Source: https://keckgeology.org/category/2018-advanced/
Timestamp: 2019-04-22 10:44:02+00:00

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Overview: This project focuses on developing the basin evolution and paleobotanical history of the Hanna Basin spanning the transition between the Paleocene and Eocene epochs. Globally a greenhouse climate state dominated the early Paleogene, and in the Western Interior of the United States the Laramide Orogeny produced a series of uplifts and sedimentary basins. Overall the paleoclimate appears to become drier and potentially more seasonal between the Paleocene and Eocene, likely related to global warming trends and changing topography due to uplift of the Rocky Mountains. In several basins there are attendant shifts in fluvial and floodplain deposition indicating drier and more seasonal conditions as well as moderate shifts in floral communities. However, the Hanna Basin of south-central Wyoming appears anomalous. Initial studies suggest this area remained largely wet and humid, defying regional trends. This uniqueness is particularly important because the basin likely contains one of the most stratigraphically-expanded records of the Paleocene-Eocene Thermal Maximum (PETM) of anywhere in the world.
The PETM is an abrupt global warming event linked to the massive release of carbon into Earth’s atmosphere and oceans (McInerney & Wing, 2011). In other basins in the Rocky Mountain region it is correlated to substantial changes in the hydrologic cycle exacerbating seasonality in rainfall and vegetation overturn (Wing et al., 2005; Foreman et al., 2012; Kraus et al., 2015). The effects in the Hanna Basin are largely unknown and may be unique in their character. Student projects will focus on one of three topics: (1) reconstruction of paleoenvironment from fluvial and floodplain deposits and recovering information on ancient landscape dynamics; (2) understanding the interacting components of the local to global carbon cycle through stable carbon isotope records; and (3) characterizing vegetation cover spanning the short- and long-term climate changes using a new plant cuticle-based proxy method.
Who: Four to six students and project leaders Dr. Brady Foreman (Western Washington University, brady.foreman@wwu.edu) and Dr. Ellen Currano (University of Wyoming, ecurrano@uwyo.edu). USGS stratigrapher Marieke Deschesne and Field Museum paleobotanist Dr. Regan Dunn will also take part in field and lab work and provide additional mentorship for student projects.
Prerequisites and Recommended Courses: Suggested (but not required) are core courses in the Geology major: Historical Geology, Structure/Tectonics, Stratigraphy, Mineralogy, Paleontology and Geochemistry. Students should have completed key cognate courses in Chemistry and Math. Experience at a field camp or in a field geology course is recommended but not required. We are particularly interested in applicants with an interest in Paleoclimates, who have a high degree of comfort in rugged outdoor settings, are able to hike several miles in warm temperatures (on occasion greater than 95° F in the high desert climate), are flexible eaters, and who want to use this work to complete a senior thesis (or equivalent) in geology. Helpful, but not required in the letter of recommendation from the on-campus sponsor is an indication of how well the applicant will function in a remote field setting with primitive camping (i.e., no running water, no bathrooms, limited cell phone service).
Participation in all project-related work during the summer (July 6-August 5, 2018).
Follow up data analysis at home institution and regular conference calls with research team throughout academic year.
Write an abstract and present a paper (poster or talk) for the Geological Society of America Rocky Mountain Section meeting in Manhattan, Kansas (conference is March 25-27, 2019).
Expected but not required: Use this work for the completion of a senior thesis (or equivalent) at your home institution.
The Paleocene-Eocene Thermal Maximum is viewed as one of the premier geologic analogs for modern, anthropogenic global warming. However, our understanding of the major terrestrial impacts is limited to one well-documented example (Bighorn Basin of northwest Wyoming) and a few other locations globally where only a few key hydrologic or temperature constraints exist. Broadly speaking, the terrestrial response to elevated carbon dioxide levels is expected to be highly variable due to topographic variability and shifting atmospheric circulation patterns under different latitudinal thermal gradients. As such it is critical to develop additional continental locations where the temperature, hydrologic, and biologic changes are constrained. The Hanna Basin is a logical extension of the work in the Bighorn Basin, and, more importantly, a key example for changes that may mimic those predicted for modern river and vegetation systems during future anthropogenic climate change.
