SEDIMENTOLOGY AND GEOCHEMISTRY OF SINIAN LOW-LATITUDE GLACIAL DEPOSITS ON THE YANGTZE PLATFORM, SOUTH CHINA, AS INDICATORS OF NEOPROTEROZOIC CLIMATE CHANGE: INITIAL RESULTS

For several decades, the geologic paradox of the low-latitude glaciations in Neoproterozoic time remains unresolved. In sections on most known continental fragments the glacial successions are conformably overlain by 'cap carbonates' connected to warm if not tropical waters. The dichotomy of glacial deposits directly overlain by tropical carbonates, together with isotope data, led Hoffman et al. (1998a, b) to propose the testable 'Snowball Earth' hypothesis of an entirely glaciated Earth. Here we present initial results of a sedimentological and geochemical study of the Sinian, probably Sturtian, glacial deposits on the Yangtze platform in southern China. The glacial successions may be divided into three units. A basal tillite unit (Changa'an Formation) is overlain by a fine-grained succession of water-lain sand- and siltstones (Datanpo Formation) interpreted as an interglacial unit. The upper part of this unit contains 10 cm to 2 m thick intercalations of limestones or limestone breccias. This unit is overlain by the thick tillites of the Nantuo Formation with a cap carbonate at it's top which gives way to black shales and carbonates (Doushantuo Formation). Preliminary results from matrix samples of the two glacial units have significantly lower average CIA values than those of the interglacial unit indicating increased chemical weathering in the latter. Very preliminary carbon isotope data on a few cap carbonate samples indeed show negative d¹³C values between -2.8 and -3.3 ‰ consistent with a Snowball Earth scenario and comparable to other carbon isotope records from cap carbonates. Interglacial carbonate samples, in contrast, show a broad range of carbon isotope values between -2.6 and +1.1 ‰. Diagenetic alteration appears to be within reasonable limits, at least based on an oxygen isotope signal which is comparable and in agreement with proposed thresholds (Kaufman et al., 1991, i.e. a d ¹⁸O > -10 ‰).