9:45 AM - 10:00 AM
[SGL22-04] Integrated stratigraphy across the Cretaceous/Paleocene boundary in the Shiranuka Hill, Hokkaido, Japan.
Keywords:K/Pg boundary, Nemuro Group
It has long been known that the Nemuro Group in the Shiranuka Hill contains Cretaceous/Paleogene (K/Pg) boundary. However, there are many faults and quartz veins at the previously identified outcrop of the K/Pg boundary in the Nemuro Group (Mokawaruppu River section). Consequently, previous K/Pg sequence in Mokawaruppu section is not adequate to study continuous paleoenvironmental changes across the Cretaceous-Paleogene boundary.
Our group explored for more complete strata containing transition from uppermost Cretaceous to basal Paleogene in the Nemuro Group, and found a 250-m-thick continuous strata containing K/Pg boundary along the Kawaruppu River section. Dip and strike of the strata are N10-20 E, 80E, respectively. Bioturbated mudstone is predominant lithology of this sequence and abundant intercalations of thin white felsic tuff bed. Although large (10-50cm) calcareous nodule occur abundantly but no macro and microfossils were identified from them.
The felsic tuff contains abundant apatite phenocrysts, but zircons are usually quite rare. However, we found two tuffs that contains abundant zircon at the uppermost part of Cretaceous and the basal part of Paleogene, respectively.
Here we performed an integrated study of calcareous nannofossil, planktic foraminifera, carbon isotope, osmium isotope, and paleomagnetic stratigraphies as well as U-Pb zircon dating of felsic tuffs for the strata around the Kawaruppu River. Although our study revealed that the K/Pg boundary is lacking by a fault in the main river of the Kawaruppu River, we found the stratigraphic interval probably containing the K/Pg boundary in one branch of the Kawaruppu River. The strata yield Cretaceous and Paleogene calcareous nannofossil and pkanktic foraminifera in the lower and upper sequence. All the stratigraphic interval we densely sampled is included in a reverse magnetic polarity zone, and a U-Pb zircon age of 65.44±0.89 Ma was obtained from the tuff intercalated near the K/Pg boundary horizon. Although we have not identified the just K/Pg boundary, further research will perform to find the boundary.
Our group explored for more complete strata containing transition from uppermost Cretaceous to basal Paleogene in the Nemuro Group, and found a 250-m-thick continuous strata containing K/Pg boundary along the Kawaruppu River section. Dip and strike of the strata are N10-20 E, 80E, respectively. Bioturbated mudstone is predominant lithology of this sequence and abundant intercalations of thin white felsic tuff bed. Although large (10-50cm) calcareous nodule occur abundantly but no macro and microfossils were identified from them.
The felsic tuff contains abundant apatite phenocrysts, but zircons are usually quite rare. However, we found two tuffs that contains abundant zircon at the uppermost part of Cretaceous and the basal part of Paleogene, respectively.
Here we performed an integrated study of calcareous nannofossil, planktic foraminifera, carbon isotope, osmium isotope, and paleomagnetic stratigraphies as well as U-Pb zircon dating of felsic tuffs for the strata around the Kawaruppu River. Although our study revealed that the K/Pg boundary is lacking by a fault in the main river of the Kawaruppu River, we found the stratigraphic interval probably containing the K/Pg boundary in one branch of the Kawaruppu River. The strata yield Cretaceous and Paleogene calcareous nannofossil and pkanktic foraminifera in the lower and upper sequence. All the stratigraphic interval we densely sampled is included in a reverse magnetic polarity zone, and a U-Pb zircon age of 65.44±0.89 Ma was obtained from the tuff intercalated near the K/Pg boundary horizon. Although we have not identified the just K/Pg boundary, further research will perform to find the boundary.