Geology topics
W.F. Cannon
Publications and source records attributed to W.F. Cannon.
Keweenaw hot spot: Geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift System
The Proterozoic Midcontinent Rift System of North America is remarkably similar to Phanerozoic rifted continental margins and flood basalt provinces. Like the younger analogues, the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad, asthenospheric, thermal anomaly which we call the Keweenaw hot spot. Great Lakes International Multidisciplinary Program on Crustal Evolution seismic reflection profiles constrain end-member models of melt thickness and stretching factors, which yield an inferred mantle potential temperature of 1500°–1570°C during rifting. Combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased, and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material. The isotopic signature of Keweenawan volcanic rocks, presented in a companion paper by Nicholson and Shirey (this issue), is consistent with our model of passive rifting above an asthenospheric mantle plume.
Crustal-scale thrusting and origin of the Montreal River monocline-A 35-km-thick cross section of the midcontinent rift in northern Michigan and Wisconsin
A structurally simple, 35-km-thick, north facing stratigraphic succession of Late Archean to Middle Proterozoic rocks is exposed near the Montreal River, which forms the border between northern Wisconsin and Michigan. This structure, the Montreal River monocline, is composed of steeply dipping to vertical sedimentary rocks and flood basalts of the Keweenawan Supergroup (Middle Proterozoic) along the south limb of the Midcontinent rift, and disconformably underlying sedimentary rocks of the Marquette Range Supergroup (Early Proterozoic). These rocks lie on an Archean granite-greenstone complex, about 10 km of which is included in the monocline. This remarkable thickness of rocks appears to be essentially structurally intact and lacks evidence of tectonic thickening or repetition. Tilting to form the monocline resulted from southward thrusting on listric faults of crustal dimension. The faults responsible for the monocline are newly recognized components of a well-known regional fault system that partly closed and inverted the Midcontinent rift system. Resetting of biotite ages on the upper plate of the faults indicates that faulting and uplift occurred at about 1060 +/−20 Ma and followed very shortly after extension that formed the Midcontinent rift system.
The North American Midcontinent rift beneath Lake Superior from GLIMPCE seismic reflection profiling
The Midcontinent rift system is a 1.1-b.y.-old structure extending from Kansas, through the Lake Superior region, and into southern Michigan. The rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments. For most of its extent it is buried beneath Paleozoic rocks but can be traced by its strong gravity and magnetic anomalies. The rocks of the rift system are exposed only in the Lake Superior region and comprise the Keweenawan Supergroup. Much of the geology of the Keweenawan is beneath Lake Superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on-shore geology. Seismic reflection surveys by the Great Lakes International Multidisciplinary Program on Crustal Evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail. The reflection profiles across the rift reveal a deep, asymmetrical central graben whose existence and magnitude was not previously documented. They show that, in addition to crustal sagging documented by previous investigations, normal faulting played a major role in subsidence of the axial region of the rift. A sequence of volcanic and sedimentary rocks, in places greater than 30 km thick, fills the graben. Thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben. Near the axis, the prerift crust is thinned to about one fourth of its original thickness, apparently by a combination of low-angle extensional faulting and ductile stretching or distributed shear. The sense of asymmetry of the central graben changes along the trend of the rift, documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments. The location of the accommodation zones is inferred from abrupt disruptions in the Bouguer gravity signature of the rift. Uplift of the central graben occurred when the original graben-bounding normal faults were reactivated as high-angle reverse faults with throws of 5 km or more in places. The Midcontinent rift has some striking similarities to some younger passive continental margins. We propose that it preserves a record of nearly complete continental separation which, had it not been arrested, would have created a Middle Proterozoic ocean basin.
