Sequence-stratigraphic comparison of the upper Cambrian Series 3 to Furongian succession between the Shandong region, China and the Taebaek area, Korea: high variability of bounding surfaces in an epeiric platform

摘要

This study focuses on the stratigraphic sequences and the bounding surfaces in the upper Cambrian Series 3 to Furongian Gushan and Chaomidian formations in the Shandong region, China. The bounding surfaces are compared with those of the coeval succession in the Taebaek area, Korea. According to the vertical arrangement of the facies associations and the identification of the bounding surfaces, three stratigraphic sequences are recognized, representing dynamic changes in accommodation versus sedimentation. The bounding surfaces can be traced in the Shandong region for about 6,000 km2 in area, but cannot be correlated with those of the Taebaek area (eastern margin of the platform, about 1,000 km apart). Surface 1 is characterized by an abrupt facies change from carbonate to shale, representing a distinct drowning surface. The drowning surface is also diagnosed in the Taebaek area but highly diachronous. Surface 2 is a cryptic subaerial unconformity, reflected by an erosion surface, missing of a trilobite biozone (Prochuangia Zone), and an abrupt increase in carbon isotope value. It is not identified in the Taebaek area where the Prochuangia Zone is present. Surface 3 is a marine flooding surface, indicated by a subtle transition from flat-bedded microbialite to domal microbialite (or grainstone). It may be correlated with that in the Taebaek area, which is, however, represented by an abrupt facies change from sandstone to limestone-shale alternation. The high variability of the sequence-bounding surfaces is indicative of variable regional factors such as topographic relief, carbonate production, siliciclastic input, and hydrodynamic conditions. It suggests that the sequence-bounding surfaces are invalid for a basin-scale correlation, especially in an epeiric carbonate platform.