Four chemically and mineralogically distinct rock suites formed in south-central Wisconsin during a major anorogenic intrusive-extrusive event 1.76 b.y. ago. This anorogenic event followed by at least 60 m.y. the terminal calc-alkalic plutonism and volcanism of the Penokean orogeny. The 1.76-b.y.-old rocks may represent the only surface expression in the Lake Superior area of a 1.69 to 1.78-b.y.-old felsic terrain that can be traced in the subsurface from Wisconsin into northern Illinois and possibly as far as western Arizona. The magma types formed during the anorogenic event are (1) a peraluminous suite of texturally variable ash-flow tuffs; (2) a meta-luminous suite containing quartz- and orthoclase-bearing rhyolites and closely associated granophyric granites; (3) a suite of low SiO 2 , high Sr, and REE depleted granites at Baxter Hollow and chemically similar rhyolite dikes on Observatory Hill and at Marquette (Baxter Hollow suite); and (4) a suite containing a diorite intrusion and numerous dikes of tholeiitic basalt and andesite (Denzer suite). Major and trace element modeling studies suggest the following scenario for the formation of these post-Penokean rocks. The peraluminous and metaluminous suiteswere probably derived by partial melting (16%) of different crustal sources of tonalitic composition having slightly different residue mineralogies. These source rocks may have formed earlier by igneous activity related to the Penokean orogeny. Compositional variation within the two suites may be related to feldspar-dominated fractional crystallization. The Baxter Hollow suite was probably generated by large-scale (66%) partial melting of a crustal source of dioritic composition. Heat for such a major melting episode may have been provided by magma of the Denzer suite rising from a mantle source. Modeling studies suggest that 10% melting of either garnet peridotite, or eclogite will provide liquid of the composition of the Denzer suite. Fractional crystallization is most probably responsible for the chemical variation observed within both the Denzer and Baxter Hollow suites.

During the last part of the Wisconsin Glaciation, central Wisconsin was occupied by a proglacial lake called Lake Wisconsin. Lake Wisconsin formed when the Green Bay Lobe of the Laurentide Ice Sheet reached the Baraboo Hills in south-central Wisconsin, damming the lowland to the north, which is now occupied by the south-flowing Wiscon-sin River. At its greatest extent the lake was 115 km long, and its greatest depth was 50 m. Lake Wisconsin occupied a number of separate but interconnected topographic basins, including a main basin and seven smaller basins to the south of the main basin. During most of its history, Lake Wisconsin drained to the northwest through one of several outlets, all of which emptied into the Black River, a tributary of the Mississippi River. Sometime before the maximum expansion of the Green Bay Lobe during the Wisconsin Glaciation, Lake Wisconsin emptied southward through the Devils Lake outlet to the Wisconsin River, a tributary of the Mississippi River. At the end of Lake Wisconsin’s existence, it drained to the southeast out the Alloa outlet, to the Wisconsin River. The southern basins drained, at various times, through various combinations of a dozen separate outlets. Wind, slope wash, soil creep, cryoturbation, and solifluction have destroyed most of the beach ridges and terraces of Lake Wisconsin. Based on elevations of the few remaining beaches plus the elevations of the lake outlets, ice-rafted erratics, offshore deposits, the break in slope at the west edge of the outwash plain on the east side of the main basin, and shore-ice collapse trenches, we have been able to reconstruct three shorelines; from oldest to youngest, these are: the Wyeville shoreline, which has been tilted S45°W at 0.1 m/km; the Johnstown shoreline, tilted S60°W at 0.6 m/km; and the Elderon shoreline, tilted S55°W at 0.3 m/km. They intersect each other along a northwest-southeast line through the northwestern outlet. The shorelines are tilted because the Earth’s crust had been depressed under the weight of the glacier but later rebounded with the removal of the weight. The Wyeville shoreline probably formed about 19,000 years ago when Lake Wis-consin first came into existence as the glacier expanded into central Wisconsin during the last part of the Wisconsin Glaciation. The Johnstown shoreline formed about 15,000 years ago when the glacier was at its maximum extent and the Earth’s crust was at its maximum depression. The Elderon shoreline formed about 14,000 years ago, just before the lake finally drained, after the crust had begun to rebound as the glacier wasted back near the end of the Wisconsin Glaciation. Tens of meters of offshore sand was deposited in the deep basins of glacial Lake Wisconsin. Much of this sand is included in the Big Flats Formation. The sand was deposited in part by density currents derived from glacial meltwater rivers. In most areas, one or more beds of offshore silt occur within the sand sequence; the uppermost one (the New Rome Member) was probably deposited somewhat before the Johnstown Phase of glaciation. The final drainage of Lake Wisconsin was probably catastrophic. As the Green Bay Lobe wasted from the east end of the Baraboo Hills, the narrow ice dam was breached by water from the lake, causing the water level in the Lewiston basin to drop tens of meters, probably within a few days. This caused water from the main basin and other southern basins to flood into the Lewiston basin through a breach in the Johnstown moraine; this flood at least partly cut the sandstone gorges at the Wisconsin Dells.X

