茨城県桜川低地における過去３２，０００年間の環境変遷

Paleoenvironments of the last 32, 000 years have been investigated along the middle to lower reaches of the Sakuragawa Lowland, Ibaraki Prefecture. Stratigraphical data, including tephras and ¹⁴C dates, and paleoenvironmental data that is mainly based on pollen analyses clarified the environmental history of the later stage of the last glaciation and the Holocene. In particular, the Shimo-Oshima Formation of 24, 000 to 17, 000 yrs BP provided information on the detailed paleoenvironments around the maximun phase of the last glaciation. The results are summarized as follows;<br>1)-32, 000 yrs BP: The Paleo-Kinu River flowed along the Kokaigawa Lowland into the Sakuragawa Lowland through the Kyowa upland, which consists of Kyowa Gravel of the Oda terrace (Tc₁), covered with the whole Tachikawa tephra formation including KP (Ikeda et al., 1977; Suzuki, 1980; Suzuki et al., 1981).<br>2) 32, 000-24, 000 yrs BP: The Paleo-Kinu River was still flowing into the Sakuragawa Lowland, and deposited Tsuchiura Gravel on the Yahagi terrace (Tc₂), which was covered with the upper part of Tachikawa tephra formation including AT.<br>3) 24, 000-22, 000 yrs BP: The course of the Paleo-Kinu River changed between the Sakuragawa Lowland and the Kokaigawa Lowland, and left the Kyowa Upland as a wind gap. River terraces of Oda and Yahagi were undercut, and a shallow valley in the middle and a deep valley in the lower reaches of the Sakuragawa Lowland were formed and buried with the Shimo-Oshima formation. The basal gravel of the Shimo-Oshima formation is called Shimo-Oshima Gravel (SG) and it is correlated with the basal gravel (BG) deposited along the valley bottom in the lower reaches. The lowest part of the Shimo-Oshima formation is characterized by cool-temperate deciduous broadleaved forest with dominant occurrence of Quercus subgen. Lepidobalanus (Pollen assemblage zone: SM-I).<br>4) 22, 000-21, 000 yrs BP: AT, one of the most wide-spread tephras in Japan, was deposited in the swampy environments of the lower part of the Shimo-Oshima formation. The vegetation was characterized by an increase in coniferous forest and Tilia and inversely a decrease in Quercus subgen. Lepidobalanus (Pollen assemblage zone: SM-IIa).<br>5) 21, 000-17, 000 yrs BP: The vegetation from the middle part of the Shimo-Oshima formation was dominated by upper cool-temperate to lower subartic coniferous forest with high percentages of Pinus subgen. Haploxylon and Picea (Pollen assemblage zone: SM-IIb). This is relevant to the coldest phase of the last glaciation.<br>6) 17, 000-11, 000 yrs BP: The upper part of the Shimo-Oshima formation consists of tuffaceous silt, and is characterized by a decrease in the coniferous forest and an abrupt increase in Alnus and Betula (Pollen assemblage zone SM-III).<br>7) 11, 000-10, 000 yrs BP: Undercutting was done by the Tsukada sand and gravel (HBG), the basal gravel of the Holocene Iida formation. This short period characterized by active fluviation may be related to the Younger Dryas event.<br>8) Holocene: The Holocene (Jomon) transgression invaded the lower reaches of the Sakuragawa Lowland to form the marine portion of the Iida formation, whose sedimentary environment was changed to a fluvial one toward the middle reaches. The late Holocene fluvial deposits, Upper Iida formation (Hojo member), are distributed along the Sakuragawa Lowland.