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The Cretaceous Miyako Group lies unconformably over various kinds of Neocomian and older sedimentary and effusive rocks which are folded and intruded by granodiorite (Miyako and Taro granodiorites) on the Pacific side or the Kitakami Mountains. Distribution of the Group is shown in Fig. 1. Fig. 2 is a schematic illustration of the stratigraphic succession of the Group in the standard area. A tuff layer is intercalated in the Group, which, in turn, is intruded by sills or rhyolitic rock. Fossil talus, remaining in a few limited parts on the surface of the basement rocks, is overlain by the probable fluvial deposits, which constitute the main portion of the Raga Formation. The Tanohata Formation lies in the main part directly over the Raga Formation and in the northern part of the area abuts against the uneven surface of the basement rocks. The Tanohata Formation, distributed in the isolated areas to the east, such as Bentenzaki and Moshi, consists mainly of the thick conglomeratic sandstone with reef-building corals and has only poorly developed sandy mudstone in the upper part. This indicates that the sedimentary environment of this part was a rocky bottom projected toward the open sea. On the other hand, the formation distributed in the western areas, such as Hiraiga, Taro and Hideshima, has well developed sandy mudstone. Mode of occurrence of corals and other fossils suggests an environment of inner side of the reef or inner bay. The Hiraiga Formation consists mainly of well sorted, medium- or fine-grained calcareous sandstone, with layers of abundant shells. It was probably formed under the shallow open sea in which sediments were much affected by currents and waves. In the Hiraiga area, calcareous sandstone changes laterally and northwardly into Orbitolina-bearing coquinal deposits, which directly overlies the basement. Accordingly the accumulation of shell fragments may be related to the topographical relief of the basement and the orientation of the current. The Aketo Formation consists of muddy fine sandstone and contains thin shelled ammonites and echinoids, among others. It was probably deposited on a bottom of rather quiet open sea. Using oxygen isotope of a belemnite, 18℃ on the average was obtained by LOWENSTAM and EPSTEIN (1954) for the paleotemperature of the Hiraiga Formation. The pachyodonts, nerineans, hexacorals, calcareous algae, larger foraminifer, etc., provide the evidence to infer that the deposition of entire Miyako Group was under the influence of the subtropical sea water. Table 1 shows the stratigraphic division and the corresponding succession of selected species. On the basis of the indicated species, the major portion of the Hiraiga Formation and the upper part of the Tanohata Formation are most probably referred to the upper part of the Upper Aptian. Lower Albian age can be suggested for the Aketo Formation.
国立科学博物館の論文
1968-03-30

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