Browsing by Subject "Randy Seeling Award"

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A Comparison of Two Archean Ultramafic Pyroclastic Rock Units from the Superior Province, Northwestern, Ontario
(1989-12) Schaefer, Stephen Jon
Two Archean komatiitic pyroclastic rock units occur on opposite sides of the Quetico Fault in northwestern Ontario. The eastern unit, the Dismal Ashrock is located 3 km north of Atikokan, Ontario on the northern side of the Quetico fault within the Wabigoon Subprovince of the Superior Province. It is part of a supracrustal sequence (the Steep Rock Group) that overlies an Archean unconformity. The Grassy Portage Bay Ultramafic pyroclastic rock unit (GUP) is located 100 km to the west on the south side of the Quetico fault, and is part of an overturned succession comprising mafic metavolcanic rocks, GUP and metasedimentary rocks. The Quetico Fault is a dextral fault with a history of transpressive tectonics. The Dismal Ashrock is steeply inclined, little deformed and has undergone greenschist facies metamorphism, and it is divided into komatiitic lapilli tuff, komatiitic volcanic breccia, komatiitic volcaniclastic rocks and a mafic pillowed flow. The GUP outcrops form an arcuate fold interference pattern. The GUP is strongly deformed and has undergone amphibolite facies metamorphism. It is divided into komatiitic lapilli tuff and komatiitic volcanic breccia. The Dismal Ashrock and the GUP contain cored and composite lapilli - unequivocal evidence for explosive volcanism. Locally some of the lapilli fragments are highly vesicular (up to 30% by volume) - greater than reported for any other komatiites. Other fragments show no vesicularity. The low vesicularity of some of the pyroclasts and association with pillowed lava flows in the case of the Dismal Ashrock indicate phreatomagmatic volcanic activity. Explosive water-magma interaction was probably initiated by modified eruption characteristics produced from exsolving volatiles. The Dismal Ashrock and GUP are similar in chemical composition and plot on the border between peridotitic and basaltic komatiites on a Jensen AFM diagram. They are high in MgO, Cr and Ni; however, they are unusually enriched in Fe, Ti, Zr, Mn, P, Ba, Nb, Rb and Sr compared to other komatiites. Several lines of evidence indicate that this unusual composition could not have been caused by alteration or assimilation, and an enriched mantle source region is the likely cause. Many of the characteristics that Dismal Ashrock and GUP share are rare or unique on a global scale, indicating that the Dismal Ashrock and GUP are correlative in some manner.
Fluvial Evolution between the Salt Wash and Brushy Basin Members of the Upper Jurassic Morrison Formation, South-Central Utah
(2007-06) Ali-Adeeb, Riyad Abdulrahim
Abrupt changes in fluvial deposition between the Salt Wash.Member and Brushy Basin Member of the Upper Jurassic Morrison Formation, in western interior North America, have been documented across a seemingly basin-wide depositional unconformity that resulted from poorly understood base level changes. Detailed field observations, paleopedologic analysis, and petrographic analysis of proximal deposits in the Henry Basin in south-central Utah, yield no depositional hiatus in the proximal basin during formation of the unconformity in the distal basin. Rather, the proximal stratal architecture suggest continuous deposition between the two members that likely resulted from a combination ofrelatively high basin subsidence rates and an increase in gravel fraction in the sediment source. Computed subsidence rates in the southern Henry Basin from recent radiometric dates document a time-averaged subsidence rate of 0.121 mm per year, while petrographic evidence suggests an up-section increase in chert-rich sediment source. Collectively these resulted in starving the distal basin of sediment and the formation of a distal depositional hiatus, while the proximal basin accumulated prograding sediments. A south-to-north transect near the western margin of the Morrison Basin documents a rapid pinch out of 150 meters of fluvial gravelly-sandstones of the Salt Wash Member between the southern Henry Basin to the south and the Emery High region 75 miles to the north. Mature paleosols at the base of the Morrison Formation in the north suggest that the lack of Salt Wash Member deposits were due to non-deposition rather than to post depositional erosion or incision. This suggests that syndepositional basin subsidence in the south channeled deposits there, while regions to the north underwent little to subsidence and deposition. The disparity in basin subsidence rates between closely spaced regions in proximal regions, combined with petrographic evidence of increased gravel-fraction in the sediment, suggests that tectonic thrusting to the west may have contributed an important role in producing continuous aggradation of sediment in the proximal basin, while contemporaneously starving the distal basin and forming a depositional unconformity. Incised valley-fill conglomerates at the contact between the two members also suggest that the change in sedimentation was influenced by a drop in base level that followed deposition of the Salt Wash Member.
High Grade Metamorphism and Partial Melting in the Bluegrass Creek Suite, Central Laramie Mountains, Wyoming
(1990-03) Spicuzza, Michael Joseph
The Bluegrass Creek Suite (BCS) comprises Archean metasupracrustal rocks which suffered at least two periods of high grade metamorphism during the Precambrian. The first, a regional amphibolite grade metamorphism (6.0 kb, 660°C; Grant & Frost, 1986) at approximately 2.6 BY significantly dehydrated the BCS. The latest event, at 1.4 BY, a high grade contact metamorphism related to the protracted intrusion of the Laramie Anorthosite Complex (LAC) at approximately 3 kb, resulted in high temperatures (locally >> 700°C) and partial melting of metapelites within 2 km of the LAC intrusive contact. Contact metamorphic mineral assemblages in the BCS record both increasing temperatures and decreasing X(H2O) towards the LAC contact. Five isograds determined using metapelites are located within 3 km of the contact whereas one mapped isograd is based on metacarbonate equilibria. An isobaric T-X(H2O) diagram (Grant, 1985) shows BCS metapelitic equilibria could exist nearly isothermally at sub-solidus temperatures at varying X(H20). However, textural evidence of partial melting in metapelitic compositions increases toward the intrusive contact. Leucosomes, restites, lathe-shaped plagioclase, orthopyroxene overgrowths, euhedral cordierite and quartz, bimodal biotite grain size and fabric, and oikocrystic K-feldspar and quartz are all consistent with the presence of melt. Phase equilibria and textural evidence (especially inclusion relations and igneous morphology) are consistent with melt-forming reactions emanating from a single isobarically invariant point. Two schematic isobaric liquidus diagrams are presented, one for X(H2O) lower and one for X(H2o) higher than the value for the invariant point. In the Bluegrass Creek suite both the temperature and the composition of the coexisting- vapor phase are important variables for melting relations. Metapelitic compositions melting at different X(H2O) results in different melting relations. These relations provide important clues to the process of anatexis at moderate crustal levels. Although garnet-bearing restites formed by incongruent melting of metapelites probably reflect a lower X(H2O) and/or higher temperature than required to produce orthopyroxene-rich restites, it appears that eutectic crystallization of orthopyroxene requires lower X(H2O) than does garnet. The vapor absent melting reaction (at the invariant point) is potentially the most important based on the scarcity of orthopyroxene coexisting with garnet in the BCS.
