Syn-Orogenic Hydrostratigraphy and Fluid Flow in the Central Appalachian Orogen: No Evidence for Regional Flushing

Evans, Mark A.
Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, PA 

Fluid inclusion microthermometric analyses and O and C stable isotopic analyses of vein minerals are used to determine the chemistry and trapping conditions of fluids present in the central Appalachian fold-and-thrust belt during the Late Paleozoic Alleghanian orogeny. In the Valley and Ridge province, the Upper Paleozoic rock section contains three regional hydrostratigraphic systems based on fluid chemistry and temperature. The Ordovician Trenton Formation through Devonian Helderberg Group was a regional aquitard, dominated by high salinity, CH4-saturated in-situ fluids. The Devonian Oriskany sandstone through the lower portion of the Chemung Formation was a regional aquifer system, and experienced an influx of 'warm' migrating fluids. The migrating fluid was a ‘warm’ (160° to >220°C) CH₄-saturated NaCl-CaCl₂ brine. The upper portion of the Devonian Chemung through Pocono Formations was also a regional aquifer, but it was dominated by an influx of meteoric water that mixed with in-situ fluids.

However, Middle and Upper Devonian rocks immediately forelandward of the Valley and Ridge, in the Appalachian Plateau province contain veins that have only liquid hydrocarbon and low-temperature brine inclusions, indicating that the ‘warm’ fluids did not significantly migrate into the rocks of the Appalachian Plateau province. Instead, the ‘warm’ fluids are interpreted to have migrated to the paleosurface as hot springs. Stable and strontium isotope chemistry of the vein minerals suggests that some infiltration of the ‘warm’ fluids into the easternmost Plateau province did occur. This indicates that wholesale flushing of a foreland during an orogeny may not be as common as originally thought.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004