AS THE WATERS COVER THE SEA: CHARACTERIZATION OF THE INITIAL MIDDLE ORDOVICIAN ONLAP ONTO EASTERN NORTH CHINA CARBONATE EPEIRIC PLATFORM

We report the initial Middle Ordovician inundated pattern of Sino-Korean (North China) epeiric carbonate platform from the lower Makgol Formation, Taebaeksan Basin, Korea, located in the easternmost margin of the Sino-Korean Block via integrating facies analysis, conodont biostratigraphy and carbon isotope chemostratigraphy.

The lower Makgol Formation is characterized by repeated cyclic packages consisting of subtidal meter-scale cycles and succeeding peritidal meter-scale cycles. In all sections, peritidal cycles rest on the hiatus, and succeeding cyclic packages contain sharp lower and upper boundaries that can be correlated across the study area. The frequency of peritidal cycles gradually decreases upward, while the subtidal cycle becomes predominant, indicating stepwise onlapping of the carbonate platform. Biostratigraphic and carbon isotope data are related to facies and type of cycles. The lower part, dominated by peritidal cycles, shows poor conodont recovery, and early Darriwilian conodonts Tangshanodus tangshanensis and Aurilobodus leptosomatus are abundantly yielded from the middle part. Carbon isotope data show variation of -1 to -6 ‰ in peritidal cycles, whereas smaller variation of -2 to -3 ‰ in subtidal cycle. These results indicate that the peritidal condition is unfavorable for conodont preservation and carbon isotope values were disturbed by meteoric influence related to subaerial exposure.

The record of Middle Ordovician inundation varies across the Sino-Korean Block. In the northern region, repeated accumulation of peritidal cycles over hundreds of meters thick are recorded above the hiatus, whereas in the western region, micritic limestone intercalated with quartz sandstone overlie the hiatus, which appears to reflect the autogenic process, siliciclastic input from nearby highland, and topographic relief.

Intraplatformal basin and archipelago models have been suggested for understanding the inundation process of the epeiric platform, but this study highlights that the inundation process cannot be simply explained by these models and can vary greatly regionally. In addition, this study demonstrates the challenges of precise dating of the initial onlap, given the nature of the geologic records.