Stratigraphy and depositional age of the Izumi Group, Niihama area, central Shikoku, Japan

Abstract

The Izumi Group, which occurs from western Shikoku to the Izumi Mountains, Kii Peninsula, comprises sediments deposited in a strikeslip basin during the Late Cretaceous. Some differences exist in the lithostratigraphy and geological structure of the group where it occurs in western and eastern Shikoku; consequently, outcrops in central Shikoku may record evidence of the transition between the two areas. We conducted stratigraphical and geochronological surveys as part of compiling the "Niihama" geological map, and defined the following new formations: the Kussaki, Isoura, and Niihama formations. Fission-track dating of felsic tuff beds in the Niihama Formation yielded an age of . . Ma. The radiolarian assemblages within mudstone of the Niihama Formation are DK assemblages, corresponding to the earlymiddle Campanian. The combined age data indicate a middle Campanian age for deposition of the Niihama Formation. The present results suggest that the Izumi Group was simultaneously deposited in western and central Shikoku. The lithology and geological structure in the study area are comparable with those in western and eastern Shikoku, indicating that the analyzed rocks represent strata deposited during a transitional stage of basin development and basin fill. Abstract

Revised radiolarian zonation of the Upper Cretaceous Izumi inter-arc basin (SW Japan)

“…The arkosic sandstone yields brackish and shallow-marine mollusks (Ichikawa and Maeda, 1963), whereas the successive argillaceous beds with ammonites and inoceramids are suggestive of outer-shelf deposits (Yamasaki, 1986). The Northern Shelf Facies is subdivided into the Kussaki, Shiroyama, Hiketa, Seidan, and Mutsuo formations that appear in this order from west to the east (Matsumoto and Morozumi, 1980;Morozumi, 1985;Itihara et al, 1986;Yamasaki, 1986;Miyata et al, 1993;Noda et al, 2010). The Kussaki Formation (Noda et al, 2010) occurs in central Shikoku.…”

“…The Northern Shelf Facies is subdivided into the Kussaki, Shiroyama, Hiketa, Seidan, and Mutsuo formations that appear in this order from west to the east (Matsumoto and Morozumi, 1980;Morozumi, 1985;Itihara et al, 1986;Yamasaki, 1986;Miyata et al, 1993;Noda et al, 2010). The Kussaki Formation (Noda et al, 2010) occurs in central Shikoku. The Shiroyama and Hiketa formations, which represent a contemporaneous heterotopic relation (Yamasaki, 1986), are distributed in Eastern Shikoku, and the Seidan Formation (Ichikawa, 1961) is found on the Awaji Island.…”

“…Many researchers have surveyed the lithostratigraphy of the Izumi Group (Nakagawa, 1961;Suyari, 1966Suyari, , 1973Miyata et al, 1993;Noda et al, 2010). The Northern Shelf and Central Turbidite Facies are well correlated by many acidic tuff intercalations.…”

Tectonics of the Ryoke-Izumi belt of the Izumi Mountains, Southwest Japan from thermochronological data

“…Since 65 Ma, the uplift has been greater towards the east in the eastern Izumi Mountains. Noda et al (2010), and this study. Error bars indicate two standard errors (2σ).…”

“…Error bars indicate two standard errors (2σ). The biostratigraphic age of the Onogawa Group (Noda, 1994) was revised following Hayakawa and Hirano (2013). The thin dashed line represents the younger limit of whole-rock ages (Yuhara et al, 2000).…”