ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) OF CENTRAL APPALACHIAN SANDSTONES: EXAMINING THE RELATIONSHIP BETWEEN LITHOLOGY, DEFORMATION MECHANISMS, AND STRAIN

Anisotropy of magnetic susceptibility (AMS) was measured for 65 Middle and Upper Devonian sandstone sites in a wide variety of structural settings in the Central Appalachians. Samples include Middle Devonian Oriskany calcite-cemented arenites, and Upper Devonian Chemung litharenites and Hampshire (Catskill) red sublitharenites.

The Chemung and Hampshire rocks have susceptibilities that range from 100x10^-6 SI to 450x10^-6 SI, indicating a low ferromagnetic contribution and that paramagnetic minerals are controlling the signal. This is verified by SEM-EDS analysis that shows abundant intergranular chlorite and other phyllosilicates, although platy hematite is common in the red rocks. The Chemung and Hampshire have primarily triaxial-oblate AMS fabrics reflecting a strong bedding normal compaction component and a slight tectonic component. This is reflected in an obvious bed-parallel grain alignment seen in both optical petrography and SEM. In addition, the Hampshire also includes many sites with a uniaxial-prolate AMS fabric that parallels regional strike. This fabric may be due to an overprinting of a moderate tectonic component with a compaction component.

The cleaner Oriskany arenites have very low susceptibilities (<60x10^-6 SI) reflecting the low abundance of phyllosilicates and ferromagnetic grains in these rocks. The dominant control on the AMS signal is interpreted to be ferroan calcite cement that can exceed 25% of the rock. Most Oriskany sites generally have a uniaxial oblate AMS fabric that is characterized by the short axis in the direction of strike, and the plane of the ellipsoid in the dip plane. This unusual fabric is interpreted as an inverse AMS fabric related to twinning deformation of the calcite cement. Where bed-parallel stylolites are present in the Oriskany, the AMS fabric is triaxial-oblate in the plane of bedding. This is due to the accumulation and orientation of phyllosilicates and the growth of new magnetite.

The different AMS fabrics in the rocks are all composite fabrics resulting from a complex interaction between rock lithology, deformation mechanisms, and strain magnitude. Although all the rocks have experienced similar deformation conditions, multiple AMS fabrics are exhibited, even from different beds in the same outcrop.