The Hanna Basin record is a particularly important complement to the record already collected in the Bighorn Basin because stratigraphic data suggest a difference in water availability between the two basins. Both the PETM interval and the entire early Eocene sequence preserved in the Willwood Formation in the Bighorn Basin contain abundant red beds, indicative of well-drained soils and seasonal precipitation (Kraus and Riggins, 2007; Kraus et al., 2015). The Bighorn Basin records the consequences of a semi-arid basin experiencing an abrupt global warming event. In contrast, the Hanna Basin sedimentary sequence remains drab-colored and coal-rich from bottom to top, suggesting that wet, swampy conditions prevailed through the PETM and early Eocene (Dechesne et al., in prep). It records the consequences of a more humid basin experiencing an abrupt global warming event. Thus, comparisons of the Hanna and Bighorn basins will allow us to disentangle the roles of temperature and water availability in driving vegetation change and the fluvial response in a dominantly humid setting. This water availability difference is likely due to the location of the Hanna Basin, farther to the east and potentially more proximal to moisture sources as compared to the Bighorn Basin (Sewall & Sloan, 2006). The Laramide Orogeny created a complex topography within the Western Interior that strongly influenced water vapor transport paths (Sewall & Sloan, 2006). In general, however, circulation models suggest the easternmost Laramide basins were wetter as moisture from the paleo-Gulf of Mexico moved north and westward. This resulted in semi-arid and dry conditions in the western and northernmost Laramide basins (Sewall & Sloan, 2006).
Figure 1: Cretaceous to Eocene sedimentary strata of the Hanna Basin, Wyoming.
The targeted field area for this study is the Hanna Basin of south-central Wyoming near the small town of Hanna. The Hanna Basin is a Laramide style basin bounded by the Rawlins uplift to the west, Sweetwater Arch in the north, the Simpson Ridge anticline in the east, and the Medicine Bow and Sierra Madre Mountains in the south. It is exceptional among the Laramide basins because of its high subsidence rate, extremely thick Cretaceous to Eocene sedimentary strata, and extensive coal deposits (Dobbin et al., 1929; Roberts and Kirschbaum, 1995; Wroblewski, 2003). The Paleocene-Eocene boundary is preserved in the Hanna Formation, which is over 2000 meters thick at the center of the basin (WOGCC, 2016; Gill et al., 1970; Wrobleski, 2003) and consists of conglomerates, sandstones, siltstones, carbonaceous shales, and coals. These deposits are interpreted as low gradient fluvial to paludal and lacustrine (Dobbin et al, 1929; Wrobelski, 2003; Lillegraven et al., 2004). The student projects will target a specific set of extensive outcrops along the Hanna Draw Road and in “The Breaks,” which have been the focus of new geologic and paleontologic work by Currano, Dechesne, and Dunn. The majority of research will focus on a 250-meter thick stratigraphic interval with pollen biostratigraphic constraints and initial bulk d13C values that indicate the PETM is preserved.
Overall the goal of the project is to characterize fluvial and vegetation changes spanning the transition between the Paleocene and Eocene epochs in the Hanna Basin of Wyoming. This is a critical time period in the history of life and the evolution of the Rocky Mountain region. Specifically, there were transitory changes in paleo-plant communities, persistent shift in the mammalian biogeography, and a general warming and drying trend in the Western Interior of the United States. The Hanna Basin potentially records the most complete history of this transition because it witnessed the fastest sedimentation rates of any Laramide basin in the region. This broad goal requires (1) refining the chronstratigraphy of the basin using pollen fossils and carbon isotope records; (2) detailed lithofacies analysis and outcrop mapping; and (3) application of new cuticle-based proxies for vegetation cover.
Student projects will be sub-divided into three discipline related groups. These groups entail Sedimentology-, Geochemistry-, and Paleobotany-themed projects. All students will receive training and gain experience in the three disciplines, and interact extensively with other students, faculty, and collaborators. However, the specific projects they pursue will fall into one of these three categories. Sedimentology projects will involve detailed lithofacies analysis and paleoenvironmental reconstructions of the river and floodplain deposits (n = 1 student project). These students will develop field datasets that address key questions regarding river channel morphology, avulsion behavior, floodplain drainage, and overbank flooding frequency. The majority of data can be derived from the detailed stratigraphy section we will measure at Hanna Draw area. A separate Sedimentology project (n = 1 student project) will focus on mapping out and describing the larger alluvial architecture of the fluvial sandbodies. This entails identifying and tracing out the amalgamation of distinct channel occupation events and the lateral migration of channel bodies. This information will yield insights into the mobility and paleo-dynamics of the ancient river systems.
