Geology of Tobishima Island, Yamagata Prefecture

Abstract

The geology of Tobishima island, Yamagata Prefecture, which is closely related with the so-called "Green Tuffs" of the oil-field along the Japan Sea Coast, was clarified as follows:1) The stratigraphic succession of the formations in Tobishima island, from the upper to the lower are as follows: Hatimanzaki formation (:320 m, -1- in thickness) : consists mainly of basic black tuff clinao-uncomformity Katsuura formation (100 m., -4- in thickness) : alternation of green Cuff and scoria tuff conformity Arasaki formation (130 m., -I- in thickness) : consists mainly of acidic green tuff(containing ligruite and silicified woods) 2) The intense igneous activity of acidic andesite which consists chiefly of dacite, isrecognized in both the Arasaki and Katsuura formations., An intense activity of basic andesite-basalt which consist chiefly of basalt, is recognized in the Hatimanzaki formation., 3) It appears that the geologic structure of Tobisluiuna island which is fundamentally controlled by the faulted structure of NW-SE din-ection and the folded structure of E-W crend, is closely related with the uplift of the island., The folded structure of E-V7 trend in the island exhibits a striking contrast to the N-Strend of the conterrtporarteous strata in the, eXkita oil-field., 4) From the rock facies, silicified woods, igneous activities and unconformaity observed, It appears that the Arasaki and Katsuura f01'1Il2Lt1O11S may correspond aj'proxurnately t0 tl18Daizima and Nishikurosawa stames, and the Hatimanzaki formation to the Onragawzt stage.,