Maximum depositional age and provenance of the Cretaceous Icheonri Formation in southeast Korea using detrital zircon U-Pb dating

The Gyeongsang Basin, Korea, is a non-marine sedimentary basin formed in an active continental margin setting by the subduction of proto-Pacific plates beneath the East Asian continent during Early Cretaceous time. The basin fill is divided into four Goups based on volcanism and plutonism. They are the Sindong (no volcanic materials), Hayang (some volcanic materials), Yucheon (mostly volcanic materials), and Bulguksa intrusive Groups. Since siliciclastic sediments were dominant in the early stages of basin evolution, it was relatively easy to classify the relative ages of strata through lithostratigraphy. However, the chronostratigraphic correlation of the Yucheon sediments is very difficult due to the intermittent development of siliciclastic deposits compared to volcaniclastics by increased volcanic activity. The Icheonri Formation, belonging to the Yucheon Group, is very difficult to correlate chronostratigraphy with other strata. Additionally, radiolarian-bearing chert pebbles, whose source rocks have not been reported in Korea, were recently found in the Icheonri Formation, raising interest in their provenance. Therefore, LA-MS-ICP-MS detrital zircon U-Pb dating was performed on two sandstone samples to constrain the maximum deposition age of the formation and obtain information about their provenance. Both samples showed a relatively wide age range, from the Precambrian to the Cretaceous. Among them, the weighted mean ²⁰⁶Pb/²³⁸U ages of the youngest age clusters were calculated to be 99.76±0.58 Ma (n=10, MSWD=1.5) and 99.86±0.38 Ma (n=25, MSWD=1.5), respectively. These ages correspond to the early Cenomanian in geological time scale. The presence of radiorarian-bearing chert pebbles and significant concentrations of Permian zircons in the Icheonri Formation raises the likelihood that some of the sediments that comprise the formation are from SW Japan. This study provides important evidence indicating that before the opening of the East Sea, the Korean Peninsula and Japan were closer together than they are now. The temporal and spatial variations of the origin and supply processes from the source to the sink, however, still require more investigation.