Palaeoenvironmental change across the Permian-Triassic boundary inferred from palynomorph assemblages (Godavari Graben, south India)

Abstract

In the present study, palynological and palaeoenvironmental investigations have been carried out on a 601 m-thick sedimentary sequence intersected by borehole MCP-8 (Sattupalli-Chintalapudi coal belt, Chinatalapudi sub-basin, Godavari Graben). These studies have been carried out for the dating and correlation of sediments to understand the biostratigraphy and climatic change documented by terrestrial plant ecosystem across the Permian-Triassic boundary interval. The palynological investigation revealed the presence of five distinct floral assemblages (Assemblages I–V). Assemblages I–III (Guadalupian-Lopingian) represent Raniganj palynoflora (591.27–157.20 m), whereas Assemblages IV–V (Induan) represent Panchet palynoflora (137.79–136.08 m). The Gondwanan flora underwent a considerable change after the end-Permian mass extinction event. The late Permian glossopterids, conifer and cordaites dominated palynoflora was replaced by the lycopsids and few forms of peltasperms and conifers during Early Triassic. The decline in the glossopterid diversity and abundance along with concomitant rise in the lycopsids suggest a gradual but obvious palynofloral and environmental change across the Permian-Triassic boundary. Palynofacies studies have been carried with regards to reconstruct palaeoenvironment and palaeovegetation. These studies predict a thick closed forest cover during the deposition of the Raniganj palynoflora (late Permian), dominated by striate glossopterid and non-striate conifer and peltasperm pollen grains, which were the main peat-forming elements and the vegetation of the hinterland. The Panchet (Early Triassic) palynoflora was dominated by cingulate cavate spore bearing lycopsids and arborescent to sub-arborescent plants of conifers bearing taeniate bisaccates forming an open forest. On the basis of recovered palynoflora and palynofacies studies, the upper Permian (Guadalupian and lower Lopingian) deposits have been inferred as fluvio lacustrine while extensive peat-forming swamp dominating conditions prevailed during Changhsingian times. The Lower Triassic deposits represent hot-arid conditions along the braided river systems.