The Sudbury impact layer in the paleoproterozoiciron ranges of northern Michigan, USA
A layer of breccia that contains fragments of impact ejecta has been found at 10 sites in the Paleoproterozoic iron ranges of northern Michigan, in the Lake Superior region of the United States. Radiometric age constraints from events predating and postdating deposition of the breccia are ca. 1875 Ma and 1830 Ma. The major bolide impact that occurred at 1850 Ma at Sudbury, Ontario, 500–700 km east of these sites, is the likely causative event. The Michigan sites described here, along with previously described sites in Minnesota and Ontario, define an extensive ejecta-bearing deposit throughout the Paleoproterozoic iron ranges of the Lake Superior region that we refer to as the Sudbury impact layer. The layer at the sites in Michigan exhibits a range of thicknesses, lithologic characters, and sedimentary settings. The diversity of rock types and internal stratigraphic details of the layer imply that several different processes of transport and deposition are represented, but the detailed investigations needed to document them are incomplete. Many of the sites had been described and interpreted previously as products of common terrestrial processes, but the presence of relict shock-induced planar deformation features in quartz indicates that the breccia layer is in fact the product of an extraterrestrial impact. At most localities, this layer also contains relict fragments of altered devitrified glass and/or accretionary lapilli. One immediate use of the impact layer is as an ultraprecise time line that ties together the well-known stratigraphic sequences of the various geographically separated iron ranges, the correlation of which has remained controversial for many decades. The Sudbury impact layer most commonly lies at a horizon that records a significant change in the character of sediments across the region. The impact layer marks the end of a major period of banded iron formation deposition that was succeeded by deposition of fine clastic rocks, commonly black shales. The impact may have produced regional, if not global, changes in the environment that resulted in this widespread synchronous change in sedimentation style.
Pilot studies for the North American Soil Geochemical Landscapes Project - Site selection, sampling protocols, analytical methods, and quality control protocols
In 2004, the US Geological Survey (USGS) and the Geological Survey of Canada sampled and chemically analyzed soils along two transects across Canada and the USA in preparation for a planned soil geochemical survey of North America. This effort was a pilot study to test and refine sampling protocols, analytical methods, quality control protocols, and field logistics for the continental survey. A total of 220 sample sites were selected at approximately 40-km intervals along the two transects. The ideal sampling protocol at each site called for a sample from a depth of 0-5 cm and a composite of each of the O, A, and C horizons. The <2-mm fraction of each sample was analyzed for Al, Ca, Fe, K, Mg, Na, S, Ti, Ag, As, Ba, Be, Bi, Cd, Ce, Co, Cr, Cs, Cu, Ga, In, La, Li, Mn, Mo, Nb, Ni, P, Pb, Rb, Sb, Sc, Sn, Sr, Te, Th, Tl, U, V, W, Y, and Zn by inductively coupled plasma-mass spectrometry and inductively coupled plasma-atomic emission spectrometry following a near-total digestion in a mixture of HCl, HNO 3 , HClO 4 , and HF. Separate methods were used for Hg, Se, total C, and carbonate-C on this same size fraction. Only Ag, In, and Te had a large percentage of concentrations below the detection limit. Quality control (QC) of the analyses was monitored at three levels: the laboratory performing the analysis, the USGS QC officer, and the principal investigator for the study. This level of review resulted in an average of one QC sample for every 20 field samples, which proved to be minimally adequate for such a large-scale survey. Additional QC samples should be added to monitor within-batch quality to the extent that no more than 10 samples are analyzed between a QC sample. Only Cr (77%), Y (82%), and Sb (80%) fell outside the acceptable limits of accuracy (% recovery between 85 and 115%) because of likely residence in mineral phases resistant to the acid digestion. A separate sample of 0-5-cm material was collected at each site for determination of organic compounds. A subset of 73 of these samples was analyzed for a suite of 19 organochlorine pesticides by gas chromatography. Only three of these samples had detectable pesticide concentrations. A separate sample of A-horizon soil was collected for microbial characterization by phospholipid fatty acid analysis (PLFA), soil enzyme assays, and determination of selected human and agricultural pathogens. Collection, preservation and analysis of samples for both organic compounds and microbial characterization add a great degree of complication to the sampling and preservation protocols and a significant increase to the cost for a continental-scale survey. Both these issues must be considered carefully prior to adopting these parameters as part of the soil geochemical survey of North America.
