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ABSTRACT

CM grain-size diagrams of clastic deposits were introduced in 1957. Since that time a large number of diagrams representing most environments were constructed. Discussions of parameters C and M by the writer and by others showed that these parameters are indicators of hydraulic conditions under which sediments were deposited. In a 1975 paper Vandenberghe discussed the CM diagram of the Boom Clay, Belgium, and excluded that CM diagrams gave indications on the mechanism of sediment deposition.

In order to answer Vandenberghe's assertions, the writer systematically compared the numerous CM diagrams representing suspension deposits with the characteristics of the environments. Result of this comparison is that CM diagrams give coherent indications on deposition which agree with the known environment characteristics. The Boom Clay diagram is no exception.

Another result of the present study is that, in environments as a whole, two types of currents can be recognized, distinguished by the relation between bottom turbulence and grain-size of sediment in suspension. Deposits of these two current types and distribution of bottom turbulence are indicated by CM diagrams. Characteristics of these currents vary with the environment and help the reconstruction of ancient environments.

Information given by CM diagrams has limits and should be integrated with other evidence on the environment. However, contrarily to the information given by ecology or sedimentary structures, the evidence provided by grain-size has the advantage of being universal, as it can be obtained in all clastic deposits. This evidence therefore is a means for comparing and classifying clastic deposits as a whole.