The Sedimentation and Petrology of the Lower Proterozoic McCaslin Formation, Northeastern Wisconsin

Abstract

The McCasin Formation of northeastern Wisconsin is exposed along the McCaslin Range, at Deer Lookout Tower Hill, and at Thunder Mountain. The McCcaslin overlies the Waupee Volcanics of uncertain age, and has been intruded by the Hager Rhyolite and High Falls Granite of the Wolf River Batholith, dated at 1500 million years. The McCaslin Formation is at least 1220 meters thick. It consists of a basal quartzose metaconglomerate and a metmorphosed quartz arenite (quartzite). Clasts include vein quartz, iron-formation, jasper, and chert. Detrital heavy minerals include hematite, zircon, red rutile, pyrite, and tourmaline. The only feldspar present in the quartzite occurs in contact with the High Falls Granite. The formation was metamorphosed to the hornblende hornfels facies by the Wolf River Batholith. Principal metamorphic minerals inc1ude andalusite, sillimanite, andradite, chloritoid, and epidote. Substantial recrystallization has removed most evidence of original grain boundaries. The McCaslin has been folded into a syncline that probably plunges about S20°W at about 30 degrees. The southern branch of the McCaslin Range has been overturned. Deer Lookout Tower Hill is probably separated from the McCaslin Range by a major east-northeast trending fault. The McCaslin appears to have been deposited as part of a braided alluvial system. Evidence of braided fluvial deposition includes poor sorting; small scale, predominantly trough cross-bedding; thin, discontinuous conglomerates; and the scarcity of both shale layers and ripple marks. The type of sedimentation was influenced by the lack of land vegetation, by intense weathering, and by probably aeolian conditions. Paleocurrent data indicate the direction of sediment transport was mostly from west to east, with substantial local variation. Clast lithologies and heavy minerals indicate multiple sources of sediment. The tectonic environment was generally stable with slight but steady subsidence. The McCalsin probably correlates with other quartzites in the region, including the Baraboo, Waterloo, Barron, Flambeau, and Sioux Quartzites. Radiometric ages indicate deposition of these formations between approximately 1630 and 1760 million years, or during Early Proterozoic time. The minor radioactivity of the McCaslin Formation is due to placer-like deposition of zircon grains. Since the McCaslin-Waupee contact is not exposed and has not been studied, an unconformity-type uranium deposit can not be ruled out.