Environmental Dynamics of Organic Accumulation and Preservation for the Principal Paleozoic Organic-Rich Sedimentary Rocks on the Yangtze Block, South China

Daizhao Chen¹, Jianguo Wang¹, Detian Yan², Hengye Wei¹, Hao Yu¹, and Qingchen Wang^1
1Key Laboratory of Petroleum Resource Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
²Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan, China

On Yangtze Block, there are three regionally-distributed organic-rich sedimentary rock horizons: the Lower Cambrian, Upper Ordovician-Lower Silurian and Permian (Middle and Upper Permian). The Lower Cambrian organic-rich rocks mainly consist of the lower black chert (Liuchapo Formation) and upper siliceous and carbonaceous black shales (or stone coals) (Niutitang Formation). During the Early Cambrian, extensive hydrothermal activities occurred on the Yangtze platform margins extending ca. 2,000-3,000 km long, which could have released vast amounts of silica-rich fluids into the ocean, inducing extensive silica precipitation off the platform margin through basin. Simultaneously, a large quantity of H2S and greenhouse gases (CO2, CH4) and metalliferous anoxic fluids were vented into oceanic waters and atmosphere, leading to an apparent elevation of atmospheric CO2 concentration and large-scale rise of sea-level. In this case, nutrient-rich deep water masses were carried through the upwelling currents into the shallower shelf, which could have aggravated the anoxic state of water masses and enhanced the primary production, organic burial and subsequent preservation in the context of sulphate depletion in seawaters.

The Upper Ordovician-Lower Silurian organic-rich rocks mainly comprise black shales and siliceous shales of Wufeng and Longmaxi formations, which formed during marine transgressions under a warm climate during the Ordo-Silurian transitional time. With the accelerated amalgamation between Yangtze and Cathaysian blocks to the east, a large epicontinental sea with subbasins (or sags) linked to a largely expanded landmasses was developed on the Yangtze Block, forming silled subbasins within which water circulation was restricted and oceanic stratification was intensified, favorable for organic accumulation. The enhanced chemical weathering could have increased nutrient flux into the basins, thereby increasing the nutrient supply and primary productivity. However, greatly increased continental clastic flux into basins could have diluted the organic matters in sediments, lowering the organic enrichment as seen in the upper part of Longmaxi Formation.

The Middle Permian organic-rich rocks are mainly present in the Qixia Formation, especially in the lower part; they deposited in the intrashelf depressions (or subbasins). The organic abundance in rocks varies episodically in accordance with the tune of stratal cyclicity, therefore it could have been controlled by the fluctuations of primary productivity triggered by the variable-order sea level fluctuations. The superimposed sea level rises in different order are the most favourable interval for organic accumulation and preservation. The Upper Permian organic-rich rocks mainly occur in the Dalong Formation; they also deposited in the intrashelf depressions (or subbasins), within which water masses were poorly circulated and stratified, leading to fairly euxinic (sulfidic) oceanic condition in the presence of excessive sulphate in seawaters, favourable for organic accumulation and preservation. This condition is different from those in the Early Cambrian and Ordo-Silurian transition.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013