堆積機構の基礎的軒究(その5) : 波状堆積について
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概要
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Sediments having wavy surface are typified by sand ripples, sand waves, and sand bars (or sand dunes)., For the sake of convenience, such morphological characteristics are included into the name of "wavy sediments"., From experimental studies and field observations on the factors producing such features, the writer has obtained the following conclusions., (1) The production of wavy sediments is mainly determined by the mutual relation between current velocity and size distribution of sediment grains., (2) Conditions necessary for the construction of current ripple marks are given as follows; a bed is composed of sorted sand, the diameter of which is about 0.,08〜1.,3 mm., and a current velocity near the bottom is larger than the critical velocity required for traction of the bed grains and less than that for saltation of the same (see Parts 3 and 4 of this series of studies)., In this case, if the grain size is about 0.,12〜0.,3 mm., the ripples are especially liable to be produced., (3) When the bed is poorly sorted and the grains comprising it are coarser than those required for ripple marks and the current velocity sufficient for their transportation, sand waves are produced., In this case, if the size of the coarse grains corresponds to "second stable granularity" (see Parts 3 and 4), regressive sand waves (the so-called anti-dune phase)are produced., (4) Sand bar is a kind of layer composed of well sorted grains., Accordingly, sand ripples liable to be produced on the surface of a sand bar., (5) The grain size of 0.,2〜0.,3 mm., in diameter is most liable to be transported by traction., Therefore, it is probable that sand grains of such size are transported to considerable distances from the coast to form sand layers there., Under ordinary conditions sandy sediments which are produced by traction display wavy or lenticular) forms., Therefore, if sandy sediments exhibit a flat layered condition, it can be said that either the producing process of the wavy sediments has been interupted by forced fixation (for example, as by the covering of suspended materials), or the sediments have been dispersed during the progress of sorting.,
- 日本地質学会の論文
- 1954-12-25