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

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

Shallow marine to fluvial transition in the Siwalik succession of the Kameng River section, Arunachal Himalaya and its implication for foreland basin evolution

International audienceAn understanding of the depositional environment and paleogeography of the Siwalik foreland basin are crucial in interpreting the basin configuration, sediment transport pathways and its evolutionary history. This study examines the sedimentology of the Siwalik succession of the Kameng River valley, Arunachal Himalaya, northeastern India. The facies characteristics of the fine-grained, well-sorted sediments of the Dafla Formation and its complex, polymodal paleocurrent pattern in this section, reveals deposition in a variety of open marine to deltaic environment. The overlying Subansiri Formation, characterized by coarse-grained, thick, multistoried sandstone, and showing more consistent SW-ward paleocurrent, indicate deposition from a large, axial braided river system. The proposed redefinition of the boundary between the Lower Siwalik Dafla and the Middle Siwalik Subansiri formations implies their transition at around 7.5 Ma, instead of 10.5 Ma, suggested earlier. The revised age of the transition is consistent with the age of arrival of the Transhimalayan sediments at 7 Ma and also denotes the time of marine to fluvial transition in this area. Presence of marine sediments in the Kameng section, with similar records further west, indicates the existence of an extensive seaway in the eastern Himalaya during the lower Siwalik time. The extant paleodrainage reconstructions have been recast on the basis of new data on the sedimentology and paleocurrent from this section. It is inferred that the changing sea level, uplifting Shillong Plateau and drainage evolution in the eastern Himalayan foreland during the middle Miocene time controlled the marine to fluvial transition in the basin

Detailed map of the Siwalik rocks of the study area around Churanthi River, showing the bedding plane orientations and fossil locality.

<p>The detailed sedimentological log (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0150168#pone.0150168.g002" target="_blank">Fig 2A</a>) was measured along the red line.</p

Depositional Environment of Mio-Pliocene Siwalik Sedimentary Strata from the Darjeeling Himalayan Foothills, India: A Palynological Approach - Fig 7

<p>Photomicrographs of fungal spores recovered from the Siwalik succession:A, B, F. <i>Hypoxylonites</i> sp.C, G. <i>Inapertisporites elongates</i>. D, J, K, L, O, T. <i>Inapertisporites</i> sp.E. <i>Inapertisporites ovalis</i>.H. <i>Inapertisporites solidus</i>. I. <i>Inapertisporites kedvesii</i>.M, N. <i>Inapertisporites nodulus</i>.P. <i>Monoporisporites</i> sp. Q, W, X. <i>Pluricellaesporites</i> sp. R. <i>Dyadosporites dyadosporus</i>.S. <i>Dyadosporites</i> sp.U. <i>Multicellaesporites ellipticus</i>. V. <i>Dyadosporites elsikii</i>. Y, Z. <i>Microsporonite</i>s sp.A1. <i>Mediaverrunites</i> sp.B1, C1. Unidentified.</p

Stratigraphy of the Siwalik succession of Darjeeling foothills modified after Acharyya [36].

<p>Stratigraphy of the Siwalik succession of Darjeeling foothills modified after Acharyya [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0150168#pone.0150168.ref036" target="_blank">36</a>].</p

Trace fossils and sedimentary structures from Churanthi River section.

<p>A. Bioturbated ripple laminated silt-claystone. (<i>Planolites</i> marked by black arrows and <i>Skolithos</i> marked by red arrows). B. <i>Palaeophycus</i>; C. Low-angled cross stratifications and plane parallel strata; D. <i>Rosselia</i>, sectional view: funnel shaped burrow, occurring in very fine grained sandstone (F5), 318 m north of measured section; E. <i>Rosselia</i>, bedding plane view: (Note the concentric rings around sandy core).</p

Percentage of palynotaxa in Middle Siwalik sediments of Churanthi River section of Darjeeling foothills.

<p>Percentage of palynotaxa in Middle Siwalik sediments of Churanthi River section of Darjeeling foothills.</p