Abstract:

Recycled detrital quartz grains are unequivocally identified if there are overgrowths on abraded overgrowths. We argue that the presence of such recycled grains even in small quantities in successions of quartz arenites indicates derivation from one or more sedimentary units from below an intrabasinal unconformity or an extrabasinal unroofing surface, as do detritus from characteristic rock fragments in polymict clastic rocks. In fact, detrital grains with inherited overgrowths are sedimentary rock fragments. We apply this petrographic criterion to evaluate the presumed unconformities that bound the Singhora, Chandarpur, Raipur, and the Kharsiya groups of the essentially Mesoproterozoic Chhattisgarh Supergroup in the Bastar craton – a critical Ur-craton vis à vis Columbia and Rodinia. Optical examination of about 6000 grains in 12 thin sections reveal few recycled quartz grains in sandstone samples of the Chandarpur Group but about 4% recycled quartz grains with inherited abraded overgrowths in a sandstone unit of the Raipur Group. Characteristic rock fragments from a tuffaceous unit at the top of the Raipur Group are found in the basal sandstone of the Kharsyia Group. The data indicate that the presumed unconformities below the Kharsyia Group and the Raipur Group are real but the one above the Singhora Group cannot be confirmed on the basis of recycled grains. Published age spectra of detrital zircons from these sandstones are compatible with the above inferences drawn from detrital-quartz and rock-fragment petrography.

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