Continental-scale patterns in soil geochemistry and mineralogy: results from two transects across the United States and Canada
In 2004, the US Geological Survey (USGS) and the Geological Survey of Canada (GSC) initiated a pilot study that involved collection of more than 1500 soil samples from 221 sites along two continental transects across Canada and the United States. The pilot study was designed to test and refine protocols for a soil geochemical survey of North America. The two transects crossed a wide array of soil parent materials, soil ages, climatic conditions, landforms, land covers and land uses. Sample sites were selected randomly at approximately 40-km intervals from a population defined as all soils of the continent. At each site, soils representing 0 to 5 cm depth, and the O, A, and C horizons, if present, were collected and analyzed for their near-total content of over 40 major and trace elements. Soils from 0–5 cm depth were also collected for analysis of organic compounds. Results from the transects confirm that soil samples collected at a 40-km spacing reveal coherent, continental- to subcontinental-scale geochemical and mineralogical patterns that can be correlated to aspects of underlying soil parent material, soil age and climate influence. The geochemical data also demonstrate that at the continental-scale the dominance of any of these major factors that control soil geochemistry can change across the landscape. Along both transects, soil mineralogy and geochemistry change abruptly with changes in soil parent materials. However, the chemical influence of a soil’s parent material can be obscured by changing climatic conditions. For the transects, increasing precipitation from west to east and increasing temperature from north to south affect both soil mineralogy and geochemistry because of climate effects on soil weathering and leaching, and plant productivity. Regional anomalous metal concentrations can be linked to natural variations in soil parent materials, such as high Ni and Cr in soils developed on ultramafic rocks in California or high P in soils formed on weathered Ordovician limestones in central Kentucky. On local scales, anomalous metal concentrations recognized in soil profiles, such as high P in soils from animal confinement sites, are consistent with local anthropogenic disturbances. At a larger scale, the distribution of Hg across the west to east transect demonstrates that it can be difficult to distinguish between natural or anthropogenic contributions and that many factors can contribute to an element’s spatial distribution. Only three samples in a subset of seventy-three 0–5 cm depth soil samples from the north to south transect had organochlorine pesticides values above the method detection limit, apparently related to historic usage of the pesticides DDT and dieldrin.
Soil geochemical signature of urbanization and industrialization – Chicago, Illinois, USA
The concentrations of 45 elements in ambient (not obviously disturbed) surface soils were determined for 57 sites distributed throughout the city of Chicago, Illinois in the upper Midwestern United States. These concentrations were compared to soils from 105 sites from a largely agricultural region within a 500-km radius surrounding the city and to soils collected from 90 sites across the state of Illinois. Although the bulk composition of the Chicago urban soils reflects largely natural sources, the soils are significantly enriched in many trace elements, apparently from anthropogenic sources. The median concentration of Pb in Chicago soils is 198 mg/kg, a 13-fold enrichment compared to regional concentrations. Zinc (median 235 mg/kg), Cu (59 mg/kg), and Ni (31 mg/kg) are also enriched from 2- to 4-fold in Chicago soils and all four elements show strong mutual correlations. These elevated concentrations are most likely related to vehicular and roadway sources and represent uneven distribution across the city as airborne material. Other airborne particulate material from a combination of fossil fuel combustion, waste incineration, and steel production may contribute to apparent elevated concentrations in Chicago soil of Fe (median 2.9%), Mo (5 mg/kg), V (82 mg/kg) and S (0.09%). Chicago soils are enriched from about 1.6- to 3-fold in these elements. Enrichments in P and Se may be caused by direct addition of phosphate fertilizer to parklands, lawns and gardens. The density of the sampling (1 site per 10 km 2 ) is inadequate to define the distribution of the observed enrichments within the city or to predict soil compositions for most of the areas between sample sites, but does provide a statistically significant signature of the history of urban and industrial activity within the city in contrast to the surrounding agricultural lands.
Extraterrestrial demise of banded iron formations 1.85 billion years ago
In the Lake Superior region of North America, deposition of most banded iron formations (BIFs) ended abruptly 1.85 Ga ago, coincident with the oceanic impact of the giant Sudbury extraterrestrial bolide. We propose a new model in which this impact produced global mixing of shallow oxic and deep anoxic waters of the Paleoproterozoic ocean, creating a suboxic redox state for deep seawater. This suboxic state, characterized by only small concentrations of dissolved O 2 (???1 ??M), prevented transport of hydrothermally derived Fe(II) from the deep ocean to continental-margin settings, ending an ???1.1 billion-year-long period of episodic BIF mineralization. The model is supported by the nature of Precambrian deep-water exhalative chemical sediments, which changed from predominantly sulfide facies prior to ca. 1.85 Ga to mainly oxide facies thereafter. ?? 2009 Geological Society of America.
