Palynology of the tertiary subcrops of upper Assam.

Abstract

A brief account of the palynological studies carried out on the subcrops of Upper Assam during the last decade and more, mostly by one of us (D.B.), is presented. One representative well, each from Borholla, Naginijan, Teok, Amguri, Disangmukh, Rudrasagar, Geleki, Naz-ira and Lakwa structures, in Sibsagar district of Upper Assam, have been incorporated in the present

report. On the basis of the palynofossils recorded, the subsurface sequence has been divided into three or four broad palynological zones, depending on the formations penetrated. The lowest zone, Paly. Zone IV, has been dated as Eocene (Mid. to Upp.) on the basis of the common occurrence of Retialetes spp., Monolites mawkmaensis, Palmaeplloenites eocenicus, Palaeorubiaceaepites sp., Palaeocaesalpiniaceaepites eoceniea, Polyeolpites spp., Nothofagidites spp., Sehizosporis spp., Hystrichospheridium spp. etc. In Geleki, Nazira and Lakwa structures, wells have not been drilled deep enough to encounter this zone. Sylhet and Kopili Formations are included in this zone. The next higher zone, Paly. Zone III, has been dated as Oligocene and includes the Barail Formation. The palynomorphs of common occurrence are Meyeripollis sp., Foldexina inaperturata, Cieatrieosisporites sp., Disuleites sp., Palaeoeaesalpiniaeeaepites sp., Polyeolpites spp. (not more than 5-6 colpi), Simsangia trispinosa etc. Paly. Zone II has been dated as Miocene (Mid. to Upp.) and is characterised by the occurrence of Dieksoniaeeaesporites sp., Cieatricosisporites spp., Pinuspollenites spp., Podoearpidites sp., Castaneapollenites sp., Alnipollenites sp., Erieipites sp., Stephanoeolpites sp., etc. Tipam Formation is included in this zone. The Surma Group, if present, has not been confirmed so far; Bhuban Formation is not represented in the sequence studied so far. The topmost zone, Paly. Zone I, includes Girujan, Namsang and younger formations. This zone has been dated as Plio-Pleistocene to Recent and the taxa occurring commonly are Pteridacidites, Scabratriletes, Sporites circulus, Cyathidites, Graminidites, Juglanspollenites, Compositoipollenites, Umbelliferaepites, Impatiensidites etc.

The distribution of the taxa in different zones is more or less the same in all the wells under report, which suggests the prevalence of largely similar ecological conditions during deposition of successive zones. Some of the important phenomena noted are:

1. the frequent occurrence of typical Barail microflora in the Namsang,
2. the consistent occurrence of typical Lr. Gondwana (Permo-Carb.) micro flora in the Miocene; and
3. the decrease in the frequency of microplanktonic elements from west to east of the area.

From the distribution of the different taxa, it has been concluded that the basin was deeper towards the northeast from Borholla, the westernmost well. Largely shallow-marine to brackish water conditions existed during Paleogene times and there was luxuriant vegetation growing in swamps, fresh-water lakes, along the coast and inland near-shore areas in a warm, humid, tropical to sub-tropical climate. The Neogene flora was essentially inland terrestrial, growing in moist, shady lowlands and cool uplands in a temperate to sub-tropical climate. This change in the microfloral contents in the Paleogene and Neogene is interpreted as indicative of orogenic activities in the northern parts of the area and regression of the sea southwards.