Geological Setting

Kokemushi Paradise, otherwise known as the Utasai Section, is a small quarry situated in woodland close to the Buna Center, Kuromatsunai (N46°38’56’“, E140°18’36”). About 6.5 metres of the upper part of the Nakasato Conglomerate Member of the Setana Formation were formerly exposed (Takashima et al. 2008) before the main face of the quarry was graded. The age of the Nakasato Conglomerate Member, which forms the basal part of the Setana Formation, is estimated to be 1.2-1.0 Ma (Takashima et al. 2008). The member is located at the overlap between the ranges of the planktonic foraminifers Neogloboquadrina pachyderma and N. incompta, and within the CN13b calcareous nannofossil zone.

The sediment comprises weakly cemented, coarse bioclastic sandstone containing beds of clast- or matrix-supported conglomerate. Clasts are of pebble to cobble grade and mostly consist of various types of volcanic rocks. There are no aragonitic macrofossils present, presumably due to leaching from groundwater passing through the porous sediment, although small amounts of aragonite do remain in some of the bimineralic bryozoan species (Taylor et al. 2012). The dominant macrofossils are bivalves, including Mizuhopecten yesoensis, Swiftopecten swiftii, Chlamys islandicus and Acirsa ochotensis (Suzuki 1989). While evidence from these bivalves has been taken to indicate a shoreface depositional environment (Suzuki 1991), there is contradictory evidence to suggest a deeper water, more offshore setting (Taylor et al. 2012). Modern stylasterids mostly live at depths exceeding 100 m (e.g. Tendal et al. 2005; Lindner et al. 2008) noting that 89% of modern stylasterid species live in water deeper than 50 m. Furthermore, there is a conspicuous absence of the calcareous algae that are present elsewhere in the Setana Formation of the Kuromatsunai region.

Figure. Schematic stratigraphic profile of the Plio-Pleistocene formations in the Kuromatsunai district.

A narrow strait between the Sea of Japan to the northeast and the Pacific Ocean to the southwest during deposition of the Setana Formation was possibly at Kuromatsunai which today lies at a relatively low altitude. Mixing of water masses and strong currents in this strait may have resulted in high levels of phytoplankton for the bryozoans. In addition, regular influxes of pebbles and cobbles apparently provided suitable hard substrates for bryozoan colonization. The clasts are of volcanic origin and are rounded.  It is thought that they were transported into deeper water from a shallow source area and became encrusted afterwards.

Cold-water conditions dominated during deposition of the Nakasoto Conglomerate Member, in the lower part of the Setana Formation (Suzuki 1989; Nojo et al. 1999). According to Suzuki & Akamatsu (1994), the lower Setana molluscan fauna, including Yabepecten tokunagai, Chlamys islandicus and Acirsa ochotensis, belongs to the subarctic province and is indicative of sea surface temperatures of 12-20°C in the summer and 0-2°C in the winter. The bryozoan fauna also indicates cold conditions, as it contains several genera typical of Arctic or northern Boreal latitudes today. The seas around northern Japan today are mainly influenced by two currents, the warm-water Tsushima Current flowing northwards through the Sea of Japan, and the cold-water Oyashio Current flowing southwards in the Pacific (Kitamura and Kimoto 2006). It seems possible that the Oyashio Current prevailed during the interglacial highstand when the Nakasoto Conglomerate Member was deposited. In spite of evidence for a generally cold climate, MART (mean annual range of temperature) analysis using zooid size variability in two bryozoan species (Porella concinna (Busk) and Escharoides hataii Hayami) indicates a highly seasonal climatic regime, with an average temperature range of almost 12°C (Dick et al. 2008b). Flushing of the inferred depositional channel by the cold-water Oyashio Current in the winter and the warm-water Tsushima Current in the summer may have pertained.

Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith