Abstract
A shell-concentrated sandstone bed (50-53 cm thick), containing sporadic vesicomyid and thyasirid shells up to 10 cm in diameter, occurs in the Lower Pleistocene Nojima Formation, which is a forearc basin fill on the Miura Peninsula, Pacific side of central Japan. The sandstone bed consists of three units (in ascending order): Unit 1, a cm-thick reverse-graded, coarse-grained to pebbly coarse-grained sandstone; Unit 2, a 25-cm-thick normally graded, pebbly coarse-grained to fine-grained sandstone; and Unit 3, a 10-cm-thick parallellaminated, fine-grained sandstone. Units 1 and 2 contain abundant small molluscan shells, commonly with a maximum length of less than 1 cm and including a wide variety of shallow- to deep-water species, whereas molluscan shells are rare in Unit 3. Vesicomyid and thyasirid shells occur mainly in Unit 2, and rarely in Unit 3. The molluscan shells throughout bed are commonly fragmented and abraded.
We measured the shell orientations of bivalves and gastropods in units 1 and 2, excluding shell fragments. In Unit 1, the axes of the gastropod shells and the commissure planes of the bivalve shells are randomly oriented, but those in Unit 2 commonly dip southward.
We interpreted the sandstone bed as having been deposited by both a debris flow (Unit 1) and a turbidity current (Units 2 and 3) occurring in sequence during a single gravity flow event with a north-ward flow direction. This sediment gravity flow originated in shallow-water sediments containing shallow-water molluscan shells and subsequently entrained mollusks living in a wide variety of environments, including a cold seep site where vesicomyids and thyasirids flourished. These results indicate the existence of active cold seepage not at the study exposure, but south of the exposure, at the time of the sediment gravity flow event.
