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Student Research 2000

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Jody Brandrup

Faculty Advisor/Collaborator: J. Brian Mahoney

Geochemical Analysis of Glaciofluvial Sedimentation, Puget Lowland, Washington: Constraints on Episodic Provenance Shifts

Quaternary sediments in the Puget Lowland comprise a complex succession of intercalated glacial, glaciofluvial, and glaciomarine sediments derived from two distinct source regions. Sedimentation patterns were alternately dominated by 1) the Puget Lobe of the Cordilleran Ice Sheet, which provided detritus rich in metamorphic and plutonic debris from southern British Columbia, and 2) smaller alpine glaciers to the east, which provided detritus dominated by volcanic detritus from Tertiary to Recent volcanism in the Cascade Range. Each source region provided sediment with distinctive petrologic characteristics and heavy mineral composition, but the bimodal nature of sediment supply, complex interfingering and diachronous sedimentation make basin wide correlation and comprehensive basin analysis difficult. Geochemical analysis of fine-grained glaciofluvial sediments provides a rapid, semi-quantitative method of discriminating lateral and vertical variations in provenance, and, to a lesser degree, identifying episodic sediment mixing and/or recycling. Major and trace element geochemical analyses of sediments from throughout the Puget Lowland indicate that samples comprise two distinct geochemical groups: 1) low silica (< 65% SiO2) with relatively high levels of Al, Ti, Fe, K, and 2) high silica (>70% SiO2) with corresponding low values of Al, Ti, Fe, and K. The most simplistic interpretations suggests that the low silica group corresponds to sediments derived from the Cascade volcanic carapace, whereas the high silica group represents sediment derived from the metamorphic/plutonic complexes in British Columbia. However, differences between major element values between proposed source rocks and the sediments, and anomalously high concentrations of some trace elements (Nb, Th, Y, V, Zr) in the low silica group suggests sediment composition is not controlled entirely by bedrock composition. We suggest that sediment composition is a function of both the composition of the source area and the intensity and duration of weathering in the source area prior to transport. Verification of this assumption is underway, and constraining the primary controls on the ultimate sediment geochemical signature will provide a powerful tool for analysis of Quaternary basin evolution in the Puget Lowland.

Lauren Buchholz and Sarah Tietz
Faculty Advisor/Collaborator: Bradford R. Burton

Computer GIS-based Geologic Map of the Harrison Pass and Franklin Lake 7.5 Minute Quadrangles, Nevada

The spatial distribution and structural attitude of geologic units are the fundamental data used in geological interpretation. Accurate field mapping is vital to the scientific method in the geosciences. Recent advances in computer-based Geographic Information Systems are revolutionizing traditional mapping methods. Most government agencies and private publishers now require digital map submissions. This project tested the application of digital mapping methods to a complex geological terrane previously mapped by traditional methods. The Harrison Pass and Franklin Lake Southwest 7.5-Minute Quadrangles in northeast Nevada expose upper-crustal plutonic and metamorphic rocks in the footwall of a classic Cordilleran metamorphic core complex. Mylar base maps prepared following traditional US Geological Survey publication procedures were digitized using ArcView¨ GIS to produce digital geologic maps. The study shows that digital mapping methods produce a higher-quality map product than traditional drafting methods, and digital maps can be updated, as new data become available. However, the application of digital mapping methods to complex geological terranes is not yet optimized. Digital mapping methods are initially more time consuming and the computer-interface limits the geologist's ability to devise new map symbols to represent phenomena not anticipated by the software developers. The final maps will be published digitally by the Nevada Bureau of Mines and Geology as colored geologic quadrangles.

