研究業績

To reevaluate the Neogene stratigraphy, geological mapping was undertaken in the Shichiyama area of eastern Tottori Prefecture, Japan, coupled with detailed chronological and petrological analyses. To refine depositional ages and lithological characteristics, zircon U–Pb dating and K–Ar dating on plagioclase and hornblende were conducted, together with major-element geochemical analyses. The Neogene strata in the Shichiyama area comprise the Iwami Formation (ca. 17.1–16.4 Ma) and the Shichiyama Formation (ca. 15.3–15.1 Ma). The Shichiyama Formation consists of felsic pyroclastic rocks, intrusive rocks, and lava, and unconformably overlies the Iwami Formation. The Iwami Formation consists of marine deposits, whereas the pyroclastic facies of the Shichiyama Formation indicate deposition in a terrestrial environment. These observations indicate a marine regression during the early Middle Miocene, which has been widely recognized in the San'in region and is now corroborated by geological data from the eastern Tottori region. Our findings suggest that the regression associated with widespread folding in the San'in region is not consistent with the stratigraphic and chronological data from the eastern part of the region, indicating that the regression may not have resulted from crustal shortening. To further evaluate the causes of this regression, further precise data on the timing, magnitude, and spatial extent of the Middle Miocene uplift are required.