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Nilkanth Formation; Upper Cretaceous; mixed carbonate–siliciclastic; bryozoan; mass flows.

The Upper Cretaceous Nilkanth Formation awaits a process-based depositional model despite being a topic of discussion between stratigraphers, palaeobiologists and structural geologists over the last few decades. Sedimentary facies analysis of a $\sim$50 m thick section along a $\sim$2.8 km long section along Rishikesh–Tal Bidhashini in Pauri Garhwal district of Uttarakhand allowed documentation of mixed carbonate–silicicalstic facies types, dominantly consisting of sand- and pebble-sized carbonate debris mixed with siliciclastics in a proximal to distal facies tract. Ten different facies types that include matrix rich and matrix-poor shelly conglomerate, mixed clastic-carbonate wackestone, packstone, impure calcirudite and calcarenite, biomicrite and ferruginous sandstone are documented. Delineation of facies association and documentation of facies stacking pattern provide a post-Santonian mixed carbonate– siliciclastic sedimentation history of the Nilkanth Formation, deposited in the form of mass flows of varied rheology on a barred low- to moderate-gradient carbonate ramp, formed at the leading edge of the India plate before its collision with the Kohistan–Ladakh arc. Carbonate clasts comprising bivalves, crinoids, algae, bryozoan, etc., were produced in a narrow high-energy transgressive coastline and supplied across shelf along with reworked siliciclastics from clastic shoreface bar. It is argued that the reworked fossils, including the bryozoa Ceriocava Nilkanthi, present within massflows may not justify fixing of an absolute age for the formation but may definitely help in providing an age range.

SHRUTI R MISHRA^{1 2} ADITI SHARMA³ PARTHA P CHAKRABORTY³ SARADA P MOHANTY² SATISH C TRIPATHI⁴

1. Geological Survey of India, Northern Region, Lucknow, India.
2. Indian Institute of Technology (ISM), Dhanbad, India.
3. Department of Geology, University of Delhi, Delhi 110 007, India.
4. Geological Survey of India, Southern Region, Hyderabad, India.

Published

25 May 2020

Manuscript received

4 December 2019

Manuscript revised

8 February 2020

Accepted

10 February 2020

Early published

Unedited version published online

Final version published online

22 May 2020