LINKING CLEAVAGE REFRACTION TO MINERALOGY IN DEFORMED QUARTZITES AND PHYLLITES

The Baraboo Syncline in Baraboo, Wisconsin contains interbedded quartzite and phyllite layers that formed refracted cleavage during deformation associated with the Mazatzal Orogeny. The layers have gradational contacts that provide opportunities to study the relationship between relatively simple mineralogy and cleavage refraction. In our study, we analyzed transects of gradational layers from both the north and south limbs of the syncline. The angle between bedding and cleavage varies between approximately 89° in the most competent layers and 15° in the least competent layers. We used X-ray diffraction (XRD) to determine the mineralogy of the samples. The most competent layers that resist cleavage formation contain approximately 99% quartz and 1% pyrophyllite. The least competent layers have a well-developed cleavage and contain approximately 82% quartz and 13% pyrophyllite. Where cleavage forms, it is typically defined by the preferred alignment of pyrophyllite. In comparing the angles between cleavage and bedding with the XRD results, we note that relatively small variations in mineralogy make a significant difference in competence determined by cleavage/bedding angles. Where possible, we determined viscosity ratios using the Treagus (1999) cleavage refraction method. Preliminary cleavage/bedding angle data suggest that the effective viscosity ratios range between 1.0 – 3.7, which are values comparable to other studies.