Colour Variation in Triassic Red Beds of Indian Gondwana

ABSTRACT The Triassic red mudstones from the four master basins of Indian Gondwana occur in spectacular colours ranging from green to purple, including all shades of red. The purple colour of mudstones is caused by a coarser particle size compared to red mudstones, which coagulated the mud grains, intensified the hematite coating and increased the redox fluctuations to produce the purple colour. Whole rock geochemistry of the mudstones indicated similar major oxide ratios, high LREE/HREE ratio, flat HREE pattern, pronounced but variable negative Eu anomaly and felsic source for all mudstones. These mudstones contain a large number of vertebrates, which show size variation within certain hues of mudstones. Larger, robust vertebrates are discovered from purple mudstones of Middle Denwa, Satpura Basin, while red mudstones of Upper Denwa, Maleri and Tiki Formation have numerous small, fragmentary vertebrates. The purple and red mudstones have similar permanganometric and clay fraction makeup, but due to the coarser particle size of the purple mudstones, their permeability is higher. This resulted in greater water seepage ability, which formed a favourable niche for the vertebrates to grow.