Abstract This volume of 25 field guides plus one paper on field instruction was prepared in conjunction with the 2011 GSA Annual Meeting in Minneapolis, Minnesota. The diverse slate of field trips spans a geologically broad range of topics, including the Precambrian geology of the southern Canadian Shield; the economic geology of the Lake Superior region; Phanerozoic strata in Minnesota, Wisconsin, Iowa, and North Dakota; glacial geology; hydrogeology and limnology; undergraduate and K12 geoscience field education; archaeological investigations in the upper Mississippi River valley; and geology by bicycle.

Abstract This field trip examines deformed Archean basement, and variably metamorphosed supracrustal rocks and an exhumed midcrustal batholith of late Paleoproterozoic age in east-central Minnesota. Collectively, these rocks reveal an approximately 100 m.y. geologic history of crustal growth and stabilization of this part of the craton. The Penokean orogen in Minnesota consists of a northern foreland basin (the Animikie basin), a medial fold-thrust belt, and a southern high-grade metamorphic and plutonic terrane, representing two major orogenic events: the Penokean (geon 18) and Yavapai (geon 17) orogenies. The 1870–1830 Ma Penokean orogenic rocks are part of a belt of juvenile crust accreted onto the southern margin of Laurentia-Baltic continent during the late Paleoproterozoic. Metamorphism along the southern margin of the Archean Superior province has been historically attributed to the Penokean Orogeny, in a corridor of amphibolite-facies rocks which record 1.86–1.80 Ga (geon 18) metamorphic ages that correspond to the culmination of arc accretion. However, a widespread geon 17 amphibolite-facies metamorphic overprint is also recorded along the regions of greatest thickening of the Penokean crust, which corresponds to the tectonically buried Archean-Proterozoic continental margin. This was also the locus of emplacement of the voluminous east-central Minnesota batholith, composed of some twenty separate intrusions that range from mafic to dominantly felsic-intermediate compositions. Most of these are Yavapai in age, with emplacement ages between 1787 and 1772 Ma.

ABSTRACT This field trip is an overview of Precambrian terranes in northeastern Minnesota using some of the most illustrative and accessible exposures—the term "classic" refers to the fact that many were the basis for earliest geologic study of the Precambrian and continue to be exemplary. The geology is presented in the context of major orogenic, rifting, and meteorite impact events during evolution of the North American continent. The Archean rocks are the products of three periods of orogenesis: the ca. 2695 Ma Shebandowanian orogeny that created major folds and thrust stacks; the ca. 2680 Ma Minnesotan orogeny that produced regional transpressive fabrics, folds, and metamorphism to greenschist-amphibolite grade; and a third event that produced localized faulting and folding of earlier structures and fabrics. The Sheban-dowanian may represent collision of the Wawa subprovince with the composite Superior superterrane to the north. The Minnesotan can be attributed to oblique collision of the Minnesota River Valley subprovince with the Superior superterrane. Structures bounding major components of the Superior Province are locally inferred to be thrust faults that formed during terrane assembly. Their vergence and offset histories are derived from seismic surveys in Minnesota and extrapolation from Lithoprobe and NATMAP transects in adjacent Canada. The Paleoproterozoic rocks are the products of three orogenic and rifting events, reflecting continued continental growth at Geons 18, 17, and 16. Mesoproterozoic rocks result from Geon 11 continental rifting, producing volcanic and sedimentary rocks of the Keweenawan Supergroup and plutonic rocks of the Midcontinent Rift Intrusive Supersuite.

