Neoproterozoic fluvial succession of Ramgiri Formation, Pranhita-Godavari Valley, India: implications for sheet braided versus deep channelized model of pre-vegetation fluvial system

Tapan Chakraborty ^1* Suchana Taral ²

• ¹ Geological Studies Unit, Indian Statistical Institute, Kolkata, India
• ² Department of Earth Sciences, School of Physical, Chemical and Applied Sciences, Pondicherry University, Pondicherry, Puducherry, India

In recent years, deposits of deep channelised fluvial systems have been documented from some of the pre-Silurian successions, casting doubts about the exclusive sheet braided models of pre-Silurianvegetational fluvial sedimentation. In this study we explore the Neoproterozoic Ramgiri Formation: (i) to understand the surface processes in a pre-vegetation fluvial system, (ii) to examine the existence of sheet-braided or channelised architecture in this succession, and (iii) to understand possible process control that might have resulted in either a sheet braided or channelised architecture in these deposits. This study examines a transect of ~50 km strike length of the outcrop belt, subdividing the succession into ten facies and three facies associations supported by sedimentological logs and paleocurrent data. Facies association I consists of sheet-like units of weakly channelized, stratified pebble-coble conglomerate alternating with trough cross-stratified pebbly sandstone. Facies association II consists of comparatively finer, pebbly, coarse-grained, sheet sandstones comprising small, trough crossstrata and capped by thin mudstones. Both the associations show an abundance of dm-thick, mass flow or hyperconcentrated flow deposits. The regional paleocurrent patterns of these two associations is dominantly to NE, transverse to the basin axis, and are represent proximal (FA I) and distal (FA II) megafan deposits. The facies association III, in contrast, contains >3 m deep channel scours, thick simple and compound bedforms and bars. The overall paleoflow of FA III is towards SE, parallel to the basin axis, and represents deposition from a lower-gradient, perennial, sandy braided river. We infer that the flows were sheet-like on the megafan because of its steeper gradient resulting in flow attenuation and widening on a non-cohesive substrate, whereas, lower-gradient, higher discharge axial drainage of FA III gave rise to deeper, channels bedforms similar to the Phanerozoic fluvial deposit. The geophysical and tectonic data indicate the Godavari Basin developed as a half-graben with an active normal fault on its NE margin and a roll-over anticline on the SW. The Ramgiri megafans developed on the roll-over margin and the adjacent low-gradient basin axial plane was occupied by deeper channels resulting in the simultaneous development of contrasting architectures.