Impacts of Municipal and Commercial Usage of a Shallow Unconfined Aquifer, Nags Head Area Maritime Forest and Wetlands, Atlantic Coast Barrier Islands, North Carolina
(1999-05) Holmes, Mark Alan
A three-dimensional finite-difference digital model was used to simulate groundwater flow within the Nags Head area maritime forest and wetlands. The study area is 16 km2 (4000 acres) on the North Carolina Barrier Island. The unconfined aquifer is part of the North Carolina Coastal Plain Aquifer System. The model was developed from a hydrogeologic framework that is based on geology, sediment analysis, aquifer tests, high resolution groundwater analysis, input fluxes, and appropriate boundary conditions. The model was calibrated by comparing observed and simulated water levels. The model calibration was achieved by adjusting model parameters such as hydraulic conductivity, recharge, and storativity until the simulated water levels were within 0.5 feet (15 cm) of observed water levels. Analysis indicates that the model is sensitive to decreases in hydraulic conductivity. The model is only slightly sensitive to changes in storativity, recharge, increased hydraulic conductivity, and vertical hydraulic conductivity. Precipitation is the only source of recharge to the unconfined aquifer, and it averages approximately 12 inches/year (30.5 cm/yr.). Flow is primarily horizontal with the water table exposed as shallow ponds and puddles within inter-dunal lows. Hydrologic analyses of the flow system, using the calibrated model, indicate that pumping conditions from Fresh Pond create a cone of depression around the pond and alter the groundwater flow. The cone of depression impacts the surrounding areas, especially west of Fresh Pond, and is magnified during times of reduced recharge to the aquifer. Declines of groundwater levels, which are a result of water taken from Fresh Pond, are extensive in some areas and minimal in other areas. Decline of hydraulic heads of more than 2 to 4.5 feet (0.6 to 1.4 meters), depending on temporal variability in recharge and pumping rates, have been predicted from the calibrated model. The area with the most significant impacts is west of Fresh Pond at the groundwater divide within Nags Head Woods Ecological Preserve. Other areas impacted are to the east and south of Fresh pond. Artificial recharge from a small sewage treatment plant near Nags Head could be pumped to an appropriate engineered wetland and provide some limited benefits to the aquifer. Additional commercial wells pumping groundwater east of Fresh Pond provide additional drawdown to the pond area water table.
A Landform-Based Approach to the Estimation of Groundwater Recharge in Complex Glacial Topography
(1994-06) St George, Lynette Marie
The estimation of recharge in complex glacial topography is difficult to determine. This study utilizes an inverse numerical groundwater model and applies a landform-based approach to estimate this parameter in the stagnation complex of the Itasca moraine in north-western Minnesota. The landform assemblages were defined based on their glacio genie origin as well as topography, sedimentology, and stratigraphy. The Itasca moraine was chosen based on the numerous sedimentological and topographically distinct landform assemblages present. Eight separate landform assemblages were delineated and used as fundamental hydrostratigraphic units, each assigned specific parameters based their individual characteristics. The numerical flow simulation was set up in MODELCAD and run under steady state conditions with MODFLOW. The model was calibrated by varying the spatial distribution of recharge over each landform assemblage. A recharge value and range of recharge was determined for each landform assemblage.
Paleoenvironmental Variability in the Southeast African Tropics Since the Last Glacial Maximum: Molecular and Isotopic Records from Lake Malawi
(2007-04) Castañeda, Isla Sarita
This study utilizes molecular and isotopic techniques to examine past variability in terrestrial and aquatic tropical ecosystems from southeast Africa. Two different timescales are investigated, the past 23 cal ka to examine glacial-interglacial climate variability, and the past 730 years to examine decadal to centennial scale climate variability. Carbon isotope measurements of plant leaf waxes provide a sensitive indicator of aridity and document dry conditions in southeast Africa during the Last Glacial Maximum, the Younger Dryas cold period, and during the Little Ice Age. Peak wet conditions are observed at 13.6 and 5 cal ka, and a shift to wetter conditions is also noted from 1800 AD to the present. Arid conditions in southeast Africa are associated with southward migrations of the mean latitudinal position of the Intertropical Convergence Zone (ITCZ) during Northern Hemisphere cold periods. In contrast to studies that have suggested Holocene climates were relatively stable, the Holocene in southeast Africa was characterized by extreme and abrupt changes in moisture availability, which likely affected human and faunal migrations as well as the development and collapse of human civilizations. In addition to affecting aridity in southeast Africa, southward migrations of the ITCZ also influenced algal productivity in Lake Malawi. Lipids of aquatic algae indicate a major increase in the primary productivity at the Pleistocene/Holocene boundary, which is likely related to a northward migration of the ITCZ over Africa at this time. The Younger Dryas stands out as a major feature in the records of nearly all algal lipids and is marked by an abrupt increase in algal productivity, which can be attributed to increased northerly winds over Lake Malawi. During the past 730 years there is also evidence for changes in algal productivity with decreasing abundances of diatom lipids and increasing abundances of dinoflagellate lipids noted over the past few centuries.
Paleohydrology of the Western Outlets of Glacial Lake Duluth
(1996-09) Carney, Scott James
Glacial Lake Duluth occupied the western end of the Lake Superior Basin, dammed between the retreating Superior lobe and a series of moraines. Lake Duluth is identified by a series of discontinuous strandlines observed throughout the western portion of the lake basin. Two prominent outlets have long been recognized, the Portage outlet in Minnesota, which drained southward along the Kettle channel, and the Brule outlet in Wisconsin, which drained along the St. Croix channel. However, the relative role of each outlet in the drainage of the lake has never been adequately explained. The Brule and Portage outlets formed early during ice retreat and they drained small ice marginal lakes. Further ice retreat allowed the small lakes to coalesce forming Lake Duluth. Because of isostatic tilting, the Lake Duluth strandlines rise about 0.5 meters per kilometer eastward between the Portage and Brule outlets from 323 m to 335 m. After adjustment for rebound, the Brule outlet is estimated to be about 10 m below the elevation of the Portage outlet. The paleodischarge of the outlets and their associated channels was estimated using the U.S. Army Corps of Engineers water surface modeling package, HEC-2. Inputs to the model included topographic cross-section of the outlets and channels constructed at 1500 meter spacing from the lake outlets to a distance of 60 km downstream. Analysis of glaciofluvial sediments were used to estimate a range of Manning's roughness coefficients. The model was run with various discharges to construct stage/discharge relations. Maximum lake discharges were detennined by minimum channel cross sections. To check the validity of the discharge estimates, an atmospheric energy-balance approach utilized to estimate potential maximum meltwater availability. Peak channel discharges estimated using HEC-2 and peak summer meltwater production form the energy-balance approach are in remarkably close agreement, ranging from 30,000- 45,0000 m3/s. The Brule outlet served as the primary drainage channel for Lake Duluth. Stage/discharge relations indicate that the Portage outlet could only have been active during the peak of seasonal meltwater production or because of extraordinary inputs of water.
