Glacial origin of the Cryogenian Nantuo Formation in eastern Shennongjia area (South China): Implications for macroalgal survival

The Cryogenian Period was characterized by global glaciations, imposing extreme environmental stresses on early multicellular organisms. Globally distributed diamictite deposits are the major evidence for these glaciation events, but the glacial origin of at least some of these diamictite deposits has been disputed, complicating interpretations of the environmental conditions influencing the evolution of life in the Cryogenian (~720–635 Ma). In this study, we conducted a systematic sedimentological investigation of the Marinoan-age (~650–635 Ma) Nantuo Formation in six sections representing a ~20-km-long, east–west transect in the eastern Shennongjia area of South China. The Nantuo Formation, which ranges in thickness from ~2 m in the east to ~300 m in the west, comprises diamictite, conglomerate, lonestone-bearing sandstone, green siltstone, and black shale facies. The eastern sections (e.g., Longxi and Wushanhu) consist mainly of diamictite facies exhibiting micro-scaled rotation structures, microshear, grain stacking, grain-to-grain contacts and water-escape structures, supporting their deposition in a subglacial setting. In contrast, the western sections (Shijiahe, Liziping and Songluo) consist of massive and graded diamictite and conglomerate facies, consistent with an origin as glacially induced submarine debris flows. Furthermore, the lonestone-bearing sandstone facies in these sections contain abundant dropstones and mud pellets within finely-laminated turbidites, indicating an ice-rafted origin with locally reworking by turbidity currents. The green siltstone and black shale facies lack glacigenic features and may represent late deglacial deposits. Based on thickness and facies variations among the study sections, we infer that the Nantuo Formation records glacial sedimentation on a complex paleotopographic surface that was incised into the underlying bedrock, with subglacial deposits to the east transitioning into glaciomarine deposits to the west of study area. The distal Songluo section likely records three cycles of glacial advance and retreat in the form of a facies alternation between diamictite and black shale, consistent with a dynamically fluctuating ice margin. In summary, our sedimentological investigation supports a glacial origin for the Nantuo Formation in the eastern Shennongjia area. The occurrence of the benthic macroalgal-dominated Songluo Biota in the black shale facies at Songluo suggests that strong glacial retreats provided niches for macroalgal survival in the Cryogenian glacial ocean.