Abstract With its wide variety of geological features and phenomena packed into a small area, the Baraboo of south-central Wisconsin is among the most visited parts of the Midwest by geology students. This guidebook, the first comprehensive look at the area in decades, covers the spectrum of geological features present in the area, and it is useful as a teaching tool. An exceptional outdoor classroom, the Baraboo area contains a spectrum of geology, including excellent examples of geomorphology, glacial geology, structural geology, petrology, stratigraphy, and sedimentology. Ages of the strata range from 1.7-billion-year-old Precambrian to the Quaternary. The area has been studied for about a century, but it still holds surprises for professionals and students alike.

The Midcontinent Rift System is a major tectonic feature that formed about 1.1 to 1.2 b.y. ago in Precambrian crust that had a long and complex history but that was a rigid crustal block for several hundred million years prior to rifting. The rift formed at a high angle to the generally east-northeast-trending structural grain of the older rocks and crossed the boundary between two very different geologic provinces. The northern part of the rift, including all the rift in the immediate vicinity of Lake Superior, formed on rocks of the Superior Province or where Superior Province rocks are basement for Proterozoic supracrustal rocks. Available geochronologic data suggest that these rocks are not older than 3.1 b.y. and that they have remained a rigid crustal block since 2.6 b.y. ago. To the south, the rift developed in rocks that may be as old as 3.8 b.y. and that have since undergone repeated orogenesis at 2.6, 1.85, and 1.76 b.y. ago. Intrusion of the 1,5-b.y.-old, non-orogenic Wolf River Batholith in central Wisconsin marks the last major event prior to Keweenawan rifting 1.1 b.y. ago. The boundary between the Superior Province and rocks to the south seems to have had a pronounced effect on the nature of the Midcontinent Rift System. Where the rift occurs in rocks of the Superior Province, it is about 150 km wide and is complex. Where it is in rocks of the more complex terrane to the south, it is much narrower (about 90 km wide) and has a simpler shape. The width of the rift changes abruptly at the contact of the two terranes. Although orientation of the rift does not seem to be strongly influenced by the older northeast-trending fabric that it crosscuts, the orientation and location of the rift seem to have been controlled by pre-Keweenawan faults and fractures and a major intrusive body in central Wisconsin. Lineament analysis of gravity and magnetic data and Landsat imagery, as well as orientation of major dike swarms and mapped faults show that individual rift segments are either parallel to or colinear with these features. A major elliptical gravity low in central Wisconsin suggests the presence of a large body of subjacent granite. The arcuate shape of the rift encircles this gravity low, suggesting that the inferred granite served as a tectonic guide during rift formation. Some important constraints can be placed on the width and direction of rifting, even though a unique solution for the width of the rift is not available from gravity and magnetic data. Geometric limitations suggest that the rift formed by roughly north-south separation parallel to a major preexisting tectonic zone, herein named the Trans-Superior Tectonic Zone, which bisects the arc formed by the two arms of the rift. Correlation of pre-Keweenawan features across the rift suggests that maximum separation was about 50 km in a north-south direction. But the presence of compressional features along the rift, such as the Keweenawan and Isle Royale faults, makes a unique solution impossible.

