ABSTRACT

The Permian and Triassic Gondwana deposits around Bheemaram belong to six mapable rock units ("formations"): Barakar, Ironstone Shale, Kamthi, Yerrapalli, Bhimaram and Maleri. Of these, the Barakar, Middle and Upper Kamthi and Bhimaram formation are composed of profusely cross-bedded, prismatic or lenticular bodies of coarse, argillaceous sandstones. The other formations are made of structureless, thin sheets of clay stone (or shale) and silty sandstones with inter-bedded lenses of calcareous sandstones and sandy limestones. The sandy and clayey formation juxtapose, often, with interfingering contacts.

Lithology, sand, body geometry, primary structures and patterns of grain-size distribution of these Gondwana rocks are comparable to those of fluvial sediments. The coarser fractions of the rocks which were transported mainly by tractive currents, constituted point-bars and channel-bars of the Gondwana river and the finer fractions were deposited from suspension in the interchannel floodplain areas.

Cross-bedding dip directions indicate a northerly paleocurrent. In the Lower Kamtha, the measurements show large dispersion due to arcuate directions of flow at the river meanders. With time, the direction of sediment transport shifted gradually, but statistically significantly, to the west. The flow was essentially unidirectional during the later phase of Kamthi sedimentaion. Orientation of long-axes of elongate pebbles and ripple mark lee-slopes are in conformity with the local foreset dip directions. Within the limited area studied, only slight suggestion of a northerly decrease in cross-bedding thickness, indicating a northerly paleocurrent, is noted. Contrary to these observations, pebble sizes in the Upper Kamthi decrease to the west-southwest, possibly due to a local swing in t e size trends. Variations in pebble size seem to yield useful paleocurrent clues only when studied on a regional scale with the observations restricted to the same depositional plane. Pebble roundness is found to vary only as a function of pebble size, and provide no clue to the paleocurrent direction.

The study reveals an uninterrupted fluvial sedimentation during the Upper Paleozoic and Mesozoic, during which time, a number of dissimilar sedimentological units were contemporaneously deposited in the varied environs of the same fluvial system. On the other hand, several similar units were also deposited in similar environmental conditions at different times. An appropriate scheme of Gondwana classification should be developed to depict the complex relationship between the various lithological, chronological and biological units which have developed in this process.