Stratigraphical and Sedimentological Studies of the Paleogene to Miocene Strata in Southwestern Shikoku, Japan

KIMURA Koshi

Journal of science of the Hiroshima University. Series C, Geology and mineralogy

In the Hata Peninsula, Kochi Prefecture, the thick strata of Tertiary age are widely distributed, occupying a part of the Shimanto Terrain, which is situated in the southern most part of the Outer Zone of Southwest Japan. The strata can be classified into the Hata Group, the Shimizu Formation and the Misaki Formation on the basis of their lithology and geological age.
The Hata Group includes the nearly contemporaneous Hirata, Tatsugasako, Kurusuno and Hiromi Formations, ranging in age from Eocene to Early Oligocene (?). The Hirata Formation is mud-dominated sequence containing shallow-marine molluscs. The Tatsugasako and Kurusuno Formations are composed mainly of alternating beds of sandstone and shale with various sand/shale ratios. The former includes pebbly sand-stone in the Upper Member, while the latter is interbedded with acidic tuff at several horizons. The Hiromi Formation is pervasively sheared sequence which consists of chaotic deposits with minor intercalations of greenstone and red shale. The Hirata, Tatsugasako-Kurusuno and Hiromi Formations correspond to shelf facies, flysch facies and mélange facies, respectively.
The Shimizu Formation consists mainly of chaotic deposits, accompanied locally with coherent beds. The chaotic deposits contain irregular-shaped blocks and clasts of various kinds of sedimentary and igneous rocks (mainly sandstone) dispersed in scaly and cloven muddy matrix. The coherent beds are less deformed, consisting of conglomerate, sandstone and mudstone. The Shimizu Formation is referable to an olistostrome ranging in age from Late Oligocene to Early Miocene.
The Misaki Formation is represented by shallow-marine sandstone and mudstone, and shows a coarsening- and thickening-upward cycle as a whole. The formation unconformably overlies the Shimizu Formation. Its lowest horizon is assigned to upper Lower Miocene (Burdigalian) on the basis of planktonic foraminifers.
Most of the sandstones of the Tertiary strata are rich in quartz and poor in rock fragment. They belong to feldspathic wacke. The sandstones of the Hiromi Formation are, however, exceptionally richer in feldspar and rock fragment than others. The paleo-current directions in the Hata Group are clearly grouped into two systems: One is represented by longitudinal current from southwest to northeast with lateral current, and the other by southerly lateral current and westerly longitudinal current. The general sense of current flow in the Misaki Formation was dominantly to the south.
The result of the analysis of sedimentary facies suggests that a part of the Hata Group corresponds definitely to turbidite fan deposits, and that most of the group corresponds to non-fan deposits. The Shimizu Formation contains coherent olistoliths of non-marine deposits, shallow-water deposits and deep-water deposits. On the other hand, the Misaki Formation is interpreted as representing progradational deposition on beach to basin plain, affected by storm waves and tidal currents.
Lastly, the depositional history from Eocene to Middle Miocene is discussed from the stratigraphical and sedimentological viewpoints.

eng

HIROSHIMA UNIVERSITY

[ISSN] 0075-4374

[NCID] AA00706718