Abstract

The Bear Lodge igneous complex was formed by forceful emplacement of alkalic plutons along the crest of the western block of the Laramide Black Hills uplift. The intrusions produced complex structural relationships among the Tertiary bodies as well as with Archean granite and Phanerozoic sedimentary cover. Along the southeastern flank of the Bear Lodge Mountains separate, smaller intrusions emplaced in the Paleozoic and Mesozoic strata surround the core of the larger complex.

Domes, a bysmalith, and the fully exhumed laccolith of Sundance Mountain occur in this peripheral area. In plain view the domal structures are circular (Green Mountain) to elliptical (Tent Canyon) with maximum diameters of two mi and structural relief from 600 to 1500 ft. The margins consist individually, or in combinations, of monoclines, peripheral dikes (Sugerloaf Mountain), and concentric faults. The bysmalith at Sheep Mountain is encircled by major concentric faults and exhibits smaller, radial ones as well.

Igneous rocks associated with these domes, include quartz latite (Sundance laccolith and the Sugerloaf complex), foyaite (Sugarloaf complex), and trachyte, phonolite, and carbonatite (Sheep Mountain bysmalith). Tuff-filled diatremes (Sugarloaf complex) contain xenoliths of downdropped Cretaceous rocks as well as Archean granite which was carried upward at least 2300 ft. At Sheep Mountain sills occur in the Cambrian Deadwood Formation. It is likely that emplacement occurred at a similar horizon in other domes. On Green Mountain igneous rocks are not exposed, but the Pennsylvanian-Permian Minnelusa Formation is thermally altered.

A progression of structural development for the domes from an early, irregularly-shaped outline to near-perfect symmetry occurs as the laccolith grows from 400 to 900 ft relief. The early-formed arched roof may be modified by partial encirclement by a fault or perrhipheral dike to total rupture of the early-formed, circular monocline as the thickness of the laccolith increases. Alternatively, for margins of similar structural relief, faulting, as opposed to a monoclinal bend, may indicate a higher stratigraphic level of igneous emplacement or a different rate of intrusion.