Faculty Advisers/Collaborators: J. Brian Mahoney and Bradford R. Burton
Geochemistry and Petrology of the Boulder Batholith, Southwestern Montana
The Boulder Batholith of western Montana is a major, north-northeast trending intrusive complex of Late Cretaceous (70-80 Ma) age. The Boulder Batholith is unique among similar age plutons in western North America due to its emplacement in the foreland of the Rocky Mountain fold and thrust belt and subsequent involvement in thin-skinned contractional deformation. The complex interaction between deep-seated, high-temperature magmatic processes and shallow-level, low temperature deformational processes is poorly understood. The origin and evolution of the Boulder Batholith is therefore important to our understanding of tectonic processes in continental arc systems.
Initial geochemical and petrographic analysis of the main phases of the Boulder Batholith was conducted to document petrologic variation that would reflect the crystallization history of the batholith. Twelve samples were collected from nine separate phases of the Boulder Batholith. Each sample was classified using hand sample/stained slab petrography, thin section analysis, and X-Ray Fluorescence Spectroscopy. Modal analysis demonstrates that the majority of samples fall within the monzogranite to quartz monzonite compositional fields. The samples show little compositional variation-modal analysis is insufficient to discriminate plutonic phases, however, they define a weak differentiation trend from dioritic to syenogranitic compositions, typical of an evolving calc-alkaline plutonic suite. Additional field and analytical work is required to confirm this apparent petrogenetic sequence.
The relatively small sample set makes large scale geochemical interpretations difficult. However, the samples form several distinct groupings in major and minor element space. The Boulder Batholith has been traditionally considered to comprise two distinct calc-alkaline suites-the main series and the sodic series (Hamilton and Myers, 1974). Our data show distinct geochemical groupings, including a main suite represented by the Butte quartz monzonite, a high-silica suite (the pluton of Donald, alaskite dikes and the quartz monzonite of Climax Gulch), and a low silica-low sodium suite represented by the Sagebrush Park stock and the Radarsburg granodiorite). These groupings seem to define a geochemical differentiation trend from low silica-to main series- to high silica compositions-a geochmical trend that is typical of a differentiating plutonic system. Significantly, our data indicate that the entire batholith is more calcic than previously described and suggest a greater component of magma-crust interaction during generation, ascent, and emplacement than previously thought.
Thomas J. Danielson
Faculty Adviser/Collaborator: J. Brian Mahoney
Geochemistry and Tectonic Setting of the Powell Creek Group, Southwestern British Columbia
The Powell Creek Group, of southwestern British Columbia, is a thick succession (-4000 m) of Albian to Cenomanian volcanic and volcaniclastic rocks, exposed along the eastern edge of the Insular superterrane, immediately adjacent to the boundary between the Insular and Intermontane superterranes. Paleomagnetic data require -2000 km of relative displacement between the Insular and Intermontane superterranes. Thick successions of Albian-Cenomanian volcanogenic conglomerate containing volcanic clasts lithologically similar to the Powell Creek Group occur east of the Group's main outcrop belt, along the western margin of the Intermontane superterrane. The purpose of this investigation is to analyze the geochemical characteristics of the volcanic rocks of the Powell Creek Group, and compare those characteristics with the geochemical signature of coeval volcanogenic conglomerates to the east. If the Powell Creek Group can be shown to be the source of the coeval volcanogenic conglomerates, then the superterrane boundary and any proposed translational Streams in the interior of the model domain are modeled as constant head boundaries or with the MODFLOW River Package. Input values for hydraulic conductivity, storativity, porosity, pup rates, and recharge values are obtained from published literature. Steady-state conditions achieve convergence with an error criterion of 0.01 feet Calibration is to nine groundwater observation wells and to the Eau Claire County water-table map with a mean absolute error of 0.72 feet. The calibrated model is most sensitive to higher pump rates and lower recharge.
The calibrated model is used to determine one-year, five-year, and ten-year steady-state capture zones and pathlines for the three municipal wells. The results of this study are presented to the City of Augusta to help the city complete a Well Head Protection Plan. The study was funded, in part, by the University of Wisconsin-Eau Claire, the City of Augusta, and the Groundwater Committee of Eau Claire County.
