Neoformed magnetic minerals as an indicator of moderate burial: The key example of middle Paleozoic sedimentary rocks, West Virginia

In order to help unravel the thermal history of middle Paleozoic sedimentary rocks in West Virginia, a rock magnetic study was conducted with a focus on the Marcellus Shale. Vitrinite reflectance, fluid inclusions microthermometry and conodont alteration index data yield contradictory burial temperature within the range 150-250 degrees C (302-482 degrees F). The characterization of magnetite and pyrrhotite may be used as an index to track burial temperature around 200 degrees C (392 degrees F). Low-temperature and room-temperature magnetic measurements were performed in order to determine the magnetic assemblage. Three magnetic assemblages were identified that were stratigraphically distributed. The goethite and nanosized magnetite (Al) assemblage is mainly found in the Clinton Group-Oriskany Sandstone stratigraphic interval (Silurian-Lower Devonian). Nanosized fraction of magnetite and probably pyrrhotite (A3) assemblage essentially constitutes the Marcellus Shale-Chemung Formation sequence (Devonian). Microsized pyrrhotite is the typical mineral for A2 that is only identified near the Alleghenian structural front. Overall, the rare occurrence of micron pyrrhotite in our samples suggests that the study area has not experienced burial temperatures higher than 200 degrees C (392 degrees F).