The fossiliferous, marine Upper Cretaceous sequence of fine sandy siltstone or silty sandstone lithology (Upper Yezo Group) is well exposed in the lower stream of the Pombets, central Hokkaido (Fig. 1).
The age of the uppermost part of the sequence is determined as the Coniacian, because of the occurrence of Inoceramus uwajimensis and Baculites cf. yokoyamai. The main part of the Upper Yezo Group in this area is Upper Turonian, as has been hitherto determined (Fig. 3).
Throughout the sequence the Upper Turonian comprises many well-preserved ammonites which are studied paleoecologically in this paper. The Upper Turonian ammonite assemblages are roughly classified into the following three in accordance with the differences in faunal composition and diversity, i.e. the assemblages of the lower, middle and upper parts (Fig. 4). The assemblage of the lower part is characterized by the common occurrence of a baculitid Sciponoceras intermedium with associations of collignoniceratids, scaphitids, tetragonitids and others. The assemblage of the middle part is similar to that of the lower part in the major species diversity, but differs in the absence of collignoniceratids. The upper Upper Turonian assemblage is rich in collignoniceratids (Lymaniceras planulatum and species of Prionocyclus) with rare associations of heteromorph and smooth or weakly ornate morphotypes. The comparison with previous works on the Turonian ammonite assemblages in other areas of Hokkaido (Matsumoto, 1965; Tanabe et al., 1978; Tanabe, 1979) suggests the ascending biofacies change from a shallow sea fades to a less shallow and more offshore "transitional facies," and again to shallow sea fades in the Upper Turonian of the Pombets area. The upper Upper Turonian ammonite assemblage characterized by L. planulatum, Prionocyclus aberrans etc. is also distributed in the Bibai (north of Pombets) and Sentarozawa (west of Pombets) areas, but has not yet been found from the Manji-Hatonosu areas (south of Pombets) (Matsumoto & Harada, 1964; Obata & Futakami, 1975, 1977). In contrast, the upper Upper Turonian assemblage in the Manji-Hatonosu areas and also in the Katsurazawa (eastern Ikushumbets) area is represented by the abundant occurrence of Reesidites minimus, which is very rare in the Pombets area.
These lines of evidence strongly suggest a segregation in habitats between R. minimus and L. planulatum in Hokkaido in the late Turonian time.
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