The Geochemistry-themed projects will focus on developing a detailed δ13C bulk organic isotope stratigraphy through the 250 meters of section thought to contain the PETM (n = 2 student projects). This will entail a combination of stratigraphic section measuring (fieldwork), and sample preparation at the University of Wyoming. This includes powdering rock samples, acid digestion, and distilled water treatments, followed by drying and weighing steps. Time permitting students will run samples themselves on the Isotope Ratio Mass Spectrometers at the University of Wyoming Stable Isotope Facility. Two students will focus on this component of the project. One student will recover secular variation from densely sampled (every 0.5 m or finer) vertical samples distributed throughout the 250-meter target interval. A second student will focus on constraining lateral variability of the isotope signals. Since bulk organic δ13C records mix vegetative matter from several different types of plants it is important to constrain the total amount of variability across a landscape. This is rarely done in ancient strata (Magioncalda et al., 2004; Foreman et al., 2012), and this student will produce one of the most detailed evaluations of this uncertainty to date.
Paleobotany-themed projects will utilize a new proxy for vegetation cover that uses leaf epidermal cell size and shape to reconstruct leaf area index (LAI=foliage area/ground area; Dunn et al., 2015). This proxy has been effectively used to correlate changes in vegetation cover in South America to environmental changes, as well as to vertebrate evolution. The work in the Hanna Basin will be the first attempt to quantitatively document changes in vegetation structure across the Paleocene-Eocene boundary. The samples contain fragments of fossil leaf cuticle that preserve epidermal cell morphology. Students will photograph cuticle fragments using the microscope set-up in Currano’s lab, measure leaf epidermal cell size and shape using a Wacom Cintiq tablet and ImageJ, and reconstruct LAI using regression equations between size and shape parameters and LAI produced by Dunn from using a modern calibration datasets (Dunn et al., 2016). One student will analyze the vertical transect and a second will analyze a set of lateral samples by tracing out a stratigraphic marker bed over several 100’s of meters. This approach will allow both temporal and spatial changes in vegetation to be constrained.
Figure 2: Measuring section using a Jacob’s staff.
The expectations for students include (1) a positive, flexible attitude, (2) a responsiveness to the needs of the group, (3) a collaborative mentality, (4) strong work ethic, and (5) interest in developing quantitative and interpretative skills. These will be rough working conditions. Temperatures can exceed 95º F in the high desert climate of Wyoming. Strong winds and intense rainstorms are also likely. Students will be expected to have appropriate field gear (e.g., boots, hat, sunglasses, tent, sleeping bag, sleeping pad, backpack, water bottles). Students will need to be physically fit, and able to walk several miles per day on steep slopes in the heat, dig quarries that requires the use of shovels and pickaxes, and be capable of carrying and packing out over 30 lbs of rock per day over 3 miles. Days will likely be long, in excess of eight hours. Dietary restrictions will be accommodated if possible. Each research sub-group will be expected to produce a cohesive presentation for the annual Rocky Mountain Section meeting of GSA in May 2019 (location of meeting is unannounced). Faculty advisors will coordinate with the students towards this endeavor and aid in the construction of the poster presentation. All students will be expected to attend the meeting and participate in associated activities.
As with any field and lab work there will be safety concerns. Field risks include dehydration, heat stroke, exhaustion, sunburn, insect bites, poisonous snakes, and physical injuries such as sprained ankles and broken bones. Lab risks are relatively minor, but include the use of (weak) acids when preparing stable isotope samples and airborne dust respiratory concerns when handling and preparing plant fossils. Risks will be mitigated by following proper training protocols, outlining and identifying the risks, and holding several safety meetings. Faculty associated with the project have extensive experience in all pertinent field and lab methodologies, wilderness first aid training, and will coordinate with lab technicians to maintain safety standards.