Lee Delcore, Heather Golding, and Brian Hennings
Faculty Advisers/Collaborators: Robert L. Hooper and J. Brian Mahoney
Student Experience in Lithoprobe Seismic Refraction Experiment
Lithoprobe is a Canadian multidisciplinary geoscience research project conducted to investigate the three dimensional structure of the North American Craton. Lithoprobe's primary goal is to reconstruct 4000 Ma of earth history. The purpose of this survey of northwestern Canada, referred to as Slave-Northern Cordillera Lithospheric Evolution (SNORCLE), is to investigate the Early Archean crust and the evolution of the craton, the process of ocean spreading of the Proterozoic-Paleozoic rifting margin, the growth of continents by terrane accretion, and the Phanerozoic evolution of the Cordilleran orogen. Seismic refraction methods were used to study the upper 100 to 250 km of the lithosphere of northern British Columbia, southern Yukon, and the Northwest Territories. Seismic refraction is the use of penetrating sound waves and vibrations generated by a point source explosion with analyses of seismic waves propagation providing a three-dimensional subsurface image. The deployment of three strategically placed transacts depended on the geology of the area and the accessible local roads. Researchers collaborated from Canadian universities, Geologic Society of Canada (GSC), provincial and territorial geologic surveys, and mining and petroleum industry to successfully plan, deploy, and interpret the project. U.S. collaborators were from the USGS (Menlo Park, CA), IRIS - PASSCAL Instrument Center, Stanford University, and the University of Wisconsin - Eau Claire. The University of British Columbia, University of Victoria, and the Geologic Survey of Canada currently are studying the seismic data. Preliminary analysis demonstrates strong shear-wave arrivals and good signal-to-noise ratios that will provide quality images of subsurface crustal structure.
Faculty Adviser/Collaborator: Lori Snyder and Brian Mahoney
Stratigraphy and Geochemistry of Eocene Endako Group Rocks at Four Localities in Central British Columbia
This study comprises a portion of the collaborative, multidisciplinary Nechako NATMAP Project being conducted by the Geological Survey of Canada (GSC), the British Columbia Geological Survey Branch (BCGSB), numerous universities and members of the mining industry.
Over two week period in the summer of 1997, four well-exposed sections of Eocene Endako Group mafic to intermediate lava flows with minor associated hyalociastite and tuffaceous sedimentary rocks in central British Columbia were studied. Rocks at these sites were measured and described in detail and systematically sampled for geochemical analysis at the UWEC Geology Department laboratory and for Ar-Ar age dating at a GSC laboratory. The rocks are aphyric to plagioclase and pyroxene, and rarely olivine phyric and are commonly amygdaloidal. At the Nautley River and Bungalow Lake/Mount localities, the basal contact between the Endako Group and underlying Ootsa Lake Group felsic volcanic rocks is exposed. The nature of the timing and contact relationships between these two compositionally distinct volcanic units is one focus of this study. The Kenney Dam locality provides the thickest, and least stratigraphically variable section of Endako Group mafic volcanic rocks.
This study has significantly revised geologic interpretations by documenting that Eocene volcanism in the Nechako NATMAP area is a continuum of felsic through mafic magmatism with only minor compositional or temporal breaks. Geochemical signatures of Endako Group rocks are consistent with an extensional plate tectonic environment-a result which lends support to structural, stratigraphic and paleomagnetic data that also suggest Eocene extension in Central British Columbia.
Steven P. Jenson
Faculty Advisers/Collaborators: Karen Havholm and Brian Mahoney
Sedimentology and Stratigraphy of the Lone Rock Formation in Trempealeau County, Wisconsin
The Lone Rock Foundation is a thin- to medium-bedded, silty to fine-grained marine glauconitic sandstone that represents the second transgression of two transgressive-regressive successions in the late Cambrian Hollandale Embayment in western Wisconsin. Previous investigations of the Lone Rock in Dunn Co. indicate three smaller (second-order) regressive events superimposed on the overall deepening trend of the Lone Rock Fm. (Paddock et al., 1997). In order to compare the nature of the Lone Rock Fm. in Trempealeau Co. with that of Dunn Co., sixty miles to the northwest, nine partial sections of the Lone Rock Formation were measured and the stratigraphy and sedimentology described in detail. From these partial sections, two composite sections of the Lone Rock Fm. were derived, one for the Whitehall area (central Trempealeau Co.) and one for the Arcadia area (Southwestern Trempealeau Co.). Lithofacies identified include microhummocky and hummocky cross-stratified sandstone, bioturbated, laminated sandstone, trough and chaotic trough cross-stratified sandstone. These facies are interpreted to represent deposition on the inner shelf ranging from near storm wave base to fair weather wave base, respectively. The stratigraphic distribution of these facies provides evidence for two minor (second-order) regressive events superimposed on the overall transgressive trend represented in the Lone Rock Fm. Three major laterally continuous intraclastic flat-pebble conglomerate horizons indicate three major storm events that occurred during relative sea-level high stands. These results agree with relative sea-level changes found in Dunn County.
