奥尻島の海成段丘と第四紀地殻変動

Abstract

For the purpose of testifying a generally accepted hypothesis in Japan, that is, crustal movements have continued in the Quaternary Period with the same mode and the same rate, marine terraces of Okushiri Island, which is located on the west of the Oshima Peninsula, Hokkaido (Fig. 1), were surveyed precisely, because it had been said that the Quaternary crustal movements of this island had changed in a mode by some researchers. The means of this investigation were to classify marine terraces, to measure altitudes of former shorelines of the terraces by an altimeter, and to infer the mode of crustal movements on the distribution of the altitudes obtained. The results are as follows.(1) There are three higher denudation surfaces of marine origin and six marine terraces, the 1st to 6th terraces in descending order (Fig. 3).(2) The altitudes of each former shoreline of the 3rd to 6th terraces are higher in the northwestern part of the island than in the southeast. The general strike of tilting is nearly ENE-WSW (Fig. 4, 5). The inclinations of tilting increase progressively from the younger terrace to the older one (Fig. 6).(3) The equations of interrelation between the altitudes of the 3rd and 4th terraces and those of the 4th and 5th terraces are:T3=1.40T4+1.65 (1)T4=1.04T5+34.4 (2)where T is the altitude of shoreline in meter.(4) Therefore, the hypothsis of“same mode”in crustal movement seems to be accepted in this island, at least since the time of formation of the 3rd terrace. The terraces, however, have been not dated, the hypothesis of“same rate”is not checked.(5) The constants of equation (1) take the same values approximately to those of equations which have been obtained from two terraces in some other part of the Uetsu Fold Zone, a Neogene-Quaternary fold zone, which may be correlated to the 3rd and 4th terraces in Okushiri Island.(6) The mode of tilting inferred from the marine terraces is in accordance with the geologic structure of Neogene sedimentary strata in the northern part of Okushiri. Thus it is probable that such a tilting has continued since Neogene.