Justin Humenik
Faculty Advisor/Collaborator: Bradford Burton

Computer -based Mapping of Geochemical Data for Mineral Deposit Exploration, Northern Mongolia

Geochemical analysis of soil samples over an extensive grid sampling area is a proven exploration method for base and precious metals. We present new data from more than 3000 soil samples collected in two remote study areas in the Khentii Prefecture, Mongolia. The data were collected and provided for analysis by Uranerz U.S.A., Inc. during a 1998 exploration program. Computer mapping and spreadsheet analysis, coupled with geological interpretation of mapped anomalies facilitated analysis of the large data set. The data were analyzed for indicator elements Au and Ag as well as common pathfinder elements As, Sb, Zn, Pb, Hg, Cu, Bi, and Te. The computer-mapping platform allows for rapid analysis of combinations of indicator and pathfinder elements, which predict the location of two ore deposits within the study areas. In the Namarjaa Uul area, coincidental anomalies of Cu, Zn, Hg, and Bi may indicate the presence of a copper porphyry deposit in the northeast part of the sample grid. Coincidental anomalies of Cu, Zn, Ag, and Pb indicate a possible kidney shaped copper porphyry deposit in the northeast portion of the Bayan Undur Uul area. The data do not indicate the presence of massive disseminated gold deposits, which were the focus of the exploration program.

Joel D. Hyzer
Faculty Advisor/Collaborator: Kent Syverson

Glacial Geology of the Huron and Colburn 7.5' Quadrangles, East-Central Chippewa County, Wisconsin

A 132-sq-km region on the Huron and Colburn 7.5' Quads was mapped during summer 1999 to determine the surficial glacial geology. Well logs, aerial photographs, the county soil survey, and topographic maps were used for two weeks to construct a preliminary surficial geologic map. Then six weeks were spent in the field describing sediment outcrops, boring with a hand auger, and drilling to depths of 20 m in three locations with the Wisconsin Geological and Natural History Survey drill rig. Most of the study area is located within the Chippewa Moraine, a Late Wisconsinan ice-disintegration moraine deposited by the Chippewa Lobe approximately 20,000-15,000 yrs ago. The hummocky Chippewa Moraine displays 4-12 m of relief and is underlain by diamicton of the Copper Falls Fm. The northwestern part of the map area displays a gently rolling surface with 2-4 m of relief underlain by Copper Falls Fm. diamicton. Four ice-walled-lake plains are present in the northern half of map area. Low areas in the southernmost part of the map area are underlain by proglacial lake sediment containing laminated sandy silt up to 10 m thick. During the earliest part of the Late Wisconsinan Glaciation, the Chippewa Lobe flowed from the north to an area 1 km north of Stanley, WI. This event deposited a small, ice-cored moraine, stabilized, and constructed the Chippewa Moraine. Southerly melt water drainage was dammed by the earlier moraine near Stanley, and an extensive proglacial lake formed.

April D. Johnson
Faculty Advisor/Collaborator: J. Brian Mahoney

Testing Large Scale Terrane Translation: Geochemical Provenance Analysis of the Cretaceous Jackass Mountain Group, Methow Terrane, southern British Columbia

The southern Canadian Cordillera is composed of a number of allochthonous terranes assembled on the western edge of North America during Jurassic to Tertiary time. These terranes can be broadly grouped into larger superterranes based on the timing of terrane linkages. The Intermontane superterrane consists of smaller terranes amalgamated to North America by middle Jurassic time (ca. 180 Ma); the Insular superterrane to the west consists of terranes amalgamated by mid-Cretaceous time (ca. 100 Ma). One of the most significant tectonic problems in the Cordillera relates to the amount of relative displacement between the two superterranes. Paleomagnetic data suggests up to 2000 km relative displacement between the superterranes between the late Cretaceous and Eocene time (80-55 Ma). Conversely, geologic data suggests that the Insular and Intermontane superterranes were largely amalgamated by mid-Cretaceous time and subjected to less than 1000 km margin-parallel translation since that time. This investigation seeks to constrain the amount of displacement between the superterranes in southern British Columbia. Albian-Cenomanian (100-85 Ma) volcanic and chert-rich conglomerate of the Jackass Mountain Group occurs on the Methow terrane, which is firmly linked to the Insular superterrane by 90 Ma. The coarse-grained nature of these deposits indicates a limited transport distance, and geologic constraints suggest the clasts were derived from the Intermontane superterrane to the east. However, paleomagnetic data requires that the two superterranes were more than 2000 km apart during this time. If geochemical analysis of conglomerate clasts of the Jackass Mountain Group allows them to be tied to plutons of the Intermontane superterrane, then large-scale translation between the terranes is not allowable.

