Alluvial fan to storm-dominated shelf transition in the Mesoproterozoic Singhora Group, Chattisgarh Supergroup, Central India

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Mesoproterozoic siliciclastic strata at the basal part of the Singhora Group, Central India provide an example of a pre-vegetation alluvial fan and braid-delta depositional system. Detailed process-based facies and paleoenvironmental analysis have led to the identification of 15 facies types, grouped under four different facies associations, which record paleoenvironmental settings ranging between continental alluvial fan and distal marine shelf. Two formations investigated are: the Rehtikhol Formation formed in a conglomeratic or coarse arenaceous alluvial fan, braid-plain and delta setting, and the overlying Saraipalli Formation, which preserved the sedimentation record of a storm-infested inner to distal shelf setting. The preponderance of granular and sandy sediment gravity flow products with subordinate rock fall deposits within the alluvial fan system together with a pervasive west–northwesterly paleocurrent is suggestive of a gently sloping beveled source area in the east–southeast. Distal to the inferred fan, a braid-plain system developed with the decrease in the depositional slope. Streams within the braid-plain had widths up to several hundred meters. The braid-plain system at the margin of the marine basin developed a delta with the deposition of wave-reworked fluvial-dominated sediments. The crest trends of wave-generated bedforms within the inferred delta sediments depict the northeast–southwest shoreline interpreted for the Singhora Sea. The prodeltaic basinal part of the Singhora braid-delta was storm infested. The inner shelf acted as a zone of throughput with high-velocity storm flows eroding deep narrow scours (gutter casts) and is interpreted as a ‘zone of bypass’. Operation of geostrophic currents on the Singhora shelf has been inferred from the directional relationship of gutters with the inferred orientation of the shoreline. A proximal–distal relationship has been established within the shelf products based on the relative dominance of erosional and depositional features. The Singhora alluvial fan and braid-delta facies are arranged in a fining-upward sequence. Denudation of the source area, increasing bathymetry with relative rise in sea level, or a combination of both is inferred. Both extrabasinal and intrabasinal forcings possibly contributed to the deepening-upward trend.