Depositional Environment of Mio-Pliocene Siwalik Sedimentary Strata from the Darjeeling Himalayan Foothills, India: A Palynological Approach

Abstract

A rich and diverse palynoassemblage recovered from the Churanthi River section (26°53' 59.3" N, 88°34' 17.2" E), Darjeeling foothills Eastern Himalaya, has yielded 87 species assigned to 69 genera. The palynoassemblage is rich in angiosperm taxa (45.63%) followed by gymnosperms (0.45%), pteridophytes (18.49%) and fungal remains (23.88%). Based on their nearest living relatives, a wet evergreen to semi-evergreen forest under a humid tropical to sub-tropical environment during the Mio-Pliocene age has been suggested. A lot of angiosperms such as Palaeosantalaceaepites, Araliaceoipollenites, Malvacearampollis, Zonocostites, Neocouperipollis, Dicolpopollis, Palmidites, Palmaepollenites, isolated salt glands of mangrove plant leaves (Heliospermopsis) and Mediaverrunites type of fungal spores, along with ichnofossils like Planolites, Palaeophycus, Skolithos, Rosselia, Ophiomorpha and Teichichnus associated with rippled mudstone-siltstone suggest an environment strongly influenced by brackish water. Primary sedimentary structures in the associated strata indicate strong wave agitation common in shallow marine setting. Some high elevation components (5.14%) such as Alnipollenites, cf. Corylus (Betulaceae), Juglanspollenites, Engelhardtioipollenites (Juglandaceae), Quercoides, Cupuliferoidaepollenites, Lithocarpus, Castanopsis (Fagaceae), Abietineaepollenites (Pinaceae) represent hinterland vegetation possibly transported to the prograding deltaic coastline by the rivers. Reworked palynotaxa (Striatopodocarpites sp., Striatites sp., Faunipollenites sp., Circumstriatites sp., Crescentipollenites sp., Cuneatisporites sp., Parasaccites sp., Scheuringipollenites sp., Rhizomaspora sp., Marsupipollenites sp., Lophotriletes sp.) of Permian age have also been recorded in the palynoassemblage (11.55%) indicating the abundance of Permian Gondwana strata in the source area.

Late Miocene-Pleistocene evolution of India-Eurasia convergence partitioning between the Bhutan Himalaya and the Shillong Plateau: New evidences from foreland basin deposits along the Dungsam Chu section, eastern Bhutan

“…A paleoenvironmental change occurs east of Nepal where thick deltaic deposits appear in the lower and middle Siwalik subgroups (Figure b). Along the Churanthi River in West Bengal (Figure ), brackish water to shallow marine conditions are documented by palynological data and trace fossils in the undated middle Siwalik subgroup [ More et al ., ]. Farther east, our observations indicate that along the Dungsam Chu, the basal environmental units 1 and 2 correspond to deposits of a river‐dominated deltaic system and a wave‐influenced deltaic environment, respectively, associated with marine trace fossils, glauconite, and some coastal pollen taxa, suggesting a marine setting before ~5 Ma (Figure b).…”

“…There are extensive lateral variations in lithologies and thicknesses of the different sediment sequences described in those different areas, some of them located quite far (a few hundred kilometers) from the Dungsam Chu section. However, we speculate that the observations collectively suggest that the marine incursions emanating from the Bay of Bengal to the south and recorded both in the Naga Hills and the Surma Basin until the deposition of fluvial facies from ~3.9 Ma may have had a peripheral impact on the facies observed at the front of the Himalayan range in eastern India [ Chirouze et al ., ; More et al ., ] and in Bhutan until at least ~5 Ma. Our hypothesis supports a post‐Miocene full continentalization of the eastern Himalayan foreland basin, much later in time than previously suggested in this area [e.g., Alam et al ., ; Johnson and Nur Alam , ] and than farther west.…”

“…Pre‐Cenozoic sporomorphs were uncommon and listed under “corroded/indeterminate sporomorphs” in Table S3. This was not expected, given the deltaic (i.e., river‐influenced) nature of the depositional environment and the common occurrence of these forms in Siwalik deposits documented elsewhere (e.g., in West Bengal [ More et al ., ]). This suggests that sediment reworking has had no marked influence on the pollen composition of the Dungsam Chu section.…”

“…Sedimentary archives of the Neogene‐Quaternary Siwalik Group preserved in the Himalayan foreland basin have been extensively documented along the Himalayan arc from Pakistan to Nepal, providing valuable information on mountain building in space and time, past organization of drainage networks, and paleoclimate [e.g., Najman , , and references therein]. Recent studies have also focused on sections of the Siwalik Group in eastern India, in the states of West Bengal [ More et al ., ] and Arunachal Pradesh [ Chirouze et al ., , ; Lang et al ., ]; however, there is still a lack of information in the frontal Bhutan Himalaya, leaving about 400 km along the strike of the orogen of undocumented foreland basin deposits regarding ages, thicknesses, sedimentary facies, and depositional paleoenvironments.…”

“…[]. Abbreviations are MBT: Main Boundary Thrust, MFT: Main Frontal Thrust, C: Churanthi river section [ More et al ., ], and K: Kameng section [ Chirouze et al ., , ].…”