Abstract 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.

Abstract The Devonian Alamo Breccia is a thick (<30–130 m) unit, interpreted as a bolide impact deposit, which is bracketed by marine carbonates. Samples were collected within the breccia and above/below the breccia for a contact test to determine if the breccia acted as a conduit for fluids that could have caused the widespread chemical remanent magnetizations (CRMs) present in Palaeozoic Era rocks in Nevada. The carbonates above, below and in the breccia contain a Cretaceous Period syn-tilting CRM that resides in pyrrhotite and a pre-tilting late Palaeozoic Era CRM that resides in magnetite. The contact test is negative. Despite these results, diagenetic alteration by externally derived fluids is interpreted as the most likely mechanism of remagnetization. This hypothesis is supported by 87 Sr/ 86 Sr values in the breccia and surrounding rocks that suggest alteration by fluids with a radiogenic signature. The fluids were not localized in the breccia but are interpreted to have moved pervasively through the rocks. The results differ from some other studies that found that fluids caused localized CRMs around fluid conduits.