New Mexico Geological Society Annual Spring Meeting — Abstracts

Fort Union National Monument, located in the Mora Valley in northeastern New Mexico, features the ruins of the Fort Union military post and ruts of the Santa Fe Trail. At the request of the National Park Service, we studied mortar samples from the ruins in order to characterize their mineralogy and assess if the materials came from a local geologic source. The samples are pale colored, brittle, and porous (averaging 18%). They contain 1.0-3.0 mm lithic fragments embedded in a very fine-grained calcite-clay matrix. Each mortar sample contained approximately 40% quartz grains. The quartz is 0.25-0.50 mm in size, rounded to subangular in shape, and coated with iron oxide. Half of the quartz population showed straight extinction and half the population showed undulatory extinction. Other major phases included 15% lithic fragments (polycrystalline quartz, quartz arenite, and micrite) and 2% opaque grains. Accessory (<1%) minerals included fine-grained microcline, hornblende, biotite, and muscovite. Low-field susceptibility versus temperature experiments yielded a very narrow spectrum of response, indicating the presence of a low-Ti ferrimagnetic (s.l.) mineral phase, likely low-Ti titanomagnetite, within the mortar samples.

The major mineral phases identified in the mortar samples are non-unique. They are the most common rock-forming minerals in the region’s sedimentary rock formations. In particularly, the quartz-rich Cretaceous Dakota sandstone is a likely source for the abundant quartz grains and quartz-rich lithic fragments. The Graneros shale and the Greenhorn Limestone are probable sources for the calcite-clay matrix and calcarous lithic fragments. While the mortar mineralogy does not point to a specific geologic outcrop for resource procurement, the similarity in terms of percentages of phases present among the mortar samples suggests that the raw materials were obtained from the same location(s). A larger sample set is needed to confirm this hypothesis and to make conclusions about resource procurement for the construction of the Fort Union compound.