Carrie Rowe and Lisa Sobczak
Faculty Advisers/Collaborators: Robert L. Hooper and J. Brian Mahoney
Transport Pathways of Heavy Metal Contamination in Fluvial Sediments, Coeur d'Alene River, Northern Idaho
The Coeur d'Alene River Valley in northern Idaho has been subject to sulfide contamination from mine tailings from the Bunker Hill Mining District for the past 109 years. Due to severe health effects to humans and wildlife from heavy metal contamination, the area was declared a Superfund site in 1986. The mechanics of heavy metal transport in fluvial environments are poorly understood, making remediation and hazard mitigation difficult. This study involves analysis of bedload, floodplain and wetland samples from a transect 25 miles downstream from the mining district. These samples were analyzed through geochemical sequential extraction and scanning electron microscopy.
Scanning electron microscopy was used to evaluate heavy metal distribution in samples collected from fluvial, floodplain and wetland environments in the Coeur d'Alene River Valley in the summer of 1997. Detrital grains are ubiquitously coated with Mn-Pb-Fe oxide coatings; detrital sulfide minerals are relatively rare. These coatings indicate sulfide breakdown occurs very early in the transport process, suggesting heavy metals are traveling as both bedload (oxide coatings) and as colloidal suspensions Significantly, SEM has identified a substantial biologic component acting to fix heavy metals in both the floodplain and wetlands environments. Various forms of bacteria have been identified authigenically fixing sulfide minerals (sphalerite (ZnS)) in both environments, indicating that these bacteria may play an important role in restricting heavy metal transport.
Sequential extraction geochemistry from two geographically separate levee drill cores demonstrate a distinct vertical partitioning of Ph and Zn contamination. Near-surface samples display a high concentration of contaminants in easily exchangeable ionic form, whereas samples below 30-40 cm contain Ph and Zn within bound Fe-Mn oxides. This vertical gradation suggests recent deposits contain easily mobilized Ph and Zn sediment derived from fluvial sedimentation during recent floods. These easily mobilized contaminants become progressively less mobile through time due to oxidation. In addition, samples from below 30-40 cm contain significant quantities of Zn in the residual fraction, suggesting authigenic sulfide precipitation may be fixing Zn below the surface. The two sample sites demonstrate lateral variability in heavy metal partitioning that may indicate downstream modification of transport mechanisms and resultant bioavailability. Further studies will clarify lateral variability in transport mechanisms.
Michael J. Schmidt
Faculty Adviser/Collaborator: J. Brian Mahoney
Geochemistry of the Spences Bridge Group, British Columbia
There are two conflicting theories about the tectonic evolution of the Southern Canadian Cordillera. The Canadian Cordillera is comprised of several tectonic terranes which are broadly grouped into larger assemblages called superterranes Paleomagnetic data from the Insular and Intermontane superterranes in British Columbia suggests significant northward translation prior to 50 Ma. Paleomagnetic data has been used to suggest northward displacement of ~3000 km for the Insular superterrane, and ~1100 km for the Intermontane superterrane. These values require a relative displacement between the Insular and Intermontane superterranes of up to 2000 km. However, geophysical and geochemical data collected by other researchers requires substantially less relative displacement, and is only permissive of ~500 km.
The Spences Bridge Group of the Intermontane superterrane is a subduction-related volcanic arc formed during the Mid-Cretaceous (ca. 100 Ma) period. Paleomagnetic data collected from the Spences Bridge Group suggests ~100 km of displacement. However, previous workers (Mahoney et. al. 1996) have documented a thick succession of Upper Cretaceous chert pebble conglomerate overlying volcanic rocks correlated to the Spences Bridge Group to the north of the main outcrop belt of the Spences Bridge Group. These chert pebble conglomerates are stratigraphically tied to rocks of the Insular superterrane, suggesting they have been transported ~3000 km. The depositional link between the volcanic rocks and overlying conglomerate indicates that volcanic rocks displaying paleomagnetic signatures requiring -1100 km northward translation (Spences Bridge Group) are unconformably overlain by chert-pebble conglomerates linked to rocks displaying ~3000 km displacement. Since displacement must have occurred between 90-50 Ma, the existence of a ~100 Ma stratigraphic link between the two superterranes is problematic.
