A Brunhes-age viscous remanent magnetization (VRM) carried by magnetic and hematite in the El Paso and Montoya dolomite formations has unblocking tempertures significantly higher than those predicted by single-domain theory. The samples appear to contain a substantial fraction of pseudo-single domain magnetite, suggesting transdomain processes may be responsible for the stable VRM. A strong inverse correlation between the occurrence of high unblocking temperatures and high isothermal remanence fractions acquired above 0.3 Tesla suggests that the hematite grain size may also be important in determining the range of observed temperatures. Larger grains are inferred to have lower coercivities but higher unblocking temperatures. An older, reverse-polarity component is also observed, and appears to be a record of late Cretaceous-Tertiary tectonic activity.