Detailed examination of the conglomerate clast geochemistry is underway. Preliminary data suggests that the majority of clasts were derived from a single source. Trace element patterns suggests a calc-alkaline destructive plate margin source, similar to plutons on the Intermontane superterrane. However, definitive linkage of conglomerate clasts to their plutonic source will require multiple geochemical ties.

Melissa Klinger
Faculty Advisors/Collaborators: J. Brian Mahoney and David L. Kimbrough (SDSU)

Geochemical Provenance Analysis of Conglomerate Clasts from Valle Group Cretaceous Forearc Basin Strata, Baja California

The Cretaceous Valle Group (VG) is widely exposed across the Vizcaino Peninsula of Baja California and constitutes an important record of forearc basin tectonics and sedimentation associated with Mesozoic subduction beneath western North America. The presumed source of VG strata is the adjacent Peninsular Ranges batholith (PRB); a presumption based mostly on evidence from sandstone petrography, conglomerate clast compositions, and paleocurrent data. At its present erosional level, the PRB is divided into distinct western and eastern zones based on petrologic as well as age and geophysical parameters. The precise relationship between the unroofing of the arc and basin evolution is unclear. This investigation seeks to correlate lateral and vertical variations in conglomerate clast lithology and geochemistry with progressive uplift of the arc sequence in order to develop a comprehensive model of basin evolution.

Whole rock major and trace element X-ray fluorescence analyses are reported here from 38 representative plutonic VG conglomerate clasts as part of an effort to more critically test provenance models for VG strata. Major element chemistry indicates clasts may be subdivided into two rough groupings, including one with intermediate SiO2 (~60-68%), with corresponding high values of Ti, Mg, Ca, Al, P, V (Group I) and one with higher SiO2 values (>70%), with lower values of these elements (Group II). Segregation is also evident among trace elements. In particular, distinct high Sr (Group I) and low Sr (Group II) clast suites are distinguishable. The bimodal distribution of clast geochemistry corresponds to the distinct east-west geochemical zonation of the arc assemblage. High Sr values (Group I) correspond to the eastern arc assemblage, low Sr values (Group II) correspond to the western arc assemblage.

Joshua D. Kohn and Karl Beaster
Faculty Advisor/Collaborator: Karen Havholm

Wind or Water? Paleoenvironment of the Proterozoic Hinckley Sandstone, Northeast Minnesota

The Proterozoic Hinckley Sandstone is a fine-grained, mineralogically mature sandstone representing late stage Keweenawan rift fill sedimentation. Previous work (Tryhorn & Ojakangas, 1972) interpreted the Hinckley Sandstone as a shallow water lake deposit. Recent recognition of adhesion structures suggest it may be partially eolian (wind-deposited). The purpose of this project was to reevaluate the environment(s) of deposition of the Hinckley Sandstone. The study area is located in Pine County, MN where five sections were measured and described in detail along the Kettle River. Four major facies were identified: cross-stratified sandstone (CSS), trough cross-stratified sandstone (TCS), pebbly trough cross-stratified sandstone (PTS), and planar bedded sandstone (PS). Fine grain size, large set size, and details of stratification style indicate an eolian origin for CSS facies. Coarser grain size and characteristics of trough strata indicate a subaqueous origin for TCS and PTS facies. Planar beds (PS) include complexly interbedded crinkly and mottled (adhesion) strata, rippled (subaqueous) laminae, and pin-striped (wind-ripple) laminae, indicating a changing environment. Overall, Hinckley strata record a depositional environment that alternated between wet (subaqueous) and dry (eolian) conditions. Correlation between measured sections was difficult because Hinckley Sandstone exposures are incomplete and widely separated. This limits regional paleoenvironmental interpretation.

