Drift Pebble Study Tomahawk Road Area, Lake Co.

Abstract

This MDNR project was aimed at testing the usefulness of using glacial drift pebble composition to determine the type of underlying bedrock in drift-covered areas. Pebbles > 1/4" in 81 surface drift samples from west-central Lake County were separated and examined, and each was assigned to one of 19 rock types (12 Keweenawan, 3 Animikie, 3 Archean, 1 "unknown". The 50 (or more) largest pebbles were counted in each sample; this number was found to give reproducible results. Each sample was then assigned to one of seven Drift Pebble Assemblages, which were plotted on a digitized map. No significant differences were found between samples classed as subglacial (basal) and "reworked" (supraglacial, meltout) till.

Meanwhile rock outcrops and the six DDH cores from the area were examined both megascopically and in thin section, and a revised geologic map was constructed. Four (and possibly 5) bedrock units are discernible: anorthosite in the eastern 2/5, olivine gabbro and troctolite of the Bald Eagle Intrusion in the north-central part, and one or two troctolite units (including the South Kawishiwi troctolite) in the western half of the area. Some large gaps in outcrop control, however, make some contacts poorly constrained.

In general, the most abundant pebble type in these samples corresponds to the underlying bedrock type, suggesting that this technique can be useful for "remotely sensing" bedrock types in covered areas. However, in the eastern 1/4 of the area the drift is dominated by lithologies (Archean, Animikie, Keweenawan lavas, granophyre) that have been transported for long distances (several lO's of km) from the E, ENE, or ESE. This must have been carried by the Superior Lobe and is clearly not basal till (directly overlying bedrock). Elsewhere in the study area, ice transport has produced some gradations or transition zones in the drift pebble assemblages, compared to the bedrock contacts. Also, since glacial transport in the Rainy Lobe (dominant here) was primarily roughly parallel to the main bedrock contact (anorthosite vs troctolite), the pebble assemblage at any sample site may have come largely from a few km up-ice. Thus the technique will be most successful when the drift is relatively thin and its stratigraphy is known well enough to exclude the existence of an upper drift sheet that is not in contact with local bedrock, and where rock boundaries are at large angles to ice transport direction.