Collapse and Shogidaoshi Structure(Morphology and formative mechanism of late Cenozoic sedimentary basins, especially of tilting basin)

Abstract

In this paper the writer maintains the following four points of view; 1. During the geological times sediments were always concentrated and laied down in limited areas, i.e., sedimentary basins. All the present coastal plains and intramountain basins in the Japanese Islands are final products of the lowland that has been developed since the Cretaceous. Thus, sedimentary basin, coastal plain and intramountain basin in this case are included into a same category. 2. Two types of generative process of sedimentary basin are generally known. One is collapse, i.e., downthrowing of basement block to produce a depression bounded by faults. The other is subsidence, i.e., down-warping of basement to form a basinal depression. Giving several examples, the writer pointed out previously that down-warping of sedimentary basin was nothing else than a surface phenomena of underground fracturing or collapsing. In this respect, generation of a sedimentary basin due to subsidence would be synonymous with occurrence of a collapse under the ground. 3. The category of "Shogidaoshi (Fujita, 1951)" does not involve any genetic definitions, since it is a term defining a configuration of sediments of which depocenter shifts unidirectionally. Some people who do not accept the term "Shogidaoshi" want to replace it with the other terms such as "tilting block movement" or "Shogidaoshi" (unidirectional shifting of upheaval to bring migration of depcenter)". They, however, must have misunderstood the original definition of the term excluding its mechanism. 4. The law of "Shogidaoshi" is applicable to a developing stage of pre-existing sedimentary basin, not to a generative stage of new sedimentary basin. Incidentally, it rarely happens that a new sedimentary basin of exotic nature is generated just in front of unidirectional shift of a "Shogidaoshi". Such a case is accidental, being out of applicable extent of the law of "Shoeidaoshi".