ROCK STRAIN, ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS), AND MAGNETIC MINERALOGY OF THE MIDDLE DEVONIAN ORISKANY SANDSTONE, WEST VIRGINIA

Oriented samples were collected from the Oriskany sandstone across the Patterson Creek anticline (PCA) and the northern Willis Mountain anticline (WMA) of northern West Virginia. In order to determine total rock strain, Pressure Solution Shortening (PSS, which determines grain to grain pressure solution strain) analysis was performed on mutually perpendicular sections of 5 samples that contained clear grain interpenetrations. Strain data are compared with Anisotropy of Magnetic Susceptibility (AMS) data for each of the samples. Maximum shortening ranges from 3-11%. The maximum shortening axis is generally oriented SE-NW and is within the plane of bedding, suggesting a tectonic component. In contrast, stylolitic pressure solution indicates predominantly bed normal compaction strain. In terms of AMS, the samples from PCA show a good correlation between the intermediate shorting axis and the kmin axis of the AMS ellipsoid. In contrast, samples from the WMA show a correlation between the maxim shorting axis and the kmin axis. Bulk susceptibility (Kmax) values range from 10 to 60 x 10^-6 SI. Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray (EDX) data indicate that samples from both anticlines contained framboids (5-20mm in diameter) of iron sulfides (pyrite) and/or iron oxides (magnetite?). The presence of magnetite is supported by rock magnetic studies. In addition, iron oxides occur as clusters of octahedral crystals (100-500 nanometers) that are found within stylolite zones. Samples containing iron oxides have correspondingly high Kmax values and good clustering on the AMS plots. Whereas samples containing mostly pyrite grains and have low Kmax values and chaotic AMS plots. Paleomagnetic data indicate that the Oriskany sandstone was remagnetized during the Carboniferous. It yields a synfolding (78% unfolding) magnetization with a mean of Dec=161.9°, Inc=-7.1° (k=48). The morphology of the iron oxides suggests an authigemic origin that may be related to grain growth during deformation.