Three types of bedding parallel rock failures, sometimes distinguished in folded rocks in nature, are classified into two series of forma -tive mechanisms. These are the "shear-fault series" and "yield-flow series" which are regarded as low ductility facies and high ductility facies respectively.


Dans la nature on pear discerner trios types differents de rupture paralleled à la stratification dans les roches plisses. lies sont classes dans deux serie de mechanisms formatives differents: la serie de cisaillelDent et la serie de fluage plastique. Ells sont regarded come represents de fancies de la base ductility et de la ductility levee respectivelDent.


Dried Shorten von Schichtungs-parallele Entkraftung der Ge Steine, manchmal erkennbar in gefalteten Gesteine in der Nature, send klassifizierend in zwei Gruppen von Bildungsmechanismen, Serie des "Scher-Ver -werfung" und Serie des "Ergebung-Flies", aufgeteilt warden. Diese Group- pen send ales "Niederdukutilitatsfaziens" bzw. "Hohedukutilitatsfaziens".


Bedding parallel failure is one of the most fundamental aspects in rock structures sometimes accompanied With folding in nature. The mechanism of rock folding was so far elucidated by flexure, shear and flow of rocks. It was not until the proposal of mechanical classifi -cation by Dobath & Parker (1964) that folding mechanism was discussed in terms of ductility of rocks. According to them, there are two types of movements of materials at folding, namely slip and flow, and both is divided into flexural type parallel to the bedding and passive type whithout regular relation to the bedding. As a result of examination on the ductility of rocks at deformation, they divided the folds into four basic types. Later on, the concept of deformation facies was introduced (Demure. 1982). It makes possible to discuss the deformational condition of different kinds of deformed structures with two incises on ductility. They are "mean ductility" and "ductility contrast", which represents the deformation grade and deformation series. respectively. Accordingly, the problem at present is deformation facies analysis of bedding failure with examples in nature and experimental results.


In the East Kabuki area, Niigata Prefecture, northern Central Japan, the later Miocene to Pliocene sedimentary series are widely distributed forming some branchy-anticlines and -synclines around the fold axes of ENE--WSW in direction (Fig. 2.). They consists mostly of sandstone, mudstone, and alternation of them with pyroclastics, volcacnic rocks, and conglomerite. The wave shapes of folds shown in cross sections are mostly parallel type with slightly similar one. According to Ramsay's classification of wave shape (Ramsay, 1967), they are included in Class 1B to 1C. It suggests that these folds has been formed through the flexural slip to slight flexural flow in mechanisms. The cause of folding is elucidated by lateral compression and differential uplifting of the basement blocks as well (Demure, 1989; 1900).


The following three types of bedding failures are found in the sedimentary strata above described. (l) Bedding fault. In the alternation of sandstone and mudstone, the faults along bedding plane are well developed. They are recognized as the dark gray coloured clay films in general. They are not only found along the boundaries between sandstone and mudstone, but also along the most of lithologic boundaries. They would be originated as layer parallel Shears, and then developed to the bedding faults (Fig. 3.). (2) Bedding parallel flow. This is mostly developed in thick and mas- sive argilaceous rocks. A number of small scale bedding parallel fissilities are developed to fracture the rocks into small fragments as well observed on the weathered exposures (Fig.3). This type of bedding failure is macro- scopically regarded as a kind of cataclastic flow (Fig.3)

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