Geochemical analyses of the volcanic rocks underlying the chert pebble conglomerate have been conducted in an attempt to link these volcanic rocks to the Spences Bridge Group, located approximately 100 km to the south. If the volcanic complex under the chert pebble conglomerate has the same geochemical signature as the Spences Bridge Group, large scale displacement between the superterranes is not permissible. Approximately 25 samples from the main outcrop belt of the Spence Bridge Group allow characterization of the geochemical signature of the Spence Bridge Group itself, and will provide a basis of comparison with rocks underlying the chert pebble conglomerate. The Spences Bridge Group is a medium K subalkalic, calc-alkaline andesite suite of volcanic rocks with a distinct subduction-related signature. The rocks display trace element abundances characterized by enrichment in large-ion-lithophile (LIL) element, and distinctly spiked elemental abundances indictative of volcanic arc magmatism. Major element oxides form distinct linear arrays on Harker variation diagrams, demonstrating a cogenetic origin of the entire rock suite.
Amy Jo Steffen
Faculty Advisers/Collaborators: Kent M. Syverson and Robert Hooper
Clay Mineralogy of Pre-Late Wisconsinan Till Units, Western Wisconsin
Several Pre-Late Wisconsinan lithostratigraphic units have been defined in western and central Wisconsin. These include older, calcareous, gray, fine-grained till units (Pierce and Marathon Fms.) overlain by younger, red, sandy till units (River Falls and Lincoln Fms.). Grain size, magnetic susceptibility, carbonate percentage, color, clay mineralogy, and depth of weathering have been used by other workers to identify till units. Different size-separation techniques, mounting methods, data reduction techniques, and mineral intensity factors have been used to study till clay mineralogy, so quantitative comparisons between published data sets are inappropriate. This pilot study seeks to determine if standardized methods for clay analysis improve the usefulness of clay mineralogy for till unit identification.
Valid clay mineralogy comparisons may be made for pre-Late Wisconsinan till units using internally consistent lab procedures to collect data. Pre-Late Wisconsinan till samples have been obtained from each type section and reference section, drilling during the summers of 1996 and 1997 in Chippewa County, and Robert Baker (UW-River Falls, collected in the early 1980s). Three to five samples from each red till unit are being analyzed for clay mineralogy using methods described by Moore and Reynolds (1989). Pre-treatments are being kept to a minimum and applied to all samples.
Clays from fifteen till samples have been separated, mounted, and scanned from three members of the River Falls and Lincoln Formations. Preliminary results indicated that the <1 um fraction reduces interference from quartz and provides better peak intensities than the <2 um size fraction, so the <1 um fraction was used for quantitative analysis.
Faculty Adviser/Collaborator: J. Brian Mahoney
Geochemical Distinction of Glaciofluvial Deposits of the Puget Lowland
The Puget Lowland of northwestern Washington represents a Tertiary basin filled with Quaternary glaciofluvial deposits derived from transgressing and regressing glacial lobes of both alpine and continental origin. Source regions for the early to late Pleistocene glacial and interglacial sediments in the Puget Lowland have been interpreted as alternating between the Cordilleran Ice Sheet from the north and alpine glacial lobes originating in the Cascade range to the east. Sediments from the Cordilleran Ice Sheet are characterized by gneiss, schist, quartzite and distinctive heavy metals from granitic and metamorphic terranes of British Columbia, whereas Cascadian sediments are typically derived from andesite, basalt, diorite, and granodiorite. The bimodal nature of sedimentation and the complexly interfingering stratigraphy make basin-wide correlation of stratigraphic units difficult. Sedimentologic examination of the glaciofluvial sediments and discrimination of different source areas may provide a detailed record of glacial and interglacial events within the Puget Lowland. Unfortunately, traditional analysis of grain composition is prohibitively time-consuming. Lithologic differences between the two proposed source regions should produce distinct geochemical signatures of sediments derived from each source. Geochemical analysis of the Pleistocene glaciofluvial strata may document a source region-dependent geochemical signature, and therefore provide an estimate of the relative contribution from each source area through time. Determination of the relative contribution from each source region through time will provide important constraints on the origin and evolution of ice sheets supplying detritus to the Puget Lowland during the Pleistocene.
