伊勢湾周辺の中部更新統 その分布と断層地塊運動

Abstract

The area around Ise Bay has been broken into many blocks by faultings. As subsiding blocks, Ise Bay and Nobi Plain are surrounded by 30 to 200 meters high hilly blocks and by upheaved mountain blocks ranging from 500 to 1200 meters high.The mountain blocks are made up of pre-Tertiary basement rocks. The hilly blocks are composed mainly of the Tokai Group; deposited in“Lake Tokai”, a sedimentary basin from Pliocene to Early Pleistocene time. The hilly blocks are extensively dissected by rivers and only a series of hill tops at approximately the same height is left as a remnant of the erosional surface in early stage. Older fluvial deposits, “the highest gravel”, unconformably overlie some of the hill tops. Younger fluvial deposits, the higher and lower terrace sediments, are developed along major valleys. Since these fluvial deposits consist mainly of gravelly sediments and the relative altitudes of surfaces of the terraces are modified by the movements of local fault blocks, detailed stratigraphic positions of these were not clear.Cenozoic sediments underlying the Nobi Plain attain 1500 meters in thickness. In the sequence of the sediments, a remarkable unconformity is recognized. The upper sediments, more than 350 meters thick, consist of alternating marine and fluvial sediments, suggesting that they were deposited under the influence of the glacial sea level oscillations. The lower sediments corresponding to the Tokai Group are unconformably overlain by the upper sediments. Microfossil analyses of a large number of drilling cores have clarified the stratigraphic succession of these sediments. The upper sediments are divided into four units: The lower Middle Pleistocene Yatomi Formation, the upper Middle Pleistocene Ama Formation, Upper Pleistocene formations, and Holocene sediments.The biostratigraphic information about the upper sediments furnishes valuable clue to the stratigraphic position of the fluvial deposits on land. Thus, the distribution pattern and altitude of these upper Quaternary sediments reveal the differential movements of each fault block surrounding Ise Bay.The Nobi Plain block has rapidly subsided since early Middle Pleistocene. Subsequently the gentle uplift in the hilly blocks occurred in the later Middle Pleistocene time, the stage of“the highest gravel”, and followed by the dissection of the hilly area. The subsidence of Ise Bay block may have started later than that of the Nobi Plain block, though the subsidence rates of both blocks have been of the same order since late Pleistocene.