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Himalayan Geology, Vol. 45 (1), 2024, pp. 138-154, Printed in India

Evidence of fluvial to marine transition in the Siwalik rocks of the Itanagar area, Arunachal Pradesh, India: Implication for the regional paleogeography


1Geological Studies Unit, Indian Statistical Institute, Barrackpore Trunk Road, Kolkata -700108, India

2Department of Geology, University of Calcutta, 35, Ballygunge Circular Road,Kolkata -700019, India

*Email (Corresponding author):

Abstract: Sedimentological study of the Middle Siwalik Subansiri and lower part of the Upper Siwalik, Siji Formation, from the Itanagar area evidence for marine incursions within an established continental depositional setting. The braided fluvial deposit of the Subansiri Formation is overlain by the shallow marine fan or braid delta deposit of the Siji Formation, recording for the first time the fluvial to marine transition from this area. In the Subansiri Formation isolated large cross strata of the bar platform, downcurrent accreting cross sets of bar of supra-platform, bar top low-stage channel scour fills and rare, thinly rippled silty flood plain deposits were recognized. These assemblages constitute 5-12 m thick sheet sandstone bodies stacked up, forming about a km-thick Subansiri sandstone succession. In the overlying Siji Formation, abundant wave- and combined flow-ripples, well-developed hummocky and swaley strata, and brackish water trace fossils (Lingulichnus, Arenicolites) indicate a marine depositional regime. The depositional domains of the Siji Formation include mudstone dominated prodelta, alternating planar and cross bedded mudstone-sandstone units of the lower delta front; channelized, cross stratified pebbly sandstone and conglomerate of delta mouth bars, and fine sandstone units with tidal bundles from shallow coastal embayment. The Shillong Plateau is a unique feature in the eastern Himalaya whose deformation pattern influenced the depositional system and makes it different from the Western Himalaya. Further a comparison is carried out of the distinctive features of the western Himalayan Siwalik with those of the eastern (Itanagar) region and emphasize that two distinct tectono-geomorphic regimes characterized the foreland basin system.

Keywords: Himalayan Foreland Basin, Braided River, Fan-delta, Marine transgression, Itanagar

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