Permalink : http://hdl.handle.net/2115/38440

KEYWORDS : Bryozoa;Pleistocene;Setana Formation;Kuromatsunai;Hokkaido

The Pleistocene Setana Formation is exposed along the axial zone of the Kuromatsunai district, southwestern Hokkaido Island, and is divided into the Nakasato Conglomerate Member and Soebetsu Sandstone Member. The Nakasato Conglomerate Member consists of medium- to coarsegrained sandstone that accumulated in shoreface, delta, and channel systems. This member is correlated with the overlapping interval between the CN13b calcareous nannofossil zone and the Neogloboquadrina pachyderma (s) / Neogloboquadrina incompta planktonic foraminiferan zone, which indicates an age of 1.2-1.0 Ma. The Soebetsu Sandstone Member is made up of sandy siltstone to very fine-grained sandstone and includes several beds of shell concentrations. This member is correlated with the CN14a calcareous nannofossil zone and is thought to range in age from 1.0-0.6 Ma. The Setana Formation contains abundant and well-preserved molluscs, bryozoans, and planktonic and benthic foraminifera. Paleoclimatic reconstructions based on the molluscan and planktonic foraminiferan faunas, and on oxygen isotope ratios from planktonic foraminifera, indicate that the basal part of the Nakasato Sandstone Member was deposited during a cooler climate than that of the present Kuromatsunai district, while the lower and middle parts of the Soebetsu Sandstone Member were deposited when the climate was warmer than at present. The Setana Formation, together with other Pleistocene deposits in Japan, has the potential to become a model system for studies on the effects of climatic change on the composition and diversity of assemblages of benthic marine animals; on how community-level parameters modulate these effects; on the responses of individual taxa to climatic change; and on comparison of various proxies used to assess paleoclimate and climatic change.