Natural remanent magnetization characteristics of the upper unconsolidated sedimentary sequence of a drill core (20HCL04) from the Jeokjung-Chogye basin (impact crater), South Korea: Preliminary implication of multiple possible geomagnetic excursions within persistent normal-polarity state and notes toward magneto-chronostratigraphic contribution

Abstract In the Jeokjung-Chogye Basin, southeastern Korean Peninsula, presumed to be a Quaternary meteorite impact crater, an azimuthally non-oriented drill sediment core (called 20HCL04; up to 66.0 m depth) was retrieved. We conducted paleomagnetic and mineral magnetic investigation on samples from the unconsolidated sedimentary sequence of 14.0–42.0 m-depth interval of the core (collectively, 747 discrete subsamples with high subsampling density), to provide independent constraints on the chronologic framework for the core. Comprehensive mineral magnetic analyses suggest the existence of low-coercivity magnetic minerals, probably magnetite and greigite, as major remanence carriers, which contribute to natural remanent magnetizations (NRMs) in parts of the horizons. Fortunately, careful analyses of the progressive alternating field (AF) demagnetization results allow the determination of a reasonably stable characteristic remanence (ChRM) component. Relative paleointensity (RPI) can be putatively estimated by adopting the ratio of the NRM intensity after 30 mT AF demagnetization normalized by the initial magnetic susceptibility as the RPI proxy candidate. A reasonable magnetostratigraphic record consisting of ChRM inclinations and putative RPI values could be reconstructed by conservatively excluding data that might be potentially greigite-carried due to the large uncertainty in remanence acquisition. The reconstruction allows us to recognize that the entire analyzed sequence corresponds to a single persistent normal-polarity chron, in which there are multiple, large-amplitude directional swings associated with remarkable RPI lowering, indicating the recording of geomagnetic excursion events. Currently, at least six separate excursion events are considered to be recorded. Combined with current knowledge on geomagnetic excursions, particularly from the Brunhes period, our results can reasonably estimate the timescale of the 28 m-long sediment formation, which is likely in the order of one- to several-hundreds of kyr. Although the age-depth relationship for the core remains to be ascertained, current data recording large-amplitude fluctuations associated with geomagnetic excursions provide a valuable basis for establishing the age-depth relationship and the ultimate aim of high-fidelity chronologies of basin-filling sediments and basin-forming impact cratering. Moreover, the current data and further magnetic investigations could potentially highlight the ancient geomagnetic field nature during and across a single excursion event, in addition to regional paleoclimate and paleoenvironmental changes.