Abstract The Ordovician rocks of the conterminous United States (US) have a complex history, spanning multiple ancient basins, shifting palaeoclimate and evolving tectonic regimes. The US portion of the palaeocontinent of Laurentia occupied a relatively stable and isolated position around the southern tropics during the Ordovician. In general, Lower Ordovician rocks form a vast autochthonous blanket of fine-grained (tropical) carbonates that covered much of Laurentia, named the ‘Great American Carbonate Bank’. Outboard, ribbon carbonates and graptolitic shales are found in allochthonous fragments of the ancient continental margin. Middle Ordovician strata are more lithologically diverse, including the addition of several regionally distributed sandstones of the inner detrital belt, mostly overlying the Sauk–Tippecanoe unconformity. Upper Ordovician strata show the greatest lithologic and faunal diversity, reflecting steepening topography resulting from regional compression along the south Laurentian (Appalachian) margin. Recent advances in the interpretation of the US Ordovician come primarily from studies of carbon and oxygen stable isotopes, sequence stratigraphy, palaeoecology, tephrochronology, redox geochemistry, strontium isotopes and geochronology.

Abstract The study of Ordovician tephras yields a wealth of valuable information about regional tectonism, sedimentation, stratigraphic correlation, and process rates. As such, these layers are prized by geologists and are the subject of a rich literature. Ordovician tephra studies were pioneering, particularly in the development of chemical fingerprinting to improve precision in tephrochronology. Modern radioisotope geochronology utilizes zircons and other phenocrysts from these layers to generate eruption ages with uncertainty on the order of a hundred thousand years. When integrated with biostratigraphy, chemostratigraphy and astrochronology, tephra ages provide an unparalleled opportunity to constrain process rates. Fifty such Ordovician tephra ages have been published over the last decade from U–Pb analysis of individual zircon phenocrysts, providing geochronological coverage across all stages of the Ordovician. Laurentia dominates this coverage (24) followed by the Baltic Basin (12), North Gondwana (11), Cuyania (four) and the Siberian Tungus Basin (one). Future tephra studies should seek to fill the numerous remaining gaps in the Ordovician time scale.

ABSTRACT Pinnacle reef tracts are geomorphic features of carbonate systems that originated in the early Silurian and display an episodic distribution into the Cenozoic. Detailed study of Silurian pinnacle reefs of the United States midcontinent demonstrates repeated motifs, but most enigmatic is the coincidence of carbonate carbon isotope (δ 13 C carb ) excursions and reef pulses. Silurian δ 13 C carb excursions were associated with environmental changes and extinctions, and reefs appear to mark a resurgence of conditions favorable to biomineralizers following those extinction events. Previous workers in the region identified six discrete reef origination events in the United States midcontinent during the Silurian. Our reevaluation of outcrops and cores, conodont collections, and the generation of considerable new chemostratigraphic data across the region are clarifying the age relations of these events and their relationships to perturbations of the global carbon cycle.

ABSTRACT The Cincinnati Arch region of Ohio, Kentucky, and Indiana is an icon of North American Paleozoic stratigraphy, as it exposes strata ranging from Ordovician to Pennsylvanian in age. In particular, the highly fossiliferous Ordovician, Silurian, and Middle Devonian successions have been extensively studied since the nineteenth century, and continue to serve as a crucial proving ground for new methods and models of biostratigraphy, chemostratigraphy, and sequence stratigraphy in mixed clasticcarbonate depositional settings. These strata are locally capped by Middle Devonian limestones with their own diverse fauna and unique depositional history. Outcrops near Louisville, Kentucky, provide an excellent opportunity to examine these strata firsthand and discuss sequence stratigraphy, chemostratigraphy, sedimentary environments, and paleoecology. A series of new roadcuts south of Mount Washington, Kentucky, exposes the lower to middle Richmondian Stage (Upper Ordovician, Cincinnatian) and presents a diverse suite of marine facies, from peritidal mudstones to offshore shoals, coral biostromes, and subtidal shales. These exposures are well suited for highlighting the revised sequence stratigraphy of the Cincinnatian Series, presented herein. Nearby outcrops also include much of the local Silurian succession, allowing an in-depth observation of Llandovery and Wenlock strata, including several chemostratigraphically important intervals that have improved regional and international correlation. Supplementary exposures east and north of Louisville provide context for subjacent and superjacent Ordovician-Silurian strata, as well as examples of lateral facies changes and unconformities. Additionally, the Falls of the Ohio at Clarksville, Indiana, features an exceptional outcrop of the overlying Middle Devonian succession, including an extensive and well-preserved biostrome of corals, sponges, and other marine fauna. These fossil beds, coupled with significant exposures in local quarries, are critical for understanding the paleoecology and stratigraphy of the Middle Devonian of the North American midcontinent.

