Maximum depositional age and sediment provenance of the Cretaceous Seongpori Formation (Geoje Island, Korea) constrained by detrital zircon U–Pb geochronology.

East Asia, including the Korean Peninsula, has been located along an active continental margin since the Mesozoic, where magmatism and fault activity have been primarily controlled by changes in subduction dynamics. In this region, numerous Cretaceous sedimentary basins of various scales developed along the eastern margin of the Eurasian Plate. In particular, from the mid-Cretaceous to the early Paleogene, intensive magmatic activity occurred along the subduction-related margin, forming the Gyeongsang Volcanic Arc that extended across the present-day Korean Peninsula and proto-Japan. The development of such a volcanic arc is expected to have exerted first-order control on the reorganization of source-to-sink systems in adjacent sedimentary basins.

The Gyeongsang Basin, the largest Cretaceous basin in Korea, is located in the southeastern part of the peninsula. The Seongpori Formation on Geoje Island in the southeastern part of the basin, was deposited adjacent to rocks related to the Gyeongsang Volcanic Arc. To constrain the depositional age and sediment provenance of the Seongpori Formation, we conducted detrital zircon U–Pb dating using LA–ICP–MS on six samples (four from the northwestern island and two from the southeastern island).

Maximum depositional ages were estimated using the maximum likelihood age (MLA) method. The resulting MLAs range from 98 to 93 Ma for the northwestern samples and from 106 to 105 Ma for the southeastern samples. Provenance analysis indicates that the common major sediment sources of the Seongpori Formation were Paleoproterozoic and Mesozoic rocks exposed in the Yeongnam Massif, which borders the western and northern parts of the Gyeongsang Basin. In addition, some northwestern samples contain Neoproterozoic and Paleozoic zircons, which are interpreted to have been derived from more distal sources, such as the Okcheon Belt located in the northwestern part of the Yeongnam Massif, or possibly from the Gyeonggi Massif farther to the northwest.

The Cretaceous detrital zircon age spectra show clear spatial variations. Northwestern samples are dominated by 100–90 Ma zircons, whereas southeastern samples are characterized by an older and broader age range of 115–100 Ma. These differences are interpreted to reflect spatial variations in magmatic activity within the Korean Peninsula and proto-Japan areas of the Gyeongsang Volcanic Arc, as well as a change in sediment provenance. We infer that the sediment sources of the Seongpori Formation varied spatially and that a shift in the source-to-sink system occurred at ~100 Ma. This timing coincides with the development of an andesitic stratovolcano at ~99 Ma in central Geoje Island, suggesting a possible reorganization of the drainage system induced by volcanic arc construction.