TAPHONOMY AND OBRUTION FORMATION OF BRYOZOAN – FORAMINIFERA THICKETS FROM THE MIDDLE MIOCENE TANOSAWA FORMATION OF NORTHEASTERN JAPAN

A wave-cut platform, raised by an earthquake in 1793, at Odose on the northwestern coast of Honshu, Japan exposes fine to very coarse sandstones of Tanosawa Formation (early-middle Miocene). The uplifted seafloor exposes a field of bryozoan frameworks thickets, many appear in living position and distribution, and bryozoan-foraminifera concentrations (occurring as nodular regions with circular bedding plane patterns) associated with the thickets. Rounded granules, grading to pebbles, of scoria occur in the thickets indicating volcaniclastic input. Several modes of formation have been suggested, including: concentrations resulting from liquefaction/fluidization during sediment dewatering; the accumulations are biogenic, but indirect from activity during the formation of the ichnotaxon Piscichnus waitemata, which has been interpreted as the feeding pattern of sting rays; or as we assert bryozoan colonies stabilizing a portion of the Miocene seafloor facilitating baffling that entrapped foraminifera and scoria during a higher energy events. The in living position habit along with the preservation of the patchy distribution of the bryozoans favors an interpretation that the deposit by obrution of the sea floor, perhaps during a seismic/volcanic event.