九十九里平野北部の沖積世干潟層の花粉分析的研究

Abstract

The Higata Formation, laid down in the north of the Kujukuri coastal plain, Chiba Prefecture, eastern Japan, is lithologically divided into two members of the Lower and the Upper Member. The Lower Member measures about 15-20 meters in thickness, has its upper surface about 4 meters above the sea level, and consists mainly of sandy and silty sediments which include marine molluscan shells. Abundant molluscan shells with warm water species and poorly the corals of Oulastrea crispata are contained in the upper shell bed of this member, called the Higata Shell Bed. The Upper Member measures about 2 meters in thickness and consists of peaty silt and peat in which a few ancient boats of the prehistoric men are contained.Radiocarbon datings on the molluscan shells from the Higata Shell Bed indicate ages of 5, 940±180ys. B. P. and 6, 100±190ys. B. P., and on the peat materials from. the lowermost and the middle parts of the Upper Member indicate ages for the former of 5, 470±110ys. B. P. and for the latter of 3, 660±130ys. B. P.A pollen analysis was made on samples from the columns of the four sites YK-3, 4, 6 and 7. For the studies of the paleoenvironments in the area, four pollen assemblage zones are differentiated on the basis of the prosperities of the main genera of pollen in pollen diagrams.Lithological and palynological evidences of the formation suggests a paleogeographical change at about 5, 000ys. B. P. from the bay filled with warm sea-water and its seaside salt marsh in which the halophytes grew thick, to the eutrophic lake in which the hydrophytes attached to the substrate and free-floating ones lived. The peatland has developed along the inner margin of the lake from about 4, 000ys. B. P. The forest vegetation on the hills changed at about 3, 700ys. B. P. gradually from the forests dominated by Quercus and other deciduous genera, to the forests dominated by Gryptomeria and Quercus. At about 1, 500ys. B. P., there were increases of Pinus and Gramineae, which were considered to be influences of the catastrophic destruction of the former natural forests and the agricultural activities of the prehistoric men.