McNamara, Missoula County, Montana, USAi

Regional Level Types
McNamara	Formation
Missoula County	County
Montana	State
USA	Country

The locality of McNamara is several miles east of the town of Bonner, Missoula County, Montana. The geographic name refers to a historical settlement that has only a few remaining buildings on the north side of the Blackfoot River in the vicinity of the “McNamara Bridge.” Montana Highway 200 (“MT-200") follows the river from Bonner to McNamara Bridge. East of the bridge, MT-200 separates from Blackfoot River, following instead Union Creek that flows over more open terrain. An unpaved road continues in the canyon of Blackfoot River, allowing access to additional outcrops. The west end of McNamara Bridge is indicated by the latitude and longitude values provided for the locality.

The site of McNamara is the type locality of the McNamara Formation, and the formation is exposed along much of this stretch of the Blackfoot River and MT-200. The McNamara Formation is part of the Missoula Group in the Belt Supergroup of Mesoproterozoic age (1,600-1,000 million years). Argillite, siltite (weakly metamorphosed shale and silstone, respectively), and quartz sandstone. This largely detrital interval also contains thin siliceous, phosphoritic, and calcareous layers in the upper part. Karstic features indicate presence of carbonate intervals in the upper part of the Missoula Group, and those may be Cambrian in age. Gabbro and diorite sills, apparently of Mesoproterozoic age, are also exposed in the area. The McNamara Formation is notable for the dark-colored clay chips and nodules of chert that it contains. The chert apparently replaces gypsum/anhydrite that indicates arid
conditions during deposition of the McNamara.

Other Regions, Features and Areas containing this locality

References