Abstract
The Miocene Kawabata Formation consists of turbidites and associated coarse clastics, and shows an N-S trending elongated distribution in the western Yubari Mountains, central Hokkaido. The Kawabata Formation is divided into two members. The lower, Amagiri Sandstone and Mudstone Member is characterized by monotonously interbedded sandstone and mudstone with sporadically intercalated conglomerate beds. Pebbly mudstone is present at two horizons of this member. The upper, Higashiyama Sandstone and Conglomerate Member consists of thick-bedded conglomerate and sandstone with sand-rich alternation of sandstone and mudstone. Based on the correlation of five tuff beds (K1-K5, in ascending order), the lithostratigraphic boundary between these two members is younger in the south than the north. By the fission-track method, the age of tuff bed K5 is 13.2±0.9 Ma. Diatoms from beneath the K4 are indicative of the Crucidenticula nicobarica Zone to Denticulopsis praedimorpha Zone (13.1-11.5Ma). Diatoms from a horizon 400m above the K5 are indicative of the Denticulopsis dimorpha Zone (10.0-9.2Ma). The depositional age of Kawabata Formation thus ranges up to Late Miocene. The change in lithology from mud-rich alternation (the Amagiri Sandstone and Mudstone Melnber) to thick-bedded con-glomerate and sandstone (the Higashiyama Sandstone and Con-glomerate Member) was earlier in the northern area. Pebbly mudstone of the Amagiri Sandstone and Mudstone Member thickens northward. These suggest southward progradation of the basin-fill deposits. In contrast to the Miocene formations in northern Hokkaido and the Hidaka coastal area, gravelly deposits were continuously supplied until Late Miocene in the Yubari Mountains.