ABSTRACT Outcrops within the broad expanse of the Minnesota River Valley in southwestern Minnesota mark the southernmost exposures of the Archean Superior Province of the Canadian Shield. Despite their relatively restricted exposure, the Meso- to Paleoarchean gneisses in the Minnesota River Valley have received considerable attention due to both their antiquity and their complexity. The rocks exposed include the migmatitic Morton and Montevideo granitic gneisses, schistose to gneissic amphibolite, metagabbro, and paragneiss. The units have undergone upper amphibolite to granulite facies metamorphism, multiple periods of folding, and intrusion by a weakly foliated Neoarchean granitic unit (the Sacred Heart Granite) and Paleoproterozoic mafic dikes and adamellite granite. Classic geochronologic studies of the Minnesota River Valley gneiss terrane from the 1960s through the 1970s used K-Ar, Rb-Sr, and U-Pb zircon isotopic techniques to establish the antiquity of the gneisses and general aspects of the geologic history of the terrane. However, more recent U-Pb SHRIMP (sensitive high-resolution ion microprobe) zircon geochronology has considerably refined our understanding of the complex history of the gneiss terrane. These studies indicate that the oldest units in the Minnesota River Valley terrane crystallized ca. 3500 Ma, but the rocks subsequently saw new zircon growth associated with events at ca. 3440, 3385, 3140, and locally 3080 Ma. The Archean history of the terrane culminated with high-grade metamorphism ca. 2619 Ma and intrusion of the Sacred Heart Granite at 2604 Ma. In addition to visiting classic outcrops of the Morton and Montevideo Gneiss, this field trip includes stops at each of the major gneissic rock units in the Minnesota River Valley. We will examine field relationships that are the basis for both our general understanding of the deformation and metamorphic history of the gneiss terrane and the sampling strategies for our recent geochronologic and ongoing isotopic studies.

ABSTRACT The Baraboo District includes an exceptional array of outcrops that have provided geological enlightenment to students and professionals, alike, for 150 years. In the late nineteenth century, several fundamental structural principles were developed here, such as criteria for determining stratigraphic facing and the significance of cleavage-bedding relations. More recent studies of deformational features in the folded Baraboo Quartzite, such as crenulation cleavage and quartz fabrics, have yielded insights into the kinematics of folding in the District and the significance of regional tectonics in the context of the Proterozoic assembly of North America. Additional petrologic, geochemical, and isotopic studies have established the age of the Baraboo Quartzite (≤1700 Ma), identified a Paleoproterozoic weathering profile, confirmed the supermature composition of the Baraboo Quartzite, established the presence of geon 14 hydrothermal alteration, and elucidated the Proterozoic tectonothermal evolution of the District, all of which bear importantly on Proterozoic tectonic, atmospheric, and climatic conditions in the southern Lake Superior region. By Late Cambrian time, the Baraboo Quartzite was a ring of islands, which was abutted by spectacular conglomerates deposited by tropical storms. These were surrounded by more distal sandstones and were eventually buried by Ordovician dolomite and sandstone. During the field trip, we will visit eleven localities, which have been selected to illustrate the key geological features of this North American classic.

Abstract The western Upper Peninsula of Michigan is well known for hosting significant concentrations of copper in copper-dominated deposits. Most of the copper is hosted by rocks of the Mesoproterozoic Midcontinent Rift. Copper deposits in the western Upper Peninsula can be subdivided into two overlapping world-class copper mining districts. The Keweenaw Peninsula native copper district produced 11 billion lbs of copper and a lesser unknown but significant quantity of silver. Native copper deposits in this district are stratiform and hosted by tops of rift-filling subaerial basaltic lava flows and interflow coarse clastic sedimentary rocks. These deposits are interpreted to be the result of mineralizing hydrothermal fluids derived from rift-filling basaltic volcanic rocks that migrated upwards, driven by late Grenvillian compression of the rift some 40–50 million years following cessation of active rifting. The Porcupine Mountains sediment-hosted copper district produced or potentially will produce 5.5 billion lbs of copper and 54 million ounces of silver. These stratiform/stratabound deposits are hosted in rift-related black to gray shale and siltstone and dominated by chalcocite rather than native copper. Chalcocite is interpreted to be the result of introduction of copper-bearing fluids during diagenesis and lithification of host sediments. At the now-closed White Pine Mine, the chalcocite mineralizing event was followed by a second stage of native copper deposition that demonstrates a spatial and temporal overlap of these two world-class mining districts. While these two districts have been dormant since 1996, favorable results from recent exploration at Copper-wood suggest a revival of the mining of copper-dominated deposits in the western Upper Peninsula of Michigan.