Petrographic Evidence for the Recycling of Late Proterozoic - Early Paleozoic Quartzose Arenites, Southeastern Minnesota - Southwestern Wisconsin
(1997-12) Galarowicz, Amy Marie
The Late Proterozoic - Early Paleozoic arenites of southeastern Minnesota - southwestern Wisconsin are all mature, quartzose arenites. The Late Proterozoic Hinckley Sandstone was deposited as a rift fill sandstone. With the transgression of the Sauk Sea during Cambrian time, the continent was flooded. In southeastern Minnesota, the sea entered the Hollandale embayment. During this time of transgression and regressions, the quartzose Cambrian Mt. Simon Sandstone, Eau Claire Formation, Galesville Sandstone, Ironton Sandstone, Franconia Formation, Jordan Sandstone, and the New Richmond Sandstone of the Shakopee Formation were deposited. Following a major regression at the end of Shakopee time, the sea reentered the continent. The Tippecanoe transgression was responsible for the widespread deposition of the St. Peter Sandstone during Middle Ordovician time. All of the arenites of this study are quartzose in nature and highly mature. Almost 95 percent of all the framework grains consist of monocrystalline quartz; potassium feldspar and plagioclase are minor components. Two units, the Eau Claire Formation and the Franconia Formation, contain significantly higher amounts of feldspar, generally 11 to 15 percent. These two quartzose feldspathic units are in general finer grained than the other more quartzose units, an observation noted earlier by other workers. Other detrital minerals include polycrystalline quartz and multicycle monocrystalline quartz. A decrease in the amount of polycrystalline quartz stratigraphically upward in the column suggests that this unstable mineral portion was eliminated by abrasion. Multicycle quartz grains with abraded quartz overgrowths are present in all nine of the units. The presence of an abraded quartz overgrowth indicates that the grain was once previously cemented by quartz, and that through erosional processes, the grain was removed from its original environment of deposition and incorporated into a stratigraphically younger unit. The grain, with the original overgrowth intact, is then recemented with quartz, resulting in a euhedral overgrowth enclosing the abraded one. The amount of unaltered and altered feldspar types can also be used to support the recycling of the quartzose arenites. Peaks of unaltered feldspar occur during Eau Claire and Franconia time, indicating that influxes of fresh sediment may have occurred during these times. Further evidence of the recycling of the quartzose arenites in relation to the feldspar concentrations can be found in the Ironton Sandstone, in which unaltered feldspar of Eau Claire time was reworked and recycled to the point that during Ironton time, the amount of altered feldspar surpassed the amount of unaltered feldspar. A high mineralogical maturity in the arenites is also evidenced by the heavy mineral concentrations. All of the units contain similar heavy mineral concentrations, consisting mainly of rounded zircon, tourmaline, rutile, and garnet. As further evidence of the recycling of the arenites, all nine units also contain tourmaline grains with abraded overgrowths. Abraded tourmaline overgrowths form in the same manner that quartz overgrowths do, and are indicative of recycling. In summary, it has long been speculated that the Late Proterozoic - Early Paleozoic quartz arenites are multicycle in origin because of the presence of clean quartz sand (e.g. Matsch and Ojakangas, 1982, p. 65; Datt and Batten, 1982, p. 283; Datt and Prothero, 1994, p. 236). This study now provides petrographic evidence that erosional processes during the Late Cambrian - Early Ordovician marine transgressions and regressions played a substantial role in the creation of these quartz arenites, thereby recycling grains from older exposed units within the Hollandale embayment.
Petrography and Diagenesis of the Upper Cambrian Mt. Simon Sandstone, Southeastern Minnesota
(1994-06) Uribe A, Ruben Dario
The Mt. Simon Sandstone (or Mt. Simon Formation) is the lowermost formation of the Dresbachian Stage (Upper Cambrian). The unit rests unconformably on a wide variety of Precambrian rocks in southeastern Minnesota and its average thickness is near 260 feet. The Mt. Simon Sandstone is dominantly a white to gray, medium- to fine-grained quartz sandstone (Q95.4F4.1L0.5 ) with minor intercalated thin shale beds. A noticeable feature of the unit is the abundance of authigenic potassium feldspar. Petrography and microprobe analysis were used to identify the framework and matrix constituents of the Mt. Simon Sandstone and to determine the diagenetic textures, paragenetic sequence and chemical composition of authigenic feldspars. Almost 95 percent of the framework grain components consist of monocrystalline (common) quartz, potassium feldspar and plagioclase, with the feldspar content higher in finer sandstones. Other minor detrital minerals include polycrystalline quartz, vein(?) quartz, recycled quartz, rock fragments, micas, collophane fossil fragments (brachiopods), and glauconite. Potassium feldspar is the most common authigenic mineral in the Mt. Simon Sandstone. The feldspar is present in almost all the samples as euhedral to subhedral overgrowths on detrital grains, ranging from 0.2 to 23.5 percent. The overgrowths are not in optical continuity with the detrital cores, and are usually pure orthoclase in composition, as evidenced by electron microprobe analysis. Minor quartz cement is present along detrital quartz contacts and in very few cases as obvious overgrowths. Authigenic dolomite exhibits two different textures: a) As euhedral, sand-sized rhombic crystals with considerable zonation showing iron-rich (red) and iron-poor dolomite (clear), and b) As rhombic finely crystalline subhedral crystals replacing illitic matrix. Poikilotopic calcite is found in a few samples, partially replacing detrital and authigenic minerals. Calcite is also present as isolated patches filling pore spaces. Kaolinite cement is most commonly found as vermicular aggregates filling pore spaces. Other authigenic minerals include hematite, pyrite and leucoxene, present as grain coatings and pore fillings. A high mineralogical maturity in the sandstones is evidenced by the presence of rounded to subrounded zircon, tourmaline, garnet and rutile. Other non-opaque detrital heavy minerals found include apatite, amphibole, pyroxene, epidote, diaspore and staurolite. The sandstone has undergone a varied diagenetic history, which includes precipitation and dissolution of authigenic minerals, dissolution of unstable detrital grains, and compaction. The paragenetic sequence in the Mt. Simon can be summarized in three stages: an early diagenesis stage marked by the presence of leucoxene and iron oxides; a burial diagenesis stage marked by the precipitation of quartz, potassium feldspar, kaolinite, illite, dolomite and siderite(?); and finally a late diagenetic stage which includes pyrite and calcite. The total average porosity for all samples is 15.5 percent; part of it is secondary porosity, as evidenced by the presence of partially dissolved grains, cements and matrix.