Bjorn Lysne
Faculty Advisor/Collaborator: J. Brian Mahoney

Provenance of the Rosario Formation and late Cretaceous basin evolution in San Diego, CA

The Rosario Formation is a 120m thick succession of late Cretaceous coarse grained clast-supported conglomerate and fine-grained sandstone. It is discontinuously exposed along the Pacific coast of Southern California and Northern Baja California. These strata were deposited along the eastern side of the Peninsula Range Batholith (PRB) as turbidites into a submarine fan-valley-levee. A suite of 45 conglomerate clast samples was collected throughout the vertical and lateral extent of the Rosario Formation in San Diego. The vertical and lateral variation in conglomerate clast lithology and geochemistry will be documented in order to evaluate basin evolution and source area uplift through time. Major and minor element concentrations of the samples will be assessed to document the geochemical character and variability of the source area. The results will then be compared to the existing geochemical data of the PRB to determine individual source characteristics. Correlation of distinct groupings in conglomerate clasts with specific phases of the batholith will constrain the tectonic evolution of the region in the late Cretaceous. Preliminary trace element analysis suggests the presence of three distinct geochemical subpopulations, based on variations in Zr, Nb, Y, V and other trace elements, that may correlate with east-west variations in arc geochemistry.

Tyler W. Mace and Tim R. Cummings
Faculty Advisor/Collaborator: Kent M. Syverson

Computer Database to Analyze Glacial Till Samples in Western Wisconsin

Many county-scale glacial geology mapping projects have been conducted in western Wisconsin during the past 25 years, most samples having been analyzed at the UW-Madison Quaternary Geology Lab. We have collected till data from previous workers and analyzed approximately 200 new till samples from recent drilling in Chippewa County, Wisconsin. We have developed a database to organize the data in a standard format and conduct data searches by lithostratigraphic unit and sediment type. A user submits spreadsheet data files via a web interface form. Once submitted, a Perl script interprets the spreadsheet and adds the data to the database. Once information is in the database, a user accesses the database through a web interface, conducts searches using lithostratigraphic formation, member, sediment type and/or county key words, and generates a spreadsheet file with the results of the search. In addition, the user can connect to the database with a GIS program such as ArcView. This will allow a user to study how till properties change spatially over a region not limited by county boundaries. This database to more completely characterize the physical properties of till units is part of Syverson's glacial stratigraphy research project in western Wisconsin. This continuing study is funded by the Wisconsin Geological and Natural History Survey and the Chippewa County Land Conservation Dept.

Jean M. Morrison and Carrie E. Rowe
Faculty Advisors/Collaborators: Robert L. Hooper and J. Brian Mahoney

Sequential Extraction Applied to Heavily Contaminated Lead and Zinc Mine Tailings: What Really Happens During Sequential Extraction?

Sequential extraction is a step-by-step chemical procedure designed to mimic environmental conditions that may result in mobilization of heavy metals. Previous studies have shown that results are extremely dependant on laboratory technique. Sediments heavily contaminated with up to 10,000ppm lead and zinc were subjected to two different Sequential Extraction (SE) procedures to compare the results and effectiveness of metal removal. Samples from three distinctive redox environments were extracted using conventional (Tessier) and microwave procedures that involve sequentially removing metals held in exchangeable, carbonate, oxide, sulfide/organic matter, and residual fractions. Solution extracts were analyzed using Atomic Absorption and samples from each extraction step were examined using SEM and x-ray spectroscopy to determine the impact of the treatments on the metals. Conventional and microwave techniques yield parallel results for the exchangeable, organic/sulfide, and residual fractions but provide different speciation predictions for both the carbonate and oxide fractions. The conventional SE procedure is less effective at removing carbonates and more effective at removing fine-grained oxides than the microwave method. SEM analysis indicates that in both extraction methods, Fe-Mg and Zn carbonates survive essentially intact after the carbonate step. Some small-cation detrital (Fe,Mg) carbonates even survive the sulfide/organic extraction step (pH 2.0) and therefore account for a significant increase in metals recorded from the residual step. In reduced samples with abundant microcrystalline authigenic sulfide there is a tendency for premature extraction of Pb and Zn beginning in the carbonate fraction. A few larger detrital Fe and Zn-sulfides persist through the organic/sulfide step, thereby releasing metals in the residual fraction.

