SPATIAL AND STRATIGRAPHIC CHARACTERIZATION OF PALEOFLUID CHEMISTRY IN THE CENTRAL APPALACHIAN VALLEY & RIDGE PROVINCE DURING THE ALLEGHENIAN OROGENY

Fluid inclusion microthermometry and O and C stable isotope analysis of fracture-filling minerals are used to determine the chemistry fluids present in the central Appalachian fold-and-thrust belt during the Late Paleozoic Alleghenian orogeny. Since most fractures formed prior to folding, the data generally reflect a pre-folding hydrostratigraphic structure that was highly stratified by fluid chemistry, but also spatially variable.

Lower Ordovician carbonate rocks in the cores of major anticlines contain abundant CH₄-saturated NaCl-CaCl₂-rich aqueous fluid inclusions. Fluid salinities are 18 to 26 % in the western PA salient but are 14 to 20 % in the central part of the salient. In the western Valley & Ridge in WV, salinities range from 11 to 19 %. Upper Ordovician - Silurian clastic rocks have salinities of 20 to 26 % in the northern and central salient, whereas in the southern salient and WV, the salinity is 16 to 19 %. Silurian and Devonian carbonate rocks throughout the region have remarkably consistent salinities in the 20 to 24 % range. In all carbonate units, host rock isotopic values are strongly formation dependent, and vein isotopic values are typically close to those of their host rocks indicating closed fluid systems.

Devonian Marcellus – Brallier Fm. clastic rocks are dominated by CH₄±CO₂ fluid inclusions along with less common moderate- to low-salinity (12 to 15 %) aqueous inclusions. Salinities reach a maximum in the eastern salient of 20 to 25 % then fall off markedly into the Anthracite Belt (<10 %). The Chemung Fm. on the other hand, has rare CH₄±CO₂ inclusions, but common low-salinity inclusions (8 to 16 % salinity) with a band of high-salinity inclusions (20 to 22 %) in southern PA. In WV and the western salient, the Catskill and Pocono Fms. have salinities of 6 to 21 %, while in the eastern salient and Anthracite Belt the salinities are <14 %. Spatial variation in fluid chemistry may be due to tectonic fluid migration and/or diagenetic reactions.