Abstract
The Kissawa Formation distributed in Oiso Hills, ca 150, 000-120, 000 FT y.B.P. is subdivided into two plant fossil assemblage zones, the Kissawa-I and -II in ascending order. The boundary between them is situated just over the horizon of the volcanic ash layer named Klp-2, ca 130, 000 y.B.P. The Kissawa-I is characterized by such warm temperate taxa as Sapium sebiferum, Vitex cf. cannabifolia, Aleurites cordata, Stephania japonica, Paliurus, Illicium, Mallotus japonicus and Firmiana, and of many temperate broadleaf trees. The Kissawa-II contains no elements of warm temperate taxa, and is characterized by a fair amount of Cryptomeria japonica, Alnus japonica and Berchemia. The changes of the plant fossil assemblages support the previous opinion on the paleogeography of the sedimentary basin, especially the opinion that the maximum stage of the Shimosueyoshi Transgression is situated nearly the horizon of the volcanic ash layer named Klp-1.
