2011 Volume 117 Issue 2 Pages 79-94
Anjo City is divided topographically into the Hekikai terrace (which represents the main area of the city), the Yahagigawa lowland (eastern area), and the Aburagafuchi lowland (southwestern area). The city is underlain by Holocene sediments, the Hekikai and Koromo formations, the Tokai Group, and basement rocks, in descending order. The Jomon transgression, which deposited a marine clay bed in the lower half of the Holocene sediments, caused inundation up to the midstream parts of narrow valleys carved into the Hekikai terrace. The Hekikai Formation, which forms the Hekikai terrace, consists of a sand member (upper half of the formation) and marine clay member (lower half), and is 16–36m thick. The marine clay and sand members correspond with the bottomset and foreset beds of a delta, respectively. The age of the Hekikai Formation ranges from about 120 to 70ka. The transgression that deposited the marine clay member of the Hekikai Formation started from Kinuura Bay (southwestern part of the Nishimikawa Plain) and inundated the western half of the city area. The upper part of the Koromo Formation consists of alternating beds of sand and mud, and is well consolidated compared with the Hekikai Formation. The basal surface of the Koromo Formation is located at an elevation of about –100m a.s.l. in the southwestern part of the city, –45m in the central part, and –25m in the northeast. A generalized geological cross-section through the Nishimikawa Plain is proposed as a tentative model of the local geology.
