Subsurface Stratigraphy in the Obama Lowland on the South Coast of the Central Wakasa Bay

Abstract

　This study investigates the characteristics of the Pleistocene-Holocene deposits in the Obama Lowland, central Japan, based on the analyses of existing borehole logs and deposits. The main results are summarized as follows: 1） The incised-valley-fill deposits since around the Last Glacial Maximum （LGM） are conveniently divided into three layers: basal gravel （BG）, middle sand and mud （MSM）, and upper sand and gravel （USG） in ascending order. 2） The alternation of gravel and overlying mud layers is observed several times in the incised-valley-fill deposits and their lower part, suggesting subsidence of the lowland over 10⁴-10⁵ years. 3） The area near the present river mouth was drowned by the Holocene sea-level rise, though the distribution of the shallow sea was very limited. 4） The Kita River and its tributaries with large riverbed gradients supplied coarse-grained sediments and buried the shallow sea.