Integrated sequence-, bio- and chemostratigraphy of the terminal Proterozoic to Lowermost Cambrian "black rock series" from central South China

Abstract

Integrated litho-, bio-, and chemostratigraphic study from the stratigraphic interval of the Nantuo post-glacial to Lowermost Cambrian in the central South China suggests that five sequences of third grade could be recognized in the central Yangtze platform (CYP) and its Jiangnan marginal slope (JMS) or basin (JMB) and Xiang-Qian back-arc basin (XQBB). Each sequence turnover is always related to the occurrence of "black rock series" (cherts, shales, dolomites and limestones) accompanied by an oxidizing TST following a shallowing upward HST of the last sequence. These "black rock series" are thought to be formed in a relatively low-energy but current-influenced subtidal environment, regardless of their difference in paleogeographic locations, and characterized by high original organic matter contents, probably derived from primary rather highly diversified megaalgae, vendobiota, soft-bodied metabiota and prokaryotic bacteria during the Neoproterozoic III, and later from multiphyletic small shelly fossils and planktonic trilobites during the lowest Cambrian. The differentiation of organic matter contents for different black rock types of the same horizon is related to paleo-weathering activities and to the hydrothermal activities in association with the impregnation of highi sulfide-related rare and precious metal elements caused by subsequent tectono-thermal events. For the most cherts of the Upper Sinian Liuchapo Formation or coeval beds the silicification is interpreted to have resulted from prediagenetic hydrothermal replacement of primary black muds. The apparent "anoxia" expressed by these "black rock series" does not mean the absence of oxygen in the original oceanic environment during that time or in the shallow water column, but rather virtually the depletion of oxygen occurs at the sediment/water interface due to sedimentary organic surplus stockpiling and disruption as well as disintegration of organic matter during burial and also during early phases of diagenesis. The occurrence of four main biotic assemblages i.e. the Miaohe biota, Vendotaenia with Ediacara-type soft-bodied metazoans, multiphyletic small shelly fossils and trilobites in the terminal Precambrian to earliest Cambrian "black rock series" is related to shallow-water originally oxidizing transgressive environments in the CYP and JMB. The main reason for the usual absence of most of these organic-walled non-skeletal biota in the "black rock series" of same age in the XQBB would probably have been influenced by volcanically derived hydrothermal activities leading to a rising seafloor water temperature during that time, and coincidentally to a relative deep "anoxia" environment as well. Two obvious changes in the distribution of REE, EBP and Sa/Ar are observed between sequence 3 and 4 and between sequence 4 and 5 in XQBB. These two sequence boundaries would separately be equivalent to those between the Precambrian and Cambrian and between the Yanjiahe and Shujingtuo Formations in the Yangtze Platform.