Stratigraphic correlations and occurrence patterns of two sets of Lower Cretaceous black shales in coastal southeastern China and geological implications: insights from zircon U–Pb ages

Abstract

Lower Cretaceous black shales in coastal southeastern China are of significance to the geological study of the Pacific tectonic domains. However, the stratigraphic correlations and occurrence patterns of the shales have not been well constrained so far. To address these limitations, 131 new zircon U–Pb dates were obtained from the tuff layers, which were interlayered with the shales from four outcrops. In combination with previously reported geochronological data, the stratigraphic correlations and occurrence patterns of the black shales were discussed. Results show that the black shales in the Shuidishan Formation (~145 Ma) in the Dayawan section, northern Guangdong, are older than those in the lower part of the Bantou Formation (~132 Ma) in the northward Yong’an section, southern Fujian, whereas they are roughly isochronous to the base of the Bantou Formation (~144 Ma). In contrast, the black shales in the upper part of the Bantou Formation (~117 Ma) from the Yong’an section in northwestern Fujian and the black shales in the Shixi Formation (~117 Ma) from the Yiyang section in northeastern Jiangxi can be isochronously correlated with the black shales in the Bantou Formation (~117 Ma) from the Chong’an section in northwestern Fujian, and they are slightly older than those in the Guantou Formation (~113 Ma) from the Shangzhang and Xiahuyuan sections in western Zhejiang. Black shales in Units I and II of the Shipu Group (~113–109 Ma) in northeastern Zhejiang can be isochronously correlated with those in the Guantou Formation (~113–106 Ma) in western Zhejiang. These new stratigraphic correlations indicate that the Lower Cretaceous black shales in coastal southeastern China can be divided into two regional-scale sets. The first set was deposited during the early Early Cretaceous (Berriasian–Hauterivian) and is diachronous (i.e. 144 ± 2 Ma in the Dayawan section and 132 ± 2 Ma in the Yong’an section). The second set of black shales was deposited during the later Early Cretaceous and was roughly isochronous (~117 Ma). These results imply that the processes of the Pacific plate subduction in coastal southeastern China during the Early Cretaceous varied in two distinct stages that are identified by two volcanic–black shale cycles (i.e. ~145–120 and ~120–100 Ma). The distribution of the first set of black shales may extend to offshore basins in the South China Sea, whereas the second set may extend to the Taiwan Strait. Both sets are likely to be potential petroleum prospects. Copyright © 2016 John Wiley & Sons, Ltd.

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