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Rock magnetism and identification of remanence components in the Marcellus Shale, Pennsylvania

Abstract With increasing interest in the middle Devonian Marcellus Shale as a gas play in the Appalachians, a study of the rock magnetic characteristics and remanence components was undertaken. Samples were collected from outcrops of the Union Springs Formation and the underlying Onondaga Formation in a syncline immediately east of the Broadtop synclinorium in the Valley and Ridge province in Pennsylvania. The rocks contain an intermediate-temperature (IT) component with south-southeasterly declinations and shallow up inclinations that was removed by 310–350 °C, interpreted as a chemical remanent magnetization (CRM) that resides in pyrrhotite. At higher temperatures (350–480 °C) a component with more southerly declinations and shallow down inclinations, interpreted as a CRM in magnetite, is removed. Low-temperature treatments resulted in more stable decay during thermal demagnetization which allowed the IT component to be more easily identified. Cumulative log-Gaussian analysis of an isothermal remanent magnetization (IRM) and triaxial thermal decay of the IRM indicate the presence of two dominant minerals: pyrrhotite and magnetite. Low-temperature saturation IRM experiments show the 32 K Besnus transition and saturation of magnetization between 200 and 250 K, indicative of pyrrhotite. Some specimens showed the Verwey transition at 120 K, indicating magnetite.