The faculty members involved in this project and collaborators collectively have over 30 field seasons of experience working in the region. They have published over 15 scientific articles and reports on the field area and equivalent rocks in the Laramide basins of the western United States. All have led several field crews of undergraduate and graduate students under similar circumstances. Foreman and Dechesne are certified in wilderness first-aid, and we will be maintaining a list of nearby medical facilities in case of emergency. In terms of professional expertise Dr. Brady Foreman (WWU) and Marieke Dechesne (USGS) are sedimentary geologists who have worked extensively in the surrounding Laramide and Sevier foreland basin deposits. Dr. Ellen Currano (University of Wyoming) and Dr. Regan Dunn (Field Museum) are paleobotanists who have worked extensively on cuticle and plant macrofossil records in the region. Dechesne, Dunn, and Currano have collected the initial datasets upon which this proposal is based, and have identified specific areas in the basin to target for this study. This will maximize the chances of research success for the students. Foreman and Dechesne will advise students with sedimentologic projects, paleobotany projects will be advised by Currano and Dunn, and geochemistry projects will be advised by Foreman and Currano.
Figure 3: Cnemidaria fern fossil.
We plan to take all participants to the GSA Rocky Mountain section meeting in Manhattan, Kansas (25–27 March 2019). We hope most students will be first author on one paper, and probably secondary authors on others due to the collaborative nature of the project.
Bataille, C.P., Watford, D., Ruegg, S., Lowe, A., Bowen, G.J. 2016. Chemostratigraphic age model for the Tornillo Group: A possible link between fluvial stratigraphy and climate. Palaeogeography, Palaeoclimatology, Palaeoecology 457: 277-289.
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Clechenko, E.R., Kelly, D.C., Harrington, G.J., Stiles, C.A. 2008. Terrestrial records of a regional weathering profile at the Paleocene-Eocene boundary in the Williston Basin of North Dakota. GSA Bullletin 119: 428-442.
Dechesne, M., Currano, E.D., Dunn, R.E., Higgins, P., Hartman, J., Chamberlain, K. (in preparation). Climatic and tectonic responses of the fluvial to paludal strata of the Hanna Formation around the Paleocene-Eocene Boundary, Hanna Basin, Wyoming.
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Dunn RE, Barclay RS, Currano ED. 2016. Using leaf epidermis to unlock the ancient forest reconstruction enigma. Geological Society of America Annual Meeting Abstract, Denver, CO.
Foreman, B.Z. 2014. Climate-driven generation of a fluvial sheet sand-body at the Paleocene-Eocene boundary in northwest Wyoming (U.S.A.). Basin Research 26: 225-241.
Foreman, B.Z., Clementz, M.T., Heller, P.L. 2013. Evaluation of paleoclimatic conditions east and west of the southern Canadian Cordillera in the mid-late Paleocene using bulk organic d13C records. Palaeogeography, Palaeoclimatology, Palaeoecology 376: 103-113.
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WOGCC , Wyoming Oil and Gas Conservation Commission well log database: http://wogcc.wyo.gov/ (July 2016).
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Overview: This six-student project focuses on the geology of the Chugach-Prince William terrane in southern Alaska, and is part on an ongoing study of the tectonic history of the western North American Cordillera. The Chugach-Prince William terrane is a thick accretionary complex dominated by Campanian-Paleocene (c. 75-55 Ma) trench fill turbidites that were likely derived from the Coast Plutonic Complex (CPC) in British Columbia as indicated by sandstone provenance, isotopic data, and detrital zircon ages (Sample and Reid, 2003; Haeussler et al., 2005; Bradley et al., 2009; Amato and Pavlis, 2010; Garver and Davidson, 2015; Davidson and Garver, 2017). These rocks are interbedded with and intruded by mafic volcanic rocks (pillows, sheeted dikes, and gabbro), and were intruded by near-trench plutons of the Sanak-Baranof belt (63-47 Ma) and younger Eshamy Suite of plutons (37-39 Ma). For this project, we plan build on the results from key field areas in Prince William Sound and extend the reach of a critical area we visited with Keck projects in 2011 and 2014 (Garver and Davidson, 2012; Davidson and Garver, 2015). Student projects will focus on the provenance of these rocks including U/Pb dating and Hf isotope studies of detrital zircon, sedimentology and stratigraphy of turbidites and associated conglomerates, igneous petrology of the interbedded mafic rocks, and igneous petrology, U/Pb dating, and Hf isotopic studies of the Eshamy plutons.
