Optical anisotropy of coals as an indicator of tectonic deformation, Broad Top Coal Field, Pennsylvania
The anisotropic “reflectance fabric” of coal is used as a finite strain marker in the structural analysis of deformed strata in the Broad Top Coal Field of central Pennsylvania. Oriented blocks of coal were collected at various field locations. The reflectances of three orthogonal surfaces were used to estimate the magnitudes and orientations of the three principal reflectance axes. Broad Top coals are shown to have three principal reflectance values, in contrast with most bituminous coals, which have only two. Reflectance values can be represented by a biaxial indicatrix that may be either prolate or oblate. The oblate indicatrices, showing flattening in the bedding plane, are associated with gently deformed strata. The prolate indicatrices, elongated parallel to the fold axes (N32°E), are associated with more intensely folded strata and show the influence of strong lateral compression superimposed upon gravitational loading. The reflectance indicatrices maintain a constant spatial orientation, regardless of the inclination of the enveloping bedding. The minimum principal reflectance axis plunges steeply toward the west, indicating that during deformation the direction of maximum compression was plunging toward the northwest. These results suggest that in the Broad Top field maximum coalification and, therefore, maximum burial were penecontemporaneous with the later stages of deformation, postdating most of the folding.