三浦半島平作川低地の完新世の古地理変遷

The Hirasaku River, which is the longest river (ca. 8km) in the Miura Peninsula has a drainage area of 26.8km², and flows southeastward into the Tokyo Bay. The Holocene lowland along the Hirasaku River is underlain by marine deposits, a sand barrier, and fluvial valley bottom sediments plain (Fig. 1). The Quaternary active Kinugasa Fault, strikes southeastward in the hills to the southwest of the lowland. South of the Kinugasa Fault postglacial marine terraces (ca. 6, 000yrs BP) are about 20m high, and the average rate of uplift is estimated to be 3m/ka.<br>The purpose of this study is to reconstruct environmental changes in the study area based on shallow and deep borings, facies observation of the Holocene deposits, their radiocarbon ages (Table 1), and analysis of molluscan assemblages (Table 2). The marine transgression surface is well developed in the Holocene lowland along the Hirasaku River. A southeastward-sloping V-shaped deep valley underlies the lowland. This valley is called the Paleo Hirasaku Valley (Fig. 2). It is filled with the thick, unconsolidated, Holocene deposits, which are divided into three sedimentary types: basal gravelly bed (HMm·BG); thick clayey or silty deposits (HMm) (Hirasaku mud member of Kanie and Ishikawa, 1976); and thin sandy deposits (HSm) (Hirasaku sand member, ditto). Radiocarbon ages on mollusks of 6, 520 and 6, 180yrs BP from the Hirasaku mud member, and 2, 140 and 2, 210yrs BP from the Hirasaku sand member have been obtained (Figs. 3 and 4). Marine transgression resulting in deposition of the Hirasaku mud member is correlated with the postglacial (Jomon) transgression. During the transgression, the Paleo Hirasaku Valley was drowned and a deep embayment (the Paleo Hirasaku Bay) existed. A sand barrier comprising the Hirasaku sand member is interpreted as regressional.<br>Radiocarbon ages of the molluscan assemblages in the study area are shown in Fig. 5. The distribution of radiocarbon ages indicates progressive emergence of the inner part to the Paleo Hirasaku Bay, and coastal progradation.<br>Four stages in the palaeogeographic evolution of the Holocene lowland along the Hirasaku River are depicted in palaeogeographical maps (Fig. 6). Stage I is the time of the culmination of the Jomon transgression. The sea rapidly invaded inland due to the rise of the sea level from at least 9, 000yrs BP, and it reached about a relative sea level of 5m amsl at ca. 6, 500yrs BP. Thick clayey or silty deposits (Hirasaku mud member) were deposited during the transgression. Shallow marine mollusks such as Crassostrea gigas and Dosinella penicillata in the Hirasaku mud member indicate that a tidal flat and inner bay environments were widespread in the inner part of the Paleo Hirasaku Bay. Stage II commenced at the beginning of a regression at ca. 4, 000-5, 000yrs BP. At that time the bay narrowed, and sand barrier I formed of the bay mouth of Paleo Hirasaku Bay, resulting in the retreat of the sea. Inland of the sand barrier, the inner bay environment became a tidal flat, and the molluscan assemblage consists chiefly of tidal flat species such as Crassostrea gigas. At ca. 2, 000yrs BP (Stage III), the sea retreated to the middle reach of the bay, and sand barrier II began to be formed in the river mouth. Sand barrier I continued to develop, and a tidal flat was widely formed behind it. At 2, 000-300yrs BP (Stage IV), the sea continued to retreat and the bay progressively narrowed. Though two sand barriers had developed, they did not completely block the bay mouth. A narrow tidal flat remained along the present Hirasaku River, which, except for the immediate river mouth, was reclaimed at 1667.