An integrated paleomagnetic, rock magnetic, and diagenetic study of the Devonian Marcellus Subgroup from threefolds in the Valley and Ridge province in Pennsylvania and West Virginia indicates that the unit contains an intermediate temperature chemical remanent magnetization (CRM) with south-southeast declinations and shallow negative inclinations residing in pyrrhotite and a CRM with more southerly declinations and shallow positive inclinations residing in magnetite. Rock magnetic results confirm the presence of pyrrhotite and magnetite. Tilt tests indicate that pyrrhotite and magnetite CRMs are syntilting to posttilting, and paleopoles for both CRMs are similar and plot on the Permian part of the apparent polar wander path for North America. The Marcellus Subgroup has a complex paragenetic sequence which includes bitumen deposition and vein formation. The magnetite and pyrrhotite CRMs formed in the Permian, probably due to burial diagenetic processes and not orogenic fluids. The base of the unit contains the highest total organic content (TOC) values and highest magnetic intensities, both of which decrease up section. This connection between TOC and magnetic intensity suggests that the magnetite formed as a result of the maturation of the organic matter although more work is needed to test this hypothesis. The pyrrhotite may have formed as a result of thermochemical sulfate reduction involving organic matter during the production of methane in this gas reservoir. The similar Permian CRM ages may be explained by thrusting which resulted in rapid burial of the Marcellus Subgroup as it passed through the oil window and into the gas window quickly producing magnetite and pyrrhotite, respectively.
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.
A palaeomagnetic study of four oriented cores was conducted to better understand the timing of diagenetic events in the Mississippian Barnett Shale, a primary source rock and the unconventional gas reservoir in the Fort Worth Basin, Texas. Thermal demagnetization removes a present-field modern viscous remanent magnetization (VRM) as well as a chemical remanent magnetization (CRM) that has shallow inclinations and streaked south–SE-directed declinations. The VRM was used to orient the CRM data for one well and it produced a similar streak of directions. The streaking of directions could represent a mixing trend between two or more CRMs. Specimens from bedding-parallel and NE subvertical mineralized fractures contain a CRM that is interpreted to be of Pennsylvanian age and to have formed in response to burial diagenetic processes. NE- and NW-oriented vertical fractures are common to rocks that contain late Permian to Triassic CRMs. Sr and sulphur isotope results from vein minerals around NE fractures suggest the CRM could be related to fluids sourced from the Ouachita front. The SE directions in the streak could be explained if the northern part of the basin experienced a component of anticlockwise rotation of up to 20° in the Pennsylvanian.
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