Miocene Stratigraphy around Matsushima Bay, Miyagi Prefecture, Japan

The stratigraphic succession of the Miocene sedimentaries distributed around Matsushima Bay, Miyagi Prefecture, Northeast Honshu, Japan, are described and an attempt is made to correlate them with the Miocene Formations developed in the different areas of Shichigahama, Shiogama-Matsushima, Tetaru-Nobiru and the Miyato-Urato areas. The Tertiary sediments in those areas are composed of marine and pyroclastic sediments of Miocene age. In the Shiogama-Matsushima area, the Miocene sediments are superposed with distinct angular unconformity upon the Rifu Formation (Daonella-bearing) of Triassic age and the granodiorite of Cretaceous age. The stratigraphic sequence of the different units and correlation of them in the present area are given in Table 1 and Fig. 1. The lowest part of the Miocene deposits are characterized by the extensive and good development of pyroxene andesite, and their pyroclastics are found in the Shiogama and Miyato-Urato areas where they are superposed on the basement of Pre-Tertiary rocks. The former is named the Shiogama Pyroclastics, the latter is the Katsugi-ga-Ura Pyroclastics. When the rate of supply of the pyroclastic materials is relatively small, finer clastic sediments such as sand and silt may be deposited upon the above mentioned pyroclastics. In the Shiogama-Matsushima area, the Sauramachi and Ajiri Formations which attain a thickness of more than about 300 meters were deposited, whereas, in the Miyato-Urato area, there are also found the finer clastic sediments of the Murohama Formation which measures about 30 meters in thickness. The differences in distribution, lithology and thickneess changes in lithology strongly suggests that the conditions under which sedimentation proceeded was different in respective areas. The Miocene marine transgression in the present area was surrounded by upwarping which was accompanied by rather acidic volcanic extrusions of the Nirayama Andesite and Toguhama Pyroclastics in the Shichigahama area, the Matsushima Tuff in the Shiogama-Matsushima area, the Nobiru Tuff in the Tetaru-Nobiru area and the Satoura Tuff in the Miyato-Urato area. Those pyroclastics can be traced over wide areas. In the north-western and western parts of the present area, that is, the Matsushima and Shichigahama areas, the pyroclastic deposits are found to contain variously shaped huge blocks of the same lithology as the essential blocks in the pyroclastics of the other areas. These pyroclastics directly cover with unconformity the subjacent Sauramachi and Ajiri Formations and the Triassic Rifu Formation. Thus, from the depositional aspect of the pyroclastics in the area metioned above, it is inferred that the depositional environment might have been terrestrial. In the Shichigahama area, the pyroclastics and finer clastic sediments (Minatohama Siltstone Member) are in intimate relation, at places, they interfinger with one another. The Minatohama Siltstone Member can be traced downwards into the Ajiri Formation with conformity in the Shiogama area. Two or three layers of breccia facies intercalated with numerous of cobble to boulder size blocks are interbedded in the siltstone member. The siltstone facies with breccia layers can be traced eastwards into the Urato Formation in the Miyato-Urato area where marine Miocene molluscan fossils have been found, and it covers the Satoura Tuff with conformity. The formation is correlated to the Rikuzen-Otsuka Formation in the Tetaru-Nobiru area. This formation is composed of finer clastic sediments, mainly, of an alternation of compact siltstone and sandstone intercalated with some pumiceous pyroclastic layers instead of the breccia layers mentioned above. The Hatsuhara Formation in the Matsushima area and the Rikuzen-Otsuka Formation are underlain with conformity by the Matsushima Tuff which corresponds to the Nobiru Tuff in the Tetaru-Nobiru area in stratigraphic position, and is covered with conformity by the Shirasakayama Tuff in this area. The Hatsuhara Formation comprises cross-laminated tuffaceous sandstone, which grades laterally into finer clastic sediments in the Rikuzen-Otsuka Formation. The sandstone of the Hatsuhara Formation has yielded many marine molluscan fossils of Miocene age. The distribution of the Hatsuhara Formation is restricted to the west of the "the Rifu-Nagamachi Tectonic Line". The Shirasakayama Tuff consists of acidic pumiceous tuff, and is covered by the superjacent Bangamori Formation, which is composed almost of sandy sediments. The Bangamori Formation can be traced into the Aoso Formation in the area north of Sendai City. In the present area, it seems evident that deposition of the sediments proceeded during continued uplift of the sedimentary basin, and that a terrestrial environment was experienced during a certain phase of the Late Miocene or Pliocene Epoch in the area