Abstract SE¼, Sec.3, T.22N., R.3W., Hatfield 7½-minute Topographic Quadrangle.Outcrop along the Black River below the eastern half of Arbutus Dam (Fig. 1). Approach is on a 0.2-mi-long (0.3 km) gravel road that intersects Clay School Road 0.1 mi (0.2 km) west of the Green Bay and Western Railroad tracks.Additional outcrop occurs for 0.6 mi (1 km) downstream from dam.

The Penokean orogeny (1,900 to 1,800 m.y. ago) was a major early Proterozoic tectonic event in central Wisconsin, involving deformation, metamorphism, volcanism, and plutonism. Penokean isoclinal F 1 folding affected both Archean and lower Proterozoic rocks, producing a penetrative axial planar foliation that dips steeply throughout the terrane, implying significant horizontal compression during the Penokean orogeny. Foliation has been complexly reoriented by open to tight F 3 folding. Fold axes and penetrative mineral lineation are generally colinear, plunging steeply at all but a few localities. Regional metamorphism accompanying deformation is predominantly lower or middle amphibolite facies. Penokean tonalite to granite plutons are synkinematic to postkinematic; the earliest plutons intruded during the late stages of F 1 folding. F 1 , F 2 , and F 3 folding is estimated to have occurred over the interval 1,880 to 1,810 m.y. ago. Although Archean deformation and metamorphism may have affected the terrane, no unequivocal evidence for such events has been recognized. The existence of large-scale Proterozoic shear zones in central Wisconsin is contradicted by evidence which indicates that the proposed zones consist of metavolcanic rocks rather than mylonites. Spatial separation, differing ages, and contrasting tectonic histories suggest that the Archean gneisses of central Wisconsin do not correlate with the Archean gneisses of Minnesota and northern Michigan.

An Archean, early Proterozoic, and middle Proterozoic sequence of metamorphic and plutonic rocks is exposed along and near the Wisconsin River in central Wisconsin, between Stevens Point and Wisconsin Rapids. Major lithologic units, from oldest to youngest, consist of (1) banded tonalitic gneiss with inter-layered amphibolite, (2) 2,800-m.y.-old migmatite, (3) diorite gneiss, (4) 1,840-m.y.-old foliated tonalite, (5) a series of 1,820-m.y.-old lineated tonalites, (6) granitic aplites and pegmatites, (7) amphibolite dikes, and (8) diabase dikes. All units except the diabase dikes have been metamorphosed under amphibolite-facies conditions and possess a steeply plunging, penetrative mineral lineation. Deformation during the Penokean orogeny produced three steeply plunging sets of folds ranging from isoclinal to open in style. F 1 folding is interpreted as culminating prior to 1,840 m.y. ago, but also extending beyond this time. Foliated tonalite appears to have been emplaced during the waning stages of F 1 deformation. F 2 and F 3 folding occurred after the intrusion of 1,820-m.y.-old lineated tonalite, but younger limits on the age of folding are not yet available. All three fold sets are coaxial and parallel to the penetrative mineral lineation. Although not conclusive, there is evidence that suggests that an older deformation, perhaps Archean in age, affected the terrane prior to the Penokean orogeny.