A Paleogene deep-sea methane-seep community from Honshu, Japan

Abstract

An isolated limestone deposit occurs within late Eocene to early Oligocene submarine fan deposits of the Tanamigawa Formation in the outskirts of Kushimoto Town in southern Honshu, Japan. The petrography, the very negative carbon isotope signature of early diagenetic cements, and the abundance of chemosymbiotic bivalves, namely thyasirids, vesicomyids and bathymodiolins, clearly identify this carbonate block as the first Paleogene methane seep deposit in Honshu. From a biogeographical point of view, distinctive features of Paleogene seep faunas in Japan are the apparently endemic vesicomyid Hubertschenckia ezoensis and the scarcity of lucinid bivalves, whereas the bivalves Conchocele bisecta and Bathymodiolus spp., and the gastropod Cryptonatica were widespread in North Pacific seep communities of this age. Although Cenozoic seep communities generally consists of members of the same families that inhabit seeps today, marked differences in the genus-level composition of the major chemosymbiotic bivalve families such as Vesicomyidae and Lucinidae, and the subfamily Bathymodiolinae are noted when Paleogene seep communities in Japan are compared to those of early Neogene age on the one hand, and to Paleogene seep communities elsewhere on the other.