Abstract
Well preserved specimens of Lockeia siliquaria James 1879 are found in the Miocene Tatsukushi Formation, southwestern Japan. This bivalve escape traces are subvertical structures, filled with convexdown, chevron-like laminae, having an almond-shaped or elliptical cross-sections. Based on depositional facies analysis of the trace fossil-bearing interval, the trace fossils are interpreted to have formed in laterally accreted point-bar deposits. Although laterally accreted point-bar deposits are uncommon in braided fluvial systems, they do occur in such environment, and sedimentologic evidence indicates that the trace fossil-bearing unit represents a sandy braided-river system. In response to the aggradation of the river bed, the trace-making bivalves probably escaped upward in the sediment leaving chevron-like laminae. Many individuals of L. siliquaria are found on a current-rippled surface of a sandstone block. Most of the specimens show preferred orientation with their long axes arranged parallel to the paleocurrent direction, which is inferred from the dipping directon of foreset laminae of the ripples. Rounded ends of the almond-shaped traces point in the down-stream direction. Such a mode of trace fossil occurrence is interpreted as the result of the trace-maker's response to unidirectional water flow. The bivalves oriented their commissure plane parallel to the current direction and pointed the exhalent (siphonal?) area on the down-stream side in order to avoid recycling of water that was expelled from the shell of the bivalve. Thus, this trace fossil can be used as a reliable indicator of the paleocurrent direction.
