Abstract

The microscopic effects of deformation in an experimentally deformed oölitic limestone were studied to determine whether they could be used to define the macroscopic modes of deformation. Thin sections of the deformed specimens were subdivided into zones; and grains, oölites, and cement within each zone were placed in one of 25 categories, depending on the kind(s) of deformational feature(s) present or absent in them. A sample of the number and type of components present in each zone was obtained with a Chayes point counter, and the data were subjected to multivariate analysis.

Results of the analysis indicate that only 9 of the 25 categories are indicative of the mode of deformation. These nine categories of rock components were combined into four groups of similar features: (1) oölites and cement with no visible lamellae, fractures, or faults; (2) calcite grains, oölites, and cement with lamellae; (3) calcite grains and cement with lamellae and fractures or faults; and (4) intergranular discontinuities. The compositions of individual zones within the specimens were calculated in terms of the four groups, and analyzed to determine the nature of behavioral variation within specimens. Whole-specimen compositions were also calculated, and variations among specimens were compared. Analysis revealed two sets of specimens, characterized by homogeneous and nonhomogeneous deformation, respectively, that could be subdivided into four fields of similar mechanical behavior. On a diagram of total strain versus confining pressure, the distribution of specimen compositions defines the deformational mode fields recognized by Donath and others (1971). The microscopic criteria thus provide a means of designating macroscopic modes in a quantitative manner, and a possible way of relating aspects of natural deformation to experimental results.

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