Named presumably for Seminoe Mountains, Carbon Co, WY in the Northern Rocky Mountain region. Source of geographic name and type locality not given. Unconformably overlies unnamed andesitic greenstones and dark-green hornblende schists. Seminoe is infolded with them in highly contorted layers. Consists of banded jaspery iron-bearing quartzite. Ranges in thickness from a few ft to slightly more than 300 ft. Where well developed, Seminoe shows changes in composition from strongly ferruginous jasper at base through siliceous in middle to a layer rich in hypersthene at top. Section measured on east bank Deweese Creek, SW1/4 sec 19, T26N, R85W, is 275 ft of gray, jaspery quartzite thickly banded with black magnetite separated 125 ft above base by a 25 ft thick dark-green amphibolite schist and quartz-chlorite schist unit. Gray color attributed to presence of finely disseminated magnetite grains. Is resistant to erosion; forms knobs and ridges. Thought to be of sedimentary origin. The extremely fine grained silica and interlayered fine-grained iron oxide may indicate formation is a metamorphosed hot-spring deposit. Of pre-Cambrian age.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Age modified from Precambrian to Late Archean on basis of an age obtained from a quartz-phyric rhyodacite flow of 2.7 Ga (U-Pb zircon method) that is intercalated with metasedimentary rocks older than Seminoe. Is a banded iron-formation as much as 120 m thick found at the north end of the Seminoe Mountains, Carbon Co, WY in the Northern Rocky Mountain region.

Source: GNU records (USGS DDS-6; Denver GNULEX).