Shallow Subsurface Geology of the Ohmi Basin

Abstract

This study clarified the stratigraphy and sedimentary facies of the shallow subsurface strata of the Ohmi Basin, an inland lake basin in central Japan. Stratigraphic divisions in the basin are based on regional tephra and widely distributed humic layers. The shallow subsurface strata of the basin are divided into two units. Unit II comprises middle to upper Pleistocene terrace deposits. The overlying Unit I comprises uppermost Pleistocene to Holocene postglacial deposits. Although the lithofacies of Unit II vary horizontally and vertically, the uppermost part consistently consists of humic soils containing Aira-Tn tephra (AT). This distinctive tephra is widely distributed throughout the Japanese islands. Humic soils were deposited at ca. 16.5 to 25 ka according to 14C dating. Unit I, which conformably overlies Unit II, and is mainly a repetitive succession of sand, mud, and gravel. Each layer is discontinuous in terrestrial areas, where distribution patterns of lithofacies are controlled by topography. The depth distribution of AT reflects tectonic subsidence induced by reverse faulting in the Biwako-Seigan active fault zone on the western side of Lake Biwa. Thus, tectonic movement was an important factor during sedimentation. Periodic small-scale subsidence (less than several meters per event) due to slips along faults of the Biwako-Seigan system caused discontinuities in late Quaternary fluvial deposits.