Abstract In the writer's preceding paper published in this journal (Vol. 32, No. 6), he discussed 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 fossil-coenoses, and the geological range of the important species of molluscan fossils found in the Cenozoic strata in the two peninsulas was tabulated according to the writer's opinion.
However, still much remains to be solved of various problems related to the correlation. In the present paper, some fundamental problems are discussed based on the writer's recent opinion.
(1) Judging from the changes in lithologic and paleontologic facies, especially from those in sedi-mentary environments considered on the application of the HDM and the VDM characteristic curves for analyzing the molluscan fossil-coenoses, it is to be said that the repeated transgressions and regressions had alternated during the time ranging from the Upper Pliocene up to the Upper Pleistocene of the two peninsulas. The greater part of these transgressions and regressions may had probably been caused under the influence of the eustatic changes in the interglacial and the glacial stages respectively. The changes in sea-level inferred from those in lithologic and paleontologic facies of the respective strata are intimately related to the differences in thermal condition of sea-water presumed by the characteristic features of the marine fossils. It should be noted, however, that the thermal conditions indicated by the benthonic molluscan fossils seem not to harmonize with those by the benthonic or planktonic foraminiferal fossils.
(2) Although no incontrovertible direct paleontologic evidence is yet known, the approximate po-sition of the Miocene-Pliocene boundary of the two peninsulas may be considered to correspond with the so-called “Kurotaki unconformity” placed at the base of the Kazusa group. Therefore, the Miura group may possibly be presumed as belonging to the Upper Miocene and older in geological age.
On the other hand, no major stratigraphic break is found near the transitional stage between the Upper Pliocene and the Lower Pleistocene throughout the two peninsulas. This boundary is possibly placed near the boundary between the middle and the upper parts of the Umegase formation in the Bôsô peninsula, and also near the boundary between the Kosiba and the Nakazato formations in the Miura peninsula.
(3) It has long been accepted undoubtedly by almost all Japanese and foreign geologists that the sedimentary basin was mostly enlarged at the optimum of transgressional stage. Judging from the state of distribution of the strata and the sedimentary environment of the fossiliferous beds, however, the sedimentary area where the strata were once deposited must be enlarged to the greatest extent in the colder water and lower sea-level stage (=regressional stage) rather than in the warmer water and higher sea-level stage (=transgressional stage) so far as the marine Pleistocene strata developed in the Bôsô and the Miura peninsulas are concerned.
From the accounts mentioned above, the writer is inclined to consider that in the higher sea-level stage the shallow sea bottom near the intertidal and the euneritic-mesoneritic zones was an erosional or a nondepositional area because of the steep inclination of bottom and the strong erosion caused by bottom currents and sea-waves, but in the lower sea-level stage the whole sea bottom (or the greater part of the sea bottom) was a depositional area and the sea became shallower and shallower with the progress of regression

View full abstract