PRELIMINARY CHRONOSTRATIGRAPHIC CONSTRAINTS ON PROTEROZOIC SUCCESSIONS OF THE INNER LESSER HIMALAYAN SEDIMENTARY BELT OF NORTH INDIA

The Lesser Himalaya lithotectonic unit is divided into two sedimentary belts known as the “inner” (iLH) and the “outer” zones. Firm age constraints have been established for the strata of the oLH, primarily the Blaini-Krol-Tal succession (Neoproterozoic-Cambrian), whereas age constraints for the iLH remain problematic. Original age assignments of Paleo-Mesoproterozoic for the Damtha-Deoban Groups (iLH) have been questioned by recent reports of putative Neoproterozoic-Cambrian microfossils from the Gangolihat Dolomite (equivalent of the Deoban Formation). The stratigraphic position of the Berinag Group (iLH) has also been subjected to dramatically different interpretations with suggestions that it sits both stratigraphically below, or conversely, above the Damtha-Deoban Groups. Here we present preliminary results of an integrated sedimentological, paleobiological, and geochemical investigation of strata in the iLH. U-Pb ages of detrital zircons from the Berinag and Damtha Groups yielded no grains younger than ~1600 Ma, whereas a spectrum from the Neoproterozoic Blaini Diamictite shows a high concentration relatively young grains (760-1200 Ma). This supports Paleo-Mesoproterozoic ages for the Berinag and Damtha Groups. Distinctive spectrum from the Berinag Group is similar to spectra from the Kuncha Formation of Nepal and the Shumar Formation of Bhutan, with spectrum for the Ratguara Formation (Damtha Group) being similar to strata from the Nawakot Unit that overlies the Kuncha Formation. This suggests the iLH is part of a thick laterally continuous sedimentary belt traceable along strike in the north Indian margin, with the Berinag Group underlying the Damtha Group. The Gangolihat Dolomite overlies the Rautgara Formation and contains a unique deposit of phosphoritic stromatolitic dolostone that yielded a microbiota of presently undetermined affinities. This horizon is lithologically identical to the well dated Paleo-Mesoproterozoic Tirohan Dolomite from the Vindhyan Supergroup in central India. Both the similarity in age and lithology of these deposits suggest that the lower Vindhyan Supergroup and iLH might be continuous deposits from an epicontinental sea that covered the north Indian craton during the Paleo-Mesoproterozoic.