ABSTRACT

Two red-clay units, the Hanson Creek and Douglas members of the Miller Creek Formation, occur in Wisconsin along the bluffs of Lake Superior. The red clay is nonlaminated, consists of 60 to 90 percent clay, contains few pebbles, and occurs dominantly basinward of the shorelines of ancient glacial lakes. These characteristics have led previous workers to interpret the red clay as either lake sediment or subglacial till. Glacial striations, flutes, macrofabric, an areal distribution characteristic of till, grain-size trends, shear planes, and outcrop characteristics indicate that the red clay is subglacial till and not lake sediment.

Microfabric analysis of the red clay reveals a preferred orientation of elongate sand grains. This preferred orientation is related to ice-flow direction but in a way different from that shown by most analyses of till microfabric. Nearly half of the 25 samples analyzed have the major mode of sand-grain trends oriented transverse to regional ice-flow direction as indicated by flutes, striations, and macrofabric. In the remaining samples, the sand-grain trends are parallel to ice flow. The development of a transverse or parallel orientation may be influenced by clast content and subglacial shearing.

Linear streaks of silt and clay, referred to as microfoliation, occur in the matrix of the red clay. The microfoliation bands are planes that strike transverse to ice flow and dip in the up-ice direction, The structure of the microfoliation and its orientation relative to ice flow suggest that they may be shear planes caused by overriding ice. The microfoliation shows a more consistent relationship to ice flow than the sand-grain trends.