Fossil cold seep assemblages in shallow water deposits do occur in regressive facies !?

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Fossil chemoautotrophic assemblages in Japan can be divided into three types, based on species composition, mode of occurrence, and palaeobathymetric distribution. Type 1 is dominated commonly by vesicomyid bivalves and/or tube worms, and characterised by its allochthonous or parautochthonous mode of occurrence in deep-water settings, along with tectonically-or sedimentologically-induced structures such as faults, slumps, diapirs, or debris flows. Some Type 1 assemblages are comparable to modern cold-seep communities around Japan at depths of more than about 500 m, living on scarps and talus deposits, whereas other Type 1 assemblages are seemingly related to slope failures and sediment collapses caused by the drastic decomposition of gas hydrates. Type 2 assemblages are also dominated by vesicomyid bivalves, but usually the species differ from those in Type 1. In contrast with Type 1 assemblages, Type 2 assemblages are characterised by autochthonous occurrences in muddy sediments in le … More ss than 1,000 m water depth. Type 3 assemblages are dominated by Lucinoma and/or Conchocele and are characterised by autochthonous occurrences in muddy sediments at water depths of less than 300 m. No living community comparable with Types 2 and 3 assemblages has yet been observed in the seas around Japan. It is noteworthy that no tectonically-or sedimentologically-induced structures, such as faults, slumps and/or debris-flow deposits, have been observed in and around Type 2 and 3 fossil localities, although the muddy sediments sometimes show brecciated facies that suggest an explosive effusion of methane. Occurrences of Types 2 and 3 may have been synchronous with sea-level changes that triggered methane-hydrate decomposition and/or the upward migration of methane gas. This working hypothesis may also explain why there in no comparable modern community made up of these types : present-day sea level is stable compared to the ancient sea levels when Type 2 and 3 fossil assemblages flourished. Less