NEOPROTEROZOIC DIAMICTITES AND STABLE CARBON ISOTOPE CHEMOSTRATIGRAPHY OF THE QURUQTAGH SERIES, NW CHINA

To understand the frequency and geochemistry of Neoproterozoic glaciations, we studied the 4.5-km-thick Quruqtagh Series in NW China. The Quruqtagh Series is bracketed by Mesoproterozoic dolomites and Lower Cambrian small shelly fossils. Massive diamictites occur in three formations of the Quruqtagh Series: in ascending order, the Bayisi, Tereeken, and Hankalchoug formations; these diamictites have been interpreted as glacial deposits. Our field observations confirm the glaciogenic origin of the Tereeken and Hankalchoug diamictites, but the origin of the Bayishi diamictite remains elusive. The Tereeken Formation contains several diamictite members, separated by siltstontes, calcareous mudstones, carbonates, and centimeter-thick laminae of calcitic crystal fans. Stratigraphically scattered carbonate samples from the Tereeken Formation have a restricted range of d13C (-4.7 per mil PDB, std.=0.9, n=19) and very light d18O (between -18 and -16 per mil). The Hankalchoug diamictite is separated from the Tereeken Formation by 1000-m-thick interbeded sandstones-siltstones and 80-m-thick thin-bedded microsparitic limestones that record a rising d13C trend (from -11 to -1 per mil). The Hankalchoug diamictite is overlain by a 3-m bed of calcareous mudstone and micritic carbonate. This 3-m bed is characterized by very light d13C (<-10 per mil) but normal d18O (mostly between -4 and 0 per mil). Such an extremely negative d13C excursion is not a feature of Sturtian or Marinoan cap carbonates, but have been reported from Neoproterozoic carbonates in Australia, Death Valley, India, Iran, and Oman. Although synchroneity of these extremely negative excursions may not be assumed, it is possible that they are all bracketed between the Marinoan glaciation and the Neoproterozoic-Cambrian boundary. If true, the Hankalchoug tillite conformably underlying an extremely negative d13C excursion supports the claim of more than two Neoproterozoic glaciations. Diagenesis does not seem to provide a satisfactory explanation for this extremely negative carbon isotopic excursion. Sustained C-12 contribution from oxidation of organic matter or gas hydrate may be responsible for such light d13C signatures.