Megafloral diversity of the upper Gondwana sequence of the Rajmahal Basin, India

Abstract

The upper Gondwana rocks of the Rajmahal Basin include a volcano-sedimentary sequence and are represented by the infratrappean Dubrajpur Formation and the trap-intertrappean Rajmahal Formation. In this paper, the megafloral content of the sequence is documented. Four distinct megafloral assemblages are described. The oldest (first) assemblage is found in the upper part of the Dubrajpur Formation, sampled at Khatangi Hill. It is dominated by cycadophytes. The significant occurrence of Culcitites madagascariensis indicates an Upper Jurassic age and an affinity with Madagascar's flora of this age. The overlying Rajmahal Formation includes three distinct megafloral assemblages. The oldest (first) of these is represented in the first to third intertrappean beds and is dominated by cycadophytes and pteridophytes, followed by conifers. Pteridosperms, Ginkgoales and Pentoxylales are relatively uncommon. This assemblage is Neocomian in age because of the presence of the Australian Neocomian index species Phyllopteroides laevis. The second assemblage of the Rajmahal Formation is found in the classical Nipania locality of the fourth intertrappean bed. This assemblage, which appears to be of Late Neocomian age, is dominated by Pentoxylales and Coniferales; pteridophytes and cycadophytes are relatively rare. The youngest megafloral assemblage of the Rajmahal Formation has been recovered recently at the Sonajori locality. The Sonajori Assemblage is characterised by the dominance of Pentoxylales and Coniferales as well as a few pteridophytes. The significant appearance of angiospermous remains and the disappearance of Bennettitales in the Sonajori Assemblage are indicative of a late Early Cretaceous age (Aptian). This age is corroborated by radiometric dating of Rajmahal Traps. The megafloral assemblages of the Rajmahal Formation show a close affinity with the Early Cretaceous flora of Victoria (Koonwarra), Australia. Thus, the upper Gondwana megafloral diversity of the Rajmahal Basin resembles the Upper Jurassic flora of Madagascar and the Early Cretaceous flora of Australia and Antarctica.