Climbing ripple structure and associated storm-lamination from a Proterozoic carbonate platform succession: Their environmental and petrogenetic significance |
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| Authors: | Asru K Chaudhuri |
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| Institution: | (1) Geological Studies Unit, Indian Statistical Institute, 700 108 Kolkata, India |
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| Abstract: | The Mesoproterozoic Pandikunta Limestone, a shallow water carbonate platform succession in the Pranhita-Godavari Valley, south
India, displays well developed climbing ripple lamination and storm deposited structures, such as HCS, wave ripple-lamination,
combined-flow ripple-lamination and low angle trough cross-stratification. Different types of stratification developed in
calcisiltite with minor amounts of very fine quartz sand and silt. The climbing ripple structures exhibit a complex pattern
of superposition of different types (type A, B and S) within cosets pointing to a fluctuating rate of suspension depositionversus bedform migration, and an unsteady character of the flow. Close association of climbing ripple structures, HCS with anisotropic
geometry, wavy lamination and combined-flow ripple-lamination suggest that the structures were formed by storm generated combined-flow
in a mid-shelf area above the storm wave base. The combined-flow that deposited the climbing ripple structures had a strong
unidirectional flow component of variable magnitude. The climbing ripple structure occurs as a constituent of graded stratified
beds with an ordered vertical sequence of different types of lamination, reflecting flow deceleration and increased rate of
suspension deposition. It is inferred that the beds were deposited from high-density waning flows in the relatively deeper
part of the ancient shelf. The structures indicate that the Pandikunta platform was subjected to open marine circulation and
intense storm activities.
The storm deposited beds, intercalated with beds of lime-mudstone, consist primarily of fine sand and silt size carbonate
particles that were hydrodynamically similar to quartz silt. Detrital carbonate particles are structureless and are of variable
roundness. The particles were generated as primary carbonate clasts in coastal areas by mechanical disintegration of rapidly
lithified beds, stromatolites or laminites, and the finest grade was transported to the offshore areas by storm-generated
currents. |
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| Keywords: | Climbing ripple-lamination hummocky cross-stratification carbonate tempestite combined-flow carbonate silt |
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