Concentrations of polycyclic aromatic hydrocarbons and inorganic constituents in ambient surface soils, Chicago, Illinois: 2001-2002
Samples of ambient surface soils were collected from 56 locations in Chicago, Illinois, using stratified random sampling techniques and analyzed for polycyclic aromatic hydrocarbon (PAH) compounds and inorganic constituents. PAHs appear to be derived primarily from combustion of fossil fuels and may be affected by proximity to industrial operations, but do not appear to be substantially affected by the organic carbon content of the soil, proximity to nonindustrial land uses, or proximity to a roadway. Atmospheric settling of particulate matter appears to be an important mechanism for the placement of PAH compounds into soils. Concentrations of most inorganic constituents are affected primarily by soil-forming processes. Concentrations of lead, arsenic, mercury, calcium, magnesium, phosphorus, copper, molybdenum, zinc, and selenium are elevated in ambient surface soils in Chicago in comparison to the surrounding area, indicating anthropogenic sources for these elements in Chicago soils. Concentrations of calcium and magnesium in Chicago soils appear to reflect the influence of the carbonate bedrock parent material on the chemical composition of the soil, although the effects of concrete and road fill cannot be discounted. Concentrations of inorganic constituents appear to be largely unaffected by the type of nearby land use. Copyright ?? Taylor & Francis Group, LLC.
Thermal history of low metamorphic grade Paleoproterozoic sedimentary rocks of the Penokean orogen, Lake Superior region: Evidence for a widespread 1786 Ma overprint based on xenotime geochronology
Paleoproterozoic strata in northern Michigan, Wisconsin, and Minnesota were deposited between 2.3 and 1.75 Ga within the rifted margin and subsequent foreland basin of the Penokean orogen. These strata show evidence for multiple regional metamorphic events previously attributed entirely to the Penokean orogeny (1875-1835 Ma). Metasandstones from the Marquette Range Supergroup and the Animikie, Mille Lacs, and North Range Groups were sampled at multiple localities across Minnesota, Wisconsin and Michigan for metamorphic xenotime suitable for in situ SHRIMP U-Pb geochronology. All samples are from the northern Penokean foreland basin where the metamorphic grade is greenschist to sub-greenschist and the strata are virtually undeformed. Xenotime U-Pb ages in these samples have a bimodal population with means of 1786 ?? 4 Ma (n = 32) and 1861 ?? 10 Ma (n = 9). Xenotime of both ages are contained in metasandstones from the basal Chocolay Group in Michigan and Wisconsin and the Mille Lacs Group and North Range Groups in Minnesota. The older age records a regional low-temperature thermal event that is slightly older than the overlying Menominee Group in Michigan and the Animikie Group in Minnesota and Ontario. This 1861 Ma event coincides with regional uplift that led to the formation of the unconformity between the Menominee Group and the overlying Baraga Group in Michigan; hence xenotime growth must have occurred at shallow burial depths. Younger units from the Menominee and Baraga Groups in Michigan and the Animikie Group in Minnesota, record only the 1786 Ma event. A dominant 1800-1790 Ma metamorphic monazite population that overprints Penokean-interval monazite has been documented within amphibolite- to granulite-facies rocks immediately north of the Niagara Fault Zone within the vicinity of gneiss domes and granitic plutons. In contrast, the 1786 Ma xenotime ages are from low-grade, virtually undeformed rocks 50-150 km from the high-grade zones and thus do not appear to reflect a local thermal imprint. Rather, the geographic extent of the 1786 Ma xenotime growth event suggests that it reflects a basin-wide, subtle thermal pulse. It is proposed that the xenotime ages record widespread subtle heating triggered by renewed subduction along the orogen due to Yavapai-interval convergence. The 1800-1700 Ma Yavapai terrane forms an accretionary belt throughout the central and southwestern U.S. and truncates the southern part of the Penokean orogen in central Wisconsin and southeastern Minnesota, about 200 km south of the sample sites. Alternatively, an 1800-1765 Ma interval of gravitational collapse of overthickened crust of the Penokean orogen immediately north of the Niagara Fault Zone may have driven a northward flow of hydrothermal fluids which subtly but pervasively altered the northern parts of the Penokean foreland and resulted in xenotime growth. ?? 2007 Elsevier B.V. All rights reserved.
