Low latitude floral assemblage from the Late Oligocene sediments of Assam and its palaeoclimatic and palaeogeographic significance

Abstract

There are many outcrops of the Tertiary sediments in northeast India and a large number of plant megafossils have been described from there. Among all, the Late Oligocene sedimentary basin of Makum Coalfield is very important because there is no other exposure having such a rich palaeofloral assemblage not only from northeast India but also from the whole country. The assemblage is also important to confirm whether the suturing between the Indian and Eurasian plates was complete to facilitate plant migration or not. All the reported plant fossils clearly indicate the prevalence of tropical climate in the region during the deposition of the sediments. As the majority of taxa occur in tropical evergreen to moist deciduous and littoral and swampy forest, a warm and humid climate may be envisaged in Upper Assam during the Late Oligocene. The abundance of palms and pantropical megathermal plant families in the Makum Coalfield indicate that the cold month mean temperature (CMMT) was not less than 18°C. The quantitative palaeoclimate reconstruction indicates a monsoonal climate during the period with the same intensity as that of the modern day. The absence of Southeast Eurasian elements in the fossil assemblage provides clear evidence that suturing between the Indian and Asian plates was not complete till the Late Oligocene. Several modern analogues of the fossil taxa are now endemic to the Western Ghats which lies in the same palaeolatitude.