Clay orientation in some Upper Carboniferous mudrocks, its relationship to quartz content and some inferences about fissility, porosity and compactional history |
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| Authors: | C D CURTIS S R LIPSHIE G OERTEL M J PEARSON† |
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| Institution: | Department of Geology, the University, Sheffield, S1 3JD, Los Angeles, California, U.S.A.;Department of Earth and Space Sciences, University of California, Los Angeles, California, U.S.A.;Department of Geology and Mineralogy, Marischal College, The University, Aberdeen, AB9 1AS, Scotland |
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| Abstract: | The degree of preferred orientation of clay minerals in a series of fifteen fine-grained sediments has been precisely measured by transmitted X-ray goniometry. This varies systematically with quartz content. By contrast it is apparently unrelated to fissility since the one fissile sample (a ‘paper shale’) occurs at the mid-point of the sample series ranked in order of degree of preferred orientation. In this series fissility corresponds with the presence of fine-scale lamination. Estimates of compaction strain using the theory of March (1932) and assuming random initial clay orientation are consistent with the view that these samples started life with porosities similar to those of present-day muddy sediments. On this basis it is concluded that preferred orientation in clay-rich sediments results almost entirely from compaction strain. The degree to which this can be attained is limited by the presence of nonplaty particles (such as quartz grains) which prevent planar fabric development in their immediate vicinity. |
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