2016 Volume 125 Issue 5 Pages 763-773
There are two geomorphic surfaces (terraces) in the northwestern Kanto Plain: Takasaki Upland (Takasaki Surface) and Inokawa Lowland (Ino Surface). It is considered that about 11,000 years ago the Inokawa Mudflow traveled down from the west, forming the Takasaki Upland as a depositional surface. However, opinions are divided and another theory is that the Inokawa Lowland is also the depositional surface of the Inokawa Mudflow. Therefore, this study examines the subsurface geology around the Inokawa Lowland, based on an expanded dataset of existing borehole data obtained from our portable hand-operated drilling survey. The results clearly show that the Inokawa Lowland is basically a depositional surface of the Inokawa Mudflow. Reflecting upon this conclusion, how the terrace scarp between the two surfaces was formed becomes problematic. There are two possible causes to be clarified by future investigations:
(1) Mantle bedding of the Inokawa Mudflow on the pre-existing fluvial scarp,
(2) As-yet-unrecognized vertical faulting after full deposition of Inokawa Mudflow deposits in this region, probably related to the Fukaya fault system.