[S-GL19] Frontier research on chronostratigraphic unit boundaries

[SGL19-P02] Magnetostratigraphy and paleomagnetic rotation of the Nemuro Group in the Shiranuka Hill area, eastern Hokkaido

*Hiroyuki Hoshi¹, Yota Kitano¹, Ichiha Mita¹, Reishi Takashima², Keiichi Hayashi³ (1.Aichi University of Education, 2.Tohoku University, 3.Hokkaido Research Organization)

Keywords:K-Pg boundary, Shiranuka Hill, Nemuro Group, Magnetostratigraphy, Paleomagnetic rotation, Paleolatitude

Multiple lines of evidence suggest the presence of a Cretaceous-Paleogene (K-Pg) boundary within a marine clastic sequence of the Nemuro Group exposed in the Shiranuka Hill area, eastern Hokkaido. We employed a multidisciplinary approach, including detailed mapping, radiometric dating, biostratigraphy, and paleomagnetism, to refine the boundary's location and understand the tectonic history of the area. This study focuses on paleomagnetic results, aiming to establish the magnetic polarity stratigraphy and assess tectonic movements. Stepwise demagnetization experiments were conducted on samples from 45 sites within the Kawaruppu and Tomikawa formations. Our preliminary findings, presented at the JpGU meeting last year (Hoshi et al., 2023), indicated a reverse-normal-reverse polarity sequence in an approx. 800-meter-thick section. However, further analyses revealed a single, persistent reverse polarity throughout the section. The previously reported normal polarity was likely secondary in origin. Therefore, the potential K-Pg boundary lies within this reverse zone. Reliable site-mean magnetization directions were obtained from 14 sites. These directions display a significant (approx. 100°) clockwise deflection compared to those reported for the Nemuro Group in the Nemuro-Kushiro area in southeast Hokkaido. This suggests an approx. 100° clockwise rotation of the crust beneath the Shiranuka Hill area relative to the Nemuro-Kushiro area since the Danian (early Paleocene). Interestingly, the site-mean inclination shallows systematically with increasing dip angle, suggesting inclination shallowing due to stratal deformation. However, inclinations from gently dipping strata remain comparable to the expected geomagnetic inclination for the present-day latitude of Hokkaido. This indicates that the Nemuro belt has not experienced significant latitudinal movement since the Danian.