Reflectance anisotropy of Carboniferous coals in the Appalachian Foreland Basin, Pennsylvania, U.S.A.

Abstract Vitrinite in coal is optically anisotropic in reflected light, because of a preferred orientation of its component aromatic lamellae. This molecular fabric is interpreted to have formed in response to differential stresses ambient during coalification, with maximum reflectance developing incrementally in the minimum compressive stress direction, and minimum reflectance developing incrementally in the maximum compressive stress direction. As a consequence of this interrelationship, coal reflectance fabrics can, under some circumstances, provide a record of stress and strain events occurring during coal metamorphism. Patterns of vitrinite reflectance anisotropy in coal are represented by a three-dimensional graphical form resembling an ellipsoid, termed the vitrinite reflectance indicating surface (VRI), which is analogous in some respects to the finite strain ellipsoid. VRI shapes are measured by estimating the magnitudes of the three principal reflectances, R max , R int , R min , either on oriented blocks or non-oriented grain mounts (Levine and Davis, 1989a). Spatial orientations of the principal axes are determined by reflectance measurements on oriented polished cores. Standard deviations of the principal reflectance axes, measured on oriented cores, average around 0.09% in magnitude and 5° in orientation. Measurements are highly reproducible between pairs of oriented cores. Estimates of the principal reflectance magnitudes, as measured on oriented vs. non-oriented samples, agree on average to within 0.04% for R max , 0.08% for R int , and 0.20% for R min . VRI shapes are graphically depicted on axial ratio diagrams, which plot R max / R int on the vertical axis and R int / R min on the horizontal axis. Axial ratio diagrams are also used to depict the progressive change in shape of VRIs during coalification and structural deformation. VRIs of coals from the central Appalachian basin, Pennsylvania, range in shape from biaxial negative to biaxial positive, depending on their coalification history and tectonic setting (Levine and Davis, 1989b). Coal-bearing strata in the North-Central fields (medium-volatile bituminous to semianthracite) in northern Pennsylvania exhibit structural evidence of only mild tectonic deformation. Coals from this region have biaxial negative reflectance, with R min axes roughly perpendicular to bedding, and are only slightly anisotropic in the bedding plane. This VRI geometry is indicative of coalification in a predominantly burial-strain environment with only mild tectonic deformation. With increasing rank toward the southeast, VRIs for this suite of coals plot along a straight-line coalification pathway on the axial ratio diagram. This pathway comes from near the origin and extends through the biaxial negative deformation field, close to the horizontal axis. Maximum reflectance orientations for North-Central field coals are polymodal and aligned with polymodal Alleghanian fold axes (Levine and Davis, 1989b). Low-volatile bituminous coals from the Broad Top field, in the more intensely deformed Valley and Ridge Province, south central Pennsylvania, exhibit much stronger tectonic reflectance fabrics than rank-equivalent coals from the North-Central fields. VRIs are substantially elongated parallel to the fold axes, and some are prolate rather than oblate in shape. Broad Top samples also bear evidence of two superimposed reflectance fabrics, one predating the folding and a stronger one postdating the folding, which indicates that the final rank was attained only after folding was complete in this part of the fold-and-thrust belt. VRIs of coals from the intensely folded Western Middle anthracite field exhibit a wide range of shapes and orientations, which reflects their more complex textonic history. R max axes of sporinite are aligned parallel to bedding, suggesting that the strata were still flat-lying, and not subject to tectonic forces when the coals passed through the first and second coalification “jumps”. In their present-day orientation, vitrinite R min axes of Western Middle field coals are scattered about a great circle distribution, the pole to which is precisely coincident with regional fold axes. When beds are unfolded to a flat-lying orientation they become significantly less scattered, indicating that most coalification here proceded the final phase of Alleghanian folding. A suite of 7 coal samples from the Pioneer Tunnel (underground) coal mine in the Western Middle field ( R mean approximately 4.3%) provides additional constraints used in reconstructing the coalification history. Coal rank increases very slightly down section, but R min decreases down section, opposite to the expected trend. Pioneer mine coals probably attained anthracite rank prior to the principal phase of Alleghanian folding, but at least come coalification continued after folding, as suggested by reconstructed coalification pathways on the axial ratio diagram. During this final phase, the stratigraphically upper beds were coalified more than the lower beds, possibly reflecting thermal re-equilibration of the strata. This latter, syn- to post-tectonic phase of coalification, in the presence of a tectonic stress field, produced elongation of the R max and R min axes, causing the stratigraphically upper beds to attain a prolate space geometry while the stratigraphically lower beds retained their original oblate form.