FRACTURE DEVELOPMENT, FLUID HISTORY, AND PALEO-OVERBURDEN OF THE DEVONIAN SHALE SEQUENCE, SOUTHERN PENNSYLVANIA VALLEY AND RIDGE

Fluid inclusion microthermometry of vein minerals provides information on the fluid pressure, temperature, and composition history. This data in turn can provide insight into paleo-overburden present during deformation. Veins were sampled from the Devonian shale units across the southern part of the Pennsylvania Valley & Ridge province.

The earliest-formed vein minerals are blocky calcite and dolomite, and include bitumen and light-brown tinted hydrocarbon inclusions. Later stages of calcite contain Na-Cl-CaCl₂ aqueous inclusions that homogenize between 95° - 115°C and have 20.2 - 21.7 wt. % equiv. salinity. Oxygen and carbon stable isotope values of the carbonate minerals have a wide range, with oxygen values ranging from 15.4 – 24.0 ‰ V-SMOW and carbon ranging from -16.5 - 0.1 ‰ V-PDB indicating a fluid system open to fluids from multiple sources.

In all veins, the latest mineral is quartz, which has abundant CH₄±CO₂ inclusions and less common aqueous inclusions. The CH₄±CO₂ inclusions contain between 0-4% CO₂. Homogenization temperatures vary both within single quartz crystals, and across the fold-and-thrust belt. For example, a quartz crystal from the central part of the belt shows homogenization values from the core to the rim decreasing from -86° to -106°C, indicating progressive burial to 6.0 - 6.3 km. Quartz veins from the eastern part of the Valley & Ridge have inclusions that homogenize at values as low as -110°C indicating trapping pressures of 148 - 177 MPa (7.1 – 7.4 km burial). On the western margin of the Valley & Ridge, minimum homogenization temperatures of -100°C indicating trapping pressures of 95 - 110 MPa (4.4 – 4.8 km burial). Aqueous inclusions from the same area have trapping temperatures of 150°- 200°C and salinities of 15.0-20.2 wt. % equiv. salinity.

In general fluid inclusions indicate a decrease in paleo-overburden across the fold-and-thrust belt from over 7 km toward the hinterland to approximately 4.5 km toward the foreland.