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

Abstract

In the Himalaya of central Nepal, uncertainty in the absolute depositional ages and the relative stratigraphic positions of several formations in the Proterozoic part of the Lesser Himalayan series has hindered structural mapping, east-west correlation of units and structures, and interpretations of basin architecture during both the Proterozoic and Paleozoic. At the stratigraphic base of the Lesser Himalayan series in central Nepal, 441 new U/Pb isotopic ages of detrital zircons from the previously defined type Kuncha, Kushma, and Fagfog formations constrain the maximum possible depositional ages for these clastic units to be c. 1900, 1770, and 1810Ma, respectively. 182 detrital zircon U/Pb isotopic ages confirm the identity of another sample of the Fagfog Formation, and field relations indicate that this unit rests depositionally on the Kushma Formation with no obvious unconformity or lithologic difference. This interpretation, combined with the recognition that the term “Kushma Formation” historically has been applied incongruously to apparently different units in different parts of Nepal, leads us to recommend abandoning the term “Kushma Formation” and replacing it with “lower Fagfog Formation.” The overlying part of the Fagfog then becomes the “upper Fagfog Formation.” Because the 1770Ma constraint on the maximum depositional age comes from the lower part of the unit, the entire Fagfog Formation must have been deposited after 1770Ma. The depositional ages of both the lower and upper parts of the Fagfog must be younger than the Kuncha Formation because it was intruded by a granite at 1878±22Ma (2-sigma). Thus an unconformity lasting at least 86M.y. probably separates the Kuncha Formation (deposited between 1900 and 1856Ma, considering uncertainties) and the Fagfog Formation (deposited after 1770Ma). The lower Fagfog Formation is present in the Kali Gandaki region but apparently pinches out to the east because it is not present structurally beneath the Kathmandu nappe.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 500km 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 300m 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. 1300Ma. Upper and lower bounds for deposition of the Proterozoic part of the Lesser Himalayan series thus are c. 1300 and c. 1900Ma, though actual deposition likely occurred during only a portion of this interval. Further, a profound unconformity representing at least 900M.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.