Microfacies of Ediacaran Glacial Marine Carbonates Along Southern Margin of North China and Climato-Environmental Implications

Cryogenian glaciations have attracted great attention of multidisciplinary scientists worldwide, yet the glacial record of this period has not been recognized in North China Craton (NCC). The glacial deposits of the Luoquan Formation and its equivalents, which are well documented along western and southern margins of NCC, have often been cited to support the Great Late Ediacaran Glaciation hypothesis, although its maximum age remains poorly constrained. Despite its climatological significance, detailed microfacies analyses of the Luoquan Formation are still lacking. New outcrop observations and microfacies analyses of the Luoquan Formation in the Shangzhangwan section in Luonan, Shaanxi Province of China, have led to a reappraisal of its sedimentary model. Two major sedimentary facies have been identified: (1) Thick-bedded dolomitic breccial facies, previously interpreted as a massive basal diamictites, appears as a lenticular geometry with a concave-up base, flat top, and normal graded bedding, which are wedged within the laminated calcareous siltstone facies with dropstones. Therefore, the breccial facies is reinterpreted as debris flow deposits filling a channel. (2) The laminated facies with dropstones, previously interpreted as glaciolacustrine varves, consist of alternating Ca-Mg-rich, Al-K-rich, and Fe-Mn-rich millimeter to centimeter thick laminae. The Ca-Mg laminae are calcareous siltstones with graded bedding, representing turbidite deposits; the Al-K laminae are dominated by lime mud and clay, interpreted as suspension settling deposits; the Fe-Mn laminae are rich in carbonaceous clasts and pyrite, denoting biogenic deposits. Dropstones, dump structures, and soft-sediment deformation within the laminated facies indicate a glacial environment. The continental-scale distribution and absence of lacustrine indicators suggest a glaciomarine setting. Taken together, we propose that the Luoquan Formation here was deposited in an off-shore, open ocean ice-berg zone, receiving sediments from suspension, ice-berg delivery, debris flows, turbidity currents, biogenesis, and authigenesis during the deglacial period. This study provides an examples of detailed facies analysis of the Luoquan glaciation in NCC. In particular, the presence of carbonates in Ediacaran glaciomarine deposits has important implications for understanding climatic and environmental changes during the rise of animals.