Print this pageCambrian Sequence Stratigraphy of Western Ordos Basin, North China, and Initial Intercontinental Comparative Study of California at Riverside, Riverside, CA
LI, XIANG, and D. OSLEGER
In the Cambrian rocks of the western margin of the Ordos Basin, North China, 12 3rd-order depositional sequences are recognized, covering a time span of 560-510 Ma. Fifth to sixth order cycles dominate pure carbonate platform environments, while fourth to order cycles are the main type in mixed carbonate/clastic environments. The Lower-lower Middle Cambrian, composed of carbonate and clastic sediments deposited in varying environments, consist of four 3rd-order sequences, which are correlatable throughout the entire study area, indicating regional sea level control over their development. The remaining Middle Cambrian, composed of cycles of deeper subtidal or shelf facies, marks the widest marine transgression. The Upper Cambrian represents a 2nd order regression which comprises 3rd order sequences composed mainly of peritidal cycles. Sedimentological as well as ichnological studies are combined to define shallowing and deepening trends of higher-order cycles. These cycles stack to delineate lower-order sequences. Key surfaces which help in determining sequence boundaries include prominent erosional surfaces, paleokarst surfaces, contemporaneous dolomitization surfaces and maximum flooding surfaces associated with condensed sections.
Subsidence analysis provides some semi-quantitative insights into sea level changes, which is used in an inter-continental comparison between the North China and the North America Platforms. For example, an initial comparison shows that the unconformity separating the six Cambrian depositional sequences at the Riley/Wilberns boundary in central Texas coincides with the lower limit of the Cambro-Ordovician unconformity zone in Ordos.