Petrology of the Migmatite Complex along the South Fork of the Clearwater River, Idaho
(1981-06) Carlson, Diane Helen
A nonuraniferous migmatite complex is exposed along the South Fork of the Clearwater River in north central Idaho. The complex is situated along the northeastern border of the Atlanta lobe of the Idaho batholith in a high-grade metamorphic terrane consisting of aluminous gneiss, tonalitic migmatite, calc-silicate granofelses, quartzofeldspathic gneiss, quartzite, augen gneiss, and amphibolite. As the margin of the Atlanta lobe is approached, on the western border of the complex, the Droogs Creek granite intrudes the high-grade metasedimentary rocks in a lit-par-lit manner. Toward the east, the percentage of granitoid rocks decreases, and in situ migmatite is exposed near Dutch Oven Creek. Structures indicative of more advanced stages of migmatization increase westward through the complex. The high-grade metasedimentary rocks in the complex are steeply dipping and trend north northwest. They generally have gradational contacts, and contain abundant well-rounded zircon. Aluminous gneiss near the Crooked River contains sillimanite and garnet in addition to biotite, muscovite, feldspar, and quartz. The aluminous gneiss is on the eastern limb of an antiform cored by migmatite. The migmatite, here named the Dutch Oven Creek (DOC) migmatite, contains discontinuous pegmatite-1ike leucosomes which are enveloped by very thin biotite-rich melanosomes. Overall, the migmatite is tonalitic in composition but is granitic locally where layers contain up to 30 percent microcline. The migmatite is brecciated by quartz monzonite along its western contact and grades into calc-silicate granofelses. The calc-silicate granofelses are typically layered and contain epidote-, diopside-, and scapolite-bearing assemblages. Hornblende-rich selvages along the edges of calc-silicate xenoliths attest to local metasomatism. The granofelses grade into quartzofeldspathic gneiss and quartzite near Newsome Creek. Micaceous quartzite at Golden is folded into largescale open folds, the limbs of which contain hinges of isoclines. Augen gneiss occurs as xenoliths in the DOC migmatite, sheets in the aluminous gneiss, and as a complexly folded composite unit in sharp contact with the Golden quartzite. Biotite amphibolites are present in every unit either as blocks or sill-like lenses, and are garnet-bearing in the aluminous gneiss. Interference structures and isoclinal fold hinges on the limbs of large-scale folds suggest that the area was affected by two and possibly three progressive(?) deformational events. Tight-to-isoclinal folds in the DOC migmatite are coaxial with the larger north-south trending antiform. In the aluminous gneiss, sillimanite needles are coaxial with tight folds, whereas fibrolite is folded. Faults trend northwesterly in the eastern part of the complex and are randomly oriented in the west. Joints define at least two maxima at N83W;72S and N69W;54S. The entire complex is within the sillimanite zone of the upper amphibolite facies. The orientation of sillimanite prisms and fibrolite indicate that metamorphism and deformation were coeval. Mineral relations in the aluminous gneiss suggest that at the peak of metamorphism, temperatures between 700°-730°C and pressures exceeding 3.5 kb were attained. These conditions are within the range for partial melting to occur. Compositions of leucosomes in the DOC migmatite generally plot close to the isobaric cotectic surfaces in regions of low temperature in the Qz-Ab-Or-An-H2O system and are compatible with an origin by partial melting. Leucocratic layers that do not plot close to cotectic surfaces in areas of low temperature, may have formed by metamorphic segregation induced by(?) partial melting.
Physical Volcanology and Hydrothermal Alteration of the Archean Volcanic Rocks at the Eagles Nest Volcanogenic Massive Sulphide Prospect, Northern Minnesota
(2001-03) Hovis, Steven Terry
The Eagles Nest Volcanogenic Massive Sulphide Prospect occurs within the Lower Ely Member of the 2.7 Ga Ely Greenstone Formation located within the Vermilion District of the Wawa Subprovince of the Superior Province of the Canadian Shield in Northeastern Minnesota. The object of this study is to characterize the physical volcanology and hydrothermal alteration of the rocks of the footwall to the massive sulphide mineralization. This was done by a combination of geological mapping, diamond drill core logging, thin section petrology, and major and trace element geochemistry. The entire sequence has been metamorphosed to greenschist facies. The mineralization is not exposed at the surface and was discovered by airborne Electromagnetic (EM) and Magnetism surveys, followed up by ground EM Loop and Magnetism surveys. This defined a target that was intersected by three diamond drill holes drilled by Newmont Exploration. The footwall to the mineralization is composed of approximately 1300 meters of pillowed and massive andesite lava flows, with one lobe-hyaloclastite lava flow. The base of the footwall at Eagles Nest is intruded by the dioritic to tonalitic Purvis Lake Pluton. This contact is locally an intrusive breccia. The mineralized horizon consists of approximately 100-150 meters of oxide facies banded iron formation with thin lenses of massive pyrite +/- chalcopyrite +/- sphalerite, and is interlayered with thin, strongly altered andesite lava flows, and sedimentary rocks. The hanging wall is composed of pillowed and massive andesite, with one chemically distinctive basalt lava flow. Hydrothermal alteration mineral assemblages throughout the stratigraphic package have been defined by thin section analysis. In the footwall, these assemblages are epidote +/- quartz, quartz +/- epidote, actinolite +/- epidote, and chlorite +/- quartz +/- sericite. The epidote +/- quartz and quartz +/- epidote assemblages are visible in outcrop as meter scale, locally intense, discontinuous, pale green alteration patches of pervasive alteration, and/or as fillings of amygdules and veinlets. Footwall alteration is semi-conformable, and widespread for many kilometers east and west along strike from the Eagles Nest map area (Peterson, in prep). Mineralized horizon alteration mineral assemblages are chlorite + quartz, magnesium chlorite +/- sericite, and tremolite. Tremolite and magnesium chlorite alteration is interpreted to have occurred on, or immediately below the seafloor, and is syn- to post-mineralization in timing. Most rocks in the mineralized horizon have been intensely, to completely altered, and protolith is often impossible to distinguish. All alteration in the mineralized horizon is stratiform and no evidence of crosscutting alteration has been observed. Alteration mineral assemblages in the hanging wall consist of epidote +/-quartz, quartz+/- epidote, actinolite + epidote +/-quartz, chlorite +/- epidote +/-quartz. This alteration is younger than, but very similar in character to, that of the footwall. Grant's isocon method (1986) was used to define changes in rock mass and the concentration of individual components as a result of hydrothermal alteration. The Eagles Nest Prospect is interpreted to have been located on the flank of a subaqueous volcanic center during the time hydrothermal activity was precipitating the oxide and sulphide minerals. Water depth at Eagles Nest is estimated to have been 500 meters or less, based on observations of amygdule size and percentage within the andesite lava flows. The Purvis Lake Pluton was likely the synvolcanic intrusion driving volcanism at Eagles Nest and the surrounding area, but has intruded upwards in the stratigraphic sequence since the mineralizing event. Composition of oxide and sulphide minerals, alteration mineral assemblages, results from isocon analysis, and experimental work done by Seyfried and Janecky (1985) on hydrothermal fluids beneath mid-ocean ridges, all suggest that hydrothermal fluids in the Eagles Nest footwall never reached temperatures high enough to mobilize and concentrate significant amounts of Cu and Zn. At Eagles Nest the mineralized horizon does not contain significant Cu - Zn sulphides. Epidote +/- quartz alteration in the footwall is not a good exploration guide for VMS mineralization in the Lower Ely Greenstone.
