Shallow subsurface geology based on analysis of drilling cores and borehole data obtained in the Aizu basin, Northeast Japan

Abstract:

The Aizu Basin is one of tectonic basins aligning with north-south direction in the south part of Northeast Japan. Along the west and east margin of the basin, active reverse faults stretches respectively. To understand the basin development since middle Quaternary, we discuss shallow subsurface geological structure in the Aizu basin based on drawing many geological sections by using drilled cores and 1,200 borehole log data. This result also can be useful for understanding activities of fault systems and assessing groundwater flow system and potential of ground-source heat energy.

Quaternary systems in the Aizu basin are consist of gravel layers, mud layers, and volcanic sediments. Gravel layers most widely and thickly distribute, over 200 m thickness in northern part of the basin. A few meter layers of mud sediments pinches between gravel layers, but in the central region, mud layer of 10-20 m thickness widely lies at the depth of 5-10 m and covers gravel layer underlying. Volcanic sediments consist of pyroclastic flow sediments and fall ash sediments. Pyroclastic flow sediments are deposited during early Pleistocene and detected at over 100 m depth in southern part of the basin, but distribution in whole basin has been still uncertain because of lack of deep borehole logs.

In the central region, tephra layers (deposited at ca. 220 ka) were detected from a drilling core at ca. 81.5 m depth in the east margin of the basin. An average accumulation rate of sediments since 220 ka in eastern part of basin was ca. 0.37 m/ ky, comparable with a contemporary rate (ca. 0.2-0.5 m/ ky) in western part of basin calculated by previous studies. It is suggested that average vertical slip rates of an eastern active fault is comparable with a western active fault if accumulation rate depends on tectonic activities.