Tectonic and eustatic control on the distribution of black-shale source beds in the Wufeng and Longmaxi formations (Ordovician-Silurian),South China |
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| Authors: | Su Wenbo Li Zhiming Ettensohn F R Johnson M E Huff W D Wang Wei Ma Chao Li Lu Zhang Lei and Zhao Huijing |
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| Institution: | (1) School of Earth Sciences and Resources, China University of Geosciences, Beijing, 100083, China;(2) Faculty of Earth Sciences, China University of Geosciences, Wuhan, 430074, China;(3) Department of Earth & Environment Sciences, University of Kentucky, Lexington, KY 40506-0053, USA;(4) Department of Geosciences, Williams College, Williamstown, MA 01267, USA;(5) Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA |
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| Abstract: | 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.
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Translated from Earth Science—Journal of China University of Geosciences, 2007, 32(6): 818–827 译自: 地球科学—中国地质大学学报] |
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| Keywords: | Ordovician-Silurian source rocks black shale in South China lithospheric flexure eustasy plate accretion |
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