| Paleomagnetic and geochemical results are used to evaluate the origin of an apparent synfolding chemical remanent magnetization (CRM) in the Belden Formation on an asymmetric fold in northwestern Colorado. Limestones in the Belden Formation contain three components: a modern viscous magnetization, an intermediate temperature Tertiary magnetization, interpreted as thermoviscous in origin and a characteristic northeast and steep down component removed by heating to 580;Tests, however, have failed to establish a link between the mineralizing fluids in the Belden Formation and the characteristic CRM. Lithology, rather than structural position or fluid alteration, controls the magnetization. Fecal pellets rich in clay minerals and organic material are common in beds with high NRM intensity. These pellets contain alteration rims of magnetite. Selective demagnetization has established that the fecal pellets carry the characteristic magnetization. The replacement is interpreted as a burial driven event which occurred at temperatures less than 150;Greater levels of strain on the steep limb and differences in the magnetic susceptibility and anhysteretic susceptibility anisotropy elipses suggest that the magnetization on the steep flank of the anticline was deflected during deformation. Such a deformation related alteration of a preexisting remanence could influence the fold test result.;Certain beds lack the late Cretaceous/Early Tertiary CRM which provides an opportunity to magnetically characterize the TVRM and the CRM. Specimens with the CRM have higher isothermal remanent magnetization (SIRM) acquisition levels, higher NRM/IRM values, and acquisition of a partial anhysteretic magnetization over a broader coercivity range. Hysteresis properties suggest a finer grain size for magnetite in the beds carrying the CRM. While this fine component carries a stable component (CRM) which can be dated, the coarser magnetites carry a TVRM and the timing of their formation is problematical. |