The Polymetamorphism of the Little Willow Formation, Wasatch Mountains, Utah
(1980-05) Kohlmann, Nickolas Alfred John
The Little Willow Formation, of apparent Middle Precambrian age, crops out along the Wasatch Fault Zone, in the foothills of the Wasatch Mountains, approximately 13 miles south-southeast of Salt Lake City, Utah. This metamorphic complex, exposed at the intersection of the Uinta and Wasatch tectonic trends, is composed of quartzofeldspathic gneisses, pelitic mica schists, amphibolites and a heterogeneous migmatitic unit. The formation is unconformably overlain to the east by quartzites and phyllites of the late Precambrian Big Cottonwood Formation; intruded on the southeast by the Tertiary Little Cottonwood quartz monzonite stock; and is covered and bounded on the remaining sides by Quaternary glacial and alluvial sediments. The migmatite is believed to be part of the Little Willow Formation, not the Big Cottonwood. Both the migmatite and the typical Little Willow are characterized by spaced schistosity, crenulated foliation, similar orientation of the S1 foliation, recrystallized granoblastic textures and moderate-grade metamorphism. The Big Cottonwood displays a minor phyllitic foliation which is subparallel to bedding, minor recrystallization and except for thermal effects close to the stock, a low-grade metamorphic assemblage. Metamorphic phenomena of several regional events are recognizable within Little Willow rocks. Spaced schistosity, crenulated foliation and rotated porphyroblasts of andalusite and garnet are among the evidence for the Middle Precambrian regional metamorphism. During the Laramide Orogeny (late Cretaceous-early Tertiary) the unconformable contact between the Little Willow and Big Cottonwood was apparently the locus of thrust movement. This has resulted in a cataclastic texture in many of the rocks. Intrusion of the Little Cottonwood quartz monzonite stock, 24-31 million years ago, thermally metamorphosed the surrounding rocks. Estimated metamorphic pressures of approximately 4 kb and temperatures in excess of 600°C appear to have allowed preservation of some regional features while initiating some new ones. Characteristics of this thermal event include recrystallization of the earlier cataclastic textures, growth of second-generation andalusite, growth of sillimanite (possibly second generation) and replacement of foliated sillimanite by muscovite. Wasatch normal faulting, which occurred about 20 million years ago, and subsequent erosion are responsible for the uplift and exposure of the Little Willow Formation and apparently formed localized zones of rebrecciated mylonite within the recrystallized rock. Migmatitic structures, foliated sillimanite and evidence for possible anatexis found in the southeastern portion of the Little Willow area are easiest explained by the regional metamorphic event; however some question remains, since their development during the later thermal event is also a possibility.
Quaternary Geology of the Itasca - St. Croix Moraine Interlobate Area, North-Central Minnesota
(1982-12-03) Norton, Arthur Randolph
During the St. Croix Phase of the Late Wisconsinan Substage, two lobes of the Laurentide Ice Sheet terminated and formed an interlobate junction in Cass County, Minnesota. Coarse textured, yellowish- to reddish-brown supraglacial sediments (Brainerd Till) were deposited as the north-south trending St. Croix Moraine by a southwestward advance of the Brainerd Sublobe of the Rainy Lobe. Brainerd Till is characterized by a high percentage of reddish crystalline rocks derived from a source area to the northeast and also by a low percentage of carbonate, probably derived by incorporation of underlying carbonateÂ·-rich till. The terminal position of the Brainerd .Sublobe is also marked by a continuous fosse and dump ridge at the head of the westerly grading Oshawa outwash plain. A second outwash plain was formed behind the St. Croix Moraine during retreat of the Rainy Lobe at the end of the Itasca-St. Croix Phase. A contemporaneous advance of the Wadena Lobe deposited the Lower Red Lake Falls Formation, a light olive brown to gray sandy loam till which contains moderate amounts of carbonate clasts and sparse northeast-source rock types. The bulk of the sediments were deposited as the east-west trending Itasca Moraine. Within that moraine are numerous sets of transverse compressional ridges with a broad curvature to the northeast, reflecting a south-southwesterly course for the Wadena Lobe at its terminus. An advance of that glacier to a position some 25 km south of the Itasca Moraine is indicated by extra-moranic ti11 and pitted outwash. Outwash sediments from the Wadena Lobe dominate the proglacial and interlobate areas. The surfaces of two pitted outwash plains grade south and southeastward from the Itasca Moraine and bury the northern part of the St. Croix Moraine. Sediment texture in both the Itasca and St. Croix moraines becomes coarser to the southeast across the study area. Carbonate content decreases and northeast-source rock types increase in the same direction. These variations are best explained as the result of incorporation of underlying till during several glaciations of the Wisconsinan Stage prior to and including the Itasca-St. Croix Phase. The Hewitt Till, deposited by a southwestward advance of the Wadena Lobe during the earlier Hewitt Phase of the Wisconsinan Glaciation, is a compact, dark brown, sandy lodgement till. It is completely buried by outwash sediments close to the Itasca and St. Croix moraines, but gradually emerges to become the dominant surficial deposit as the outwash sediments thin to the southwest. The Hewitt Till surface in the study area is drumlinized, forming the northernmost exposed portion of the Wadena Drumlin Field. Trends of Wadena drumlins show a gradual shift from S 22Â° W in the eastern part of the study area to S 64Â° W in the western part. Fabric measurements on drumlins show a strong preferred orientation of elongate stones dipping to the northeast, indicating a southwest advance of the Wadena Lobe during the Hewitt Phase. Eolian sands form a thin blanket over the Wadena drumlins and outwash sediments in the southeast part of the study area. Barchan dunes developed in a small area where an abundant supply of fine-grained sand and a long northwesterly fetch were present. Eolian activity probably occurred during the mid-Holocene Hypsithermal Interval, approximately 8,000 to 5,000 B. P., when the climate was more arid and prairie vegetation dominated the area.
