<国立科博専報>岩手県宮古層群産大型有孔虫 Orbitolina

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The Miyako Group is a typical lower Cretaceous beds in our country but its extent is limited in a zone just along the Pacific coast of Iwate Prefecture, Northeast Japan. In addition to the narrow distribution, it has a monoclinic structure inclining toward the ocean so that the major part may be under the sea bottom and only the basal part remained on land. This observable part consists mainly of conglomerate to sandstone and, as well known, contains many Orbitolina, a Cretaceous index fossil, nearly throughout the strata. These larger foraminifers were studied so early as 1926 by YABE and HANZAWA who identified three species among them and suggested a trend of their stratigraphically different occurrences. The writers obtained many isolated specimens of Orbitolina from three beds of the Orbitolina sandstone facies of the Hiraiga Formation (HANAI et al., 1968,see this Memoir), at sea cliff south of the village of Aketo, Tanohata-mura. The three beds are apart from one another for 1 to 1.5m in stratigraphical distance and all contain numerous tests of Orbitolina, arrangement of which gives a remarkable bedding plane for the rock. All axial sections of these isolated specimens up to 138 in total number, however, show so very poor preservation that the writers failed to examine enough their characters, particularly of nepionic stage, by such a modern method developed recently by DOUGLASS (1960) and HOFKER (1963; 1966). A cause of the poor preservation is evidently the coarse-grained wall texture of these Orbitolina tests as seen in Plate for example. According to DOUGLASS (1960) and others, some exotic materials combined with the wall texture of Orbitolina would increase in number and in grain size, if the rock containing them was more coarsely grained. The coarseness observed in the Miyako samples may have obscured the nepionic structure. Besides this primary cause, the poor preservation may be due largely to the remarkable abrasion of tests by water action as exemplified in several pictures of Plate. Because this condition is the same for the individuals contained in several thin sections of the rocks, it is not caused by any artificial results on the occasion of sectioning isolated specimens. Almost all molluscan shells from the same beds were completely broken, too. A fact that so many sections as up to 138 failed completely to reveal the definite nepionic apparatus, however, suggests that the Miyako materials may belong to neither the form-group I of HOFKER (1963), which has a simple and large protoconch and ranges from the Barremian to Middle Aptian, nor the later ones than the Upper Albian representatives of the form-group III with huge nepionic apparatus (up to ca. 1mm in diameter). It seems that YABE and HANZAWA (1926) described three species of Orbitolina from the Miyako Group based on poorly preserved specimens, as far as a few pictures shown by them in plates are concerned. Their specific identification, therefore, depends on rather external characters, in particular, test-shape and test-size. Using the axial sections available for measurement, the writers found a distinct linear relationship between their test-height and test-diameter as shown in Fig. 1. The same measurements for "Orbitolina planconvexa YABE and HANZAWA" are also included in the linear relationship. The frequency distribution of the ratio of test-diameter to the test-height shows clearly a unimodal curve, i.e., a single population (Fig. 2). A little apart from the unimodal distribution, a very weak but distinct peak is observed in the range of the higher ratio, in which the values calculated from two measurements available for "Orbitolina japonica miyakoensis YABE and HANZAWA" fall, too. These flat forms might be microspheric tests because of their rare occurrence and their relatively larger size of test, although any "Phrygian cap" with nepionic streptospiral could not be recognized.
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<国立科博専報>岩手県宮古層群産大型有孔虫 Orbitolina

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