ABSTRACT Landscape features that formed when buried ice melted and overlying sediment collapsed are abundant and widespread in the part of Wisconsin’s Northern Highland region glaciated by the Wisconsin Valley Lobe and the western part of the Langlade Lobe. Stagnation and burial of ice of the Wisconsin Valley Lobe are documented by broad tracts of hummocky moraine topography that record the position of the maximum extent of the lobe, and by extensive pitted and collapsed heads-of-outwash and outwash plains deposited during recession. Recession of the Wisconsin Valley Lobe was characterized by episodes of stagnation interspersed with episodes of readvance, documented by small west-east–trending heads-of-outwash. Advances of the western margin of the Langlade Lobe deposited large northwest-southeast–trending heads-of-outwash characterized by extensive areas of pitted and collapsed outwash plains with obscure but recognizable ice-contact faces. Following recession of the Wisconsin Valley and Langlade Lobes, the Ontonagon Lobe advanced out of the Superior Basin and over sediment containing abundant buried ice. Permafrost and debris cover combined to delay the melting of buried ice and the formation of the postglacial landscape. Regional correlation of ice-margin positions, combined with geomorphic and stratigraphic relationships, indicates that ice buried in north-central Wisconsin persisted in some places for up to 5000 yr or more following the recession of active ice.

The Marenisco-Watersmeet area in the western part of the northern peninsula of Michigan contains a greenstone and granite terrane (Puritan Quartz Monzonite) of late Archean age on the north and a gneiss terrane (gneiss at Watersmeet) on the south. A granite and gneiss belt (collectively called the granite near Thayer) crops out between these contrasting terranes. Lower Proterozoic metasedimentary and metavolcanic rocks of the Marquette Range Supergroup are extensive. Radiometric dating of the tonalitic phase of the gneiss at Watersmeet establishes an early Archean age and a complex subsequent history. U-Th-Pb systematics provide a firm minimum age of 3,410 m.y. with the possibility of a much greater age—3,500 to 3,800 m.y. Cataclasis and recrystallization during the early Proterozoic Penokean orogeny are recorded on a regional scale by whole-rock and mineral Rb-Sr ages of 1,750 m.y. Intense cataclasis of granodioritic gneiss in the Watersmeet dome locally produced metamorphic zircon with concordant ages of 1,755 m.y. Zircons from a tonalitic phase of the granite near Thayer are dated at 2,750 m.y. Zircons from leucogranite dikes, which are abundant in the tonalitic phase of the gneiss at Watersmeet, are slightly younger at 2,600 m.y. These intrusive rocks are approximately contemporaneous with the development of the greenstone-granite terrane of late Archean age.

Sm-Nd model ages on a whole-rock sample of the gneiss at Watersmeet, Michigan, show that these rocks were added to the continental crust about 3,600 m.y. ago. These model ages are calculated by assuming that the rocks were derived from a mantle reservoir with the Sm/Nd ratio of CHUR, which corresponds approximately to the chondritic ratio. These data confirm the U-Th-Pb zircon results that indicated a minimum age of 3,410 m.y. and possibly an age as old as 3,800 m.y. This study further illustrates the utility of Sm-Nd model ages in deciphering the history of complex Precambrian terranes.

Granitic rocks ranging in composition from granite to tonalite and associated metavolcanic-metasedimentary rocks compose an east-trending belt as much as 180 km wide and 300 km long in northern Wisconsin. The granitic rocks have an initial 87 Sr/ 86 Sr of 0.7025 ± 0.0005 and a Rb-Sr whole-rock isochron age of 1,885 ± 65 m.y., which is interpreted as the time of crystallization of the granitic rocks. Rb-Sr whole-rock mineral secondary isochrons for two samples give ages of 1,655 ± 55 m.y. and 1,545 ± 55 m.y.; K-Ar ages of biotite from these samples are 1,615 ± 55 m.y. and 1,598 ± 54 m.y., respectively. These mineral ages are interpreted as resulting from isotopic resetting caused by a thermal event about 1,600 m.y. ago. The granitic rocks and associated metavolcanic-metasedimentary rocks constitute lower Proterozoic greenstone-granite complexes that are remarkably similar in pattern to the Archean greenstone-granite complexes in the Superior province of the Canadian Shield.