets in an intermontane environment; (2) red boulder conglomerates with brownish red sandstone and siltstone interbeds, which were deposited as proximal fans and longitudinal braid bars; (3) interbedded conglomerates and sandstones showing profuse development of tabular and trough cross-bedding. Siltstone layers occur in the upper part of this facies. These rocks are considered to have been deposited by megaripples and transverse bars in the middle and lower reaches of a braided river system. Structures associated with this facies indicate that bar dissection and current deflection occurred during a falling water-level. Development of extensive flood plains was inhibited by frequent lateral shift of the active channels, high discharge, and short length of the river system. The fluvial facies grades into, and is overlain by, sandstones which accumulated on an embayed, high-energy, tide-dominated coast.