1999 Volume 108 Issue 5 Pages 589-615
Construction of the Chubu International Airport has started off Tokoname City (Chita Peninsula) in Ise Bay. Various surveys such as bathymetry, seafloor drilling, sonic prospecting, and geologic examinations of recovered borehole core samples, have been performed to reveal soil engineering characteristics, submarine topography, and submarine geology at the airport site (Chubu Kuukou Chousakai=The Chubu International Airport Research Foundation, 1994). Many fruitful results on the submarine geology have been obtained in these surveys, as follows :
(1) The submarine topography is divided into the inshore “Upper submarine terrace”, “Offshore gently sloping sea floor”, and “Submarine scarp” between the two. A submarine channel is trenched southwards on the inshore submarine terrace.
(2) The bay area is underlain by the A, B, C1, C2, and T Formations, in descending order. The T Formation is Mio-Pliocene Tokoname Group, the basement of the bay area. The Tokoname Group forms a buried wave-cut platform of a peninsular shape, which extends southwards. On the east side of the peninsular wave-cut platform, a submarine buried valley stretches southwards under the submarine channel and is mostly filled with the A Formation. On the west side of the wave-cut platform runs the Ise Bay Fault, and the A, B, C1, and C2 Formations thicken offshore.
(3) The A Formation, which is divided into the A1 (upper), A2 (middle), and A3 (lower) Members, consists of marine muddy strata of the present bottom surface and is correlated with the Nanyo Formation below the Nohbi Plain (north of the Ise Bay). The A Formation is 25 to 35 meters thick, and the 14C ages range from 9, 400 to 5, 200 y. B.P. of Holocene age.The A3 and A2 Member intercalates the U-Oki and K-Ah tephra, respectively, both are widespread tephra layers in Japan.
(4) The B Formation, which is divided into the B1 (upper), B2 (middle), B3 (lower), and B4 (base) Members, consists of sand beds and gravelly beds. The B1, B2, and B3 Members are correlated with the Nohbi and First Gravel Formations below the Nohbi Plain, respectively. The B4 Member forms a buried terrace on the west slope of the peninsular wave-cut platform and is correlated with the Toriimatsu or Ohzone Formations, Lower Terrace Deposits in the eastern margin of the Nohbi Plain. The B Formation is of late Pleistocene age.
(5) The C1 Formation consists mainly of marine clay beds about 40 meters thick. The fossil pollen composition indicates that the C1 Formation is correlated with the lower part of the Atsuta Formation in the Nohbi Plain and is of middle Pleistocene age.
(6) The C2 Formation consists of sand (upper) and gravelly (lower) beds and exceeds 50 meters thick. The fossil pollen composition indicates that the C2 Formation is correlated with the Ama Formation below the Nohbi Plain and is of middle Pleistocene age.
(7) The basement T Formation (Tokoname Group) consists of alternating sand and compact mud beds and intercalates two volcanic ash layers, which are correlated with the Souri (upper, Sr) and Higashitani (lower, Hg) Volcanic Ash Layers from the conformity in index of volcanic glass. The Souri and Higashitani V. A. Ls are intercalated within the upper horizon of the Tokoname Group in the Chita Peninsula.
(8) The Ise Bay Fault (faults and flexure zone), trending in the NNW-SSE direction, extends in the western margin of the bay area. The Tokoname Group on the east side of the fault trends northwest and gently dips northeast. The Utsumi Fault, trending in the WNW-ESE dircction, extends in the southern margin of the peninsula.
The geological relationship between the Chita Peninsula and airport site in the above-mentioned results brings the following problems for further study.
