ABSTRACT

This study corroborates the earlier view that the Barakar coal measures (Lower Permian), as examined in 14 "Lower" Gondwana coalfields, is largely of cyclic nature. An idealised standard sedimentary cycle is proposed which, in the vertically upward sequence, comprises the following lithologic types: (A) pebbly very coarse sandstone, (B) coarse to medium sandstone, (C) fine sandstone, (D) interbedded assemblage of fine to medium sandstone, siltstone and shale, (E) carbonaceous siltstone and shale, (F) coal, shaly coal. Most cycles are, however, truncated, nonetheless each shows a fining upward character. Bedding types, like texture, show systematic variation within each cycle. Largescale cross-bedding which occurs profusely in the lower two units is steadily replaced by smallscale cros -bedding, wavy or parallel lamination in the succeeding units above.

The associated sedimentary characters suggest that as the deposition of a cyclothem proceeded, the flow intensity progressively decreased, and was perhaps considerably reduced during the deposition of the carbonaceous unit (E). Significantly, the next unit above is a coal seam in many cyclothems, which may suggest stagnation of the depositional area allowing mainly vegetal accumulation.

Sedimentation inevitably took place in the alluvial basin(s), and, for each cyclothem, the lower coarse grade member apparently represents a variety of lateral accretion deposits and the upper fine grade member largely overbank and lake/swamp deposition. It is suggested that the development of cycles was in all probability controlled by (1) channel or sediment wandering, and (2) intermittent tectonism in the basin which tended to convert the alluvial flood plain into coal-forming environment.