Where: Southern Alaska:  Gathering and field trips in Anchorage and staying at the University of Alaska-Anchorage,  stay in Valdez with road and boat work in Prince William Sound,  Put in a remote camp by boat in northern Prince William Sound.
Prerequisites and Recommended Courses: Suggested (but not required) are core courses in the Geology major: Historical Geology, Structure/Tectonics, Stratigraphy, Mineralogy, and Petrology. Students should have completed key cognate courses in Chemistry and Math. Experience at a field camp or in a field geology course is recommended but not required. We are particularly interested in applicants with an interest in Tectonics, who have a high degree of comfort in rough outdoor settings, are flexible eaters, and who want to use this work to complete a senior thesis (or equivalent) in geology. Helpful, but not required in the letter of recommendation from the on-campus sponsor is an indication of how well the applicant will function in a remote field setting with primitive camping.
2. Follow up sample preparation and/or analytical work that may include a one week visit to the University of Arizona Laserchron Center in late November or early December.
3. Write an abstract and present a paper (poster or talk) for the Geological Society of America Cordilleran Section meeting in Portland, Oregon (abstracts due Feb. 7, 2019; conference is May 15-17, 2019).
The Chugach-Prince William (CPW) composite terrane is a Mesozoic-Tertiary accretionary complex that is well exposed for ~2200 km in southern Alaska and is inferred to be one of the thickest accretionary complexes in the world (Plafker et al., 1994; Cowan, 2003). The CPW terrane is bounded to the north by the Border Ranges fault, which shows abundant evidence of Tertiary dextral strike slip faulting, and inboard terranes of the Wrangellia composite terrane (Peninsular, Wrangellia, Alexander) (Pavlis, 1982; Cowan, 2003; Roeske et al., 2003). Throughout much of the 2200 km long belt of the CPW terrane it is bounded by the offshore modern accretionary complex of the Alaskan margin, but east of Prince William Sound the Yakutat block is colliding into the CPW and this young collision has significantly affected uplift and exhumation of inboard rocks (Fig. 1).
Figure 1: Map of southern Alaska showing the distribution of rocks in the Chugach-Prince William terrane (green) and the Yakutat terrane (yellow), which is colliding with Alaska.
Very soon after imbrication and accretion to the continental margin, rocks of the CPW were intruded by near-trench plutons of the Sanak-Baranof belt that has a distinct age progression starting in the west (63 Ma in the Sanak-Shumagin areas far to the west) and getting progressively younger to the east (53-47 Ma on Baranof Island; Bradley et al., 2000; Haeussler et al., 2003; Kusky et al., 2003; Farris et al., 2006; Wackett et al., in revision). In western Prince William Sound, these rocks were also intruded by the 37-39 Ma Eshamy Suite of plutons (Johnson, 2012, see Fig 3).
Paleomagnetic and geologic data indicate that the CPW has experienced significant coast-parallel transport in the Tertiary (see Garver and Davidson, 2015). The CPW has apparent equivalents to the south, and this geologic match suggests that in the Eocene, the southern part of the Chugach-Prince William terrane was contiguous with the nearly identical Leech River Schist exposed on the southern part of Vancouver Island (Cowan, 1982; 2003). The geological implication of this hypothesis is profound yet elegant in the context of the Cordilleran tectonic puzzle: the CPW is the Late Cretaceous to Early Tertiary accretionary complex to the Coast Mountains Batholith Complex that intrudes the Wrangellia composite terrane and North America. Thus, the CPW is inferred to have accumulated in a flanking trench to the west and then soon thereafter these rocks were accreted to the margin. This geologic match is elegant because it suggests that the CPW accumulated outboard the Coast Mountains Batholith Complex (Gehrels et al., 2009) and that the CPW essentially is the erosional remnants of that orogenic belt. Thus, the focus of this proposal is on the very thick rocks of the CPW accretionary terrane that were intruded by near trench plutons and then translated some controversial distance along the North American margin in the early Tertiary. This late-stage translation by strike-slip faulting is of critical importance to this project.
