Sedimentology and environmental significance of the Cryogenian successions of the Yangtze platform, South China block.

Abstract

The Neoproterozoic glaciations during Sturtian (c. 720 Ma) and Marinoan (c. 600–635 Ma) times are considered events of extreme climate change of global dimension, sometimes interpreted according to the snowball Earth hypothesis. Here we present, for the first time, a detailed sedimentological description and facies analysis of the deposits of both glacial events recorded on the Yangtze platform of the South China block. The Yangtze platform was located at c. 40° latitude during the older glaciation, and close to the equator during the younger one. The studied successions of Nanhuan–Sinian age, including both diamictite-bearing Units, vary in thickness from not, vert, similar 90 m to not, vert, similar 1800 m and show some remarkable differences between the deposits of Sturtian and Marinoan age. In general, both diamictite-bearing successions are composed of diamictites, debris flow deposits, turbidites, structureless or laminated sand- and siltstones, and mudstones. However, diamictites of the lower Unit were deposited from proglacial debris flows, whereas those of the upper Unit formed as lodgement deposits of the grounded zone. Laminated sand- and siltstones containing dropstones are abundant in the lower Unit and very rare in the upper one. Deposits of subaqueous mass flows are present in both Units. The lower diamictite Unit is not overlain by cap carbonates. The upper diamictite Unit grades transgressively into 3 to 8 m thick successions of thin-bedded and partly clast-rich carbonate rocks representing the basal strata of the Ediacaran Doushantuo Formation. In view of a conformable contact between the upper diamictite Unit and these carbonate layers, and their similar lithostratigraphic position to those on other continents, we consider them to represent cap carbonates. Our sedimentological data indicate different patterns of development of the two glaciations. The Sturtian event in South China is characterized by deposition of ice-rafted debris distal to ice margins thus indicating ample open ocean space available for iceberg drift. The Marinoan record in South China registers depositional processes proximal to ice-margins including lodgement deposits. In consideration of the palaeogeographic position of the Yangtze platform in intermediate (Sturtian) and low (Marinoan) latitudes during Neoproterozoic time, there were probably significant differences in intensity and extent between the glaciations, the older one being less severe and characterized by regionally limited ice centres. There is no evidence of a shutdown of the hydrological cycle during the deposition of the two diamictite Units. In conclusion, our sedimentological data neither support a Sturtian nor a Marinoan “hard“ snowball Earth glaciation.