The Quaternary Stratigraphy and Glacial History of the Duluth-Superior Area
(1986-11) Lannon, Patrick Michael
Three major lithostratigraphic units of Pleistocene age are exposed in the Duluth - Superior area, all of which are the result of Late-Wisconsin glacial activity. The lowermost unit, a reddish-brown, sandy-textured diamicton (sand/silt/clay ratio: 52/38/10) containing abundant clasts of Precambrian red sandstone, is interpreted to be subglacial till of the Lower Cromwell Formation, which was deposited by the Superior lobe during the St. Croix phase of glaciation approximately 20,000 years B.P. Its main exposure within the study area is at the base of stream valleys and roadside cuts in the southern portion of the Esko Quadrangle. Overlying the Lower Cromwell Formation is a very compact reddish-brown, silt-rich diamicton (sand/silt/clay ratio: 30/43/27) interpreted to be subglacial till of the Upper Cromwell Formation, which was deposited by the Superior lobe during the Automba phase of glaciation, approximately 14,000-18,000 years B. P. This unit is the dominant stream valley and roadside exposure in the southern portion of the Eska Quadrangle. The Upper Cromwell Formation also contains a supraglacial facies composed of flow tills and outwash sediments deposited during the retreat of Automba phase ice. These flow tills have been previously interpreted as subglacial tills associated with a later "Split Rock" advance. This supraglacial facies is the dominant surficial deposit in the northern portion of the Esko Quadrangle. The youngest lithostratigraphic unit grades vertically from a basal laminated silt and clay into massive red clay in its upper portions, and grades laterally into sands and gravels. Topographically, it is confined to elevations below 1115 feet, and is found throughout the southern portion of the Esko Quadrangle and most of the Berea Quadrangle. Based on its areal extent, facies relationships, grain-size trends, stratigraphic relationships, geomorphic expression, sedimentary structures, and engineering properties, the entire unit is interpreted to be glaciolacustrine sediment and is assigned to the Wrenshall Formation. It was deposited in Glacial Lake Duluth approximately 10,000 years B. P. The massive clay facies of this unit is not the equivalent of the Douglas Till Member of the Miller Creek Formation deposited during a late-glacial ice expansion referred to as the "Marquette" phase approximately 9900 years B. P. That advance affected portions of northern Wisconsin and Upper Michigan, but did not reach as far southwest into the Superior Basin as the Duluth - Superior area.
The Sedimentation and Petrology of the Lower Proterozoic McCaslin Formation, Northeastern Wisconsin
(1982-03) Olson, Jean Marie
The McCasin Formation of northeastern Wisconsin is exposed along the McCaslin Range, at Deer Lookout Tower Hill, and at Thunder Mountain. The McCcaslin overlies the Waupee Volcanics of uncertain age, and has been intruded by the Hager Rhyolite and High Falls Granite of the Wolf River Batholith, dated at 1500 million years. The McCaslin Formation is at least 1220 meters thick. It consists of a basal quartzose metaconglomerate and a metmorphosed quartz arenite (quartzite). Clasts include vein quartz, iron-formation, jasper, and chert. Detrital heavy minerals include hematite, zircon, red rutile, pyrite, and tourmaline. The only feldspar present in the quartzite occurs in contact with the High Falls Granite. The formation was metamorphosed to the hornblende hornfels facies by the Wolf River Batholith. Principal metamorphic minerals inc1ude andalusite, sillimanite, andradite, chloritoid, and epidote. Substantial recrystallization has removed most evidence of original grain boundaries. The McCaslin has been folded into a syncline that probably plunges about S20°W at about 30 degrees. The southern branch of the McCaslin Range has been overturned. Deer Lookout Tower Hill is probably separated from the McCaslin Range by a major east-northeast trending fault. The McCaslin appears to have been deposited as part of a braided alluvial system. Evidence of braided fluvial deposition includes poor sorting; small scale, predominantly trough cross-bedding; thin, discontinuous conglomerates; and the scarcity of both shale layers and ripple marks. The type of sedimentation was influenced by the lack of land vegetation, by intense weathering, and by probably aeolian conditions. Paleocurrent data indicate the direction of sediment transport was mostly from west to east, with substantial local variation. Clast lithologies and heavy minerals indicate multiple sources of sediment. The tectonic environment was generally stable with slight but steady subsidence. The McCalsin probably correlates with other quartzites in the region, including the Baraboo, Waterloo, Barron, Flambeau, and Sioux Quartzites. Radiometric ages indicate deposition of these formations between approximately 1630 and 1760 million years, or during Early Proterozoic time. The minor radioactivity of the McCaslin Formation is due to placer-like deposition of zircon grains. Since the McCaslin-Waupee contact is not exposed and has not been studied, an unconformity-type uranium deposit can not be ruled out.
A Seismic Stratigraphic Study of Western Lake Superior
(1997-09) Anderson, Keri Ann
Approximately 1000 km of high resolution seismic and sidescan sonar profiles were collected in the western third of Lake Superior during the 1996 field season. The study area was bounded to the east by a line from Isle Royale to Houghton, Michigan, and extended westward to Duluth, Minnesota. Seven seismic facies are identified on seismic reflection profiles. Each has a distinct acoustic signature, and can be correlated with a representative section of Lake Superior sediments. These units are incorporated into three depositional groups. The oldest sediments, Group A, represent pre-Late Wisconsinan and Late Wisconsinan sedimentation (Isotope stage 4-2). This group contains seismic facies composed of lacustrine clays and till. Group A is followed by Group B, which includes till, red clay, red varved clay and gray varved clay. This group represents Split Rock and Nickerson phase glacial deposits (Isotope stage 2). The youngest group, Group C, contains gray clay, which represents Holocene sedimentation (Isotope stage 1). Sidescan sonar profiles revealed features covering the lake floor including circular depressions and linear grooves interpreted to be iceberg scour marks. Three moraines, found on seismic profiles within the till unit of Group B, could represent the ice margin at the time of Glacial Lake Duluth.
