Stratigraphic and tectonic implications of field and isotopic constraints on depositional ages of Proterozoic Lesser Himalayan rocks in central Nepal

Suggestions about the depositional age of the carbonate-dominated Malekhu Formation at the top of the Proterozoic part of the Lesser Himalayan series have ranged from Mesoproterozoic to late Paleozoic. These age estimates largely stem from correlation with parts of the Krol succession some 500 km to the west in northwest India, and changing interpretations of the depositional age of the Krol rocks. 104 new measurements of carbon isotope abundances in carbonate from a 300 m measured section through type exposures of the Malekhu Formation reveal a narrow range of δ13C values between −1.7 and +0.2‰ (VPDB) and a mean of −0.9 ± 0.4‰ (1 standard deviation). These values are inconsistent with correlation with the Krol succession, which preserves large amplitude positive and negative excursions in δ13C values. Comparison with known marine carbonate δ13C values through time suggests deposition of the Malekhu Formation prior to c. 1300 Ma. Upper and lower bounds for deposition of the Proterozoic part of the Lesser Himalayan series thus are c. 1300 and c. 1900 Ma, though actual deposition likely occurred during only a portion of this interval. Further, a profound unconformity representing at least 900 M.y. separates the Malekhu Formation from the depositionally overlying Carboniferous-Permian Sisne Formation. No upper Mesoproterozoic, Neoproterozoic, or lower Paleozoic rocks have been found in the Lesser Himalayan series in central Nepal. The carbon isotope stratigraphy of the Malekhu Formation in central Nepal matches that of the Buxa Formation exposed in the Ranjit window of Sikkim to the east, but does not match the chemostratigraphy of rocks correlated with the Buxa Formation in eastern Bhutan and Arunachal Pradesh, northeast India.