ABSTRACT

The Pine River quartzite conglomerate and the Keyes Lake quartzite are informally designated members of the Michigamme Slate, a formation of the Middle Precambrian Animikie Series of northern Michigan. The units crop out as steeply dipping northwest-southeast striking homoclines in Florence County, Wisconsin, and are apparently anomalous local quartz-rich bodies within a thick sequence of typically "eugeosynclinal" chloritic slates, graywackes and, basic volcanic rocks.

The Pine River member consists of a lower meta-conglomerate, middle cross-stratified quartzite and pebbly quartzite, and upper meta-conglomerate, each of which thins to the northwest from a maximum total thickness of 600 feet to 150 feet in a distance of 3 miles. The conglomerate pebbles are elongate and consist predominantly of white and blue-gray recrystallized chert, interstratified chert and specularite, mosaic quartzite, and well-rounded strained vitreous quartz; the quartzose matric contains variable amounts of specularite, magnetite, sericite-muscovite, biotite, chlorite, garnet and, locally, grunerite and chloritoid.

The Keyes Lake quartzite consists of parallel-stratified quartzite, profusely cross-stratified quartzites, and finer quartzose slates that can be traced parallel to strike for different distances. Thin persistent conglomeratic zones are found containing pebbles of rounded red and white strained vitreous quartz, with lesser amounts of quartzite, chert and very rare inter-laminated chert and specularitc pebbles. The quartzite is compositionally mature (more than 90 percent quartz) but texturally immature (more than 10 percent fine matrix), thus representing sandstone intermediate between quartz wacke and quartz arenite.

The paleocurrent flow for both members is toward the southeast quadrant. Both are postulated to have been deposited in a shallow marine environment and derived from somewhat similar source rocks, but they cannot be correlated on a stratigraphic basis. It is suggested that the typical Animikean "graywackes," which are invariably very quartzose, are not necessarily indicative of deposition in deeper water, but of deposition in lower energy, partly reducing environments; the quartzites formed in more agitated environments, perhaps closer to source areas.