Abstract
The Lower Miocene Hokusetsu Subgroup yields a magnetic polarity stratigraphy that is useful for inferring its depositional age. The sampled sedimentary section is about 760 m thick, consists mainly of mudstone and sandstone with numerous intercalations of pyroclastic material in upper horizons, and is lithologically divided into the following four formations (in ascending order) : the Kawakado, Shimoda, Tsubosawa, and Kuroze Formations. Paleomagnetic samples collected at 16 sites were measured for remanent magnetization and detailed stepwise demagnetization by alternating-field and thermal techniques. Demagnetization results were analyzed to determine magnetic polarity for the sampled sites. The magnetic polarity stratigraphy of the studied section consists of three newly defined polarity zones in ascending order : the Hokusetsu R1 reversed polarity zone, Hokusetsu N1 normal polarity zone, and Hokusetsu R2 reversed polarity zone. Based on constraints provided by radiolarian fossils and fission track dates, the magnetic polarity stratigraphy was correlated with the Early Miocene portion of Ogg and Smith’s (2004) geomagnetic chronostratigraphy, yielding three possible age models for the Hokusetsu Subgroup. We conclude that : (1) the sampled section is younger than the base of Chron C5Er (ca. 18.8 Ma) and older than the top of Chron C5Cr (ca. 16.7 Ma), and (2) the Kawakado Formation, for which there exists no direct age constraints, was deposited some time between the base of Chron C5Er (ca. 18.8 Ma) and the base of Chron C5Dn (ca. 17.5 Ma).
