Sequence stratigraphy of the Cryogenian Nantuo Formation in South China: Constraints on Marinoan glaciation dynamics

摘要

Cryogenian low-latitude glaciations mark the 'Snowball Earth' events and were thought to be dynamic on the basis of sedimentary features, especially glacial cycles. Although the glaciation dynamics and associated environmental changes can be investigated through the sedimentary records, there are sparse sedimentary studies. Studies from the Marinoan deposits in South China provide the basis for establishing a sequence stratigraphic framework that includes two complete glacial cycles across a basin-to-platform transect. Both cycles are characterized by symmetrical facies patterns, with a lower ice-advance succession and an upper ice-retreat succession. Each ice-advance succession consists primarily of a massive diamictite lithofacies association, and is dominated by proximal glaciomarine deposits. The gravel abundance and size of the deposits increase upward gradually, suggesting a progradational sequence. Each ice-retreat succession is composed of both a massive diamictite lithofacies association and a fine-grained lithofacies association, representing a mix of distal glaciomarine, proximal glaciomarine, and non-glacial marine deposits. The gravel abundance and size of these varied deposits decrease upward gradually, interpreted to reflect a retrogradational sequence. This sequence stratigraphic framework, interpreted to reflect gradual alternation from ice advance to ice retreat, indicates dynamic co-evolution of the sedimentary environments, paleoclimate, and sea level during the Marinoan glaciation. The glaciation dynamics, as they are reflected globally, highlight a gradual transition from the Marinoan glaciation to the Ediacaran greenhouse, and indicate a time lag between the onset of the deglaciation and the formation of the cap carbonates.