静岡県牧ノ原台地に発達する古谷層 (上部更新統)の堆積環境

抄録

Depositional environments of the Makinohara Upland area, Shizuoka Prefecture, are determined from detailed sedimentological, paleontological and stratigraphical data.
The results are summarized as follows:
1) The Furuya Formation (maximum thickness 30m) is composed mainly of fine-grained sediments that were deposited in shallow embayments during transgressions of the sea up narrow valleys in the early stage of a so-called Last Interglacial Age.
2) The Furuya Formation can be divided into five superposed mappable lithologic cycles (coded I-V), each of which shows a nearly complete, fining-upward, sedimentary sequence consisting of, in ascending order, a basal gravel, a sand, sandy clay, and finally clay (Figs. 5, 6).
3) The Makinohara Formation (maximum thickness 50m) comformably overlies the Furuya Formation. The sedimentary surface of the Makinohara Formation forms the terrace known as the Makinohara Upland. The Makinohara Formation is laterally divided into two lithofacies; poorly sorted gravel proximally, and well sorted sands in the distal parts of the buried valleys. The sands that were deposited near the river mouth and near the bay mouth and coastal areas of the“Paleo-Sagara-Bay” are considered deltaic and coastal sand bank deposits, respectively.
4) The history of sedimentation in the study region is interpreted as follows (Figs. 10, 11):
(a) Before the Last Interglacial Age two major valleys had been cut to Neogene basement.
(b) During the early stage of the interglacial transgression, fluvial sediments consisting of coarse sands and gravel were deposited along the distal parts of the buried valleys (Sedimentary cycles I-II).
(c) As the transgression continued, marine water soon inundated the buried valleys and the initial“Paleo-Sagara-Bay”was formed (Sedimentary cycles III-IV).
(d) During a standstill of the sea at the time of maximum transgression, a “Paleo-Sagara-Bay”became filled with fine sediments, eventually forming a marsh (Sedimentary cycle V).
(e) After the maximum phase of the transgression, the“Paleo-Sagara-Bay”was totally covered by coarse fluvial deposits and/or coastal and deltaic sand.
5) Because the last (marsh) phase of“Paleo-Sagara-Bay”was at sea level, the amount of differential crustal movement within the study area can be closely estimated. It has been determined that in the uplands (Makinohara) of the northern part of the study area the contact between the Furuya Formation and the Makinohara Formation has been uplifted at least 150m (Figs. 12, 13).