Palaeomagnetic dating of hydrothermal alteration in the Woodford Shale, Oklahoma, USA

An oriented Woodford Shale core from the eastern Ardmore Basin was sampled to test if the shale was an open or closed system to hydrothermal fluids, and to determine the timing of alteration. Mineralized fractures are ubiquitous in the core, and the shale exhibits a complex paragenesis with multiple hydrothermal minerals, including biotite, magnesite, norsethite, gorceixite and potassium feldspar present in and around the fractures. These minerals suggest that the Woodford Shale was an open system during part of its diagenetic history. Vitrinite reflectance (R-o) measurements indicate values of similar to 1.81 % (similar to 230 degrees C). Palaeomagnetic analysis reveals a characteristic remanent magnetization (ChRM) with south-southeasterly declinations and shallow inclinations that resides in magnetite. This ChRM is interpreted to be either a chemical remanent magnetization (CRM) or a thermochemical remanent magnetization (TCRM) that was acquired at 245 +/- 10 Ma during Late Permian time based on the pole position (51.0 degrees N, 115.6 degrees E). Because the palaeomagnetic specimens show evidence of extensive hydrothermal alteration, the CRM/TCRM is interpreted to date the migration of hydrothermal fluids through the shale. The agreement in timing with other studies that report hydrothermal alteration in southern Oklahoma and the Ouachita Mountains in Late Permian time, suggest that there were post-collisional fluid-flow events which accessed reservoirs of warm fluids.