ISOTOPIC INVESTIGATION OF OROGENIC FLUID FLOW IN THE ALLEGHENIAN VALLEY AND RIDGE, WEST VIRGINIA

We are investigating fluid-rock interactions and fluid sources during deformation of the Silurian-Devonian section exposed in the westernmost Valley and Ridge, testing the hypothesis of Evans and Battles (1999) that major carbonate formations (Tonoloway, Helderberg) served as relative aquitards. We have identified d¹⁸O shifts of up to 3.0‰ and d¹³C shifts of up to 5.0‰ relative to values for host rocks, within an overall d¹⁸O range of +19.5‰ to +25.6‰, and d¹³C range of -8.3‰ to +4.6‰. Most veins are depleted in ¹⁸O and ¹³C relative to host rocks. At regional scales, isotopic compositions appear rock-buffered. For example, veins and host rock in the Tonoloway Formation have low d¹³C signatures, veins and host rock in the Helderberg Group have higher d¹³C signatures, and there is little d¹³C overlap between units. At some smaller scales (single outcrop, hand-specimen), open system behavior is more evident. Kilometer-scale isotopic variation is observed between major folds and differing positions in individual folds, with the most noticeable patterns in the Helderberg Group and in the Wills Mountain Anticline. In some cases, temporal evolution in fluid composition can be deduced using textural relationships and isotopic compositions of multiple vein generations.

We suggest that relatively saline formation fluids were mobilized in early-formed, primarily bed-parallel veins. Next, pulses of “warm” fluids sourced in deeper carbonate+silicate formations resulted in the prevalent vein-host rock isotope shifts. A possible later infiltration stage involving a mixture of warm fluids and another fluid of lower d¹⁸O introduced by topographic recharge, produced some silicification. The results of this study suggest a spatial-temporal evolution in fluid migration possibly reflected in regional paleomagnetic signatures. Hydrocarbon reservoirs along the western margin of the Appalachian Basin are considered to be the result of fluid migration from the Valley and Ridge into the Plateau Province. Whereas our results do not directly demonstrate it, they are consistent with such flow, particularly along some of the more through-going, deeply-projecting structures. Other units in the section (Chemung Formation; Evans and Hobbs, 2003) apparently accommodated significantly greater fluid flux.