Carrie Rowe
Faculty Advisors/Collaborators: J. Brian Mahoney and Peter Mustard

Clastic Dykes as Paleoslope Indicators in the Nanaimo Group, Hornby Island, British Columbia

The Upper Cretaceous (91-71 Ma) Nanaimo group on the western edge of the Canadian Cordillera is composed of eleven main stratigraphic units. Hornby and Denman Islands are the northernmost of British Columbia's Gulf Islands and provide excellent exposure of the upper two-thirds of the Nanaimo Group. Comprising more than two kilometers of clastic sedimentary strata, the Nanaimo Group on these islands consists of thick bedded conglomerate and sandstone dominated formations alternating with mudstone and thin-bedded turbidite sequences. The Northumberland Formation, a mudstone unit with thin sand interbeds, is exposed on the western side of Hornby Island. Near the top of this unit, on the southeastern side of Hornby, are numerous clastic sandstone dykes and synsedimentary folds. These dykes and sedimentary folds are formed from a partially lithified unit cracking and pulling apart as it slides down a slope. Orientations of these structures are good indicators of paleoslope direction. The synsedimentary fold axes trend shallowly to the northwest (19/336, n=27). The axial planes of the synsedimentary folds strike northwest and dip northeast (334/34, n=27). Fold axes and axial plane orientations suggest paleoslope is to the southwest. Dyke orientations, which are nearly vertical but have a general northeast strike (035/86, n=60), form perpendicular to the paleoslope suggesting that the slumping direction is to the northwest/southeast. This contrasting data indicates that either the use of axial planes or fold axes are unreliable in the interpretation of paleoslope direction. Comprehension of the formation of clastic dykes as paleoslope indicators will aid in constraining the geography and geometry of formation in the Nanaimo basin.

Carrie Rowe and Jean Morrison
Faculty Advisor/Collaborator: J. Brian Mahoney and Robert Hooper

Heavy Metal Partitioning and Transport in the Coeur d'Alene River Valley, Coeur d'Alene Idaho

The lower Coeur d'Alene River Valley has been widely contaminated with heavy metals from Ag-Pb-Zn sulfide mine tailings. Contaminated mine tailings rapidly remobilize by surficial processes, primarily during flood events, and are deposited in subaerially exposed over bank levees and in adjacent wetlands. Lead and zinc levels exceed 10,000 ppm in the upper few meters of the lower river valley. Successful remediation in CDA and other sulfide mine districts requires understanding metal mobility and speciation processes during transport and remobilization. The river system is composed of three subenvironments: bedload, levee, and wetland. Oxidation/reduction conditions vary dramatically between the different subenvironments leading to a complicated system of metal migration and precipitation. The physical and chemical properties of the detrital grains are assessed by Scanning Electron Microscopy and x-ray spectroscopy. River bedload is a recurrent source of heavy metals during flood events. Flood deposit lead is primarily found as carbonates, sulfides, or coatings on detrital grains. Zinc is transported primarily as sulfides, carbonates or as Fe-Mn oxide coatings. In subaerial oxidizing environments such as levee tops, Zn-Pb-Mn-Fe oxy-hydroxides and Pb-Zn-Fe carbonates are the dominate heavy metal particulates. Fine grained, subsurface over bank levee deposits can represent transitional redox environments where metals occur in a wide variety of phases including carbonates, sulfides and coatings. In organic-rich anoxic environments, such as marshes and lake sediments, Pb and Zn tend to concentrate in colloidal, bacterially-precipitated sulfides, which are readily bioavailable. SEM analysis indicates that microorganisms play a critical role in fixation of heavy metals especially in transitional redox and reducing environments.

