Depositional processes and sequence stratigraphy of the Jigunsan Formation (Middle Ordovician), Taebaeksan Basin, mideast Korea: implications for basin geometry and sequence development

Abstract

This study focuses on the depositional processes of the Middle Ordovician Jigunsan Formation of the Taebaeksan Basin, mideast Korea, an extensive fine-grained deposit (greenish to dark gray mudstone and interbedded limestone), in view of the recent plate reconstruction of the Sino-Korean Block. The Jigunsan Formation and the bounding formations are represented by eleven sedimentary facies that are organized into sis successive facies associations (FAs). The facies successions reflect an overall development of a carbonate platform that was inundated during an initial phase of transgression (lower part of FA1) forming deep subtidal to basinal environments which were influenced by dilute turbidity currents and occasional storm-induced density currents (FAs 2,3, and 4). During a stillstand in sea level, shallow platform sediments (FAs 5 and 6) prograded over the deep subtidal to basinal area. The initial flooding surface of the platform is the top of a supratidal deposit that is underlain by a peloidal grainstone (Gp) bed. The surface is interpreted as a sequence boundary/transgressive surface. The maximum flooding zone (MFZ) occurs in the upper part of facies association 2, which is characterized by a mudstone interval. The conodont and trilobite biostratigraphy suggests that the Jigunsan Formation represents a third-order (0.5−3 My) rise in sea level. The deposit formed in the early Darriwilian in the eastern end of a large-scale (ca. 1000 km long and 50 km wide) eastward-deepening embayment, an eastward extending arm of the North China Platform across the Pyeongnam Basin, North Korea. The early Middle Ordovician sequence boundary in the Taebaeksan Basin was formed by eustatic sea-level fall During the subsequent transgression, the eastern arm was largely inundated and under a tidal regime. In the deepening eastern end of the arm (Taebaeksan Basin), however, deposition was largely influenced by the input of siliciclastic sediment and the expansion of oxygenrestricted deep water due to the proximity to the hinterland and the semi-enclosed basin configuration, respectively.