Bulletin of the Geological Survey of Japan Top Page

Bulletin of the Geological Survey of Japan Vol.66 No.3/4 (2015)

Cover photograph | Table of Contents | Abstract

Cover photograph

Geology of the Koyasan area, northwestern part of Kii Peninsula, Southwest Japan

1:Sambagawa metamorphic rocks. 2-9:Upper Cretaceous Hanazono Formation of the Shimanto Belt(2-3:Hn1 Unit, 4-9:Main part, Hn2-Hn5 Units).
1. Pelitic schist. 2. Shale including big blocks of sandstone (Hn1). 3. White bedded chert intercalated in shale (Hn1). 4. Mixed rock including blocks of sandstone (Hn2). 5. Mixed rock made of black and grayish green tuffaceous shale matrix (Hn2). 6. Dark red bedded chert(Hn2). 7. Basaltic lava showing pillow structure (Hn2). 8A. Stratified sandstone (Hn3). 8B. Shale patch in the sandstone (maximum diameter is 1cm). 9. Shale including white felsic tuff (Hn4). See text 41-79 pages for more information.

(Photograph and Caption by Chikao Kurimoto)

Table of Contents

All the pages PDF ： 66_03_full.pdf ［17MB］

Abstract

Geology and radiolarian fossils of the Upper Cretaceous Hanazono Formation in the Koyasan area, northwestern part of Kii Peninsula, Southwest Japan

Chikao Kurimoto, Katsumi Kimura and Makoto Takeuchi

　The western Kii Peninsula which is situated in the Outer Zone of the Southwest Japan, is generally characterized by E-W trend zonal structure of the Sambagawa, Chichibu and Shimanto Belts from north to south. However, the zonal structure is incomplete in the central part of Kii Peninsula and rocks of the Chichibu Belt are not cropped on the land surface. In this paper, geology of the Hanazono Formation in the Koyasan area where the rocks of the Chichibu Belt are absent is described, and radiolarian fossils from the Hanazono Formation and the adjacent Yukawa and Miyama Formations are reported.

The Hanazono Formation is in fault contact with the Sambagawa Metamorphic Rocks through the Aridagawa Tectonic Line to the north, and is in fault contact with the Yukawa or Miyama Formations through the Yanase Fault or the Yukawa Thrust to the south. The Hanazono Formation is divided into 5 units and they are called Hn1, Hn2, Hn3, Hn4 and Hn5 Units, respectively. Hn1 Unit is situated in the northern-most part of the Hanazono Formation, and characterized by obvious foliation as compared with Hn2 to Hn5 Units. Hn1 Unit is in fault contact with Hn2-Hn5 Units through the Kamiya Fault.

　The Hanazono Formation mainly consists of shale with sandstone, felsic tuff, chert, red shale, limestone and basalt, and is characterized by mixed rock which includes the blocks of sandstone, chert and basalt in shale matrix. Radiolarian fossils were obtained from chert, felsic tuff, shale and tuffaceous shale of the Hanazono, Yukawa and Miyama Formations, and 4 radiolarian fossil assemblages are recognized, that is, Holocryptocanium barbui Assemblage, Dictyomitra formosa Assemblage, Dictyomitra koslovae Assemblage and Amphipyndax tylotus Assemblage. According to the previous studies, Holocryptocanium barbui Assemblage is correlated to late Albian to Cenomanian, Dictyomitra formosa Assemblage to Turonian to Coniacian，Dictyomitra koslovae Assemblage to Santonian to early Campanian，and Amphipyndax tylotus Assemblage to late Campanian.

　Judging from the lithologic characters and imbricated structure, the Hanazono Formation is considered to be constructed as an accretionary complex. The radiolarian fossils show that the construction age of the Hn1 Unit was Turonian to Coniacian, Hn2 Unit was Turonian to late Campanian, and Hn3 and Hn4 Units were Turonian to latest Campanian and Hn5 Unit was late Albian to latest Campanian.

Elemental distribution of surface sediments around Oki Trough including adjacent terrestrial area: Strong impact of Japan Sea Proper Water on silty and clayey sediments

Atsuyuki Ohta, Noboru Imai, Shigeru Terashima, Yoshiko Tachibana, Ken Ikehara and Hajime Katayama

　Four-hundred sixty marine sediment samples were collected in the western Sea of Japan and analyzed for 53 elements for a marine geochemical mapping project associated with a nationwide terrestrial geochemical map. Grain size and chemical compositions of marine sediments vary significantly with location of origin (shelf, marginal terrace, slope, or basin). Sandy sediments distributed on the shelf do not likely reflect the geochemical features of river sediments, which are the dominant source of sands in the shelf. Most of the shelf sediments sampled are composed of relict sediments (little contribution of stream sediments) formed between the regression age and the transgression age because they contain a large amount of quartz coated by iron hydroxide and highly enriched in As. The marginal terrace is covered by modern silty sediments that are selectively deposited at the water mass boundary between Tsushima Current (surface water) and Japan Sea Proper Water (deep water). Silty sediments in the western portion of marginal terrace are highly enriched in Nb, rare earth elements, Ta, and Th, which are supplied from Quaternary alkaline volcanic rocks by a denudation process. They are carried eastward by as much as 200 km by oceanic currents. In contrast, the eastern marginal terrace is covered by silty sediments that are highly abundant in Cu, Zn, and Hg, which is attributed to biogenic remains in sediments (organic complex formation). Clayey sediments are widely distributed in the Oki Trough and basin where a hemi-pelagic environment and highly oxic conditions are found because of the influence of Japan Sea Proper Water. A thin Mn oxide layer in the uppermost 0–4 cm and extreme enrichment of V, Co, Ni, Mo, Sb, and Pb would be caused by an early diagenetic process. Thus, the spatial distribution patterns of elements in marine sediments in the study area are strongly controlled by their depositional environments.