ABSTRACT The 100-mile-long Mesabi Iron Range contains the Biwabik Iron Formation, the largest of the Lake Superior–type iron-formations in the United States, deposited on the northern edge of the Paleoproterozoic Animikie Basin. This basin has been interpreted as a foreland basin that developed north of the Penokean Fold-and-Thrust Belt (ca. 1850 Ma), or alternatively, as a backarc basin north of the Wisconsin magmatic terrane. The basal unit in the basin, the siliciclastic Pokegama Formation, was deposited upon the ca. 2700 Ma granitic-volcanic basement. It is conformably overlain by the Biwabik Iron Formation, 200–750 ft thick, which consists of four members: lower cherty, lower slaty, upper cherty, and upper slaty. There are two prominent stromatolite zones. Both of the above formations contain attributes of deposition in a tidally influenced environment. The Biwabik is conformably overlain by the Virginia Formation, a thick turbiditic sequence of interbedded black shale, graywacke, and ash beds. All three formations dip southeastward at 10°–20°. The iron-formation (taconite) generally consists of 20%–30% Fe present in carbonates, silicates, and oxides, and 70%–80% SiO 2 . Direct shipping ores, also called natural ores, were originally mined on the Mesabi Iron Range and were instrumental in making the United States an industrial giant and in the winning of WWI and WWII. These ores originated along fault zones in the iron-formation where silica was removed leaving high-grade oxidized hematite-goethite natural ore bodies of 50%–55% Fe. Processing of low-grade magnetic taconite began in 1952, passed the natural ores in tonnage in 1967, and is now totally dominant. Field trip stops will include all three formations, with emphasis on the iron-formation. The final stop is in the folded Thomson Formation, the southerly equivalent of the Virginia Formation.

ABSTRACT The 7–9-km-thick North Shore Volcanic Group (NSVG) constitutes the volcanic products of the 1.1 Ga Midcontinent Rift System in northeastern Minnesota. With close physical, chemical, and volcanological analogies to Tertiary-to-modern Iceland, these flows accumulated in a gradually subsiding basin over a mantle plume centered beneath modern Lake Superior between 1108 and 1094 Ma. They are essentially undeformed, except for local faulting and disruption associated with hypabyssal intrusions. Geochemically the NSVG is bimodal, dominated by basalts and rhyolites, but includes a complete tholeiitic Fe-enrichment suite that ranges from primitive olivine tholeiite through transitional basalt, basaltic andesite, andesite, and icelandite to rhyolite. The mafic magmas were partial melts of the plume and lithospheric mantle, variably modified by crystal fractionation in crustal chambers and by crustal interaction. Many, but not all, of the rhyolites were derived largely from partial melting of Archean crust. The volcanic rocks were erupted subaerially, primarily from fissures, though there is some evidence for central volcanoes. Some of the rhyolites are very large and widespread, and were emplaced as high-temperature lavas and rheoignimbrites that crystallized primary tridymite. During their accumulation and subsidence, these plateau volcanics were subjected to burial/hydrothermal metamorphism, resulting in secondary mineral associations that range from greenschist (epidote-chlorite-albite±actinolite) to zeolite (thomsonite-scolecite-smectite) facies. This field trip will allow participants to examine outcrops throughout the stratigraphic section of the NSVG, including structural relations, volcanology, geochemical diversity, burial metamorphism, and associated hypabyssal intrusions.

ABSTRACT This field trip examines a sequence of ejecta and deformed substrate resulting from the 1850 Ma meteorite impact. An impact origin for the Sudbury structure in Ontario has long been accepted, but knowledge of the corresponding ejecta was limited to fall-back breccia in the relict crater at Sudbury. The more distant ejecta blanket was discovered only recently near Thunder Bay, Ontario, and later in other parts of the Lake Superior region. Known informally as the Sudbury impact layer (SIL), it occurs at and near the stratigraphic top of Paleoproterozoic iron-formation. The impact-related deposits in the western Lake Superior region include (1) autochthonous material interpreted to be seismically folded and shattered iron-formation and carbonate rocks (breccia), overlain by (2) strata composed largely of allochtho-nous material (ejecta) derived in part from target rocks, and (3) irregular layers that appear to be mixtures of locally and distally derived material. Definitive microscopic evidence of an impact origin includes the occurrence of accretionary lapilli, ash pellets, spherules, devitrified glass, and quartz fragments marked by planar deformation features. The SIL exhibits extreme lithologic variability from place to place within each exposure area and between exposure areas. Nevertheless, the stratigraphic relationships that are presented by these exposures can be used to devise a sequence of deformation and depositional events that is largely consistent with experimental and empirical evidence of impact processes. This field trip will demonstrate that the stratigraphic arrangement of facies in the SIL has important temporal implications for understanding mechanisms of ejecta delivery and deposition.

