In the second part of this paper, geology and geomorphology of the strait are described. Geologic map and profiles presented here (Fig. 16) are prepared by examining rock specimens dredged from more than two thousand localities in the strait and also by interpreting figures obtained by sonic survey.
§ 2. Stratigraphy : Exposed formations on the strait seafloor are entirely of younger Tertiary deposits from Miocene to Pliocene in age and associated volcanic rocks such as basalt, andesite and rhyolite. They are underlain by so-called Paleozoic rocks that crop out around Cape Shirakami of Hokkaido and overlain by submerged Pleistocene formations and Holocene sediments.
Tertiary formations of this area are as follows in ascending order.
Fukuyama formation (Fy) : Mostly tuff, agglomeratic tuff and welded tuff and conglomerate at the base 330 meters in the thickest.
Kunnui formation (Kn) : Unconformable to the underlying formations. Thick or thin alternation of tuff (so-called green tuff in the main), tuffaceous sandstone and mudstone with occasional intercalation of agglomerate, conglomerate. It may be divided into several members (Kn1-Kn5) by lithic assemblage, although lithofacies and thikness are both variable at places, and volcanic members predominate in the southern half. Foraminifer fossils show its age to be middle Miocene. About 1, 300 meters in total thickness.
Yakumo formation (Yk) : Unconformable to the former formation. Plately bedded so-called hard shale with interbedded tuff. Pyroclastic member thiken in the south. Radioralia and diatom fossils are very common. More than 400 meters in the thickest.
Kuromatsunai formation (Km) : Massive, diatomaceous tuffaceous siltstone or sandy mudstone. Frequent intercalation of thin layers of tuff and marl. 800 meters or more in total thickness.
Setana formation (St) : Unconformable to the underlying formation. Mostly soft sandstone and partly conglomeratic. Rich in fossils of foraminifers and molluscs of upper Pliocene age. Thickness is more than 115 meters.
Volcanic rocks : Intrusion of basaltic rocks and andesitic rocks as well as rhyolite is found at places and particularly extensive in the south. Interbedded volcanics, mostly basic in nature are tracable from place to place, mainly in the south.
§ 3. Geologic structure : Essential geologic structure of the area is anticlinal, treanding from northwestnorth to southeastsouth and it forms broad saddle structure by lowering down of formations, accompanied by faults, toward the center of the strait from both lands. Consequently, younger formations are distributed on both flanks of anticline and also in the middle of the strait. The opening of the strait is due chiefly to the exposing of these soft, younger formations such as Kuromatsunai and Setana in the middle of the area. Foldings of minor order are also observed within the broad anticline, some of them being brought by faulting.
Four fault systems are recognised in relation to geologic structure as well as to time sequence. The first system is longitudinal and is followed by second dextral transversal fault group which lets the formations sink toward the center of the strait and also they shifted the northside of faults toward east. The third system, longitudinal in the south and diagonal in the middle of the area, divided the area into four major units. All these three systems are after the Kuromatsunai formation, while the last longitudinal system is younger than the Setana formation and forms horst topography at the southern half of the strait.