南関東地域の中部更新統の層序とその特徴

Abstract

The Middle Pleistocene formations of the South Kanto district attract many geologists' attention as one of the important Quaternary type successions in Japan, because of wide distribution, extremely thick sequence of aqueous and aeolian deposits and containing a large number of fossils. The geologic structure of the formations in this district, however, was complicatedly deformed during Quaternary, and detail stratigraphy were veiled till quite recently.In this paper, we proposed a preliminary stratigraphic chronology on the Middle to Upper Pleistocene in the district (Table 2), which has been compiled in some recent fruits by several authors (KANTO QUATERNARY RESEARCH GROUP, 1974; MACHIDA et al., 1974; UESUGI, 1970; KIKUCHI, 1977; NISHIMURA, 1980; etc). Some geologic characteristics of the Middle Pleistocene formations are enumerated, too.(1) Some volcanoes situated in the western region produced the enormous volume of pyroclastic fall materials during Quaternary. An almost continuous columnar section of aeolian pyroclastic materials deposited during the past 400, 000 years is compiled from several sections of the Oiso Hill adjacent to the Fuji-Hakone volcanic region, which is more than 250 meters in thickness.(2) Distribution of the Middle Pleistocene aqueous and aeolian deposits in the district is very complicated. Lithofacies of these deposits are extremely variable in a narrow area. Many characteristic pyroclastic key beds are most useful to correlate with these various deposits.(3) The marine deposits of the Middle Pleistocene are 300 meters thick in the Oiso Hill and 600 meters in the Boso Peninsula. They are divided into 8 to 10 stratigraphic units by disconformable boundaries caused by lowerings of the sea level. Formation of each unit corresponds to a stage of glacial eustatic sea level rising.(4) It is generally believed that the South Kanto is one of the rapid upheaval districts in Japan and there are several marine terraces recording the Middle to Late Pleistocene sea levels in the district. We must indicate, however, that there is no graded feature formed during the Middle Pleistocene at least except the Tama Hill area along the west coast of Tokyo Bay. The contemporaneous marine deposits accumulate one after another or intermittently with disconformities at the great part of the coastal area and do not constitute any terraces.