Paleomagnetism of Devonian red beds in the Appalachian Plateau and Valley and Ridge provinces

Abstract

Samples of red and green fluvial to marine sandstones in the Hampshire Formation/Catskill Group from regional-scale folds across the structural trend of the Valley and Ridge (VR) and Appalachian Plateau (AP) provinces of West Virginia were analyzed to test models for remagnetization of red sandstones. The red sandstones contain a dominant secondary Pennsylvanian-Permian magnetization with south declinations (154.8°-166.9°) and shallow inclinations (0.2°-19.3°) that resides in hematite and is interpreted as a chemical remanent magnetization (CRM) acquired during the Kiaman reversed superchron. Incremental fold tests for this CRM yield generally prefolding results from the AP rocks and synfolding results from the VR rocks. This suggests that the remanence acquisition in the AP may have occurred within the time span of deformation. A second, high unblocking temperature apparent synfolding CRM is also found in a few samples of the red sandstones and is distinguished by slightly steeper inclinations than the dominant component on northwest dipping beds. Specimens from green sandstone have weak intensities and are dominated by a modern viscous remanence. Thin section analysis shows the presence of authigenic specular hematite cement and submicron particle size red hematite pigment. Geochemical/fluid inclusion studies indicate that the rocks were exposed to mixed methane-saturated formational and meteoric fluids with no evidence that external warm orogenic fluids altered the rocks. A working model for CRM acquisition involves (1) methane reduction of previously formed iron phases and mobilization of iron and (2) a return to oxidizing conditions and precipitation of new authigenic hematite as a result of the introduction of meteoric fluids just prior to and during uplift. The green sandstones probably formed as a result of gleying which occurs in paleosols although they could be a relic of or preserve evidence of the reduction phase. Copyright 2005 by the American Geophysical Union.