Characteristics of the Megafossil Assemblages in the Upper Turonian (Cretaceous) of the Pombets Area, Hokkaido

Abstract

The characteristics of the megafossil assemblages in the Upper Yezo Group (Upper Turonian) in the lower stream of the Pombets, the tributary of the Ikushumbets, central Hokkaido are described in this paper, discussing their paleoecological and paleoenvironmental implications. The Upper Turonian sequence along the Pombets is represented by silty to fine sandy sediments with several thin beds of fine sandstone. Calcareous fine sandy nodules are predominate at various horizons, and some of them are rich in megafossils such as molluscs, fish and lobster, together with numerous drifted plant remains and amber fragments. The assemblages in the lower part of the Upper Turonian are characterized by the abundant occurrences of the collignoniceratids and Linuparus cf. japonicus (lobster), while those in the middle to the upper parts are rich in the heteromorph ammonites (especially scaphitids), desmoceratids and tetragonitids, with an elopoidid fish, Holcolepis sp. at two horizons. With respect to the species composition and diversity, the former assemblages are similar to those in the nearshore shallow-water facies in the Manji-Yubari areas, south of the Pombets area. In contrast, the latter ones suggest the transitional biofacies from the intermediate facies reflecting the outer shelf environment to the nearshore shallow-water one in the marine Upper Turonian sedimentary basin of central Hokkaido. Judging from the ecology of allied modern species to L. cf. japonicus and the Elopoidei, and the litho- and biofacies characteristics mentioned above, it is suggested that the fine sandy to silty sediments in the Upper Turonian of the Pombets area might have had deposited on the shallow sea bottoms of about 100 m depth (inner shelf) under the influence of warm currents. We also discuss some problems on sedimentary environments and habitat distributions of Nipponites and Madagascarites in Hokkaido in Turonian times.