FRACTURE AND BURIAL HISTORY OF THE SOUTHERN ANTHRACITE FIELD, PENNSYLVANIA

The Southern Anthracite Field is located in east-central Pennsylvania, and is a tightly folded and thrusted synclinorium. The Carboniferous coal-bearing strata in the area were deposited in the Appalachian foreland basin during the early stages of the Alleghanian orogeny. The cause of the anthracite rank attained by the coal in this area has been a subject of controversy, with both burial and/or ‘warm’ fluids called upon the anthratization. The fundamental question is: What caused the anthratization of the coal-bearing strata in eastern Pennsylvania? To address this, mineral vein samples were collected from the Pennsylvanian Pottsville and Llewellyn Formations.

Joint and vein orientations best represent structural conditions when rotated to bedding horizontal indicating that fracturing occurred prior to folding. Nearly orthogonal fracture sets of 340±10° and 050±5°, as well as 070±10° and 315±5° are observed. The 070±10° set is parallel to the anticlinal axis and the 340±10° is perpendicular, suggesting formation pre- to syn-folding. The other two sets are interpreted to have formed prior to folding. Single phase CH₄±CO₂ inclusions from quartz veins in the Pottsville Formation have minimum homogenization temperatures of less than -121 °C, and possibly as low as -130.3 °C. Based on an inferred thermobaric gradient of 25 °C km^-1 these values indicate minimum trapping pressures of 188 MPa (7.2 km depth), and up to 290 MPa (11.2 km). Aqueous fluid inclusions homogenize between 216 and 230 °C. The aqueous inclusions are cogenetic with CH₄±CO₂, and by utilizing the Isochore Intersection method, give trapping conditions of 230 °C at 240 MPa (9.2 km depth).

Based on this analysis, at least 6 to 8 km of syntectonic overburden have been eroded from the Southern Anthracite Field area. These values are consistent with those determined from the Weir Mountain syncline to the south. Realized temperatures are high enough for anthracite to form.