A STUDY OF CLAST COMPOSITIONS IN GLACIAL TILL OF THE COPPER FALLS FORMATION IN ASHLAND COUNTY, WISCONSIN

The Copper Falls Formation of Northern Wisconsin is an extensive Late Wisconsin-age till sheet deposited by the Chippewa Sublobe of the Laurentide Ice Sheet. It is distinguished by its reddish-brown color and gravelly sand texture. This study is being undertaken to characterize the mineral and rock composition in the formation in Ashland County, attempt to link clasts to their source regions, and determine whether hazardous minerals are present. Six sites have been studied along a 25-km transect approximately parallel to ice flow direction near Mellen, Wisconsin. This area is especially well suited for clast provenance studies because the bedrock contains distinctive rock types and the glacial ice flow direction was nearly perpendicular to the strike of bedrock formations. 3172 clasts have been identified in the field; sand has been studied microscopically as bulk samples and heavy mineral separates; and fine clasts have been analyzed with XRD. Lithologies in the coarse clastics are: sandstone 42% (mostly pebbles, not larger), gabbro 12%, granite 10%, gneiss 10%, quartzite 10%, basalt 8%, rhyolite 4%, others 4%. The sand is about 60% quartz, 10% each of feldspar, sedimentary rocks, igneous and metamorphic rocks, and heavy mineral grains. The fine fraction is similar but contains montmorillonite clay. As expected, for the rock fragments there is a strong correlation between persistence in till and rock hardness. This is well illustrated by hard granitic gneiss clasts that must have been transported long distances across the 250 km-wide Mid-continent Rift, distinctive non-resistant boulders of Copper Harbor Conglomerate that are abundant near their bedrock source in the Gogebic Range but are not persistent to the south, and sandstone derived from the Lake Superior syncline that is abundant in the north and decreases southward. A few sand-sized clasts containing distinctive asbestiform amphiboles have been found in the south, possibly sourced from gabbro or meta-BIF of the Gogebic Range. Results from this 25-km transect cannot be extrapolated far beyond the study area but they do demonstrate the region is well suited for clast provenance studies and that the Copper Falls Formation consists of a mixture of locally derived and resistant far-travelled clasts.