Fabric development in Proterozoic bedded chert, Penganga Group, Adilabad, India: Sedimentologic implications

Laterally persistent horizons of interstratified chert-limestone-manganese ores within a siliceous micritic limestone are interpreted to have been dep osited on a distally steepened deep carbonate ramp in the Proterozoic Penga nga Group of the Pranhita-Godavari Valley, South India. The association of intraformational limeclast conglomerate, turbidites, and pelagic micritic l imestone indicates deposit ion of the chert bearing horizons at the toe of a distally steepened ramp slope environment. The chert is characterized by a wide variety of fabrics, including cryptocr ystalline and microcrystalline quartz, equant megaquartz, and chalcedony. C ryptocrystalline and microcrystalline quartz are most common; they occur ma inly as lenses of mosaic quartz and define laminae. Megaquartz, in contrast , occurs as irregular patches, as fenestroids within cryptocrystalline or m icrocrystalline fabrics, or in complex aggregates and disrupted mosaics. The origin of the cryptocrystalline and microcrystalline quartz by maturati on parallels the morphological evolution of many deep sea cherts that form by maturation of biogenic opal-A, to quartz chert through an opal CT stage. Within the quartz stage, the microcrystalline fabric formed by pervasive s pace-centered grain growth of cryptocrystalline crystals, and porphyroid gr owth resulted in porphyrotopic fabric. The megaquartz formed directly from pore water in continuity with maturation of opal-CT. The disrupted and curd led mosaics attest to high fluid activity within semilithified sediments. T he variation in fabric types at the microscale suggests fabric evolution at low temperature. The presence of chert clasts in debris-flow conglomerates and development of a plethora of water-escape structures suggest maturatio n of silica at shallow. burial depth, Maturation at shallow burial was favo red by very low detrital content of these cherts and a Mg-rich pelagic carb onate depositional geochemical milieu. Estimated sedimentation rates from 2 to 10 mm ky(-1) closely match the average sedimentation rate from modern p elagic siliceous deposits.