RESEARCH ON FLYSCH IN CHINA AND SOME PROBLEMS CONCERNED

Fiysch occurrs extensively in strata from Precambrian to Mesozoic in China. Since the 1950's, the research on flysch has been made, and a large number of data have been gathered. On the basis of these data and the author's observation,some problems concerning the study of flysch are discussed in this paper. The first part of this paper refers to some new views on the morphology of flysch. 1. Flysch is not completely composed of unitary rhythmic strata. It is usually composed of rhythmic strata separated by rock layers with a certain thickness and without rhythmic texture. The rhythmic and unrhythmic strata are interlaced with each other and form a higher grade combination called cyclothem, that is about scores or hundreds of meters in thickness. Every flysch formation can be divided into many cyclothems, Therefore, it is necessary to study both rhythmic and unrhythmic units in flysch at the same time. 2. The basic characteristics in a flysch rhythm do not lie in whether there exists a Bouma's sequence, which itself is merely an incomplete and idealized model. In fact, the rhythmic texture in flysch is more complex and diversified. One basic characteristic is the existence of graded bedding, which is shown not only in the A interval of Bouma's sequence, but also in all the other intervals. 3. The flysch rhythm is polygraded. Every interval in Boums's sequence may be heterogeneous. So it can be divided into many subordinate rhythms. For example, the subordinate rhythmic bedding in the A interval is mainly composed of sand ( or coarse silt ) layers separated by thin mud layers.But,in the E interval it is composed of mud layers seperated by thin sand layers. Some secondary parts in a subordinated rhythmic bedding appear to be lenticular or lens. Sometimes microrhythms, which may consist of graded bedding, exist within a secondary rhythum. 4. In the fine-detrital layers with only several centimeters in thickness, normal grading, reverse grading, and the transition between them may be discovered. In general, such a texture can be observed only by microscope. 5. There are large-scale cross beddings, foreset laminae and ripplemarks, including symmetric ones,microscale ones and interference ones. And there are also bidirectionnal crossbeddings,herringbone cross beddings,lens-like beddings, and abundant contemporaneous breccia in some flysch. Fragments of silt-stone may exist in the mudstone layer, and fragments of mudstone may exist in the siltstone layer. All this reflects that the sedimentary area has a shallow water environment with bottom currents, wave and tidal processes. The second part of this paper points out that the research on the nature of the formation order and formation series in which flysch occurs is of great significance in recognizing the plate tectonic setting in flysch accumulation.The formation order and series are the combination of some formations alternating vertically and horizontally during the development of the flysch basin. Three kinds of flysch formation order in China are classified as follows; 1 ) Eruptive rocks order; 2 ) Sedimentary rocks order; 3 ) E-ruptive sedimentary rocks order(table I). Their tectonic settings are respectively inter-plate, intra-plate, and continentalcrust rifting with micro-divergence processes. The origin of flysch is discussed in the third part. Reviewing the theorios on the origin of flysch, such as turbidity current, bottom current and oscillation, and taking the flasch of Later Ordovician in western Zhe-jiang Province as an example, the author interprets the genesis characteristics of shallow water with frequent bottom current process. On this basis, the author comes to the conclusion that the origin of flysch is diversitied, not simplex, like that of many other geological bodies. It is also considered that the turbidity current flysch and non-turbidity current flysch may exist simultaneously. The textures, such as rhythum, graded bedding, and the Bouma's sequence are not the bases to identify flysch origin, but the indicators to recognize it. In order To discriminate the origin of flysch,it is necessary to further reveal its concrete characteristics in many res-Pects。Some signs of distinguishing turbidity current flysch from nontur-bidity current flysch are listed in this paper.