It is known that high-quality, black-shale source rocks occur in the uppermost Ordovician Wufeng Formation and in the lowermost Silurian Longmaxi Formation in South China. Hence, it is important to understand their lithostratigraphy and the controls on their deposition. A review of lithostratigraphic criteria for subdividing the two adjacent formations provides new regional correlations between the formations and related stratigraphic successions and facies. Both the black shales and the related, overlying flysch deposits at the Ordovician-Silurian transition in South China appear to have migrated northwestward in time and space, reflecting probable flexural control in a foreland basin that developed in response to subduction-type orogeny southeast of the Yangtze block. The black shales also contain K-bentonites from explosive, felsic-intermediate volcanism, the distribution of which also supports orogeny to the southeast. Finally, the analysis of sequence stratigraphy, which shows that the initiation of transgressive system tracts (TST) and condensed section (CS) in the related third-order sequences coincided with the two black-shale horizons respectively, indicates that the main controlling factors for the deposition of the Ordovician-Silurian black shales in South China are (1) northwestwardly migrating, foreland-basin subsidence caused by deformational loading related to episodic accretion of the Cathaysia block to the Yangtze block during this period, and (2) the anoxic, sediment-starved water column caused by rapid rise of the sea-level during the two successive phases of third-order global sea-level rise near the Ordovician-Silurian transition in South China. In future exploration for hydrocarbon source rocks in the area, it is important to consider likely flexural and eustatic causes for subsiding, deep, anoxic seas in recognizing other source rock intervals, and our understanding of the Wufeng and Longmaxi formations may serve as models for future source rock exploration.
|Number of pages||12|
|Journal||Frontiers of Earth Science in China|
|State||Published - Oct 2007|
Bibliographical noteFunding Information:
Acknowledgements This research has been supported financially by National Natural Science Foundation of China (Nos. 49802002 and 40372057) and SINOPEC Project (G0800-06-ZS-319), with contributions from many colleagues and students. We sincerely thank Professors Wang Hongzhen and Yin Hongfu, China University of Geosciences (Beijing, Wuhan), for their kind encouragement and supervision. We extend our appreciation to Professor Wang Xunlian for the helpful discussion about the hierarchy and primary criteria for depositional sequences and cycle, to Prof. Xie Shucheng for his constructive review and comments. Sincerest thanks to Mrs. Huang Yutao and Mr. Deng Mingliang for their selfless assistances to SWB.
- Black shale in South China
- Lithospheric flexure
- Ordovician-Silurian source rocks
- Plate accretion
ASJC Scopus subject areas
- Earth and Planetary Sciences (all)