Reconstruction of Fluvial Bars from the Proterozoic Mancheral Quartzite, Pranhita–Godavari Valley, India
- N. D. Smith2 and
- J. Rogers3
Published Online: 17 MAR 2009
Copyright © 1999 The International Association of Sedimentologists
Fluvial Sedimentology VI
How to Cite
Chakraborty, T. (1999) Reconstruction of Fluvial Bars from the Proterozoic Mancheral Quartzite, Pranhita–Godavari Valley, India, in Fluvial Sedimentology VI (eds N. D. Smith and J. Rogers), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304213.ch31
Department of Geosciences, 214 Bessey Hall, University of Nebraska, Lincoln, NE 68588-0340, USA
Cape Town, South Africa
- Published Online: 17 MAR 2009
- Published Print: 7 OCT 1999
Print ISBN: 9780632053544
Online ISBN: 9781444304213
- reconstruction of fluvial bars from Proterozoic Mancheral Quartzite, Pranhita–Godavari Valley, India;
- reconstruction of bars in ancient fluvial deposits - crucial in interpreting channel pattern and palaeohydraulics;
- bars - principal depositional elements within rivers;
- Pranhita–Godavari Valley Basin - one of major Proterozoic basins of India;
- reconstruction of bars in Mancheral quartzite;
- hierarchy of bounding surfaces;
- palaeocurrents and bar types;
- palaeochannel depth;
- model for shallow braided-river bars
Reconstruction of bars in ancient fluvial deposits is crucial in interpreting channel pattern and palaeohydraulics. Excellent exposures of the late Proterozoic alluvial deposits of the Mancheral Quartzite around Ramgundam (18°48′N, 79°25′E) provide an opportunity for such reconstruction. The Mancheral Quartzite is a pebbly, coarse-grained sandstone and conglomerate succession 30–76 m thick. Several braided fluvial facies and an alluvial-plain aeolian facies are well developed in the Mancheral Quartzite at Ramgundam. The planar cross-bedded bar facies overlie major erosional surfaces and is characterized by complex interlayering of planar cross-beds 30–120 cm thick and trough cross-beds 5–15 cm thick.
A number of low-angle, downcurrent-inclined cross-bed bounding surfaces indicate accretion of the bar lee surface. Thickening of the cross-bed sets in a down-palaeoflow direction, the presence of well-developed alternately coarse- and fine-grained foresets and counter-current ripple lamination at the toe of the cross-beds, denote flood-stage accretion of the bedforms into the adjacent pools. Lateral transformation of the larger planar cross-sets into cosets of smaller cross-beds, truncation by smaller trough cross-beds showing flow at high angle to those of the larger planar sets and thin muddy sandstones denote modification of the bar form during falling water stage.
Combined palaeocurrent data from all planar cross-beds show a mean south-westerly orientation, which is assumed to be the local channel direction. Palaeocurrent data from the bar deposits (both planar and trough cross-beds combined) indicate downcurrent, oblique or symmetric accretion of the bars with respect to the local channel direction and are inferred to document lateral or mid-channel braid bar deposits.
The thickness of the bar deposits suggests a shallow depth to the channels (∼2.5 m). Discharge was characterized by rapid and pronounced flow fluctuation. Internal organization of the bars is intermediate in character between topographically differentiated large braid bars and simple linguoid dunes, which commonly characterize many shallow sand-bed braided channels.