Remagnetization of the folded Belden Formation, northwest Colorado

Abstract

Paleomagnetic and geochemical results are used to evaluate the origin of an apparent synfolding magnetization in Belden Formation (Pennsylvanian) limestones on an asymmetric fold in Colorado. The natural remanent magnetizations contain three components: a modern viscous magnetization removed at low temperatures, a reversed Tertiary magnetization (declination ( D ) = 186.7°, inclination ( I ) = −54.6°) of thermoviscous origin removed at intermediate temperatures, and an apparently synfolding Late Cretaceous/early Tertiary characteristic magnetization ( D = 338.7°E, I = 62.5°, precision parameter k = 54.9, radius of 95% probability cone ∝ 95 = 5.9°, 65% unfolding) removed by 580°C. Hysteresis properties suggest that the characteristic magnetization resides in single‐domain magnetite. Low burial temperatures and the presence of authigenic magnetite are consistent with a chemical origin for this magnetization. The 87 Sr/ 86 Sr values (0.7088–0.7100) show the limestones to be radiogenic, with 87 Sr enriched halos around calcite‐cemented tectonic fractures. There is no control on the distribution and intensity of the magnetization by the veins or structural position (e.g., highly mineralized versus less mineralized rock). Lithology, rather than fluid alteration, controls the magnetization. Fecal pellets contain pyrite, with alteration rims identified as magnetite by scanning transmission electron microscope X ray diffraction patterns. Selective demagnetization has established that the fecal pellets carry the characteristic magnetization. It is not possible to unequivocally establish whether the characteristic magnetization is synfolding or prefolding. A synfolding magnetization caused by the syndeformational mineralizing fluids, however, can be ruled out. Structural modification of a prefolding component and a synfolding piezoremanent magnetization cannot be ruled out but are not considered likely mechanisms of remanence acquisition. Overprinting of a prefolding chemical magnetization by a postfolding viscous magnetization residing in multidomain grains is a likely explanation for the synfolding test result. The replacement of pyrite by magnetite is interpreted as prefolding in origin, perhaps triggered by an oxidizing fluid or by diagenesis of hydrocarbons.