The Influence of Some Tectonic Structures upon Vitrinite Reflectance

Data are reported for oriented coal samples taken on traverses oblique to two high angle normal faults in underground mine workings in the Southern Coalfield of the Sydney Basin, N.S.W. Apart from faulting and regional warping, relatively little tectonic deformation has occurred in this portion of the basin. Four oriented sections cut normal to bedding at known azimuth were prepared from each of twenty-one coal samples. Vitrinite mean maximum reflectance ($$\bar{R_{0}} max$$), using oil immersion, was measured for each section and results indicate that the vitrinite in this medium volatile coal has biaxial optical properties. Using the azimuths and $$\bar{R_{0}} max$$ values of the four sections of each sample, the true maximum reflectance ($$\bar{R_{0}} max$$) and its orientation can be determined from the elliptical calculated bedding plane section of the indicating surface (CBPSIS). The biaxial nature of the vitrinite is thought to result from asymmetrical growth of its molecular structure and to be related to stress fields which developed contemporaneously with coalification. In the vicinity of the normal faults, elliptical CBPSIS's with similar $$\bar{R_{0}} max$$ orientations occur in a pattern which is consistent for the two faults. The results suggest that anisotropic stress fields may influence coalification and that it may be possible to detect some faults using vitrinite reflectance. Vitrinite reflectance may also provide information concerning stress regimes and stress regime history in proximity to some faults. Biaxial vitrinites appear to provide direct evidence of strain even in weakly deformed terrane where indicators of strain are scarce.