THE ONSET OF SKELETAL COMPONENT: THE OLDEST SKELETAL DOMINATED CARBONATE PLATFORM IN THE SINO-KOREAN BLOCK

During the Ordovician period, the carbonate platform underwent significant changes in dominant components as a result of the Great Ordovician Biodiversification Event. Although these changes have been extensively documented on various continents, their evaluation in the Sino-Korean Block (SKB) was challenging due to the middle Floian-lower Darriwilian depositional hiatus and the subsequent thick peritidal strata. This study focuses on these changes and reports the oldest skeletal-dominated platform within the SKB.

We investigated a 68 m-thick and a supplementary 25 m-thick sections of the middle Darriwilian Duwibong Formation. A microfacies analysis, with a vertical sampling interval of less than 20 cm, was employed to visually estimate the proportion of components. The studied interval mainly comprises bioclastic-peloidal wackestone to packstone with subordinate argillaceous wackestone, lime mudstone, dolomitic mudstone, oolitic grainstone, and shale.

The bioclastic-peloidal wackestone to grainstone exhibits some variation in component proportion but is mainly composed of coated grains including ooids and oncoids, peloids, along with mollusks and echinoderms. Trilobites also contribute significantly, albeit less abundantly compared to mollusks and echinoderms. Ostracods, bryozoans, and tubular organisms such as Amsassia, and Solenopora are less common components.

The proportions and the diversity of the skeletal components described in the Duwibong Formation show a notable increase compared to the basal member of the Makgol Formation, just below the middle Floian-lower Darriwilian hiatus, where trilobites and echinoderms are significant components. The proportions of trilobites appear similar between the two formations, but there is a significant increase in the abundance of crinoids and mollusks, as well as an increase in bryozoans, Tubular organisms, and brachiopods.

The pattern of increase in skeletal components recognized in the Duwibong Formation is similar to that of Laurentia in the addition of various new components. However, it differs in that the increase was predominantly driven by mollusks and echinoderms. This study provides valuable data on the onset of the skeletal-dominated carbonate platform in the SKB during the Middle Ordovician.