An ice sheet advancing sequence at the beginning of the Cryogenian Sturtian glaciation.

The global synchronous occurrence of glacial diamictite at the boundary of the Tonian-Cryogenian suggests a rapid onset of the Neoproterozoic Sturtian glaciation. However, uncomfortable contacts between the Tonian and Cryogenian successions obscure our understanding of ice sheet dynamics during the initiation of the global glaciation. Here, we perform a detailed facies analysis of two successions (the Tonian Gongdong to the Cryogenian Chang'an Formation) from the deep water facies of the Nanhua Basin, South China. Seven main lithofacies were recognized, including massive to laminated siltstone/muddy siltstone (Fm), ripple cross-laminated siltstone (Fr), massive to horizontal sandstone (Sm), gravely sandstone (Sg), matrix-supported massive diamictite (Dmm), weakly stratified diamictite (Dms) and dropstone bearing siltstone (Fdl). These facies were deposited from turbidity currents to glacial-influenced debris flows in deepwater environments. The top of the Gongdong Formation is mainly composed of massive to laminated siltstone/muddy siltstone and thin-bedded massive sandstone. The base of the Chang'an Formation is primarily constituted by massive to stratified diamictite. The gradual transition from massive to laminated siltstone to dropstone bearing siltstone and massive diamictite at the boundary between Gongdong and Chang'an formations indicates an ice sheet advancing sequence. The presence of abundant ice-rafted dropstones and rippled siltstone at the basal Chang'an Formation suggests active glacier calving and subglacial debris flows during ice sheet advancing. Also, the advancing ice sheet and glacial-influenced deposition are also supported by frequent changes in organic carbon isotope values (ranging from −26.5‰ to −29.3‰) that result from increasing terrestrial inputs. The ice sheet advancing sequence here shows that the global glaciation started within 1 million years, according to the average sedimentation rate of the glacial turbidity deposits. These findings provide the initial comprehensive sedimentological evidence for the rapid onset of Snowball Earth. • Seven lithofacies are recognized during the Tonian-Cryogenian transition in South China. • The sedimentary successions record an ice sheet advancing sequence during the onset of the Sturtian glaciation. • Based on the average sedimentation rate of the glacial deposits, the duration of ice sheet advance is estimated to be <1 million years. [ABSTRACT FROM AUTHOR]