Jared Schmidt and Sarah Tietz
Faculty Advisor/Collaborator: Bradford R. Burton

Structural Analysis of the Vinegar Fault Zone, Northcentral Washington

The Vinegar Fault in north-central Washington separates crystalline rocks of the Intermontane Belt, to the east, from sedimentary rocks of the Methow block, to the west. It is therefore interpreted as the southern segment of the regionally extensive Pasayten Fault Zone. Based on paleomagnetic pole reconstruction, previous authors maintained that the Pasayten Fault Zone accommodated large-magnitude translation of the Methow block, displacing it northward more than 10 degrees latitude along the North American continental margin during the Late Cretaceous. This hypothesis was tested by examining field and microstructural evidence of the kinematic history of the Vinegar Fault. Structural analysis of field data and microscopic textural analysis of samples from the fault zone show dip-slip displacement and do not support strike-slip movement on this fault. The fault evolved from early-stage greenschist-grade mylonitic and protomylonitic shear, to later-stage brittle mechanical behavior in response to fault exhumation and cooling. Structural fabrics consistent with large-magnitude strike-slip translation of the Methow Block relative to the Intermontane Belt and North America were not identified in this study. These observations support the hypothesis that the Vinegar/Pasayten faults are responsible for vertical exhumation of the Coast Belt.

Michael J. Schmidt
Faculty Advisors/Collaborators: J. Brian Mahoney

Comparative Geochemistry of the Spences Bridge Group and Coeval Volcanic Rocks: Potential Constraints on Large Scale Translation

The Canadian Cordillera is a complex melange of allochthonous terranes that are broadly grouped into the Insular and Intermontane superterranes. Controversy surrounds the timing and mechanism of superterrane juxtaposition. Paleomagnetic data indicates ~1100 +/- 500 km of northward translation of the Intermontane superterrane between 90-45 Ma and 3000 +/- 600 km northward translation of the Insular superterrane during the same period. Geologic constraints, including stratigraphic, structural and paleontologic data, suggest less than 500 km of relative displacement. The Spences Bridge Group, on the western edge of the Intermontane Superterrane, is ~104 Ma volcanic arc. The northwest edge of the Spences Bridge Group outcrop belt is truncated by the dextral Fraser fault system. On the west side of the fault, approximately 115 km to the northwest, an unnamed package of Albian volcanic rocks (104.1 0.3 Ma) lithologically identical to the Spences Bridge Group is unconformably overlain by the late Albian-Santonian Silverquick/Powell Creek succession, which is structurally and paleomagnetically linked to the Insular Superterrane. Paleomagnetic arguments therefore require the presence of a ~2000 km displacement transcurrent fault between the Spences Bridge Group and the unnamed Albian volcanic rocks. The purpose of this investigation is to geochemically compare the unnamed Albian volcanic rocks with the Spences Bridge Group to test potential linkages between the two. Both volcanic successions display a subalkalic calc-alkaline geochemical signature, indicative of a destructive margin plate setting. Major element data define colinear trends in both successions, suggesting a similar magmatic evolution. Incompatible trace element data display a significant degree of overlap between the two units. Rare earth element data from both units display colinear trends with similar La/Yb ratios. Based on these geochemical similarities as well as lithologic, structural, and geochronologic data we feel the Albian volcanic rocks are a tectonically translated fragment of the Spences Bridge Group arc.

Michael J. Schmidt and Isaac J. Vandergon
Faculty Advisors/Collaborators: Lori D. Snyder and J. Brian Mahoney

Structural and Geochemical Characterization of Tertiary (?) Mafic Dikes in the Coast Belt and Western Bowser Basin, Nass River Area, British Columbia

