Studies of the complexly folded and metamorphosed early Proterozoic strata (1.8 to 2.1 b.y. B.P.; Van Schmus, 1976) of the Lake Superior region (Fig. 1) have revealed features indicating a complex sedimentary-tectonic evolution. These rocks, termed “the Marquette Range Supergroup,” and which include stratigraphic equivalents in Minnesota and Ontario (Cannon and Gair, 1970), have been divided into four groups (James, 1958), which, in ascending order, are the Chocolay, Menominee, Baraga, and Paint River Groups. In Minnesota and Ontario, the stratigraphic equivalents of the Chocolay, Menominee, and Baraga Groups are the Mille Lacs (Chocolay Group equivalent in the Cuyuna area), and lower (Menominee Group equivalent) and upper (Baraga Group equivalent) Animikie Group (Morey, 1978a) (Table 1). The stratified rocks of the Marquette Range Supergroup and stratigraphic equivalents occur in four structural settings (Fig. 1A): (1) elongate, generally synclinal, structural troughs bounded by Archean rocks; (2) extensive homoclines; (3) outliers on granitic basement; and (4) mantling rims around early Proterozoic gneiss domes (Sims, 1976). Some rims also form steep narrow troughs (for example, the Michigan River trough; Klasner and Cannon, 1974), which are here grouped with gneiss-dome rims as opposed to the other structural troughs (type 1 above) which appear to show no genetic relation to gneiss domes. Most of the data discussed herein concern the first two types of structural settings.
It is the purpose of this paper to suggest a relationship between the four types of structural settings and the mode of original sedimentation. Specifically, we propose that elongate early Proterozoic sedimentary basins existed in eastern Upper Michigan at the sites of what became structural troughs later in the early Proterozoic; that is, the eastern Upper Michigan structural troughs are simply appressed sedimentary basins. As will be discussed, the Animikie basin (as reported by Morey, 1967; Morey and Ojakangas, 1970; and Alwin, 1979) represents a more “stable” type of sediment receptacle (types 1, 2 structural exposure) with a sedimentary history broadly comparable to the eastern Upper Michigan basins. Finally, it is noted that individual basin history was complex, with some more areas showing evidence of both basinal and platformal sedimentation.