Negene thick deposits including turbidite facies beds underlain by coal-bearing formations are distributed along the Sea of Japan of northern Hokkaido. Recently, studies on the Neogene biostratigraphy (Tsucni ed., 1977; TANAI ed., 1982) and also on the getectonics of this area (KIMURA et al., 1983; OKADA, 1983) have made a great progress. Some geological problems of the Neogene sequences in this area are discussed in this report, on the basis of a recent progress of biostratigraphy and sedimentary basin analysis. 1) Neogene formations are divided into six stratigraphic units by a litho- and bio-strati-graphic examination as follows: Haboro-Sankebetsu, Chikubetsu, Kotambetsu, Wakkanai, Koetoi and Yuchi- Sarabetsu in as ascending order. Paleogeographic maps of the four stages such as Chikubetsu, Kotambetsu, Wakkanai and Yuchi are illustrated in Fig. 5. 2) The Neogane sedimentary basin of this ares was formed by the lateral-faulting movement closely related with an upheaval of the northern part of the Hokkaido Axial Zone. 3) One of the most important problems is to prove the existence in the Kotambetsu age of the "Western Lands", which are possibly supposed to be situated in the Sea of Japan, to the west of the line of Rishiri Island-Rumoi City. 4) In the Wakkanai and Koetoi ages, the main area of subsidence moved to the west through the continuous upheaval of the Hokkaido Axial Zone, resulting in the disappearance of the main parts of the "Western Lands" under the sea. 5) In the Yuchi age, the sea of the sedimentary basin became shallower than in the preceeding age, whereas the basin expanded to the east in the Takikawa-Fukagawa district, where coarse-grained sediments have covered the western part of the Kamuikotan Tectonic Belt. This discordance should be studied through an examination of a differential upheaval of the Hokkaido Axial Zone and/or a possible global sea level change.
