Date of Award
Master of Science (MS)
Two major igneous rock types occur in the Sundance, Wyoming area including foyaite of the Bear Lodge Mountain sill and quartz latite of Sundance Mountain and Sugarloaf Mountain. The igneous bodies were mapped on a scale of 1:5000 in an effort to determine petrogenetic relationships between the two rock types.
Sundance Mountain and Sugarloaf Mountain are extrusive in origin. Quartz latite occurs as subparallel units of breccia, tuff, and massive flows without any clear cross-cutting relationships. Fragmental types (breccia and cuff) constitute 41 to 76 percent of the rocks, suggesting that Sundance Mountain is a mixed cone. The quartz latite has a pre dominantly cryptocrystalline groundmass consisting of alkali feldspar and quartz. Oligoclase occurs as microlites and as zoned phenocrysts. The phenocrysts have distinct, oscillatory zones with narrow ranges in composition, suggesting that the crystals were subject to sudden changes in pressure. Sugarloaf Mountain is interpreted to be a satellite volcano of Sundance Mountain. Rocks from both igneous bodies have similar texturest structures, and compositions.
The Bear Lodge Mountain sill was passively emplaced along the con tact between the Pennsylvanian Minnelusa Formation and the Mississippian Pahasapa Formation. Foyaite dikes in the area have similar textures and compositions, and were probably emplaced during the same intrusive event, Primary analcime is the only feldspathoid present in the foyaite.
The quartz latite is interpreted as being the youngest of the two rock types, A maximum age for the quartz latite is Paleocene, based on the presumed age of plagiofoyaite(?) clasts found in Sugarloaf Mountain breccla, If the rocks are extrusive in origin the establishment of the present erosional character of the Black Hills region gives a younger age of post-early Oligocene.
Fashbaugh, Earl F., "Geology of igneous extrusive and intrusive rocks in the Sundance area, Crook County, Wyoming" (1979). Theses and Dissertations. 89.