The Geological Outline of the Undersea Portion of the Seikan Tunnel

Abstract

Both the coastal and undersea portions of the Hokkaido-side Kunnui Formation have been classified lithologically into 5 layers, (from Kn-1 to Kn-5).The undersea portion of the Honshu-side Kunnui Formation is largely tuffaceous. Accordingly, it is somewhat difficult to compare it stratigraphically to the Hokkaido-side formations. However, based on established theory, analysis reveals that the foraminifera fossil-bearing muddy layers of the Honshu-side undersea formations and the Hokkaido Kn2 layers can both be correlated to the Middle Miocene Period.The Yakumo Formation, whose special lithological feature include hard shale formations and a minimum of foraminifera fossils, can only be presumed to the Middle Miocene Period.Utilizing the sonic prospecting method (sparker) prior to construction, a syncline structure was presumed to exist at the central undersea portion of the Kuromatsunai Formation. Numerous correlations were made between the thin layers of tuffaceous rock takes chiefly from exploratory boring core samples which were extracted periodically as the excavation progressed towards the syncline center from both the Hokkaido and Honshu sides. They revealed much about the syncline's geological structure.Also, the upper layer of the axial part of the syncline was presumed to be from the Early Pliocene Period judging from the identification of the foraminif era fossils.Meanwhile, a K-Ar age determination of biotite obtained from the thin tuffaceous layers of the Kuromatsunai Formation was carried out.The result is a 7.6±0.4Ma reading at the upper regions of the strata.Based on the chronological test and the foraminiferae analysis, the Kuromatsunai Formation depositional phase has been dated from the later Miocene Period to the Early Pliocene Period.2. UnconformitiesThree unconformities have been observed between the following pairs of formations: 1) the Kunnui and the Fukuyama Formations, 2) the Kunnui and the Yakumo Formations, and 3) the Kunnui and Kuromatsunai Formations respectively.At the contact surfaces the lower strata exhibit a fragile nature due in part to weathering, while the basal layer of the overlying formation consists of various kinds of fine pebble together with thin intercalation including glauconite.