ABSTRACT

The late Proterozoic Penganga Group, Pranhita-Godavari valley, South India, is a sequence of siliciclastic-carbonate sediments that was deposited in an intracratonic rift basin. The nonvolcanogenic sedimentary succession containing listric basin-marginal faults shows abrupt facies changes from craton to trough with rich resources of stratabound microbanded manganese oxide ore hosted within the trough facies.

The cratonic sediments of the basin are mature siliciclastics with clear imprints of tide-dominated, shallow-subtidal deposition. The siliciclastics show rapid lateral transition into a thick sequence of nonstromatolitic and micritic ribbon limestone containing sediment-gravity-flow deposits in the basal part and a persistent horizon of bedded manganese ore deposit in the upper part. The limestone is overlain by a thick sequence of basinal shale facies through an interbedded sequence of limestone and shale. Uniformly thick, laterally persistent, parallel-sided and thin-bedded limestone-shale rhythmites and parted limestone rhythmites characterize the ribbon limestones. Except for widespread thin parallel lamination, structures formed by waves, tidal currents, and storm-induced current are totally absent. The limestone is brown and argillaceous in the basal part, and changes upwards to siliceous and pyritiferous light gray to dark gray varieties and finally to carbonaceous black limestone. The physical attributes of the limestone suggest complete drowning of the carbonate interval in response to higher sea level caused by tectonic subsidence and a deep-water slope and basinal environment of carbonate deposition under continued transgression.

The bedded manganese oxide ore interstratified with chert, simulating iron formation, typically shows micron-scale to millimeter-scale thin parallel laminae of almost uniform thickness, texture, and composition. The manganese ore and chert laminae chiefly have a cryptocrystalline to microcrystalline fabric consisting of grains of clay size to fine silt size. Todorokite and birnessite are the most abundant sedimentary manganese oxides; they were locally transformed to braunite and bixbyite during diagenesis. The centimeter-thick laminated chert beds are analogous to microbanded chert mesobands of the iron formations and are composed of microcrystalline quartz. The sedimentary attributes of the ore-chert sequence suggest deposition from suspension fallout in a low-energy environment. Pe econtemporaneous deformation of the ore deposit indicates slow downslope movement of the semiconsolidated sediments over the basin floor.

The sedimentology and stratigraphy of the Penganga Group suggest a sediment-gravity-flow-influenced deep-water, base-of-slope/basinal environment of manganese accumulation during the thermal subsidence stage of basin development within an extensional rift setting. On the basis of sedimentological evidence and parallel studies done on iron formations, a possible hydrothermal source of manganese is also considered. This represents a significant departure from the prevalent views of high-energy, shallow-water shelf environments of manganese deposition and terrigenous sources of metal in cratonic basins hosting several important manganese deposits, and thus it has important implications for genesis of manganese ores.