The southern part of the Nass River area is predominantly underlain by Jurassic and Cretaceous sedimentary rocks of the Bowser Lake Group and Tertiary plutonic rocks of the Coast Belt. Abundant mafic dikes cut both of these units and are the focus of this study. During the summer of 1999, approximately 25 of these dikes were systematically described, measured and sampled for geochemical analysis. Distribution of mafic dikes varies throughout the area with the greatest abundance to the west and decreasing eastward. Most commonly, dikes occur as single bodies less than 1meter wide although some are 2-6 meters; chilled margins are commonly observed. Thinner dikes are dark gray to green and aphanitic while thicker dikes often contain phenocrysts of plagioclase feldspar and minor pyroxene or pyrite. Locally in the west, dikes occur as swarms with cumulative thickness of up to 100 meters. Dikes show a consistent northeast to southwest orientation across the area and are steeply dipping. This pattern appears to be controlled by a preexisting fracture pattern in the plutonic rocks and bedding or cleavage in the adjacent sedimentary rocks. Preliminary geochemical analyses indicate that the mafic dikes are rich in Sr and appear to fall into at least two distinct groups. Ongoing analyses will be used to further investigate the characteristics of the source region(s), tectonic affinity and diversity within the dikes.

Katie Thornburg
Faculty Advisors/Collaborators: Robert L. Hooper and Kent M. Syverson

Clay Minerology of Pre-Late Wisconsinan Till, Western Wisconsin

Semi-quantitative clay mineralogy of the < 1m fraction from pre-Late Wisconsinan till is being examined to determine if clay mineralogy can be used to differentiate the various till units in western Wisconsin. Clay minerals being quantified include illite (I), kaolinite (K), vermiculite (V), smectite (S), and mixed layer illite/smectite (I/S). Sixty-nine basal till samples have been processed thus far. The Pierce Fm. (n=13) typically displays narrow, symmetrical smectite peaks, significant kaolinite, and subordinate illite vermiculite. The Wausau (n=4) and Medford (n=2) Members of the Marathon Fm. have broad, irrational x-ray diffraction patterns typical of I/S, variable but significant vermiculite, and subordinate kaolinite. Clay mineralogy of samples assigned to the Edgar Member of the Marathon Fm. (n=3) show greater variability than the Wausau and Medford Members. Initial analyses of the River Falls Fm. (n=12) indicate this till is dominantly illite (29.4%7.8), vermiculite (24.3%6.1), and I/S (< 10%I) (31.5%10.4), with sparse kaolinite (14.8%6.1). The Lincoln Fm. contains substantial vermiculite, I/S, illite, and sparse kaolinite. Illite seems to increase toward the bottom of the formation. The Late Wisconsinan Copper Falls Fm. (n=4) contains subequal illite, smectite, vermiculite and kaolinite. Semiquantitative analyses are currently being performed. Initial qualitative results indicate that a more rigorous statistical analysis of deconvoluted, integrated peak intensities for both pure minerals and mixed layer components will prove to be a reliable method for differentiating the till units.

Sarah Tietz and Jared Schmidt
Faculty Advisor/Collaborator: Bradford R. Burton

Structural and tectonic history of the Pasayten Fault Zone, Northcentral Washington

The Okanogan Range batholith (ORB) is a major plutonic complex exposed along the western edge of the Intermontane Belt in north central Washington and southern British Columbia. The Pasayten fault zone (PFZ) separates the ORB from rocks of the Cretaceous Methow Basin to the west. Previous studies showed that the ORB was a major sediment source for the evolving Methow Basin, and document the kinematic history of the PFZ from ~113 to >80 Ma, but the structural history of the PFZ was not known from 80 Ma to 60 Ma. Other workers have proposed that during this time, the Methow Basin was translated ~1700 km northward along the western margin of North America. We present twelve new apatite fission track ages that document the thermal/structural history of the ORB during this controversial time. The ORB was uplifted as much as 6 km since the Late Cretaceous (69.6 Ma (+/-3.8 Ma). Cooling of the ORB does not reflect simple block-uplift adjacent to the Pasayten Fault Zone. The data suggest that previously unidentified faults are present within the ORB. Limited data also show that uplift/exhumation of the adjacent Methow Basin may not have occurred until the Late Eocene. The data place limits on the time available for large-magnitude terrain translation to have occurred, and challenge the Baja-BC hypothesis.

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