Shallow-marine turbiditic sandstones juxtaposed with deep-marine ones at the eastern margin of the Niigata Neogene backarc basin, central Japan

Abstract

At the eastern margin of the Niigata Neogene backarc basin, two kinds of turbiditic sandstones were concurrently deposited, i.e. deep-marine turbiditic sandstones in the west area and shallow-marine turbiditic sandstones in the east area. The deposition took place in the upper part of the Lower Pliocene Kawaguchi Formation. Data based on sedimentological and heavy mineral analyses revealed that these sandstones were both supplied from the east, but from different source rocks through different routes. Sedimentary structures of the east shallow-marine sandstones mimic those of the Bouma Sequence except for the poor development of the massive sandstones and the existence of finer-grained thin parallel lamination in the lowermost part of some sandstone beds. These sandstones were supplied from the coastal area and transported offshore through many small feeder channels. Sedimentary structures of these sandstones seem to suggest the existence of shallow marine turbidity currents. Such turbidity currents might have occurred under such specific paleogeographic or tectonic conditions as backarc basins with relatively steep and narrow shelves.