Distribution of black shale in the Wufeng-Longmaxi formations (ordovician--silurian), south china: major controlling factors and implications

Review of lithostratigraphic criteria for subdividing the Wufeng (Ordovician) and Longmaxi (Silurian) formations reaffirms their integrity and subdivisions and provides new regional correlations between the formations and related stratigraphic successions and facies. Both the black shales and the related overlying flysch deposits and other successions 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, analysis of sequence stratigraphy, which shows that the initiation of transgressive system tracts (TST) and condensed section (cs) in the related 3rd-order sequences coincided with the two black-shale horizons respectively, indicates that the major 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 the 3rd-order, eustatic, sea-level rise near the Ordovician-Silurian transition in South China. In the future exploration for source rocks from the area in various periods, consideration of likely flexural and eustatic causes for subsiding, deep, anoxic seas may be important in recognizing other source-rock intervals.