GEOCHEMICAL AND LITHOLOGICAL CONSTRAINTS on THE AGE AND CORRELATION OF THE LESSER HIMALAYA AND THE CENTRAL INDIAN CRATON

The Lesser Himalaya lithotectonic unit of northern India is divided into two zones separated by the Tons Thrust, with the “inner” zone (iLH) situated to the north of this thrust and the “outer” zone (oLH) to the south. While a Neoproterozoic-Cambrian age for most of the oLH is well established, the age and correlation of strata in the iLH remains problematic. Phosphatic Bailaclia stromatolites in the Gangolihat Dolomite (iLH) are remarkably similar to well dated ~1600 Ma deposits in the lower Vindhyan sequence on the Indian craton. The similarity of these distinctive deposits may result from their being age-equivalent parts of a large, continuous carbonate platform. U-Pb dating of detrital zircons from siliciclastic units underlying both deposits yielded similar Paleoproterozoic age populations with no zircons younger than ~1600 Ma. A late Paleo-early Mesoproterozoic age for the Gangolihat Dolomite is also supported by d¹³C_carb values restricted between -1 and 0‰ over a 100m section. Upper iLH sandstones (Mandhali Formation) yielded an array of Mesoproterozoic aged grains with a large population of ~1000 Ma grains, and similar populations are seen from the major units within the upper Vindhyan sequence, as well as sandstones from drill cores of bedrock beneath the Himalayan Foreland basin (Ganga Supergroup). Furthermore, the jump from the youngest grains of 1600 Ma in the lower iLH to youngest grains of ~1000 Ma in the upper iLH suggests the presence of an unconformity within the iLH that is comparable to the unconformity separating the upper and lower Vindhyan sequences. The similarity of age distributions between the iLH, Ganga Supergroup, and Vindhyan basin, and presence of the large regional unconformity coupled with facies similarities between putative age-equivalent strata in the Himalaya and Vindhyan basin suggest these regions were in sediment continuity, with the Vindhyan basin representing the proximal region of an epicontinental platform and the Himalaya representing the more distal portion. Large populations of Neoproterozoic grains are known from all Cryogenian through Cambrian aged strata in both the oLH and Marwar Supergroup of cratonic India, and the absence of these younger grains in all iLH and Vindhyan rocks suggests their deposition prior to the Cryogenian (ca. 850 Ma).