The Penokean orogeny in the Lake Superior region
The Penokean orogeny began at about 1880 Ma when an oceanic arc, now the Pembine-Wausau terrane, collided with the southern margin of the Archean Superior craton marking the end of a period of south-directed subduction. The docking of the buoyant craton to the arc resulted in a subduction jump to the south and development of back-arc extension both in the initial arc and adjacent craton margin to the north. A belt of volcanogenic massive sulfide deposits formed in the extending back-arc rift within the arc. Synchronous extension and subsidence of the Superior craton resulted in a broad shallow sea characterized by volcanic grabens (Menominee Group in northern Michigan). The classic Lake Superior banded iron-formations, including those in the Marquette, Gogebic, Mesabi and Gunflint Iron Ranges, formed in that sea. The newly established subduction zone caused continued arc volcanism until about 1850 Ma when a fragment of Archean crust, now the basement of the Marshfield terrane, arrived at the subduction zone. The convergence of Archean blocks of the Superior and Marshfield cratons resulted in the major contractional phase of the Penokean orogeny. Rocks of the Pembine-Wausau arc were thrust northward onto the Superior craton causing subsidence of a foreland basin in which sedimentation began at about 1850 Ma in the south (Baraga Group rocks) and 1835 Ma in the north (Rove and Virginia Formations). A thick succession of arc-derived turbidites constitutes most of the foreland basin-fill along with lesser volcanic rocks. In the southern fold and thrust belt tectonic thickening resulted in high-grade metamorphism of the sediments by 1830 Ma. At this same time, a suite of post-tectonic plutons intruded the deformed sedimentary sequence and accreted arc terranes marking the end of the Penokean orogeny. The Penokean orogen was strongly overprinted by younger tectonic and thermal events, some of which were previously ascribed to the Penokean. Principal among these was a period of vertical faulting in the Archean basement and overlying Paleoproterozoic strata. This deformation is now known to have post-dated the terminal Penokean plutons by at least several tens of millions of years. Evidence of the Penokean orogen is now largely confined to the Lake Superior region. Comparisons with more recent orogens formed by similar plate tectonic processes implies that significant parts of a once more extensive Penokean orogen have been removed or overprinted by younger tectonic events. ?? 2007 Elsevier B.V. All rights reserved.
Reinterpretation of Paleoproterozoic accretionary boundaries of the north-central United States based on a new aeromagnetic-geologic compilation
The Paleoproterozoic crust in the north-central U.S. represents intact juvenile terranes accreted to the rifted Archean Superior craton. A new tectonic province map, based on the interpretation of a new aeromagnetic compilation, published geologic maps, and recent geochronologic data, shows progressive accretion of juvenile arc terranes from ca. 1900-1600 Ma. Contrary to earlier models, geon 18 Penokean-interval crust is primarily confined to a ???2100 Ma tectonic embayment of the rifted Superior craton. The newly defined Spirit Lake tectonic zone, characterized by a sharp magnetic discontinuity that marks the southern limit of Archean and Penokean-interval rocks, is here interpreted to represent an eastern analog of the Cheyenne belt suture zone in southern Wyoming. South of this boundary, geon 17 Yavapai-interval rocks form the basement upon which 1750 Ma rhyolite and succeeding quartzite sequences were deposited. Substantial portions of the Penokean and Yavapai terranes were subsequently deformed during the 1650-1630 Ma Mazatzal orogeny. The northern boundary of the Mazatzal terrane is obscured by abundant 1470-1430 Ma "anorogenic" plutons that stitched the suture with the older Yavapai terrane rocks. These data reveal a progressive tectonic younging to the south as the Laurentian craton grew southward and stabilized during the Proterozoic. Late Mesoproterozoic rift magmatism produced pronounced geophysical anomalies, indicating strong, but localized crustal modification. In comparison to the western U.S., little tectonism has occurred here in the last 1 billion years, providing a uniquely preserved record of the Precambrian evolution of the continental U.S. lithosphere. ?? 2007 Elsevier B.V. All rights reserved.
