A REVIEW OF FOSSIL CHEMOAUTOTROPHIC ASSEMBLAGES IN JAPAN

Abundant fossil chemoautotrophic assemblages occur in Japan, ranging from Cretaceous to Holocene in age, and reflect the complicated tectonic history related to subduction of oceanic plates. Forty fossil assemblages are interpreted to have been cold-seep dependent and only one is associated with a whale-fall. Hydrothermal-vent related fossil assemblages have not yet been recognized. The Cretaceous assemblages occur in forearc basin-fills and are characterized by the autochthonous occurrence of chemoautotrophic bivalves. An extraordinarily large thraciid species was one of the main constituent bivalves. The Paleogene assemblages occur in accretionary prisms and forearc basin-fills but little is known about their mode of occurrence. The Neogene and Quaternary assemblages are divided into two types. The first type occurs in deep-water settings, greater than 500 m in paleobathymetry, and characterized by its allochthonous mode of occurrence along with tectonically induced structures such as faults, diapirism, debris flows, and brecciation. This assemblage type is dominated by Calyptogena species. This mode of occurrence agrees with those of Japanese modern cold-seep communities found at depths of more than 500 m. The modern cold-seep communities live on scarp and talus deposits related to faults and slide escarpments, and on turbidites in fan valleys. Under such erosional settings, cold-seep communities have a little chance to be autochthonously preserved. The first-type of fossil assemblage depended on seepages controlled by subduction-related tectonism. The second type is autochthonous and occurs in shelf to upper slope facies, shallower than 500 m in paleobathymetry. This type is dominated by the bivalves Calyptogena, Lucinoma, or Conchocele. No tectonically induced structures are observable around the fossil assemblages and no comparable modern community with this type is known. The occurrence of the second type may have been controlled by sea-level changes that triggered methane-hydrate decomposition or upward migration of methane gas. This hypothesis can also explain why there is no comparable modern community of this type: the present-day sea level is stable relative to the ancient waters when the second-type chemoautotrophic assemblage flourished.