Orogenic Fluids and Regional Chemical Magnetizations, Belden Formation, Colorado

Sanjay Banerjee, David Fruit, R. Douglas Elmore

Migration of orogenic fluids is a commonly invoked mechanism for many chemical remanent magnetizations (CRMs) that occur in carbonates. The Pennsylvanian Belden Formation in central and northwest Colorado was studied to evaluate the role of orogenic fluids in causing CRMs. Limestones in the Belden from South Park, the Gunnison Basin and the Eagle Basin contain a late Cretaceous-Early Tertiary CRM that resides in magnetite and was acquired during both normal and reversed polarity intervals. In some areas the CRM is prefolding whereas in other areas it is apparently synfolding. The timing of the remagnetization, the locations of the rocks in an orogenic belt, and the fact that the limestone samples are radiogenic suggest that the CRM could be related to fluids activated during the Laramide Orogeny.

Detailed studies on an asymmetric fold, where the CRM is synfolding, were undertaken to test for a direct connection with orogenic fluids. Calcite in fractures from the hinge of the fold is radiogenic and there is a radiogenic halo in the limestones around the fractures. There is no corresponding magnetic halo, however, around the fractures. In addition, comparison of the magnetic characteristics in the same bed between the more highly fractured and mineralized areas of the fold to less mineralized shallow flank indicate no differences. The CRM is associated with magnetite that has replaced pyrite in organic-rich fecal pellets. These results suggest that the CRM is not directly related to orogenic fluids but is related to diagenesis (e.g., maturation?) of organic matter.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995