Hydrocarbon impregnated fenestral dolomites of the Phosphoria Formation were sampled on both flanks of Sheep Mountain Anticline, Wyoming. Demagnetization results indicate one dominant component that is either synfolding (Laramide in age) or predates folding (Cretaceous in age). Thermal demagnetization yields stable decay to temperatures between 500¿C and 550¿C and rock magnetic experiments suggest that the magnetization is dominated by a low coercivity component. Magnetic extracts contain spheres (<15 &mgr;m diameter) as well as other authigenic forms. Energy dispersive analysis demonstrates that Fe is the only detectable element present in the spheres and X-ray diffraction analysis indicates that magnetite is present in the extracts; the spheres are interpreted to be authigenic magnetite. The magnetization is interpreted as a chemical remanent magnetization (CRM) residing in this authigenic magnetite. Chemical conditions created by the hydrocarbons may have caused precipitation of the authigenic magnetite and acquisition of the associated CRM. ¿American Geophysical Union 1987