Mercury in soils, lakes, and fish in Voyageurs National Park (Minnesota): Importance of atmospheric deposition and ecosystem factors
Concentrations of methylmercury in game fish from many interior lakes in Voyageurs National Park (MN, U.S.A.) substantially exceed criteria for the protection of human health. We assessed the importance of atmospheric and geologic sources of mercury to interior lakes and watersheds within the Park and identified ecosystem factors associated with variation in methylmercury contamination of lacustrine food webs. Geologic sources of mercury were small, based on analyses of underlying bedrock and C-horizon soils, and nearly all mercury in the O- and A-horizon soils was derived from atmospheric deposition. Analyses of dated sediment cores from five lakes showed that most (63% ?? 13%) of the mercury accumulated in lake sediments during the 1900s was from anthropogenic sources. Contamination of food webs was assessed by analysis of whole, 1-year-old yellow perch (Perca flavescens), a regionally important prey fish. The concentrations of total mercury in yellow perch and of methylmercury in lake water varied substantially among lakes, reflecting the influence of ecosystem processes and variables that affect the microbial production and abundance of methylmercury. Models developed with the information-theoretic approach (Akaike Information Criteria) identified lake water pH, dissolved sulfate, and total organic carbon (an indicator of wetland influence) as factors influencing methylmercury concentrations in lake water and fish. We conclude that nearly all of the mercury in fish in this seemingly pristine landscape was derived from atmospheric deposition, that most of this bioaccumulated mercury was from anthropogenic sources, and that both watershed and lacustrine factors exert important controls on the bioaccumulation of methylmercury. ?? 2006 American Chemical Society.
Age constraints for Paleoproterozoic glaciation in the Lake Superior Region: Detrital zircon and hydrothermal xenotime ages for the Chocolay Group, Marquette Range Supergroup
A geochronological study of the Chocolay Group at the base of the Paleoproterozoic Marquette Range Supergroup in Michigan, Lake Superior Region, is attempted for the first time, Age data from detrital zircon grains and hydrothermal xenotime from the basal glaciogenic formation, the Enchantment Lake Formation, and the stratigraphically higher Sturgeon Quartzite and its equivalent, the Sunday Quartzite, provide maximum and minimum age constraints for the Chocolay Group. The youngest detrital zircon population in the Enchantment Lake Formation is 2317 ?? 6 Ma; in the Sturgeon Quartzite, it is 2306 ?? 9 Ma, and in the Sunday Quartzite, it is 2647 ?? 5 Ma. The oldest hydrothermal xenotime age in the Enchantment Lake Formation is 2133 ?? 11 Ma; in the Sturgeon Quartzite, it is 2115 ?? 5 Ma, and in the Sunday Quartzite, it is 2207 ?? 5 Ma. The radiometric age data in this study implies the depositional age of the Chocolay Group is constrained to ???2.3-2.2 Ga, which proves its correlation with part of the Huronian Supergroup in the Lake Huron Region, Ontario, and reveals the unconformity that separates the Chocolay Group from the overlying Menominee Group is up to 325 million years in duration. The source(s) of the ??? 2.3 Ga detrital zircon populations in the Enchantment Lake Formation and Sturgeon Quartzite remains an enigma because no known rock units of this age are known in the Michigan area. It is speculated that once widespread volcano-sedimentary cover sequences in Michigan were removed or concealed prior to Chocolay Group deposition. The hydrothermal xenotime ages probably reflect basinal hydrothermal fluid flow associated with the period of extension involving rifting and major dyke formation, that affected the North American provinces between 2.2 and 2.1 Ga. ?? 2006 NRC Canada.
A hydrogeologic model of stratiform copper mineralization in the Midcontinent Rift System, Northern Michigan, USA
This paper presents a suite of two-dimensional mathematical models of basin-scale groundwater flow and heat transfer for the middle Proterozoic Midcontinent Rift System. The models were used to assess the hydrodynamic driving mechanisms responsible for main-stage stratiform copper mineralization of the basal Nonesuch Formation during the post-volcanic/pre-compressional phase of basin evolution. Results suggest that compaction of the basal aquifer (Copper Harbor Formation), in response to mechanical loading during deposition of the overlying Freda Sandstone, generated a pulse of marginward-directed, compaction-driven discharge of cupriferous brines from within the basal aquifer. The timing of this pulse is consistent with the radiometric dates for the timing of mineralization. Thinning of the basal aquifer near White Pine, Michigan, enhanced stratiform copper mineralization. Focused upward leakage of copper-laden brines into the lowermost facies of the pyrite-rich Nonesuch Formation resulted in copper sulfide mineralization in response to a change in oxidation state. Economic-grade mineralization within the White Pine ore district is a consequence of intense focusing of compaction-driven discharge, and corresponding amplification of leakage into the basal Nonesuch Formation, where the basal aquifer thins dramatically atop the Porcupine Mountains volcanic structure. Equilibrium geochemical modeling and mass-balance calculations support this conclusion. We also assessed whether topography and density-driven flow systems could have caused ore genesis at White Pine. Topography-driven flow associated with the Ottawan orogeny was discounted because it post-dates main-stage ore genesis and because recent seismic interpretations of basin inversion indicates that basin geometry would not be conductive to ore genesis. Density-driven flow systems did not produce focused discharge in the vicinity of the White Pine ore district.
