Process variations of a Neoproterozoic delta system in response to the influence of mixed-energy systems and sea-level fluctuation: an insight from Simla Group, Lesser Himalaya, India

Deltas generally exhibit morphological and architectural variations due to repercussions of fluvial, wave and tidal currents. The Neoproterozoic Chhaosa Formation, Simla Group, Lesser Himalaya, a shallow-marine, mixed-energy deltaic system represents a unique example of variability of those processes at different scales. Outcrop-based facies analysis was combined with application of sequence stratigraphy in the present study to elucidate signatures of fluvial, tide and wave currents. The proximal part of the delta plain deposits (FA1) displaying fluvial attributes like sand sheets, large and small scale trough cross-beddings, channels, are combined with tidal activity. Delta front (FA2) deposits and prodelta deposits (FA3) essentially record mixed tidal and wave activity like mud drapes, flasers, lenticular, wavy beddings, syneresis cracks, tidal rhythmites, low angle cross-stratification oscillation ripples, hummocky and swaley cross-stratifications. Coastal process classification ternary plots are prepared to identify the interplay of different processes and their control in the deposition of Chhaosa delta system. The Chhaosa Formation has developed during elevated pulse rates of progradation and retrogradation, overlapped with elongated phases of progradation. A comparison between the Chhaosa delta and modern as well as ancient delta system has been made to understand the control of mixed-energy processes on sedimentation patterns throughout the global history.