1:45 PM - 2:00 PM
[MIS28-01] Upper Pliocene large lucinid communities with cold-seep carbonates from the Shimajiri Group in the Miyagi-jima Island off the central part of Okinawa-jima Island, Japan
Keywords:chemosynthesis, cold seepage, Lucinidae, Okinawa, Pliocene
Fossil chemosynthetic lucinid communities occur with pipe- and burrow-cemented concretions from the Upper Pliocene muddy sand facies of the Shimajiri Group in the Miyagi-jima Island off the central part of Okinawa-jima Island, Japan. The outcrop is a sea-cliff wall along the west coast of Miyagi-jima Island, over 30 m high and 150 m long. The cliff consists of muddy sandstone sporadically yielding outer-shelf to upper-slope molluscan fossils, with intercalation of thin fine-grained sandstone layers. We could directly observe only the lower one-third of the cliff wall, but the fossil-lucinid occurrence seems to be restricted to about a five-meter thick horizon, which is associated with pipe- and burrow-cemented concretions.
Lucioma taiwanensis is dominant and forms shell beds and autochthonous shell clusters in matrix-supported condition. In addition “Meganodontia” sp., a gigantic lucinid over 10 cm in diameter, was obtained as floatstones, but all specimens are conjoined valves and maybe fell out from sites with abundant burrow-cemented concretions.
The burrow-cemented concretions look like calianassid nests in shape, irregularly sinuous to vertical to beddings, a few centimeters in diameter and over 1 m long. They amalgamated one another to form network-like clusters. The pipe concretions tend to be recognized below the network clusters of burrow-cemented types. The pipes are cylindrical and straight over 30 cm long, nearly vertical to beddings. They are composed of core calcitic cements (2 cm in diameter, -10 to -15 ‰ of δ13 C) and surrounding dolomitic coating (4 to 5 cm thick, -20 to lower -40 ‰ of δ13 C). The δ13 C value less than -30 ‰ indicates methane seep influence in the sub-bottom of the lucinid community. The pipe concretions are also often recognized 5 m below the fossil lucinid horizons, which suggest that the pipe-like conduits of methane seep supported the lucinid communities.
The fossils of Lucinoma taiwanensis often preserve their life positions in matrices apart from the burrows and pipes. Therefore the methane-seep through focused pipe-like conduit were succeeded by into diffusive in sandy surface sediments. But some Lucinoma conjoined valves in life position are attached to outer walls of burrow-cemented concretions, which suggests that the burrows made contributions to promoting anaerobic oxidation of methane (AOM) and generating H2S.
Lucioma taiwanensis is dominant and forms shell beds and autochthonous shell clusters in matrix-supported condition. In addition “Meganodontia” sp., a gigantic lucinid over 10 cm in diameter, was obtained as floatstones, but all specimens are conjoined valves and maybe fell out from sites with abundant burrow-cemented concretions.
The burrow-cemented concretions look like calianassid nests in shape, irregularly sinuous to vertical to beddings, a few centimeters in diameter and over 1 m long. They amalgamated one another to form network-like clusters. The pipe concretions tend to be recognized below the network clusters of burrow-cemented types. The pipes are cylindrical and straight over 30 cm long, nearly vertical to beddings. They are composed of core calcitic cements (2 cm in diameter, -10 to -15 ‰ of δ13 C) and surrounding dolomitic coating (4 to 5 cm thick, -20 to lower -40 ‰ of δ13 C). The δ13 C value less than -30 ‰ indicates methane seep influence in the sub-bottom of the lucinid community. The pipe concretions are also often recognized 5 m below the fossil lucinid horizons, which suggest that the pipe-like conduits of methane seep supported the lucinid communities.
The fossils of Lucinoma taiwanensis often preserve their life positions in matrices apart from the burrows and pipes. Therefore the methane-seep through focused pipe-like conduit were succeeded by into diffusive in sandy surface sediments. But some Lucinoma conjoined valves in life position are attached to outer walls of burrow-cemented concretions, which suggests that the burrows made contributions to promoting anaerobic oxidation of methane (AOM) and generating H2S.