Geochemistry of glacial sediments in the area of the Bend massive sulfide deposit, north-central Wisconsin
Geochemical exploration in northern Wisconsin has been problematic because of thick glacial overburden and complex stratigraphic record of glacial history. To assess till geochemical exploration in an area of thick glacial cover and complex stratigraphy samples of glacial materials were collected from cores from five rotasonic boreholes near a known massive sulfide deposit, the Bend deposit in north-central Wisconsin. Diamond drilling in the Bend area has defined a long, thin zone of mineralization at least partly intersected at the bedrock surface beneath 30-40 m of unconsolidated glacial sediments. The bedrock surface has remnant regolith and saprolite resulting from pre-Pleistocene weathering. Massive sulfide and mineralized rock collected from diamond drill core from the deposit contain high (10s to 10,000s ppm) concentrations of Ag, As, Au, Bi, Cu, Hg, Se, Te, and Tl. Geochemical properties of the glacial stratigraphic units helped clarify the sequence and source areas of several glacial ice advances preserved in the section. At least two till sheets are recognized. Over the zone of mineralization, saprolite and preglacial alluvial and lacustrine samples are preserved on the bedrock surface in a paleoriver valley. The overlying till sheet is a gray, silty carbonate till with a source hundreds of kilometers to the northwest of the study area. This gray till is overlain by red, sandy till with a source to the north in Proterozoic rocks of the Lake Superior area. The complex glacial stratigraphy confounds down-ice geochemical till exploration. The presence of remnant saprolite, preglacial sediment, and far-traveled carbonate till minimized glacial erosion of mineralized material. As a result, little evidence of down-ice glacial dispersion of lithologic or mineralogic indicators of Bend massive sulfide mineralization was found in the samples from the rotasonic cores. This study points out the importance of determining glacial stratigraphy and history, and identifying favorable lithologies required for geochemical exploration. Drift prospecting in Wisconsin and other areas near the outer limits of the Pleistocene ice sheets may not be unsuccessful, in part, because of complex stratigraphic sequences of multiple glaciations where deposition dominates over erosion. ?? 2004 Elsevier B.V. All rights reserved.
Some statistical relationships between stream sediment and soil geochemistry in northwestern Wisconsin - can stream sediment compositions be used to predict compositions of soils in glaciated terranes?
Mean stream sediment chemical compositions from northwestern Wisconsin in the north central United States, based on more than 800 samples, differ significantly from mean A-horizon and C-horizon soil compositions, based on about 380 samples of each horizon. Differences by a factor greater than 1.5 exist for some elements (Ca, Mn, Mg, P, Ti, Ni, Pb, Se, Zn). A very large database of stream sediment geochemistry exists for the region (more than 2200 samples) and for the U.S. (roughly 400,000 samples), whereas data on the chemistry of soils is much sparser both regionally and nationally. Therefore, we have attempted to quantify trends in compositional differences between stream sediments and nearby soils to test whether the abundant stream sediment data can be used to predict soil compositions. A simple computational technique of adjusting the stream sediment compositions according to the ratio of means of soils and stream sediments was conducted. A variety of techniques of correction and interpolation of data were tested and indicate that repetitive testing of results allows an optimum correction to be achieved. Predicted soil compositions compared to analytically determined soil compositions show a range of results from relatively good correspondence for some elements to rather poor correspondence for others. In general, predictions are best at midranges of compositions. The technique does not predict well more extreme or anomalous values. Thus, this technique appears to be useful for estimating background soil compositions and delineating regional compositional trends in soils in situations where large amounts of stream sediment analyses and smaller amounts of soil analyses are available. The technique also provides probabilistic qualifications on the expected error between predicted and actual soil compositions so that individual users can judge if the technique provides data of sufficient accuracy for specific needs. ?? 2003 Elsevier B.V. All rights reserved.