Abstract Times of metal-rich brine discharge into ancient ocean basins, associated with the formation of sedimentary-exhalative (sedex) Zn-Pb-Ba ore deposits, coincided with short-duration positive excursions (“spikes”) in the global marine Sr isotope record. While these spikes are unexplained by conventional oceanic models, chronostratigraphic correlations, combined with mass balance evidence and oceanographic modeling, suggest that the flux of radiogenic Sr from sedex brines during ore formation is sufficient to explain these previously enigmatic 87 Sr/ 86 Sr spikes. We review existing 87 Sr/ 86 Sr data and present new data as verification of these global 87 Sr/ 86 Sr spikes and their correlations with the formation of giant sedex ore deposits. Major events include an 1 ×10 −4 (~0.7078–~0.7079) excursion contemporaneous with formation of the Rammelsberg deposit at ~389 Ma; spikes on the order of 1 to 3 × 10 −4 , coeval with formation of the Meggen deposit at ~381 Ma, several ore deposits in the Macmillan Pass district at ~379 to 375 Ma, and the Silvermines deposits at ~352 Ma; and two >6 × 10 −4 spikes coincident with formation of the giant Navan deposit at ~346 Ma and Red Dog deposits at ~337 Ma. Moreover, the timing of peak 87 Sr/ 86 Sr spikes correlates with global δ 13 C and δ 18 O spikes, deposition of metal-rich black shales and ironstones, metal-induced malformation (teratology) of marine organisms, and mass extinctions. The relationships among these features were poorly understood, but our new model explains how the flux of key biolimiting nutrients and metals contained in sedex brines, demonstrably equivalent to or exceeding that of the total modern riverine flux to the ocean, spurred ocean eutrophication, which, ultimately, through a series of positive feedback mechanisms, may have triggered global chemical and biological events. If, as we hypothesize, sedex hydrothermal systems are recorded in the global marine isotopic, geologic, and biological records, our findings define a new approach to the study of and exploration for sedex deposits. We demonstrate that fluid inclusion solute chemistry and isotopic and stratigraphic studies of sedex deposits, coupled with chronostratigraphic correlation and high-resolution 87 Sr/ 86 Sr isotope chemostratigraphy, can be used to answer long-standing questions about geologic processes responsible for formation of these extraordinary deposits. This approach provides evidence for the age, duration, and fluxes of fluids and metals vented into the ocean by these giant hydrothermal systems. Accordingly, the marine 87 Sr/ 86 Sr curve constitutes a global exploration tool that could be applied to assess the mineral potential of sedimentary basins. To illustrate the potential of this tool to identify favorable stratigraphic ages and basins with potential for undiscovered giant sedex deposits, we highlight several spikes, on par with those characteristic of the Red Dog and Navan deposits, which have not been correlated with known metal deposits. Given these strong temporal correlations, mass balance estimates, and results of ocean chemistry modeling, our study suggests that further work is warranted to determine the extent to which periodic venting of hydrothermal basinal brines into the ocean has influenced the evolution of marine chemistry. Ultimately, these global signatures can be applied to the study of and exploration for sedex deposits.

Abstract Each year an estimated 56,000 metric tons (t) of rare earth elements (REEs), including 23,000 t of heavy REEs (HREEs), are mined, beneficiated, and put into solution, but not recovered, by operations associated with the global phosphate fertilizer industry. Importantly, the REEs in sedimentary phosphorites are nearly 100% extractable, using technologies currently employed to meet global phosphate fertilizer needs. Our evaluation suggests that by-product REE production from these phosphate mines could meet global REE requirements. For example, the calculated REE flux accompanying phosphate production in the United States is approximately 40% of the world’s total and, alone, could supply 65% of global HREEs needs. Moreover, recognition that the tonnages and HREE concentrations of some unmined phosphorite deposits dwarf the world’s richest REE deposits suggests that these deposits might constitute stand-alone REE deposits. The hypothesized genesis of these REE-rich occurrences strongly supports the long-debated suggestion that oceanic REE contents vary in a secular fashion and that associated high-grade REE abundances reflect oceanic redox state transitions during specific time periods. Here, we use this new process-based model, based on observed variations in global-secular REE abundances, to identify phosphorite horizons deposited during periods favorable for highgrade REE accumulation.

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.