STRATIGRAPHIC AND SPATIAL VARIATION IN PALEOFLUID COMPOSITION AND TRAPPING CONDITIONS IN THE PENNSYLVANIA SALIENT DURING THE ALLEGHANIAN OROGENY

Fluid inclusion microthermometry and O and C stable isotope analysis of fracture-filling minerals are used to determine the chemistry and trapping conditions of fluids present in the central Appalachian fold-and-thrust belt in the Pennsylvania Salient during the Late Paleozoic Alleghanian orogeny. Since most fractures formed prior to folding, the data reflect a pre-folding hydrostratigraphic structure that was highly stratified by fluid chemistry, but also spatially variable. For example, in all carbonate units, host rock isotopic values are strongly formation dependent, while vein isotopic values are typically close to those of their host rocks indicating closed fluid systems.

Fluid inclusions in each stratigraphic level define a hydrostratigraphy based on fluid salinity and homogenization value (Th). Lower Ordovician carbonate rocks in the cores of major anticlines contain abundant CH₄-saturated NaCl-CaCl₂-rich aqueous fluid inclusions that have Th values of 90 to 170 °C. Fluid salinities are 18.6 to 25.6 % in the western Salient but are 13.9 to 19.5 % in the central Salient. Upper Ordovician through Silurian clastic rocks have Th values of 100 to 155 °C, with salinities of 20.2 to 25.6 % in the northern and central Salient, whereas in the southern Salient the salinity is bimodal with 16.0 to 24.3 % and 1.7 to 11.7 %. Silurian and Devonian carbonate rocks in the west-central Salient have Th values of 95 to 140 °C, and salinities of 19.5 to 25.6 %. However, in the Wills Mountain Anticline, the same rocks have salinities of 22.4 to 27.4 %.

In the Broadtop Synclinorium, Devonian Marcellus – Brallier Fm. rocks are dominated by CH₄±CO₂ fluid inclusions along with less common moderate to low salinity (0.0 to 16.0 %) aqueous inclusions. The Th values are generally 150 to 240 °C. The Devonian Chemung Fm. on the other hand, has rare CH₄±CO₂ inclusions, but common low-salinity (5.5 to 13.4 % salinity), low-Th (90 to 135 °C) aqueous inclusions. The Catskill Fm. has fluids very similar to the Chemung, while the Mississippian Pocono Fm. has high-Th fluids more similar to those in the Brallier Fm. In the south-central Salient, the Upper Devonian section has salinities of 6.5 to 21.0 %, while in the eastern Salient, the salinities are < 14.0 %. Locally, inclusion data indicates fluid migration along faults from deeper (warmer) fluid reservoirs.