Stratigraphy and Alteration of the Footwall Volcanic Rocks beneath the Archean Mattabi Massive Sulfide Deposit, Sturgeon Lake, Ontario
(1984-03) Groves, David Alan
Subaqueous mafic lava flows and breccias, mafic debris-flow and felsic pyroclastic-flow deposits, and felsic lavas form a 2 km thick succession beneath the Archean Mattabi massive sulfide deposit in northwestern Ontario. The lowermost 500 m is composed of massive amygdaloidal mafic flows, flow breccias, and heterolithic debris flows. Thin (<50m) amygdaloidal felsic lava flows and felsic block and ash deposits overlie the basal mafic flow sequence. This felsic horizon thickens both eastward and westward away from the Mattabi deposit and suggests the former existence of localized felsic vents on a broad shield volcano. Rocks interpreted to be mafic debris-flow and felsic pyroclastic-flow deposits lie above the felsic horizon and represent a change in eruptive style from lava extrusion to phreatomagmatic volcanism. The change is believed to be a result of a shallowing upward sequence and/or an increasing water/magma ratio. The mafic debris-flow deposits are massive to thick-bedded, poorly graded and composed of scoriaceous to amygdaloidal mafic clasts. Felsic pyroclastic-flow deposits include a) massive basal beds and overlying bedded ash tuff, b) well-bedded., graded lapilli tuff and c) massive pumice-rich beds. Felsic pyroclastic deposits intercalate and intermix with mafic debris flow deposits west of Mattabi and together these constitute the upper 500-600 m of the footwall succession. Massive pyroclastic beds truncate mafic debris-flow deposits and mark the culmination of explosive felsic volcanism prior to the ore-forming event. Massive pumiceous pyroclastic beds and quartz-porphyritic ash-flow tuff form the immediate mine footwall strata. Alteration within the footwall strata has been divided into four major mineralogical assemblages: 1) least altered (typical greenschist facies assemblages with moderate carbonatization), 2) iron carbonate-chlorite, 3) sericite and 4) chloritoid. Least-altered assemblage rocks are largely amygdaloidal mafic lava flows and mafic debris-flow deposits which are situated 4 to 5 km west of the Mattabi deposit. Iron carbonate-chlorite alteration is confined largely to felsic pyroclastic rocks and lavas within the upper 600 m of the footwall strata; sericitization is also widespread within these rocks. Chloritoid is developed in both sericite and iron carbonate-chlorite assemblage rocks. Mass balance computations indicate that constant volume has been maintained within all altered lithologies except iron carbonate-chlorite assemblages within felsic lavas; these rocks have undergone a 10 to 20% volume reduction. Iron carbonate-chlorite assemblage rocks display elemental gains of Fe, Mn and CO2, and losses of Si. Sericitization produces marked K and Rb gains at the expense of Na. Comparisons of iron carbonate-chlorite and sericite-assemblage rocks to similar chloritoid-bearing equivalents reveal no consistent elemental trends. It is envisioned that heated connate seawater/rock interactions within mafic lava flows and breccias produced a large reservoir of metal-rich hydrothermal solutions. Synvolcanic faulting allowed the rapid discharge of fluids from the reservoir. Diffuse, semiconformable alteration zones were developed in overlying felsic pyroclastic rocks and lavas as the solutions migrated upward to the seafloor surface. Focused discharge of fluids at several locations resulted in large-scale precipitation of iron sulfides on the seafloor.
Stratigraphy and Hydrothermal Alteration of Archean Volcanic Rocks at the Headway-Coulee Massive Sulfide Prospect, Northern Onaman Lake Area, Northwestern Ontario
(1985-10) Osterberg, Steven Arvid
The Headway-Coulee massive sulfide prospect of northwestern Ontario is situated within the Superior Province of the Canadian Shield. Rocks at the prospect form part of the Archean Wabigoon greenstone belt and consist of an intensely hydrothermally altered succession of mafic and felsic volcanic and intrusive rocks. Subaqueously deposited pillowed and amygdaloidal to massive and autobrecciated mafic lava flows form a 1-2 km thick succession which is locally interlayered with, and overlies a thin sequence of felsic volcanic rocks. The felsic volcanic rocks are laterally limited (2 km) and are composed dominantly of bedded ash tuffs capped by massive to brecciated and flow-banded lavas. The tuffs are fine-grained, generally fragment-poor, and vary from laminated to thickly-bedded. An extensive polymictic diamictite deposit, which contains clasts of granite, mafic and felsic volcanic rocks, and iron formation, is interlayered with the felsic 1olcanic rocks and is believed to represent a debris flow deposit which had its source to the southwest of the study area. Based on their fine-grain size, limited lateral extent, and thin to thickly-bedded nature, the felsic tuffs are interpreted to be products of hydrovolcanic eruptions. Based on stratigraphic relationships the deposits are believed to have formed on the submerged flanks of two adjacent tuff cones. It is envisioned that the capping felsic lavas formed either under low water/magma ratio conditions as access of water to the erupting magma was restricted, and/or under high water/magma ratio conditions within a water flooded vent or on the submerged flanks of the cones. The majority of the volcanic rocks were intensely altered by hydrothermal solutions during the waning stages of felsic volcanism. Alteration in the rocks is relatively widespread and is subconcordant to stratigraphically conformable in distribution. The altered rocks have been subdivided into four distinct mineral zones. The zones, in order of formation and increasing alteration intensity, are: (1) least altered, (2) quartz-sericite, (3) iron chlorite, and (4) chloritoid. The progressive alteration of the rocks was studied by mass balance comparisons of the altered rocks and their less intensely altered, stratigraphic equivalents. These comparisons indicate that Al was generally immobile, and that volume losses during alteration range from 0 to approximately 50%; the largest volume losses occurred during alteration of the felsic ash tuffs. Major chemical trends involved in alteration of the rocks include large gains in K and loss of Na during sericitization, and generally addition of Fe, and loss of Ca and Na during formation of iron chlorite and subsequent development of the chloritoid alteration type. Based on the distribution of the alteration types as well as the alteration mineralogy and chemistry it is proposed that, by shallow circulation through porous volcanic rocks, an acidic, K-rich fluid evolved and caused widespread sericitization within the study area. Deeper circulation evolved an Fe-rich fluid which was discharged along synvolcanic faults from a pressurized reservoir at depth. The solution chemically reacted with the sericitized rocks to produce the iron chlorite assemblage, and the pre-metamorphic equivalent of the chloritoid assemblage. The chloritoid assemblage developed as pre-metamorphic, coexisting iron chlorite + hydrous Al--silicate became unstable and reacted to form chloritoid during regional greenschist facies metamorphism.