ABSTRACT A diverse range of plutonic to hypabyssal intrusions related to the 1.1 Ga Mid-continent Rift occurs in northeastern Minnesota. These intrusions are grouped into two complexes based on their structural occurrence within the North Shore Volcanic Group (NSVG, a comagmatic volcanic edifice) and their range of emplacement ages. The better known Duluth Complex is composed of gabbroic, anorthositic, and granitic intrusions that were emplaced into the base of the NSVG between 1108 and 1098 Ma. The Beaver Bay Complex is composed of a comparable range of intrusions that were emplaced into more medial sections of the NSVG between 1098 and 1095Ma. This field trip will profile the igneous stratigraphies of two of the best-studied mafic layered intrusions associated with the Midcontinent Rift. The Sonju Lake intrusion is a 1-km-thick lake intrusion associated with the Beaver Bay Complex that shows a classic Skaergaard-type cumulate stratigraphy indicative of closed-system fractional crystallization of a tholeiitic magma. The Layered Series at Duluth (DLS) is a well-differentiated, 4-km-thick sheet-like intrusion that is the type-intrusion of the Duluth Complex. Phase and cryptic layering through the DLS indicates that it evolved as a more open system due to periodic recharge and venting.

ABSTRACT The geometric, kinematic, and deformational features along the subprovince boundaries of the Archean Superior Province are keys to understanding the tectonic amalgamation of the province. This field trip investigates the structural geology along two of the subprovince boundaries—the Wabigoon-Quetico and Quetico-Vermilion—of the Superior Craton. These boundaries separate the relatively high-grade gneisses of the Quetico Belt from typical low-grade granite-greenstone terranes to its north (Wabigoon) and south (Wawa). Both boundaries are characterized by different styles of transpressional deformation and strike-slip tectonism. Along the Wabigoon-Quetico boundary, deformation is recorded by a variety of structures controlled by competence contrast of heterogeneous lithologies at a variety of scales: from weak greenstones surrounding more competent gneiss domes to deformed polymictic conglomerates. Along the Quetico-Vermilion boundary, we will emphasize the role of plutonism and pluton geometry on subsequent deformation. Lastly, we characterize multiple deformation episodes in the Vermilion district in Minnesota.

ABSTRACT This one-day field trip will showcase how modern techniques, building on more than three decades of scientific study and mapping, are changing the paradigms used by resource professionals to sustainably manage the largest groundwater resource in Minnesota. Society's views of how to manage agricultural, domestic, and industrial impacts on surface water and groundwater are beginning to incorporate new, more detailed, realistic and scientifically and politically defensible conceptual models of how karst groundwater hydrogeology interacts with human activities. Data loggers have enabled high-resolution, time-series measurements of a variety of physical and chemical parameters and have proven to be critical to any effort to realistically model the highly dynamic karst systems. Tracing work supplementing the powerful, traditional fluorescent dyes with a variety of other physical, chemical, and isotopic tracers is beginning to shed light on the "gray-box" inner working of various karst aquifers. The availability of high-resolution, light detection and ranging (LiDAR) data is a quantum step upward in the speed and accuracy of sinkhole (and other surficial karst feature) mapping. Repeated LiDAR surveys will begin to give us data on the rate of formation (and filling) of sinkholes. Downhole geophysical techniques are fundamentally changing our conceptual models of how water moves in karst aquifers (and in many aquifers previously assumed to be isotropic and homogeneous porous media).

