Stratigraphy And Paleogeography Of Ediacaran Around Huangling Anticline

The Ediacaran Period was a critical time between the "Snowball Earth" and the "Cambrian Explosion". Well-preserved multicellular eukaryotes, including animal embryos, and the evidence of the Neoproterozoic Oxidation Event were documented in the Ediacaran rocks in South China. However, the relationship between the early life and environments is less definitive, which raises requirement for a precise stratigraphic and paleogeographic framework.In this study, multiple stratigraphic methods, including litho-, sequence-, and chemo-stratigraphy, are applied to establish the Ediacaran stratigraphic and paleogeographic frameworks in the areas around the Huangling Anticline. Four particular areas are divided around the Huangling Anticline, i.e. the southern, northern, eastern and western limb areas. First, a stratigraphic framework is established in each limb area. Then a regional correlation is conducted based on some marker beds. Finally, the paleogeography is constructed for each stage according to the stratigraphic framework.The Ediacaran strata are lithostratigraphically divided into the Doushantuo Formation and Dengying Formation in the southern limb area. The Doushantuo Formation is further subdivided into four members, while the Dengying Formation includes the Hamajing Member, Miaohe Member, Shibantan Member, Baimatuo Member and Tianzhushan Member. Six comparable sequence boundaries are identified. The high resolution Ediacaran carbon isotope profile is established in the southern limb, and six significant negative carbon isotope excursions are documented.According to the drilling cores and outcrop sections, the lithostratigraphic sequence is established in the northern limb area of the Huangling Anticline. The Doushantuo Formation contains four members, and its Second Member includes four parts. The Dengying Formation is divided into the Hamajing Member, Shibantan Member and Baimatuo Member in ascending order. Five identical sequence boundaries are documented in the Ediacaran strata in the northern limb. They can bed well correlated through the northern limb area. The high resolution carbon isotope profile of the northern limb is established in the core sections. Six carbon isotope excursions are distinctively recognized in the Doushantuo Formation and Dengying Formation.The Doushantuo Formation and Dengying Formation also exist in the eastern limb of the Huangling Anticline. The Doushantuo Formation includes four members, and the Dengying Formation contains Hamajing Member, Shibantan Member, Baimatuo Member and Tianzhushan Member. Five identical sequence boundaries are found in the eastern limb area. In the well-exposed Maohu section, a prominent carbon isotope excursion occurs in the top of the Third Member of Doushantuo Formation.Based on lots of field investigation, a complet Ediacaran sequence has been established in the western limb of the Huangling Anticline, including the Doushantuo Formation and Dengying Formation. The lithostratigraphic and sequence-stratigraphic successions can be correlated with those in the southern limb. Two carbon isotope excursions are documented in the Doushantuo-Dengying transitional interval, which relate with the SB4 and SB5.Marker beds, Doushantuo Member IV and Miaohe Member are conducted for the regional correlation. The siliceous shale unit that hosts the Ediacaran Miaohe biota referred to as the Miaohe Member. This unit was regarded to be time-equivalent with the Doushantuo Member Ⅳ. New bio-, sequence- and chemo-stratigraphic studies demonstrate that the Miaohe Member is significantly younger than the Doushantuo Member IV, most likely time-equivalent with the lower part of the Shibantan Member in the Dengying Formation. The new stratigraphic framework indicates that the top of the Doushantuo Formation should be much older than 551 Ma, and the top of the upper Doushantuo carbon isotope excursion should be much older than 551 Ma, likely≥560 Ma. The ocean oxygenation event documented from the Doushantuo Member IV black shales should also be older than 560 Ma and predated the Miaohe Biota for more than 10 Myr. The lithology of Doushantuo Member IV is regionally various. Sequence- and chemostratigraphic studies indicate the Sequence 3 may be missing in the eastern and northern limbs of the Huangling Anticline.Based on the comprehensive study of the biotic and environment events, the Ediacaran System is chronostratigraphically divided into two Series and four Stages. The previous names of the Xiadongian and Yangtzean are adopted here for the lower and upper series, respectively. But the boundary between two Series is here placed at the SB3. The Xiadongian Series is subdivided into two Stages. Stage 1 is characterized by the lower assemblage of acanthomorphic acritarchs, while Stage 2 is by the upper acritarch assemblage. The boundary between Stagel and Stage 2 is marked by the beginning of carbon isotope excursion CN3, which is coincident with the disappearance of the lower assemblage. The Yangtzean Series is also subdivided into two stages. Stage 3 is characterized by the permanent carbon isotope excursion CN4 and no fossils have been observed yet in this stage. Stage 4 contains micro-fossils such as multicellular algae and animals. The base of Stage 4 is defined at the first appearance of the Miaohe Biota.According to the new stratigraphic framework, the Ediacaran Period is divided into 13 intervals to reconstruct the paleogeography throughout the Huangling region in order to precisely illustrate the evolution of the sedimentary paleogeography and environments during the critical period. The serial reconstruction of paleogeography through the time indicates that the northeast limb was shallower than the southwest limb of the Huangling Anticline. The sedimentary basin had been clearly restricted and the ocean might be strongly stratiform since the formation of the Cap Dolomite at the beginning of the Ediacara, but it became an open carbonate facies after 551 Ma.In conclusion, this study has established the Ediacaran stratigraphic framework across the Huangling Anticline region and reconstructed the evolutionary history of the paleogeography through the time. The stratigraphic framework and paleogeographic evolution provide a firm base for the further study of the relationship between the Ediacaran life and environments.