Stratigraphy, Physical Volcanology, and Hydrothermal Alteration of the Footwall Rocks to the Winston Lake Massive Sulfide Deposit, Northwestern Ontario
(1993-09) Osterberg, Steven Arvid
The Winston Lake Zn-Cu-Ag massive sulfide deposit is situated above a sequence of metamorphosed Archean calc-alkaline volcanic and volcaniclastic rocks. A detailed mapping, petrographic, and chemical study was undertaken to evaluate the stratigraphic and hydrothermal development of the footwall rocks with regard to depositional environment and spatial controls on metasomatism and mineralization. The footwall rocks are dominated by interlayered successions of metamorphosed volcaniclastic and volcanic rocks that have been extensively intruded and block faulted. Volcaniclastic-sediments were deposited at the base of the stratigraphy where they were interlayered with felsic pyroclastic deposits and/or their turbiditic equivalents. Locally massive sulfide and cherty exhalative beds were deposited. A relatively thick section of interlayered felsic and mafic lava flows were erupted and deposited above the basal volcaniclastic rocks; minor interflow elastic and base metal-poor exhalative sediments accumulated during pauses in mafic volcanism. An upper elastic succession accumulated above the lava flows; basinal volcaniclastic-sediments were deposited and were overlain in part by felsic pyroclastic material that was erupted from a distant, extraneous source. Interlayered mafic lava flows and volcaniclastic rocks cap the footwall stratigraphy and host the Winston Lake deposit and stratigraphically equivalent mineralized occurrences. Facies analysis of lava flows, along with the basinal distribution of volcaniclastic-sediments indicates the Winston Lake footwall stratigraphy developed in a subsiding, subaqueous rift environment. Subsidence was focussed in the rift axis; associated stresses resulted in development of synvolcanic faults within and distal to the rift axis. The dominance of passive eruption products indicates volcanism occurred in relatively deep water beneath the volatile fragmentation depth. Approximately 50% of the footwall stratigraphy has been hydrothermally altered in subconcordant to cross-stratal zones. Interaction of the rocks with metasomatic fluids, followed by isochemical metamorphism has resulted in unusual modal abundances of tremolite/actinolite, biotite, sillimanite, staurolite, anthophyllite/gedrite, chlorite, and quartz relative to metamorphosed primary compositions. Microprobe analyses indicate extreme Fe/Mg enrichment offerromagnesian silicates near the base of the stratigraphy. Mass balance analysis indicates variable enrichment of MgO, Fe2O3T, and K2O, and depletion of CaO and Na2O in altered rocks; TiO2 and Al2O3 were relatively immobile. Overall mass losses, indicative of metasomatic leaching, dominate alteration towards the base of the stratigraphy, whereas both gains and losses occurred in the upper portions of the section. Mg enrichment occurred in stratiform zones through shallow circulation of seawater-based hydrothermal fluids during progressive stratigraphic growth. Minor associated base metal-poor exhalites developed during intermittent pauses in volcanism and sedimentation. Substratiform zones of iron-aluminous-potassic alteration developed as chemically evolved fluids, which originated at depth, interacted with permeable lithologic units through which they buoyantly migrated. The distribution of alteration indicates that chemically-evolved fluids rarely reached the sea floor environment but were generally confined beneath impermeable stratigraphic units. Metalliferous fluids periodically passed through the footwall rocks to the sea floor; no distinct chemical or mineralogical fingerprint of their passage is evident in the rocks, suggesting the metalliferous fluids were similar to chemically-evolved fluids except in metal content. The metalliferous fluids reached the sea floor during at least two stages of stratigraphic growth in which metals were deposited as massive sulfides. The first stage was at the Pick Lake deposit, near the base of the stratigraphy and the second stage was at the Winston Lake deposit at the top of the section. The distribution and composition of alteration and associated base metal sulfide and cherty exhalative occurrences indicates the Winston Lake hydrothermal system was multistaged and involved multiple hydrothermal fluids. Stratigraphic development in a subsiding rift environment spatially controlled the movement of buoyant hydrothermal fluids through permeable lithologic units. Periodic synvolcanic faulting released metalliferous fluids to the sea floor where base metal sulfides were deposited.
Structural Analysis of Archean Metasedimentary Rocks, Jardine, Montana
(1990-05) Jablinski, Joseph David
The structural history of the Precambrian deformational events in the vicinity of Jardine, Montana, has been reconstructed in some detail. Three phases of folding, as evidenced by minor planar and linear structures, have been recognized in the metamorphosed greywacke and mudstone sequences. The folds developed during the first phase of folding (F1), were isoclinal and recumbent and likely reached nappe-like scale. A pervasive S1 schistosity, found in all rock types throughout the region, was formed during, and axial planar to, F1 folding. S1 schistosity is a continuous type 1 schistosity that is subparallel to bedding (So). The folds developed during the second phase of folding (F2) were close and nearly upright, with gently to moderately plunging fold axes that trend northeast-southwest. An S2 zonal crenulation cleavage (symmetric or asymmetric) was formed axial planar to F2 folds. The folds developed during the third phase of folding (F3) were open and nearly upright, with gently to moderately plunging fold axes that trend northwest-southeast. A weak S3 zonal crenulation cleavage (symmetric or asymmetric) was formed axial planar to F3 folds. Metamorphic mineral growth did not occur during the F2 and F3 fold events. The S2 and S3 crenulation cleavages are defined by the axial planar alignment of microfold hinges or mica-rich domains. S2 and S 3 axial surfaces are nearly orthogonal to each other and their intersection is nearly normal to S1 schistosity. Rare, post-F3 kink bands were observed. The superposition of the three phases of folding resulted in the formation of a dome and basin (Ramsay type 1) fold interference pattern, observed in the S1 foliation surface. So displays the same dome and basin pattern, but in addition, small domains near the F 1 fold hinges display a combination of Ramsay type 2 and type 3 fold interference patterns. Calculations from electron microprobe analyses of metamorphic mineral assemblages indicate a peak metamorphic pressure and temperature of approximately 2.9 kb and 560° C. This pressure corresponds to a depth of burial of approximately 10 km. Microfabric observations show that peak metamorphic conditions occurred during or shortly after the first phase of folding. Brittle deformation features (e.g. fault zones, shear zones, joints) observed in the region were formed during the Laramide Orogeny (Late Cretaceous to Early Tertiary).

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