ABSTRACT The Pleistocene Okanogan lobe of Cordilleran ice in north-central Washington State dammed Columbia River to pond glacial Lake Columbia and divert the river south across one or another low spot along a 230-km-long drainage divide. When enormous Missoula floods from the east briefly engulfed the lake, water poured across a few such divide saddles. The grandest such spillway into the Channeled Scabland became upper Grand Coulee. By cutting headward to Columbia valley, upper Grand Coulee’s flood cataract opened a valve that then kept glacial Lake Columbia low and limited later floods into nearby Moses Coulee. Indeed few of the scores of last-glacial Missoula floods managed to reach it. Headward cutting of an inferred smaller cataract (Foster Coulee) had earlier lowered glacial Lake Columbia’s outlet. Such Scabland piracies explain a variety of field evidence assembled here: apparently successive outlets of glacial Lake Columbia, and certain megaflood features downcurrent to Wenatchee and Quincy basin. Ice-rafted erratics and the Pangborn bar of foreset gravel near Wenatchee record late Wisconsin flood(s) down Columbia valley as deep as 320 m. Fancher bar, 45 m higher than Pangborn bar, also has tall foreset beds—but its gravel is partly rotted and capped by thick calcrete, thus pre-Wisconsin age, perhaps greatly so. In western Quincy basin foreset beds of basaltic gravel dip east from Columbia valley into the basin—gravel also partly rotted and capped by thick calcrete, also pre-Wisconsin. Yet evidence of late Wisconsin eastward flow to Quincy basin is sparse. This sequence suggests that upper Grand Coulee had largely opened before down-Columbia megaflood(s) early in late Wisconsin time. A drift-obscured area of the Waterville Plateau near Badger Wells is the inconspicuous divide saddle between Columbia tributary Foster Creek drainage and Moses Coulee drainage. Before flood cataracts had opened upper Grand Coulee or Foster Coulee, and while Okanogan ice blocked the Columbia but not Foster Creek, glacial Lake Columbia (diverted Columbia River) drained over this saddle at about 654 m and down Moses Coulee. When glacial Lake Columbia stood at this high level so far west, Missoula floods swelling the lake could easily and deeply flood Moses Coulee. Once eastern Foster Coulee cataract had been cut through, and especially once upper Grand Coulee’s great cataract receded to Columbia valley, glacial Lake Columbia stood lower, and Moses Coulee became harder to flood. During the late Wisconsin (marine isotope stage [MIS] 2), only when Okanogan-lobe ice blocked the Columbia near Brewster to form a high lake could Missoula floodwater from glacial Lake Missoula rise enough to overflow into Moses Coulee—and then only in a few very largest Missoula floods. Moses Coulee’s main excavation must lie with pre-Wisconsin outburst floods (MIS 6 or much earlier)—before upper Grand Coulee’s cataract had receded to Columbia valley.

New geologic and geochronologic data clearly establish that the two Archean basement-rock terranes recognized previously in Minnesota also occur in northern Wisconsin and Michigan, on the east side of the midcontinent gravity high. Greenstone-granite complexes (about 2,700 m.y. old) that are remarkably similar to those in the greenstone terrane of northern Minnesota are present on the south side of the Gogebic Range in northern Wisconsin and adjacent Michigan and in the northern complex of the Marquette district. Migmatitic gneisses and amphibolite, which are similar to the high-grade rocks in the gneiss terrane in southern Minnesota, are sporadically exposed south of the greenstone terrane and appear to compose the basement in most of northern and central Wisconsin and northern Michigan. The gneisses in Wisconsin and Michigan have been dated at two widely separated localities; they have radiometric ages of more than 3,000 m.y. and, for one rock type, an age of approximately 3,500 m.y. The boundary between the greenstone and gneiss terranes is covered by younger supracrustal rocks, but is interpreted to trend approximately eastward across northern Wisconsin to the vicinity of Marquette, Michigan, and beyond. Subsequent to its welding, probably 2,600 to 2,700 m.y. ago, abundant mafic dikes of Precambrian X and Y ages were emplaced in a wide zone parallel to the boundary, which indicates that it was predominantly a zone of extensional tectonics throughout much of post-Archean–pre-Phanerozoic time.

Glaciers originating in the central Brooks Range extended south into the lower Alatna Valley during three major episodes of Illinoian and Wisconsin glaciation. During the oldest glaciation, coalescing glaciers formed a piedmont ice sheet that extended 60 miles south of the range and covered most of the Koyukuk Lowlands. The later Kobuk Glaciation, subdivided into two stades, was marked by a smaller lobe that terminated in the Koyukuk Lowlands, receded, then readvanced to a frontal position within the Alatna Valley 25 miles closer to the Brooks Range. Drift of the Itkillik Glaciation, which marks the final period of ice advance into the lower Alatna, is subdivided into four moraine belts. Siruk Creek moraines, the oldest and most extensive Itkillik deposits, form an arcuate belt that extends 30 miles south of the range front. The later Chebanika moraine marks a brief readvance or stillstand of glacier ice farther upvalley. Ice then stagnated in the Helpmejack Lakes area, and massive bodies of ice-contact stratified drift were deposited in a broad belt 8 to 15 miles south of the Brooks Range. Moraines at the range front represent the last major event of the Itkillik Glaciation. Subdivisions and correlations of the glacial sequence are based on extent and position of the drift sheets, inferred glacier regimens, postglacial modification of glacial deposits, radiometric dates, and ice limits within the upper Alatna Valley. The four episodes of Itkillik glaciation within the Alatna drainage system appear to be equivalent to four Itkillik stades in the Anaktuvuk Valley, and radiocarbon dates from both areas indicate a late Wisconsin age for Itkillik deposits. The preceding ice advance, correlated with the Kobuk Glaciation farther west, may represent the early Wisconsin. The oldest glaciation is correlated with an ice advance of Illinoian age that filled the Kobuk Valley and extended west into Kotzebue Sound.