Search

We describe the modes of occurrence of a vesicomyid-dominated fossil assemblage in the lowermost Pleistocene Urago Formation, a forearc basin fill on the Miura Peninsula, Pacific side of central Japan. The assemblage consists mostly of Calyptogena (Archivesica) kawamurai (Kuroda) with a minor amount of Conchocele bisecta (Conrad). The shells occur in cross-bedded and massive sandstones of an outcrop that is approximately 25 m wide and 10 m high. The sandstones are interpreted to have been formed by migration of dunes under northward- to eastward-directed bottom currents, judging from the dips of their foreset laminae. Many of the shells are disarticulated and show evidence of reworking by bottom currents, as indicated by their convex-up positions with their commissure planes dipping southwestward (up-current direction) or concordant with the cross laminae. In the massive sandstones, some articulated vesicomyids are preserved in their life position, that is, perpendicular to the bedding plane wit...

Shell microstructures of five solemyid species from Japan were characterized by scanning electron microscopy. All five species examined had outer and inner shell layers, and were categorized into four groups according to the shell microstructure composition. Group 1 consist of Solemya (Petrasma) pervernicosa and Solemya (Solemya) tagiri. The outer layer of their shells is characterized by radially elongate simple prismatic structure (RESP) type A, and the inner layer by irregular prismatic structure. Group 2 is represented by Solemya (Solemya) pusilla. The outer layer of its shell has RESP type B structure, and the inner layer structure is homogeneous. Acharax japonica is in Group 3. Its outer shell layer has RESP type C, and the inner layer is characterized by laminar, homogeneous, and irregular complex crossed lamellar structures. Group 4 is represented by Acharax johnsoni. Its outer shell layer has a reticulate structure and the inner layer has a cone complex crossed lamellar structure. Retic...

We described the shell microstructure of the cold seep-associated bivalve Conchocele bisecta using the acetate peel method and scanning electron microscopy, and revealed complex microstructural changes with ontogeny. The shell of the bivalve has outer, middle, and inner layers that consist of aragonite. The outer layer consists of spherulites and spindles of various sizes and shapes; these microstructures are identified as spherulitic, planar spherulitic, spherulitic prismatic, and spindle-like structures. The middle layer is characterized by cone complex crossed lamellar structure in the outer part and crossed lamellar structure in the inner part. The inner layer is composed of cone complex crossed lamellar, fine complex crossed lamellar, and irregular prismatic structures. On the basis of the observations from the umbo to the ventral margin of each shell layer, we recognized two growth stages that are divided by microstructural distributions in the outer and inner layers and the positions of d...

A cold-seep invertebrate assemblage consisting mainly of the large bivalve Lucinoma sp. is exposed at the Kuge Shrine and surrounding area in Shintomi Town, Koyu County, Miyazaki Prefecture, Kyushu Island, southwestern Japan. The outcrops with this assemblage are in the upper part of the Upper Pliocene Takanabe Formation of the Miyazaki Group. The Takanabe Formation exposed in Shintomi Town is a cyclical sequence of upward-coarsening and upward-fining sediments, which reflects sea-level change: the paleobathymetry inferred from molluscs implies a shallowing upward from 160 m–200 m to 50 m in water depth, followed by a deepening back to 160 m–200 m. The cold-seep assemblage occurs in the coarsest (muddy sandstone) and bathymetrically shallowest (50 m in water depth) horizon, an outer shelf environment near the average storm-wave base, in this shallowing and deepening cycle. The cold-seep assemblage around the Kuge Shrine occurs sporadically in area of about 100 m in a north-south direction and abo...

A cold-seep assemblage is present in slope to outer-shelf strata of the Upper Pliocene to Lower Pleistocene Ofuna Formation and the Lower Pleistocene Koshiba Formation of the Kazusa Group in Yokohama City, on the Pacific side of central Japan. Four cores were taken in order to learn the three-dimensional extent of the assemblage: one core was parallel to bedding and three were normal to bedding. Study of the cores and outcrops shows that: (1) the assemblage is at least 37 m thick stratigraphically and extends across the boundary between the Koshiba Formation and the underlying Ofuna Formation; (2) parallel to bedding, the assemblage is at least 16 m wide in a north-south direction and 30 m long in an east-west direction; (3) the assemblage consists mostly of large (up to 10 cm in maximum diameter), articulated bivalves of the genera Lucinoma, Conchocele, and Acharax, in aggregations or as sporadic shells; (4) authigenic carbonate cement in the matrix enclosing the assemblage ranges from dense to ...