Hiroto Ohira

Paleomagnetism and fission-track ages of Oki-Dogo Island in Southwest Japan

Zircon fission-track (F.T.) ages and paleomagnetic directions were measured on the Neogene series of the Oki-Dogo Island. The results of F.T. dating on the the Tokibariyama Formation show ages of 23.7 ± 1.3 and 20.0 ± 2.3 Ma, indicating the early Early Miocene. Paleomagnetic measurements of the Tokibariyama, Kori Formations (24–17 Ma) and the Oki Trachyte-Rhyolite (6.8 Ma) were done to reveal the movement of Oki-Dogo Island during the Japan Sea opening. The Tokibariyama and Kori Formations show an average declination of 39.9°. The Oki Trachyte-Rhyolite has the same average geomagnetic direction as the present day value. These results show that a clockwise rotation of 40° ± 15° of the Oki-Dogo Island occurred between 17 and 6.8 Ma which is similar to the main land of southwest Japan.

Provenance Ages of Cretaceous Strata in the Songliao Basin, Northeast China Inferred from Fission Track Data

The Songliao Basin in northeast China is an intra-continental fossil rift basin mainly developed from the latest Jurassic and throughout the Cretaceous. The Daqing oil field in the basin is one of the largest hydrocarbon exploitation areas in China. A fission track (FT) study has been carried out on sandstone well samples in the Songliao Basin to constrain the depositional ages of Cretaceous strata and to ascertain their source region. FT dating also has been carried out on welded tuffs in the Dahinganling Mountains to determine their eruption ages. The detrital zircon FT age suggests that the depositional age of the middle Quantou Formation is younger than 109 Ma. Zircon FT ages for Cretaceous strata (from the Nenjiang Formation to the Quantou Formation) suggest that the source region has a fairly homogeneous cooling history at ∼120-100 Ma. Eruption ages of welded tuffs in the Dahinganling Mountains show an age range from 110 to 92 Ma. The regional geology and FT data suggest that volcanic rocks in the Dahinganling Mountains were eroded soon after their deposition and were reworked toward the Songliao basin.