Some Problems Concerning the Geological Age and Correlation of the Miura Group on the Bôsô and Miura Peninsulas, South Kantô, Japan, Viewed from the Mollusca Faunistic Sequence

Abstract

Defferent opinions concerning the geological age and correlation of the Cenozoic strata developed on the Bôsô and Miura Peninsulas, South Kantô, Japan, have hitherto been expres-sed by many Japanese geologists and paleontologists. The Cenozoic strata developed on the B ôs ô and Miura Peninsulas are divided lithologically into two groups, the lower, the Miura, and the upper, the Narita. In the present paper, the writer deals with some problems concerning the geological age and correlation of the Miura group. 1) From the viewpoint of the Molluscan faunistic sequence, the Takoegawa sandstone and conglo-merate and Zusi mudstone on the Miura Peninsula may safely be correlated to the Mane mudstone, Motona sandstone and conglomerate, Siroiwa alternation and Kiyosumi sandstone on the B ôs ô Penin-sula, and the Kanazawa superformaton on the Miura Peninsula may be correlated to the Seki subgroup on the B ôs ô Peninsula. 2) No incontrovertible direct paleontologic evidence is yet known, however, the approximate position of the Miocene-Pliocene boundary in the Bôsô and Miura Peninsulas can be determined by the biochronologic analysis of the Molluscan fossils collected from var;ous horizons in the Miura group. According to the recent opinion of K. HATAI, the Miocene-Pliocene boundary in the Bôsô Penin-sula corresponds with the so-called “Kurotaki unconformity” placed at the base of the Kurotaki formation and its correlatives. However, the writer is inclined to consider that this boundary in the Bôsô Peninsula is possibly placed below the Kiyosumi sandstone and its correlatives from which the so-called Dainiti-type Molluscans are collected. In the Miura Peninsula, the Takoegawa sand-stone and conglomerate and Zusi mudstone may be considered as ranging from the Upper Miocene up to the Lower Pliocene. Therefore, no major stratigraphic break is found near the transition stage from Miocene to Pliocene time throughout the B6s6 and Miura Peninsulas. 3) The position of the Pliocene-Pleistocene boundary in the B ôs ô and Miura Peninsulas is currently one of the most controversial problems in Japanese Cenozoic stratigraphy. Although a great number of marine Molluscan fossils have been found in various horizons of the Pliocene and Pleistocene strata in the Bôsô and Miura Peninsulas, it is extremely difficult to determine where the Pliocene-Pleistocene boundary is, and opinions are still conflicting with one another on this problem. According to the writer's opinion, however, the Ôtadai alternation of sandstone and mudstone Umegase sandstone and alternation of sandstone and mudstone and their correlatives in the Bôsô Peninsula as well as the Kosiba tuffaceous sand in the Miura Peninsula may be considered to be Lower Pleistocene. The percentages of the living species of Molluscan fossils collected from these formations show tolerably high values.