Abstract

Finer-grained components of fluvial sediments are usually deposited in overbank environments, and their preservation in the rock record is generally subordinate to that of the coarser-grained channel fills. However, though the fossil record and lithological characteristics of the Late Triassic Maleri Formation, Gondwana Supergroup of the Pranhita–Godavari continental rift basin, India, indicate that it is a fluvial deposit, the formation is characterized by thin sandstone bodies vertically separated by thicker fine-grained sediments. With the help of petrographic and sedimentological features, this study characterizes the fluvial system that produced a deposit dominated by fines. These syn-rift sediments provide valuable clues towards understanding the spectrum of fluvial processes operating in continental rift-basins.

We observe that the formation is dominated by stratified siltstone (mudrocks) rather than massive mudstones. These mudrocks contain a large quantity of silt to fine-sand-size pedogenic mud aggregates, and the preserved primary structures indicate transportation by traction currents. The internal organization of the mudrock units reveals that an admixture of pedogenic mud aggregates along with other sand-grade siliciclastic grains were transported through the channelized and unconfined reaches of a network of discontinuous ephemeral streams constituting the axial drainage of the rift basin. However, evidence for accumulation of fines in swamp-like environments is preserved in a few pedogenically modified mudstones that occur in between much thicker intervals of stratified mudrocks. On the other hand, the small bodies of cross-bedded carbonate grainstones and laminated marls indicate that the conditions suitable for precipitation of freshwater carbonates prevailed in some of these swampy areas.

The sedimentary features of the thinner sheet-sandstone bodies associated with the mudrocks suggest that though the overall character of the fluvial system remained unchanged the sandstones were deposited during the phases of higher discharge. We suggest that the basin was persistently fed with a sediment load rich in mud aggregate produced in the vertic soils forming on the shales and limestones in the source area under a warm climate and seasonal rainfall.