Vertical sequences of Lithofacies in point bar and natural levee deposits, Tons River, Allahabad, U.P., India
Department of Earth & Planetary Sciences, University of Allahabad, Allahabad
The recent sediment of Tons River at Allahabad, Uttar Pradesh is studied for lithofacies variations in vertical sections of sixteen point-bar and two natural levee deposits. Altogether, seven lithofacies, namely Gcs, St, Sp1, Sp2, Sr, Sh and Fm: One gravel, five sand and one mud sub-lithofacies have been identified. The point-bar deposits exhibits lithofacies succession of Gcs, St, Sp2, Sp1, Sr, Sh and Fm; whereas levee deposits exhibit only Sp2, Sh and Fm. The Fm lithofacies, muddy deposits often show desiccation cracks. Gcs and St are characteristic lithofacies of point-bar deposits only. Facies Gcs is a product of deposition from steady traction flows, formed by foreset deposition along an avalanche face at the downstream end of a bedform (gravel transported as bedload sheets). Lithofacies St is formed by the migration of 3D dunes and or infill of minor channels or depressions. The facies Sp is connected with foresets from avalanche faces of advancing 2-D sand dunes during lower flow regime. Sand is transported up by the flank of the bed forms by traction and intermittent suspension and deposited at the crest, where the bed shear stress drops at the point of flow separation. The low angle planar foreset (Sp1) suggests transitional bedforms between high and lower flow regime. The facies Sr is a product of deposition from waning traction flows in the lower flow regime. Lithofacies Sh is the product of deposition of sand from traction flows near the transition phase between lower and upper flow regime. The facies Fm is interpreted as a product of deposition at low energy condition (quiet water) from suspension during waning flow conditions. The vertical successions studied in point-bar and levee deposits are in conformity with the standard models of fluvial deposits. The sediment grain-size distribution showed systematic downstream fining and development of log-normality. The study also showed that the sediment of Tons River was transported by rolling and suspension currents.
Keywords: Lithofacies, point-bar, natural levee, tons river, sand dune, grain-size.
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