Maximum depositional age of the sedimentary succession in the southwestern margin of the Ulleung Basin from zircon age dating of conventional drilling cores

Abstract Biostratigraphic research has been conducted over the last four decades in the southwestern Ulleung Basin and has provided a chronostratigraphic framework to understand basin evolution. However, absolute geologic ages for the basin are almost absent owing to the lack of deep-drilled cores. To obtain the absolute age from the zircon, we extensively sampled and measured five deep-drilled conventional cores previously obtained for hydrocarbon exploration from the southwestern shelf of the Ulleung Basin. Maximum Depositional Ages (MDAs) for each cored interval are determined by the youngest age groups of zircons and are calculated to 12.1 ± 0.1 Ma for Gorae I-1, 12.9 ± 0.2 for Gorae I-2, 10.2 ± 0.8 for Gorae V-3, 11.4 ± 0.7 for Gorae V-4, and 16.9 ± 0.3 for Dolgorae V-1. All age groups exhibited relatively narrow errors (avg. ± 0.4 Ma), indicating good accuracy of MDA. The MDAs for the cores were first obtained by Laser Ablation-Multi Collector-Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS), and the youngest zircon grains were subsequently reanalyzed by Sensitive High-Resolution Ion Microprobe (SHRIMP) to prevent age underestimation due to possible Pb loss. The differences in absolute ages of Gorae I-1 and I-2 between LA-MC-ICPMS and SHRIMP methods are below than 0.8 Ma, indicating good verification of MDAs. The four seismic horizons that overlapped with the cored interval were newly established as GR I-2 MDA, GR I-1 MDA, GR V-4 MDA, and GR V-3 MDA (except for Dolgorae V-1 because of the poor quality of the seismic profiles). The newly defined MDA horizons were compared to the biostratigraphic sequence boundaries of (Yi et al., Marine and Petroleum Geology 122, 2020), indicating ages 1 to 3 Ma younger than the biostratigraphic estimates. The four MDA horizons provide a new time framework for sequence stratigraphic interpretation of the Miocene deposits as well as reconstruction of the basin-evolution model in the southwestern shelf of the Ulleung Basin.