Criteria for incipient slaty and crenulation cleavage development in Tertiary flysch of the Helvetic zone of the Swiss Alps

Two parameters of incipient cleavage development are compared in a series of Tertiary flysch from the Helvetic zone of the Swiss Alps: 1) the overall relative phyllosilicate orientation as obtained from the X-ray diffraction intensity ratio of their 00l reflections in cleavage- and bedding-parallel slabs (I_{10Å (0.5°)} C/B) and 2) the dimensional aspect ratios of fissility fragments, the 'cleavage/bedding fissility ratio' (c/b). These two parameters show a roughly linear power relationship with a wide scatter. Appreciably better fits are obtained if populations showing different cleavage morphology (closely vs widely spaced cleavage; crenulation vs non-crenulation cleavage) and lithology (silt and fine-sand size quartz content; carbonate content) are plotted separately. In contrast, there is no clear link between the two criteria of cleavage intensity and the degree of incipient metamorphism as expressed by illite 'crystallinity'. Samples with crenulation cleavage show both lower (I_{10Å (0.5°)} C/B) and c/b cleavage/bedding fissility ratios than most of those with non-crenulation domainal cleavages with closely spaced cleavage domains; these lower ratios reflect the presence of an earlier, largely bedding-parallel, cleavage fabric. This difference in the ratios between these groups of cleavage morphologies proves that the cleavage/bedding fissility ratio is not only dependent on the overall phyllosilicate orientation, but also on the cleavage microstructure, in particular the morphology and spacing of the cleavage domains and the persistence of a bedding-parallel cleavage fabric in the microlithons of the crenulated samples. The relationship between the c/b cleavage/bedding fissility ratio and the I_{10Å (0.5°)} C/B ratio can provide a useful indication of the type of incipient cleavage fabric developed, and help in detecting the presence of an earlier slaty cleavage. Additional measurement of the intersection/bedding fissility ratio i/b on the fissility fragments is likely to help in assessing the effect of lithology, particularly the effect of the initial bedding anisotropy. The differences in the illite/chlorite ratios in the cleavage and bedding planes, as determined from the I_10Å/I_7Å C/B ratio, provides information regarding the relative formation of illite and chlorite in the cleavage domains and the chlorite-mica 'stacks' during development of the cleavage fabric.