KOREA INSTITUTE OF GEOSCIENCE AND MINERAL RESOURCES

First Finding of Impact Cratering in the Korean Peninsula by KIGAM

Korea Institute of Geoscience and Mineral Resources (KIGAM), as the government-supported Geoscience R & R&D organization, our priority is to enhance Korea's geoscience research capabilities, which is known as ‘K-Geoscience,’ and expand it to the world. In 2020, despite the difficult situation of on-site investigations such as COVID-19, KIGAM was able to find the first meteorite impact in basins on the Korean Peninsula. Since the last January 2020, the group of Geologic team had conducted on-site field surveys and analyses of the study basin and coring site, Jeokjung–Chogye Basin in Hapcheon (35°32′57.02” N, 128°16′7.59″E), located in the southeastern Korean Peninsula.

The story begins with the fundamental question of how we prove the evolution of this third planet, Earth, from the sun that we stand on. Here, we report the first direct evidence of impact cratering from this basin, which occurred about 50,000 years ago based on our research investigations on 142m of deep core drilling in the basin. The 7 km-diameter Jeokjung–Chogye Basin in Hapcheon, southeastern Korean Peninsula, is well-known for its bowl-shaped geomorphology. For many years, this rather ill-matched geomorphological feature has the inspired expectation that this site witnessed a meteorite collision a long time ago, but no direct evidence has been found, which has remained a longing for the Geology Communities in Korea.

When a meteorite collides, a strong shock wave is generated, forming a huge pool underground. The shock-metamorphic effects remain in the existing rocks and minerals due to the impact of the generated tremendous shock wave, and it leads to the possible links to determine whether there has been a meteorite impact in the past by tracking its lithological and geochemical transformation of the basin structures.

Last year, KIGAM’s impact crater research team confirmed the evidence of microscopic mineral deformation and macroscopic rock deformation produced by unique shock waves caused by meteorite impacts through sedimentary analysis of the Jeokjung–Chogye Basin, rewriting the birth story of the bowl-shaped basin.

The formation of the basin has been attributed to differential erosion, a meteorite impact, and bedrock weathering along fault lines due to ground motion. However, this basin's origin remains equivocal and required confirmation through a careful lithological study using a drill core in the inner basin. Our team selected a drilling point and then drilled a borehole, named core CR05 (35°32′57.02” N, 128°16′7.59″E), to 142m depth inside the crater, and collected unprecedented lithologic information in Korea. To determine when the sedimentary layers in the cores were deposited, charcoal in the lacustrine sediments was used for radiocarbon dating, and the analysis was performed at the accelerator mass spectrometry facility of KIGAM.

Our team found impact-driven metamorphic features. As a microscopic evidence, planar deformation features (PDFs) in quartz minerals were found from the impact breccia. As a marcoscopic evidence, shatter cones were collected in the 6-cm-long shale clast found at 130 m depth of the drilled core. Furthermore, the impact likely happened during the last glacial period (~50,000 years ago) based on the radiocarbon dates of charcoals in the lacustrine sediments.

Currently, there are about 200 meteorite impact craters officially recognized worldwide. The Jeonzhong-Guangdong Basin is the second in East Asia after China's Xiuyan crater was announced in 2010. Dr. Jaesoo Lim, the first author, identified the Jeokjung–Chogye Basin, which has remained a mystery of the geological world, as the first meteorite impact area on the Korean Peninsula.

- Literature Supervised by Jaesoo Lim (limjs@kigam.re.kr, Geology Division)