On the Tertiary Formations of Tomikusa in South Nagano Prefecture

Abstract

The Neogene Tertiary Formations of the Sansin mountainland along the Tenryu River in the southern district of Nagano Prefecture, are rather noteworthy as the most eastern representative of the Neogene basins of the Setouti type. The Tomikusa Formations, named by the writer for this Tertiary, of middle to upper Miocene age, occupy a small area of 22×12km, mainly distributed in the villages of Tomikusa, Osimozyo, Simozyo, Yasuoka and Tiyo. These Formations are the northwardly extended part of the Hatiraku Formations of Sidara basin in Aiti Prefecture, about 330-535m in total thickness, and bearing rich fossils. The Formations distributed in the type district of Tomikusa-Osimozyo are divided as follows in descending order: Upper. 6. Waseda Formation (Wa): White tuff and tuffacecus shale, about 40m in maximum thickness, with Yoldia laudabilis, Euspira meisensis, Tymolus and Linthia etc. 5. Awano Formation (Aw): Grey to brown coarse sandstone, light green massive sandstone, pumiceous sandstone, grit, conglomerate tuff etc.; about 60m in maximum thicknrss, bearing rich fossils as cited in Table 6 (p. 88) at Konakoo (Loc. 1), indicating the ecological condition of subneritic fascia. 4. Arakida Formation (Ar): White tuff and partially alternation of sandstone, grit, silt and lignite, about 40-60m in total thickness, carrying a fossil bank of Trapezium modilaeformis at Arakida (Loc. 18), which contains Carcharodon megalodon, Isurus hastalis and Terebratulina, Lucinoma acutili-neata, Callianassa sp. aff. inornata and Liquidambar formosana etc. are also contained. 3. Osimozyo Formation (O-O_1・O_2・O_3): Grit, pumiceous sandstone, silt and shale, about 40-70m in total thickness, bearing molluscan fossils as cited in Table 5 (p. 85) indicating the ecological condition of bathyneritic fascia; O_1 (lower) mainly consisting of grit, O_2 (middle) of massive sandstone and O_3 (upper) of grit. Middle. 2. Nukuta Formation (N-N_0・N_1・N_2・N_3): Very varied in rock facies; grit (N_0), alternation of tuff, silt, sandstone and shale, carrying lignite (N_1), grit, arkose sandstone and sandstone with lignite (N_2) and alternation of shale and silt (N_3); about 50-100m in total thickness, containing molluscan fossils as cited in Table 2 (Nukuta, Loc. 10), Table 3 (Azano, Loc. 16) and Table 4 (Obira, Loc. 19) (pp. 83-84), of which Locs. 10 and 16, belonging to N_2, indicate the ecological condition of euneritic-mesoneritic fasciae and Loc. 19, belonging to N_3, indicates those of bathyneritic fascia; at Nukuta and Azano are found rich flora such as Taxodium distichum miocenum, Acer sp. cf. ezoana, Alnus sp., Diospyros sp., Carpinus subcordata, Lindera (?) sp., Magnolia sp., and Liquidambar formosana etc.; fossil banks of Glycymeris sp. aff. nakamurai, Ostrea gigas, Patinopecten and Balanus etc. are also found in sandstones. Lower. 1. Watino Formation (W-W_1・W_2・W_3): Conglomerate and sandstone of 100-300m in thickness, covering uneven surfaces of Ina granitic rock massif; W_1 (lower) consists of boulder conglomerate, W_2 (middle) of sandstone and W_3 (uppor) of conglomerate. The Formations in Yonekawa district are divided as follows in descending order; 3. Yonekawa Formation (Y): White to yellowish grey compact tuffaceous shale and fine vitric-crystal liparitic tuff, of above 50m in thickness, bearing very rich fossils as cited in Table 9 (p. 92) at the cliff behind the Tiyo Middle School in Yonekawa (Loc. 28), which indicates the ecological condition of bathyal facia; all fossils in exceedingly good preservation and include some plant impressions as Acer, Quercus, Buxus (?) and algae etc. This formation perpaps may be correlated with the Arakida Formation. 2. Tazikara Formation (T-T_1・T_2): Grey shale, sandstone and alternation of shale, sandstone and tuff, of about 20m in thickness, bearing rich flora as cited in Table 8 (p. 90); also are contained some marine fossils as Lima yagenensis, Thyasira bisecta, Aslropecten latespinosus and Amphiophiura ponderosa etc. The formation may be correlated with the Nukuta Formation. T_1 (lower) consists of sandstone and alternation, and T_2 (upper) of grey shale. 1. Tiyo Formation (Ti): Boulder conglomerate and grey massive sandstone of above 50m in thickness, barren in fossils and correlated with the Watino Formation. The Tomikusa fossil fauna generally belonges to the Lower Kadonosawa fauna of upper F_2 to F_3 of Ikebe (Middle to Upper Miocene). The shallow seas of Nukuta and Awano Formations were controled by warm water currents as designated by Vicaryella and Telescopium etc. The flora of Tomi-kusa Formations is a kind of Liquidambar flora corresponding to those of the Kobe Formation (Upper Miocene). Hatiraku Formations of Sidara basin are all except their uppermost part (Horaizi Member) correlated with the main activity of the Horaizi volcano could not affect these Formations. The Yamanouti Formation of the Akiyo Member of the Doki basin in Eastern Gihu Prefecture also may be related to the Tomikusa Formations, The Miocene Formations of the Fossa Magna have no direct relationship with the Formations. The Formations are unconformably covered by the Simozyo Formation (sand and gravel of Uppermost Pliocene to Lowest Pleistocene in age) at their northern border. At their western border the Formations are up thrusted by the granitic rock massif with the Tomikusa thrust fault. The eastern margin of the Formations runs as a very linear border line of a NE-SW direction, paralleled to the median dislocation line of the Western Akaisi massif. The Formations are cut by Waseda-, Yonekawa- and Nukuta faults (or fault groups). The movement of these faults and thrust fault emerged after the deposition and probably uplifting (Pliocene in age) of the Formations, which are not folded and gently inclined with general dip of 10°-20°, After the ground morphological structure was constructed, the Pleistocene sand and gravel Formation (Ina Formation) was deposited, then wide scaled successive peneplanation emerged, and in the Sansin mountainland we can see nine peneplanes or terraces. These planes can be correlated in their dissection degrees and ages to the Tama and Musasino planes in the Kanto District (Late Pleistocene-J_2). The writer tentatively measured each formation, as to the maximum thickness (a), numbers of rock kind (b), frequency number of fossils (c), horizontal distance of N-S direction (g) etc. (Tabel 12); h (a×g) is assumed as reflecting the deposition capacity of the each formation, and the writer treated b×c/h as formation density (J), indicating the richness of contents of the unit capacity (1km^2) of a formation. The frequency number of fossils is given by the richness of specific and individual numbers of them; the former is tentatively shown by index 3 (above 10 species), 2 (above 5 species) and 1 (below 4 species), and the latter is by those of 3 (abundant-much), 2 (common) and 1 (rare) (Table 11). The maximum thickness of a formation is considered as reflecting the degree or depth of subsidence. The writer treated a/b (i) as the subsidence degree of an unit sediment and shows the relationship of i and j in Fig. 3 (p.97