In the west-coast of Ise Bay, the graveliferous formations in early and/or midddle Pleistocene time, the so-called Highest gravels, are classified into three types, valley-filling, veneers of gravels (fans and fluvial terraces) and talus deposits. The Kentôyama, Ôtaniike and Tôzenji formations represent those types of deposit, respectively, and they overlie unconformably the Agé group of Plio-Pleistocene and are flanked with lower terraces. Stratigraphic relations among these beds are indeterminable, but, considering with the geomorphological character of the beds and the relation to the Agé low-relief erosional surface, the Kentôyama formation may be somewhat older than the others. A designation of the Highest gravels is unsuitable when the formations are recognizable as poly-cyclic ones geomrphologically, so the name should be changed to the Kentôyama formation and its equivalents or the highest terrace deposits in strict sense.
The author has engaged in the clay mineralogy of the bottom sediments under different environments (prodelta, continental shelf, and continental slope) in the Ishikari bay, as well as the suspension of Ishikari river. Thirty-eight samples from the bay and one sample from river were investigated by x-ray diffraction methods, and chlorite, montmorillonite and illite were identified with all samples. The clay mineral assemblage was divided into the following three types. Type I contains more than 50% of chlorite, and is mainly distributed on the prodelta and its surround (A area in Fig. 9). Type II contains less than 50% of chlorite and less than 30% of montmorillonite, and is mainly distributed on the inner neritic zone (B area). Type III contains more than 30% of montmorillonite, and is mainly distributed more offshore (C area). There is very little difference between the clay mineralogical composition of the river-suspension and that of the prodelta sediments. As a whole, variation in areal distribution of each clay mineral constituent indicates seaward decrease of chlorite, seaward increase of montmorillonite on the contrary, and no significant change in illite. The dominant factor controlling such a distribution of these constituents appears to be preferential flocculation, and not to be diagensis nor current sorting. However, it is qustionable to suppose that all samples (not in mass property but in mud part) are of recent sediments.
The sonic survey by “SPARKER” revealed the following facts concerning the relations between the submarine topography and the youngest deposits in Osaka Bay. 1) The unconsolidated deposits of Osaka Bay are divisible into A and B layers (Fig. 2). B is infered to be the latest Pleistocene sediments deposited in the stage when the sea level retarded at 20m below the present level about 10, 000 years ago. This stage may be identified with the Alleröd oscillation. A is the Holocene sediments formed by the quick transgression occurred in the next stage. 2) Submarine flat planes are classified as follows. a) -10m; wave-cut terrace formed at the present sea level. b) -10∼-20m; depositional surface of A. c) -40∼-50m; approximate depositional surface of B. Generalized profile of Osaka Bay shown in Fig. 5 indicates the relations between the change of sea level and the deposition of A and B. 3) The surface of the base of the unconsolidated sediments is contoured in Fig. 3, on which a system of buried river channels can be recognized. It is named Paleo-Osaka River, which may have been entrenched in the regression stage of maximum phase of Würm Ice Age. The dotted lines in Fig. 3 express the subsiding movement of Osaka Bay since that age. 4) Akashi and Kitan straits have their origin in the Paleo-Osaka River system, but their sea caldrons reaching more than -100m have been formed by the submarine erosion after the deposition of B.
A. The Narugo volcanic area The original form of Kata-numa which seems as explosion crater is supposed to be a paleo-lake formed by the crustal deformation. The mudflow materials and volcanic ashes erupted out under the fluviatile-lake environments. The volcanic ashes from the Narugo volcanoes covered the mudflow materials and terrace plains except the present river bed. These ashes are classified into two main volcanic ashes depending upon the soil colors. One of them is the black volcanic ash, and the other is the brown volcanic ash. The soil color of the black volcanic ash is presumed to have been derived from the colored minerals and humus. B. The Onikobe volcanic area The original form of the Onikobe basin was formed by the crustal deformation, and deposited the lacustrine sediments in the paleo-lake. The first eruption of the volcanic ejecta occurred under the fluviatile-lake environments. The volcanic ashes from the Onikobe volcanoes accumulated on the outer mountains, inner volcanic bodies and terrace surfaces except the present river bed. However, the volcanic ashes in the Katayamajigoku and Arayu areas include the plant leaves. These ashes are supposed to have been deposited on the bottom of the paleo-lake in early Quaternary age. Under the polarized microscope, no mineral fragments are recognized in these ashes. This is a remarkable point of the characteristics compared with the volcanic ashes of the other areas investigated. This reason is supposed that the alteration of the volcanic ashes has been accelerated by the solfataric action and subterranean heat etc.