Depositional and Diagenetic History of the Lower Siwalik Subgroup (Miocene), Northwest Himalaya, Jammu (India)

Pandita, Sundeep ¹; Bhat, Sunil K.¹
(1) Department of Geology, University of Jammu, Jammu, India.

In the Northwestern part of Himalayan Foreland Basin, the Jammu region exhibits c.6km thick sedimentary succession of Siwalik Group exposed on the northern and southern limbs of the thrust cored Suruin-Mastgarh anticline and forms a link between the Siwalik rocks exposed in Pakistan in the west and beyond the Ravi River (India) in the east. These rocks are disposed in parallel folded zones and generally dip towards the southwest or the northeast at varying angles between 80 (Lower Siwalik) to 10 (Upper Siwalik) degrees.

The Ramnagar area of the Udhampur District in Jammu and Kashmir State exposes a thick pile of the Lower Siwalik rocks sandwiched between the Murree Group and the Middle Siwalik Subgroup on both the limbs of the doubly plunging Udhampur Syncline. Detailed sedimentological study has been carried out on two stratigraphic sections (Kalaunta and Ramnagar) in this area to establish the depositional and diagenetic history. Here the rocks consist of brown, reddish brown, grey and buff sandstones, reddish brown to dark brown siltstones and light brown to reddish brown mudstones.

The stratigraphic accumulation of facies associations and evolution of fluvial style during the deposition of these rocks has developed in two stages. The stage first started with deposition of predominance of fine grained facies (mudstone and siltstone) as facies association FA1 in the Kalaunta section followed by dominance of flood flow and crevasse splay sediments characterized by lateral accreted cross stratified sandstone deposits of facies association FA2 in Ramnagar section. Facies association FA3 comprises of sequence of fine to very fine sandstones, siltstones and mudstones overlying the sand-mud dominated association and was deposited during these two stages as interfluvial deposition. These rocks are interpreted to have been deposited by a changing river system from fine grained meandering in the lower Kalaunta section to flood flow dominated meandering system in the upper Ramnagar section.

Diagenetic signatures observed in the sandstones of the area show a reduction in porosity from 42 to 15%. The major cause of the reduction in the porosity was due to cementation in the Kalaunta section and compaction in the Ramnagar section. Porosity versus depth relationship plots suggests a depth of burial in the range of 933 to 2000m for the Kalaunta section and 800 to1500m for the Ramnagar section.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.