Abstract

The Lower Cretaceous Jewel seam, which is generally about 10 metres thick, is the major economic coal seam of the Mountain Park area. The strata of the area are folded and cut by several thrust faults. Vitrinite reflectance of the Jewel seam (and other coal horizons) was measured from samples collected at outcrops and drill holes from close to one hundred evenly distributed locations. The mean maximum vitrinite reflectance of the Jewel seam ranges from 0.94 to 1.28 percent, with the highest values in the lowest thrust sheet. In addition, there is a slight east to west increase in reflectance in each thrust sheet. In order to examine relationships between coalification and deformation, vitrinite reflectance anisotropies were determined from oriented coal blocks. Several blocks have biaxial anisotropies, indicating a relation to tectonic stress fields. These biaxial coals display maximum reflectance axes parallel to nearby fold axes, which suggests a relationship between vitrinite anisotropy and local deformation. The biaxial vitrinite reflectance ellipsoids result from superposition of tectonic strains on a primary, sedimentary, burial-related, uniaxial anisotropy. These relationships indicate that coalification resulted largely from pre-deformational sedimentary burial, with components of syn-deformational coalification during the later stages. Post-deformational coalification is indicated by significant reflectance jumps across thrust faults, which cannot be removed by palinspastic restoration. Up to 25 percent of the measured reflectances may have resulted from syn- and post-deformational coalification.

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