Significance of a Mesoproterozoic tide-reworked fan delta – tidal flat transition: A transgressive-regressive model

Abstract

Records of tide-reworked fan delta facies models are rare in literature, attributed to their lower abundances due to complex sediment distribution pattern by two exclusive end-member flows, viz., a seaward directed subaerial to subaqueous debris flow, and dominant tidal current system interfering with and reworking the debris flow. Lack of appropriate analogues of tide-reworked fan deltas makes the ancient examples more significant. The analysis of such rare tide-reworked fan deltas from deep-time, e.g., Mesoproterozoic, turns out to be unique due to the gap in clear understanding of the alluvial-fluvial vs marine processes in non-vegetated land parts. The lower part of the Mesoproterozoic Rajgarh Formation, Lalsot Basin, North Delhi Fold Belt, Aravalli Craton in NW India, provides a rare record of a tide-reworked fan delta depositional system. Sedimentary facies analysis reveals eleven lithofacies grouped under three major facies associations, viz., (i) gravel-rich conglomerate facies association, characterized by stacked debris flow deposits forming a subaerial alluvial fan, representing the delta topset component, followed downslope by (ii) pebbly, cross-stratifed sandstone facies association, characterized by reworking of the subaqueously emplaced debris flows by ebb-tidal currents, to produce the mouth bar and the delta front depositional systems, and finally, (iii) sandstone-mudstone heterolithic facies association, characterized by abundant tidal signatures in pebble-free thin-bedded sandstone-mudstone heteroliths with tidal bundles, herringbone cross-strata and wave ripples, characterizing a stable platformal subtidal-intertidal flat depositional system. The lower-middle part of the succession records an aggradational stratal stacking pattern of the subaerial alluvial fan changing to progradational stratal stacking of the laterally accreted debris-rich tide-led clinoforms producing the delta topset – mouth bar – delta front transiton of a tide-reworked fan-delta under successive regressive conditions, representing syn-rift deposition near the basin margin. A retrogradational stratal stacking pattern overlies the fan-delta succession, and signifies a transgression by landward encroaching tidal flat system over the fan-delta system during post-rift phase in a stable platformal setup, where debris supply is ceased. A regressive-transgressive model characterizing the transition from a fan-delta to tidal flat depositional systems portrays fluctuations in the accommodation conditons in response to complex interplay of basin tectonics and relative sea level changes in the Mesoproterozoic riftogenic Lalsot Basin.