Preliminary major element geochemistry indicates the samples fall into two distinct geochemical groups: 1) a low silica (< 62% SiO2) group with relatively high levels of Al, Ca, Ti, Mg, Na, and other majors element; and 2) a high silica (>65% SiO2) with corresponding low values of Al, Ca, Ti, Mg, Na and other major elements. Preliminary interpretations correlate the low silica group with calc-alkaline andesitic volcanics derived from the Cascade Range, and the high silica group with an igneous and metamorphic source derived from British Columbia. However, the sediment derived from an igneous and metamorphic source, particularly a garnet bearing source, should be higher in Al, Ca, and other major elements. Trace element analysis may clarify this inconsistency, but sediment mixing (i.e. a combination of both sources) must be considered as a possible explanation. The distinct geochemical signatures suggest that sediment geochemistry may be source terranes.
Faculty Advisers/Collaborators: Karen Havholm and Harry Jol
History of Holocene Dune Activity, Coastal North Carolina & Virginia
Northern North Carolina and southernmost Virginia coastal regions are composed of a Holocene barrier spit that hosts a number of eolian back-barrier dunes, both active and stable. Large, active back-barrier dunes in a humid coastal environment are an anomaly because they typically stabilize due to vegetation. Preservation of the unique ecosystems of these barrier islands in the face of rapid economic development requires an understanding of their history in order to determine which local and regional factors have influenced dune formation, migration and stabilization. Four active dunes distributed over 100 km of the coast were studied to determine their history. Subsurface sedimentary structures were delineated by ground penetrating radar (GPR) and were confirmed with trenches and augers. GPR revealed one phase of dune activity in the two northern dunes, Penny/Lewark Hill and Snow Hill. A buried soil is preserved within the two southern dunes, Jockeys Ridge and Run Hill, indicating a period of stabilization between two episodes of dune sedimentation. Therefore, at least one phase of dune activity is limited to the southern portion of the study area. Limited horizonation in these buried soils suggests only a few centuries of stability, but distinct reddening of the buried B-horizon in Jockeys Ridge suggests a longer stable period. Finely disseminated charcoal as well as quartz grains fused to a charred stump in the redder B-horizon suggest color intensity may be the result of soil baking (via lightning or other fire disturbance) rather than additional time. Each dune sediment package was sampled for near-infrared optically stimulated luminescence dating (IR-OSL); these dates are currently being processed. Radiocarbon dates reveal that trees rooted in the paleosols died in 1650 AD or later, indicating that the latest phase of dune reactivation occurred since European settlement.
Mae E. Willkom
Faculty Adviser/Collaborator: John R. Tinker, Jr.
Physical Groundwater Modeling of Proposed Crandon Mine Site in Forest County, Wisconsin
A three-dimensional groundwater flow model, MODFLOW, estimates potential hydrologic impacts of an underground zinc-copper mine proposed by Rio Algom, Ltd. in northeastern Wisconsin. The area contains numerous recreational lakes and streams and wetlands important for wild rice production by Wisconsin Indian tribes.
The four-layer model consists of Precambrian bedrock, saprolite, and two glacial units. Streams in the interior of the model domain are modeled using the MODFLOW River Package, and lakes are modeled using the MODFLOW General Head Package. Input values for hydraulic conductivity, specific storage, specific yield, porosity, and recharge values are obtained from consultant reports. Steady-state conditions achieve convergence with an error criterion of 0.01 feet. Calibration is to 97 existing wells with a mean absolute error of 3.41 feet. The calibrated model is most sensitive to hydraulic conductivities.
Dewatering the proposed mine to a depth of 1940 feet requires a pump rate of 1490 gpm. In addition, 290 acres and 580 acres of adjacent glacial deposits are dewatered in model layers 1 and 2, respectively. Changes in water budgets from pre- to post-pumping conditions appear small for Swamp Creek, located 4700 feet north of the ore body. Changes in the water budgets of Hemlock Creek, located 9000 feet cast of the ore body, may be significant during periods of low-flow conditions. The water budgets of Little Sand Lake, Deep Hole Lake, and Duck Lake, located 1200 to 6400 feet to the south-southeast of the ore body, do not appear to be substantially affected by pumping because of the relatively high lake bed conductance required for calibration. However, a groundwater divide located two miles west of the ore body is shifted approximately 1500 feet south of its pre-pumping location, thereby increasing the water budgets of Rice Lake and Mole Lake. These lakes constitute the western boundary of a model constructed by mining company consultants. Possible impacts shown here suggest that a boundary farther to the west of this area may be more appropriate for the consultants' model.