DEVONIAN SEDIMENTARY EVOLUTION AND TRANSGRESSION-REGRESSION PATTERNS IN SOUTH CHINA

Chen Daizhao;Chen Qiying (Institute of Geology, Chinese Academy of Sciences, Beijing 100029)  

he Devonian time-factes migration patterns .in Guangxi and Hunan are quite different from that in southeastern Guizhou of South China, especially after late Middle Devonian. The landward factes migration and shoreline shiftiness which are progressive in southeastern Guizhou, indicate that the area is located on a stable platform. But they are quite sharp, and show a steplike or z-shaped pattern in Guangxi and Hunan. The factes and siliciclastic shoreline abruptly change at the breaking points of the step relief, reflecting a terraced landform. Narrow intraplatform troughs, in which siliceous rocks, black shales and nodular limestones dominate, emerged at the breaking points on the carbonate platform almost at the same time, representing a contemporaneous extensional basement faulting.A synthetical relative sea level curve for the Devonian in South China is constructed after the detailed analysis on factes distribution and migration in Guangxi,Guizhou and Hunan. It is demonstrated that two second-order transgression-regression (T-R) cycles with predominant transgressions in early Emsian and late Frasnian.two main regressions in late Eifelian and late Famennian, and at least sixteen thirdorder cycles (C1-C16), which are dated in the standard conodont zonation, were regionally developed. The most of cycles can be correlated with Euramerican cycles,and show the characterictcs of abrupt deepening followed by prolonged upward-shallowing.It is evident that the T-R patterns of Early Devonian in South China are quite similar to those in southern West Siberia, implying that the two regions had been connected with the Paleotethyan Sea at that time. The patterns of Middle and Late Devonian, however, are much similar to the Euramerican patterns, suggesting a eustatic control on the T-R cycles.It seems unreasonable that the T-R cycles in response to sea level changes are caused by glaciation because of the absence of extensive continental ice sheets during Devonian except that at the Famennian stage. The episodic basement extension and submarine volcanism, coinciding with the dominant transgression, suggest that the combination of the growth and decay of Devonian oceanic ridge systems with intraplate volcanism (or thermal activity) and intraplate stress fields could account for the Devonian T-R cyclicity and factes migration.

【Fund】： 国家自然科学基金
【CateGory Index】： P512.3

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