PALEO-FLUID HISTORY AND STRUCTURAL GEOMETRY OF THE CENTRAL JACKS MOUNTAIN ANTICLINE, VALLEY AND RIDGE PROVINCE OF PENNSYLVANIA

The Jacks Mountain anticline is an extensive single fold in the central Pennsylvania salient. The strike of the fold ranges from 020° in the south to 075° in the east. The structure is a thrust related fold that culminates an antiformal stack of Cambrian and Ordovician carbonate rocks. The sample area is the core of the structure where 20 samples of Ordovician rock containing veins of blocky calcite, dolomite, and quartz were collected. Veins are primarily NNW striking and fault zones. Fluid inclusion microthermometry and stable isotope analysis of veins provides insight on the paleo-fluid history associated with fluid-fracture interconnectivity during deformation.

Fluid inclusions provide data on trapping conditions, including pressure, temperature, and fluid chemistry. Inclusions are both methane and two-phase aqueous brine. Methane inclusions in the carbonate rock veins have homogenization values ranging from -99 to -79 °C which gives a trapping pressure of 87-98 MPa (3.4-3.8 km). Aqueous brine inclusions from the carbonate rock veins have homogenization values ranging from 128°C to 178°C reflecting a trapping pressure of 111-203 MPa (4.3-7.8 km) and ice-melting temperatures ranging from -8 to -19 °C (11.8-21.7 wt. % NaCl equiv. salinity). Aqueous inclusions in the clastic rock veins have homogenization temperatures ranging from 132.3-134.4°C (4.1-5.2 km) and 161.7-179.8°C (5.7- 8.0 km) and ice-melting temperatures at -20.9°C to -24.7°C (23.0 -25.4 wt. % NaCl equiv. salinity).

Vein isotopic data indicates that there was little fluid fracture connectivity during the formation of Jacks Mountain. Host δ¹⁸O isotope values range from 7.29 to -5.32 ‰ and δ¹³C values range from -3.33 to 1.03 ‰. Vein δ¹⁸O isotope values range from -7.29 to -5.61 ‰ and δ¹³C values range from -4.84 to 1.98 ‰. The δ¹⁸O vein isotopic values differ from the host rock at a maximum of ~1.6 ‰ and δ¹³C values differ at a maximum of ~2.3 ‰ indicating that there was little to no movement of fluid.

The data collected exhibits multiple trapping events due to the varying ranges of homogenization values. The lack of variation in isotopic data in the carbonate samples indicates a closed fluid system during formation. The salinity of the clastic samples is higher indicating a fluid stratification with little to no intercommunication.