Comparison of Late Ordovician Epicontinental Seas and Their Relative Bathymetry in North America and China

Abstract

Six widely separated areas with Upper Ordovician strata in Canada and the United States are compared with six localities in southern China. Cyclic sedimentation, including evaporites, carbonates, and phosphatic black shales, occurred in relatively shallow epicontinental seas of North America. As many as three Ashgill cycles of regionally different styles may have been coeval throughout North America in response to modest changes in sea level. Environments in South China primarily included carbonates and black shales with a far more uniform distribution. Absence of comparable sedimentary cycles confirms that the platform bathymetry of South China was consistently deeper than in North America. The almost complete exposure of North China (Sino-Korean Plate) by Late Ordovician time underscores the fact that independent cratons have different bathymetric histories. By mid-Silurian time, South China also was fully exposed. Such hypsographic variation is critical to the intercontinental correlation of features related to eustasy, water chemistry, and even the patterns of extinctions.