Meso- and macro-scale architecture of a Triassic fluvial succession: Denwa formation, Satpura Gondwana Basin, Madhya pradesh

Abstract

The Denwa Formation (∼300m thick) representing a part of the unconformity bounded Triassic (Pachmarhi-Denwa) succession of the Satpura Gondwana basin, is characterized by two different fluvial styles marked by contrasting architectural elements within the alluvial system. These fluvial styles are distinct from the underlying Pachmarhi Formation representing an alluvial plain with channel belts characterised by multiple-channel, sandy braided rivers. The lower part of the Denwa Formation is characterized by an alternation of medium to fine grained, thick, sheet-like, sandstone bodies and red mudstone intervals having centimetre to decimetre scale fine grained sandstone interlayers. The thick sandstone bodies are comprised of several groups of storeys, and individual storeys are made up of macroform strata reflecting frontal, oblique and vertical accretion. It is evident from the overall architecture that the lower part of the Denwa Formation represents' an alluvial plain characterised by sandy, braided channel belts and associated floodplain/basin. The upper part of the Denwa Formation is characterised by conspicuous absence of thick multistorey sandstone bodies. It is predominantly a mudstone dominated, sand-poor succession characterized by ribbon-shaped bodies encased within pedoturbated mudstones. Ribbon-shaped bodies internally show inclined heterolithic (sand-mud) stratification representing lateral accretion of channel bars of high sinuosity rivers. The upper part of the Denwa Formation is interpreted to represent deposits of a network of meander channels laterally separated by well-developed and stable interfluve and floodplain areas. The changes in the fluvial styles represented in the Denwa succession have been attributed to changes in the aggradation rate, avulsion frequency and climatic setting.