Abstract
Scour marks up to 1.5m deep with smooth wall, rich in fine-grained heavy grains are found in the massive division of pyroclastic-flow deposits of the Seiganji Tephra Formation in northern Kyushu. The scour marks are asymmetric with steeper slope on the upstream side, and filled with poorly sorted grains of a(p) a(i) fabric. Some massive turbidite sandstones of the Pleistocene Umegase Formation in Boso Peninsula also exhibit similar scour marks up to 0.7m deep. However, the scour marks are characterized by symmetric shape and high-angle imbrication grain-fabric. The results of open-channel flume experiments indicate that such scours can not be formed as flute marks or pot holes, but by scouring around obstacles. Mud clasts may be favorable for such obstacles in turbidites, and rip-up soil blocks or tree debris are for pyroclastic flows. Shallow flow comparable to the obstacle thickness is responsible for deep sours, especially for scours with secondary hollow and backset fillings. The estimated underflow thickness is 0.7m for turbidity current, and 1.5m for pyroclastic flow, respectively.
