Lithostratigraphy, trace fossils and palaeoenvironment of Paleogene sequences in Parwanoo-Subathu sector of Himachal Himalaya, India

The Paleogene belt of the Outer Himalaya in the Shimla Hills is represented by the Paleocene to Lower Miocene marine to fluvial rocks of the Sirmur Group which is divisible into the Subathu, Dagshai and Kasauli formations in younging order in Parwanoo-Subathu area of the Himachal Pradesh, India. The shallow marine fossil bearing Subathu Formation overlies the various Proterozoic lithostratigraphic units like the Simla Group, Krol Group, Baliana Group, Shali Group/Deoban Group and others with an unconformable contact marked by a thin layer of oxidized and ferruginised laterite, indicating prevalence of warm tropical climate before deposition of these shallow transgressive marine rocks. The Subathu Formation grades upward into fluvial deposits,comprising an alternate sequence of reddish-purple mudstone and fine to medium grained, reddish-brown sandstone of the Dagshai Formation along with multiple palaeosol horizons. The rocks of the Dagshai Formation are further gradationally overlain by the plant fossil bearing fluvial sequence of medium to coarse grained, massive, multistory, greenish-grey micaceous sandstone and minor grey-orange mudstone of the Kasauli Formation. In response to southward leading deformation front of the Himalayan orogeny, due to collision tectonics of the Indian and Eurasian plates, the para-autochthonous Paleogene fold-thrust belt exposed between the northerly dipping Main Boundary Thrust (MBT)/Krol Thrust in north and the Main Boundary Fault (MBF) in the south shows repetition of some formations of the Sirmur Group along Parwanoo-Darampur-Barog-Subathu section of the Himachal Himalaya. The abundance of vertical and cylindrical burrows of Skolithos linearis, S. annulatus, S. ingen, S. bulbus, Ophiomorpha esp., Thallasinoides paradoxides, etc. within the upper part of the Subathu Formation suggest lower intertidal to subtidal environment. This ichnofossil assemblage along with Skolithos-Thallasinoids-Ophiomorpha represent littoral zone, intertidal to subtidal environment of deposition.