On the Correlation of the Cenozoic Strata Developed in the Bôsô and the Miura Peninsulas, SOUTH Kantô Region, Japan from the Viewpoint of the Characteristic Features of the Molluscan Fosil-coenoses With Reference to Some Fundamental Problems

Abstract

The thick marine strata of the Cenozoic Erathem are developed extensively in the Boso and the Miura peninsulas, South Kanto Region, Japan. These strata have long been studied by various authors from several points of view.In the present paper, a tentative correlation of these strata is proposed by the writer from the viewpoint of the characteristic features of the molluscan fossil-coenoses, and the geological range of the important species of molluscan fossils found in the Cenozoic strata in the two peninsulas is tabulated according to the writer's opinion.(1) The Cenozoic strata excluding the nonmarine Pleistocene and the whole Holocene strata in the two peninsulas are divided lithologically into five groups in descending order as follows.5. The Narita group 4. The Sagami group 3. The Kazusa group (=the Kurotaki unconformity) 2. The Miura group 1. The Pre-Miura group (including the Mineoka and the Hota groups in the Boso peninsula and the Hayama group in the Miura peninsula)(2) Based on the marine molluscan and foraminiferal fossils as well as the land mammalian fossils, different opinions have hitherto been expressed by many Japanese geologists and paleontologists concerning the geological age and the correlation of the Cenozoic strata in the two peninsulas. However, the biochronologic method as was universally used in the past by many authors is not helpful in the marine Pliocene and Pleistocene strata because no remarkable biochronologic change in the contents of various kinds of fossils is recognized throughout the whole strata. Especially, so far as the marine Pleistocene strata are concerned such a method is not applicable in almost all cases, and it is possible to establish the accurate correlation on the basis of the changes both in sea-level and in thermal condition of sea-water.Here, it is necessary to discriminate the strata deposited in the colder water and lower sea-level stages from those in the warmer water and higher sea-level stages by the employment of the HDM (=Horizontal Distribution Means) and the VDM (=Vertical Distribution Means) characteristic curves.(3) According to the writer's recent studies, the correlation and the geological age of the Cenozoic strata excluding the nonmarine Pleistocene and the whole Holocene strata in the two peninsulas may be considered potentially as shown in Table 1.(4) Abundant molluscan fossils are yielded from the Cenozoic strata in the two peninsulas. Based on the writer's studies, however, the contents of the molluscan fossils change very gradually from the older up to the younger strata. Especially, no remarkable change of the contents of the molluscan fossils is seen throughout the Pliocene and the Pleistocene strata. Differences in the contents of the molluscan fossils found in the respective formations under consideration may be due merely to those in ecological and sedimentological conditions and not to biochronological factors.The geological range of the important species of molluscan fossils found in the Cenozoic strata in the two peninsulas is tabulated as shown in Table 5 on the basis of the present state of the writer's knowledge.