ABSTRACT New data and review of classic sections from the Middle and Upper Ordovician North American Midcontinent in the Upper Mississippi Valley provide a refined picture of the age, stable isotope geochemistry, faunal composition, and—ultimately—origin of this epeiric ramp succession. Sequence stratigraphic analysis reveals a series of unconformity-bounded, genetically related facies packages. Shallowing and deepening trends are sometimes difficult to resolve due to a paucity of hydrodynamic indicators, yet unconformity surfaces are well marked by hardgrounds and confirmed by negative C-isotope spikes. Recent conodont biostratigraphy, new U-Pb radioisotopic ages for K-bentonites, and correlation of C-isotope profiles to global trends suggest that the succession spans the Darriwilian to Hirnantian epochs. Focus on Platteville to lower Galena Group strata (Sandbian to early Katian) provides a temporally high-resolution look at the onset and evolution of a long-term (>2 m.y.) positive carbon-isotope excursion, short-term perturbations in that record, and relationship to the preservation and diversity of the enclosed fauna and strata. Major changes in authigenic mineral suites and organic carbon content throughout the Upper Ordovician Upper Mississippi Valley suggest at least three major redox cycles. The combined evidence for globally recognized, positive carbon-isotope excursions coincident with these redox cycles, as well as high-frequency, sea-level fluctuations and successive faunal turnover events, suggests far-field responses to multiple global oceanic anoxic events.

ABSTRACT The Ice Age National Scenic Trail leads hikers on a 1200-mi (1900-km) tour of glacial and other geologic features across the State of Wisconsin. This one-day field trip highlights glacial landforms of the Superior Lobe of the southern Laurentide Ice Sheet in northwestern Wisconsin. Here the Ice Age Trail features spectacular end moraines, low-relief and high-relief hummocky topography, ice-walled-lake plains, eskers, tunnel channels, striations, and water-scoured features on basalt. The field trip involves several short hikes on parts of the trail, including one on a classic esker located in a tunnel channel. We argue that there is paleoglaciological significance to differing landform assemblages on the older, low-relief Emerald Phase land surface and the younger St. Croix Phase moraine, which has numerous high-relief hummocks, ice-walled-lake plains, and tunnel channels. Large potholes from the drainage of glacial Lake Superior are present at the Interstate State Park Unit of the Ice Age National Scientific Reserve.

ABSTRACT This field guide examines the evidence for multiple readvances of the Superior lobe, as well as the morphological and sediment record of glacial lakes in the western Lake Superior basin. During each readvance of the Superior lobe, ice went a shorter distance, reached a lower elevation, and laid down a finer-grained till due to incorporation of proglacial lake sediment. There are three distinct tills, which are correlated to three readvance phases: the St. Croix/Automba, Split Rock, and Nickerson. A red clay typically caps the stratigraphy at lower elevations in the basin. This clay may be a fourth till associated with a late readvance, perhaps equivalent to the Marquette phase in eastern Lake Superior. Alternatively, the red clay may be lacustrine. At issue are the potential hydraulic connections between glacial Lake Agassiz and the Atlantic Ocean during, and after, the Younger Dryas, because a readvance would fill the western Superior basin with ice and prohibit eastern Lake Agassiz drainage. Additional stops highlight the strandline and sediment record of the youngest glacial lake phase (glacial Lake Duluth), including the inspection of Lake Superior sediment cores that are archived at the National Lacustrine Core Repository in Minneapolis. The goals of the selected stops are to underscore the current understanding of the late glacial history of the western Superior basin and to provide new insights to spark discussion.

ABSTRACT The deglaciation history of northeastern Minnesota and northwestern Ontario is outlined using geological and ichthyofaunal evidence from the continental to local scales. Both published and new data indicate the existence of eastern outlets drawing from pre-and-early Agassiz lakes. Part of the Arctic watershed was impounded between the Duluth Complex highlands and the retreating Rainy lobe. These outlets had their flows routed through the “Keating Complex” and the “Gunflint Arrow Lakes Corridor.” Discharges around the Duluth Complex’s northeast limb reached the Superior basin along Superior lobe ice, then exited down existing pre-glacial river valleys and a prominent, “valley-type” topographic bench between Hovland and Grand Marais, Minnesota.

Abstract The glacial history of southern Minnesota, based on the surficial deposits of the late glacial Des Moines lobe and older deposits correlated to Marine Oxygen Isotope Stage 16, is documented in exposures along tributaries to the Minnesota River. A recently formalized stratigraphy defines and correlates these tills on the basis of their texture, lithologic composition, and stratigraphic position. Soils developed in this glacial landscape are among the most productive in the world. Their subtle variations reflect shifting ecotones throughout the Holocene.