北上山地志津川地方のジュラ系に見られる堆積構造

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In this paper we describe and discuss the characteristic sedimentary structures observable on three exposures of the Jurassic in the Shizukawa area in South Kitakami. The selected exposures are: southwestern coast of Niranohama, eastern coast of Gongen, and northeastern coast of Shizuhama. These exposures are the best sections of the Niranohama, Hosoura and Arato Formations, constituents of the Shizukawa Jurassic. All the sequences established in these sections are exclusively composed of clastic sediments of various grain size. Sedimentary structures are well developed and suggest the environments ranging from breaking zone to base-of-slope. The Niranohama coast is the type section of the Niranohama Formation. The succession here is divided into the lower part composed of alternation of arkose sandstone and bituminous shale, characterized by wave ripples and convolute bedding, and the upper part of of sandstone-dominant alternation, characterized by wave ripples, including hummocky cross stratifications. This represents the environment from breaking zone to storm-dominated shore-face to upper offshore. The Gongen coast represents the lower and middle parts of the Hosoura Formation. The sequence observable here is composed of strongly bioturbated black mudstone intercalated with thin storm-generated sheet sandstone layers. Associated sedimentary structures suggest the base-of-slope to basin floor environment. The Shizuhama section covers a fraction of the Arato Formation. The lithology is essentially similar to that of the previous Shizuhama section. The whole sequence observable in this section is, however, more strongly affected by submarine sliding which has left a number of slump scars in the sequence. Upper slope environment is suggested for this section. The above observations lead us to the conclusion that the Shizukawa Group is the stratigraphical sequence representing a transgressive cycle beginning with high-energy breaking zone environment to low energy offshore environment. The Hashiura Group is another transgressive cycle succeeding the former, in which the Arato Formation represents the slope stage of sedimentation.
地学団体研究会の論文
1988-11-25

北上山地志津川地方のジュラ系に見られる堆積構造

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