ORIGIN OF CHEMICAL REMAGNETIZATIONS IN SHALES

Chemical remagnetization is a common phenomenon and two general end-member diagenetic models have been proposed to explain chemical remanent magnetizations (CRMs): burial processes and alteration triggered by externally derived fluids. These two mechanisms are evaluated for shales based on integrated paleomagnetic and diagenetic studies. Studies of Paleozoic shales indicate similar results to those from carbonates: Late Paleozoic CRMs residing in magnetite and in some cases in pyrrhotite. For example, carbonate-rich lithologies in the Devonian Marcellus Subgroup in the Valley and Ridge province in Pennsylvania and West Virginia contain similar Permian CRMs residing in pyrrhotite and magnetite. Rock magnetic results confirm the presence of pyrrhotite and magnetite. Tilt tests indicate that both CRMs are syn-tilting to post-tilting. Shale and other lithologies in two Marcellus cores from the Appalachian Basin in Pennsylvania contain a viscous remanent magnetization (VRM) and a CRM. An attempt to orient the cores using the VRM produced streaked directions and was unsuccessful. The CRMs in the cores have shallow inclinations which correspond to a 285-305 Ma time of acquisition based on a comparison with the expected inclinations. The paragenetic sequences for the cores and outcrops are complex and include replacement of allochems by silica, precipitation of framboidal and cubic pyrite, generation of hydrocarbons and formation of bitumen, and filling of fractures with calcite, barite, albite, dolomite, and sphalerite. The presence of these minerals can be explained by internally sourced fluids (e.g., from the smectite to illite transition) and the magnetite CRMs are interpreted to have formed from burial diagenetic processes and not from externally-derived fluids. The pyrrhotite CRM may have formed as a result of thermochemical sulfate reduction. The Woodford Shale in southern Oklahoma also contains a late Paleozoic CRM in magnetite, but in contrast to the other units, the presence of hydrothermal minerals indicates that externally derived fluids may have caused remagnetization. In summary, some shales were probably closed basinal systems to external fluids, with the CRMs related to burial processes, whereas other shales were open basinal systems with the CRMs related to external fluids.