Abstract

Laterally persistent horizons of interstratified chert-limestone-manganese ores within a siliceous micritic limestone are interpreted to have been deposited on a distally steepened deep carbonate ramp in the Proterozoic Penganga Group of the Pranhita-Godavari Valley, South India. The association of intraformational limeclast conglomerate, turbidites, and pelagic micritic limestone indicates deposition 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 cryptocrystalline and microcrystalline quartz, equant megaquartz, and chalcedony. Cryptocrystalline and microcrystalline quartz are most common; they occur mainly as lenses of mosaic quartz and define laminae. Megaquartz, in contrast, occurs as irregular patches, as fenestroids within cryptocrystalline or microcrystalline fabrics, or in complex aggregates and disrupted mosaics. The origin of the cryptocrystalline and microcrystalline quartz by maturation 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 space-centered grain growth of cryptocrystalline crystals, and porphyroid growth resulted in porphyrotopic fabric. The megaquartz formed directly from pore water in continuity with maturation of opal-CT. The disrupted and curdled mosaics attest to high fluid activity within semilithified sediments. The 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 maturation of silica at shallow burial depth. Maturation at shallow burial was favored by very low detrital content of these cherts and a Mg-rich pelagic carbonate depositional geochemical milieu. Estimated sedimentation rates from 2 to 10 mm ky (super -1) closely match the average sedimentation rate from modern pelagic siliceous deposits.

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