Subsurface geology of the Wakayama Plain, southwestern Japan based on the deep boring survey

An all-core boring reaching 625 m in depth was conducted at Negoro in the Wakayama Plain which is a tectonic basin formed under the influence of the Median Tectonic Line active fault system. At the depth of 137m, sedimentary rocks belonging to the Upper Cretaceous Izumi Group and their sheared materials (lithologic unit : NG-1) are in contact with unconsolidated sediments by a reverse fault dipping about 30° northward. Unconsolidated sediments are composed of a sequence of alternating beds of pebble, sand and silt intercalating peaty layers and a volcanic ash layer (V290). They are divided lithologically into 4 units (NG-2 to NG-5). Some plant macrofossils were collected and paleomagnetic measurements were carried out. The sequence is divided into Zone 1, Zone 2 and Zone 3 in descending order on the basis of paleomagnetic polarities, and Zones 1, 2 and 3 are correlative to the Matuyama, Gauss and Gilbert chrons respectively, as the result of the correlation of widespread tephras and biostratigraphic constraints of plant fossils. These unconsolidated sediments correspond to the beds of the Shobudani Formation distributed in the hills, east of the Wakayama Plain. It is presumed that the formation has gently tilted northward in the basin and has contacted with the basement rocks by a fault at the northernmost part of the plain.