2010 Volume 61 Issue 3-4 Pages 125-136
Dive survey by the submersible “Shinkai 6500” was conducted at a middle part of upper reaches (thalweg: water depth of 1814 m) of the Kushiro Submarine Canyon,off Tokachi,Hokkaido.The canyon indents the outer shelf along a generally straight,deeply entrenched course of 233 km in length upon the forearc basin and slope of the Kuril Trench in the Northwest Pacific.The canyon side-wall at the dive point is stepped as three outcropped escarpment and two terrace.The first escarpment is at the water depth of 1795-1770 m and is composed of alternating beds of very fine sandstone and siltstone.The second and the last escarpments are at the water depth of 1580-1550 m and 1415-1370 m,respectively.The last escarpment represents minor step-shaped outcrop.These are composed of massive siltstone.Most rock samples are semiconsolidated and composed of diatom frustules and fine-grained terrigenous clastics.The diatom and radiolarian ages suggest that the strata in the first,the second and the last outcrops were deposited in the early Late Pliocene,the late Late Pliocene,the Early Pleistocene periods,respectively.