Sedimentology, palynostratigraphy and palynofacies of the late Oligocene Makum Coalfield, Assam, India: A window on lowland tropical vegetation during the most recent episode of significant global warmth

Abstract

The sedimentology, palynofacies and pollen and spore assemblages, together with observations on the preserved megaflora, from the Tirap coal mine, Assam, India (27° 17′ 20″ N, 95° 46′ 15″ E) provide important insights into the composition and distribution of plant communities in a late Oligocene tropical delta (palaeolatitude approximately 18°N). These insights take on additional significance because the late Oligocene included the last significantly warm interval in Earth's history before progressive Neogene cooling to the present day ‘icehouse’. The lower two thirds of a 188m studied section represents lower delta plain environments with only a small degree of brackish water (marine) influence. Raised mires, autochthonous swamps and allochthonous organic matter accumulations comprise the vitrinite-rich high volatile bituminous B coals. The upper third of the section represents upper delta plain environments with high sediment pass-through. Here standing fossil ‘forests’ attest to frequent inundation and recovery of the arborescent interfluve vegetation. Throughout the period of deposition of the studied section (several 100kyr) sediment supply exceeded increases in accommodation space afforded by basin subsidence/relative sea level rise.A dominance of spores attributed to cyatheaceous and other fern taxa indicates that tree ferns and other pteridophytes were a major component of the vegetation throughout the environments represented by the section, despite the absence of ferns in the megafossil record. Nypa and mangrove megaremains together with mangrove pollen and Acrostichum spores attest to the presence of a mangrove community similar in composition to those of modern Southeast Asia. Palms and megathermal tree taxa such as those belonging to the Malvaceae, Meliaceae, Alangiaceae, Anacardiaceae and Pellicieraceae inhabited the interfluve and swamp forests. Perennial herbs of the family Gunneraceae occur throughout the section.Palynofacies analysis shows a dominance of anoxic/dysoxic environments with a small part of the lower part of the section exhibiting more oxic conditions associated with circumstantial evidence for mild marine, possibly tidal, influence. Most palynodebris fluoresce only weakly consistent with the relatively high rank for coals of this age. A small number of reworked Gondwanic (Permian) grains suggest sediment erosion from the highlands to the north.The late Oligocene tropical delta vegetation has much in common with modern lowland wet megathermal forests from Southeast Asia. However tree ferns appear more prevalent in the late Oligocene.