Eleven localities of staurolite-bearing metapelites and pelitic bands in metachert, are reported from the Ryoke metamorphic belt in the Hongu-san area (Aichi Prefecture, Japan). Staurolite occurs as inclusions within andalusite porphyroblasts and within mica-quartz aggregates in metapelites. On the other hand, staurolite in pelitic bands in metachert, occurs not only as inclusions within mica-quartz aggregates but also as porphyroblast. Petrographic observations indicate that the mica-quartz aggregates were formed by replacing staurolite porphyroblasts during prograde stage of the Ryoke regional metamorphism. Based on the widespread spatial distribution of the mica-quartz aggregates, it is suggested that staurolite was widely produced in the area. During subsequent stages of prograde and peak metamorphism, staurolite became unstable in some rocks while remaining stable in others. Thermodynamic calculations reveal that the Ti content of biotite and the Zn content of staurolite affect the stability of staurolite, thus explaining its heterogeneous preservation in the study area.
The recent revision of the stratigraphy of the Aniai district (northern Honshu, Japan) showed the necessity of re-examination of the Paleogene and Neogene stratigraphy in other areas of the Dewa Hills, northeast Japan. We carried out a detailed field survey in the eastern Dewa Hills, around the city of Kakunodate. In addition, we conducted zircon fission-track (FT) and U-Pb double dating for 4 samples from 3 formations. Based on our results, the volcanic and sedimentary succession in the study area were divided into the Yamayakawa (the Late Oligocene to early Early Miocene), Katsurabuchi (ca. 22 Ma), Shiotezawa (ca. 16-15 Ma), Hachiwari (ca. 16-12 Ma), Onnagawa (ca. 12-10 Ma) and Yamaya (ca. 8 Ma) Formations in ascending order. Granitic rocks of the Cretaceous make up the basement of the Yamayakawa Formation. The contact between the Yamayakawa Formation and the overlying Katsurabuchi Formation (ca. 22 Ma) is a sharp angular unconformity and is newly discovered in this study. The Shiotezawa and Hachiwari Formations unconformably overlie the Katsurabuchi Formation; and the lower part of the Hachiwari Formation interfingers with the Shiotezawa Formation. The Hachiwari Formation is conformably overlain by the Onnagawa Formation. The youngest unit, the Yamaya Formation (ca. 8 Ma), overlies the Yamayakawa and Katsurabuchi formations by angular unconformity. The results demonstrate that the stratigraphic succession in the study area is similar to that of the Aniai district.
Radiolarian fossils constrain the geological age of Neogene stratiform manganese deposits in the Kitaichi Mine, Fukaura District, Aomori Prefecture. A 0.5 m-thick manganese oxide layer and the underlying tuffaceous sandstone from one outcrop in the Kitaichi Mine yielded radiolarian assemblages containing Calocyclas motoyamai and Eucyrtidium inflatum. The occurrence of C. motoyamai suggests that the two strata are correlated with the upper part of the E. inflatum Zone (late middle Miocene: 12.9 to 11.8 Ma). In contrast, the presence of two key radiolarian zonal species, Larcopyle pylomaticus and Hexacontium parviakitaense, in overlying tuffaceous sandstone indicates that the strata were deposited in the L. pylomaticus Zone to the H. parviakitaense Zone (Pliocene: 5.4 to 2.7 Ma). These results indicate a major hiatus between the manganese layer and the overlying tuffaceous sandstone. Previous work has correlated the ore deposits and the overlying tuffaceous sandstone in Kitaichi Mine with the upper lower Miocene Tanosawa Formation. However, our results clearly show that these deposits are equivalent in age to the middle Miocene to Pliocene biosiliceous facies of the Odoji, Akaishi, and Maido formations, prompting a reconsideration of the regional geological history, including the formation of the manganese deposits in the Fukaura District.
Clastic rocks in the Hida Gaien Belt, located in the Hongo area, Takayama City, Gifu Prefecture, central Japan, are subdivided into the Moribu and newly proposed Douden formations. The Moribu Formation is lithologically subdivided into three parts: a lower part consisting of mudstone intercalated with sandstone and limestone; a middle part characterized by massive, thickly bedded sandstone; and an upper part composed of alternating beds of sandstone and mudstone. The Jurassic Douden Formation consists mainly of alternating beds of sandstone, mudstone, and conglomerate.
We present new U-Pb detrital zircon ages from the sandstone beds. The youngest detrital zircon age peaks from the lower-middle and upper parts of the Moribu Formation yield detrital zircon ages of ca. 263-256 Ma and ca. 250 Ma, respectively, indicating that their maximum depositional age is middle-late Permian. The depositional age of the upper part may extend into the Early Triassic. The Douden Formation probably accumulated during the Early Jurassic, based on the youngest detrital zircon age peaks of ca. 186 Ma and 181 Ma from sandstones in the lower and upper parts, respectively. The sandstones contain abundant volcanic fragments. It is therefore probable that the zircon U-Pb ages correspond to the depositional age.
The Douden Formation can be correlated with the upper part of the Jyogodani and Kitamatadani formations in the Lower Jurassic Kuruma Group, Inugadake area, and also with the upper part of the Gamaharadani Formation, Kotakigawa area. The lithofacies and zircon age distribution in the Douden Formation are similar to those of the Jyogodani and Gamaharadani formations.
We developed a new portable percussion piston corer for coring lacustrine and shallow-marine sediments from a frozen ice surface. The weight of the corer is less than 100 kg, and it can be separated into individual components. Therefore, the corer can be carried to remote sites using pack frames and allows coring to be undertaken in areas that cannot be reached with mechanical drilling equipment. The engine-driven percussion coring can obtain relatively coarse and hard sediments. The main body of the corer is composed of a transparent polycarbonate pipe with an external diameter of 76 mm, which allows us to check length and facies of the cored sediments immediately after coring. The polycarbonate pipe is suitable for preserving sediments and can be easily replaced in subsequent coring. The new corer is advantageous for lacustrine- and marine-sediment coring in extreme (cold) environments, such as Antarctica and high mountains, and may also be applied to a wide range of geoscientific fields.