For the 2018 field season we plan to extend and expand on the transect across the CPW we completed in 2014 and adjacent to work done in 2011 (Fig. 2, 3). Preliminary data from two samples collected on the Richardson Hwy northeast of Valdez show that rocks correlative to the Paleocene-Eocene Orca Group are much more extensive than originally thought (Davidson and Garver, 2017). We now know that these younger rocks occur inboard of Valdez, in the Chugach Metamorphic complex, in the Schist of Nunatak Fiord, and in the Baranof Schist (Gasser et al., 2011; Rick et al., 2014; Olson et al., 2017). This result means that Paleocene and Eocene Orca rocks occur to the west, north, and east of the Yakutat Collision zone.
Figure 2: Geology of the southern Alaska margin centered on the Yakutat terrane collision (yellow) and the Chugach-Prince William terrane (shaded in green) . Our work in the Prince William Sound area is focused on two transects (A, B). We now recognize that large-scale imbrication by strike-slip faulting plays a crucial role in shaping the distribution of rocks of the Orca Group (light green), our primary focus. Location of this proposed Keck project shown is in transect B, and our proposed field in northern Prince William Sound is show in Figure 3. Map modified from Gasser et al. (2011).
Figure 3: Geologic map of study area focused on Prince William Sound (PWS) in southern Alaska. Our previous work in PWS in 2011 and 2014 allowed us to build a framework for understanding the Orca Group (most olive in map), and part of that framework involved the recognition that strike-slip faults play an important role in juxtaposing different facies, and in driving differential exhumation. Our proposed work is partly focused on documenting facies changes (provenance) across the Western Gravina fault and the Jack Bay fault (aka Contact fault). Map modified from Wilson et al. (2015).
The primary goal of this project is to sort out the age, source, and timing of accretion of the Valdez and Orca turbidites and subsequent strike-slip motion along the Contact fault and allied structures (i.e. Jack Bay, West Gravina, Rude River) associated with the collision of the Yakutat plate in eastern Prince William Sound (Fig. 2). Ancillary goals of this project include: 1) comparing the age and geochemistry of interbedded mafic volcanic rocks of the Orca Group with the Knight Island and Resurrection ophiolites to the west and south (Fig 3); and 2) compare the crystallization age and petrology of undated Eshamy Suite plutons with those previously studied by our group to the southwest (2011 site, see Johnson, 2012).
This project is significant because it will allow students to work in units that are classic in Cordilleran tectonics, and the results will directly feed into ideas of terrane translation and development of the Cordilleran tectonic collage.
Provenance and maximum depositional ages of sandstones and conglomerates (2-3 students). Our preliminary work along the Richardson Highway north of Valdez suggests that the contact relationships between the Valdez Group (Campanian-Maastrictian) and Orca Group (Paleocene-Eocene) is much more complicated than currently mapped (as shown in Figure 3 derived from Wilson et al., 2015). One of the goals of this project is to use U/Pb and Hf isotope data of detrital zircon from two transects across the Contact fault (Jack Bay) to help define the extent of these two units (Fig. 2). Our working hypothesis is that the turbidites of the CPW are imbricated in vertical panels by strike slip faulting, juxtaposing rock packages with different MDA’s and provenance (indicated by unique U/Pb and Hf detrital zircon signatures). Cobble and pebble conglomerates containing sandstone and plutonic clasts are common in the Orca Group in this area. At least one student will determine the provenance and age of these clasts to see if sandstones from the older Valdez Group is being exhumed and shed into the basin during deposition of the Orca Group.
Core-Rim dating (1-2 students). Core-rim dating will allow us to determine high-grade metamorphic source regions because important and distinctive rims on zircon may form during metamorphism. Our initial experiments at LaserChron show we can successfully date rims with a 10 or 12 um laser spot size (Fig. 3). There are a number of reactions that drive changes in zircon in the metamorphic environment, and these include: 1) recrystallization; 2) fluid alteration; 3) subsolidus nucleation; 4) precipitation from aqueous fluids; and 5) precipitation from melts during anatexis (Hoskin and Black, 2000; Xie et al., 2009). We focus here on two commonly recognized rims that form under high-grade metamorphic conditions (upper amphibolite and granulite). One type of rim typically appears dark in CL images, shows little internal structure, and has relatively high uranium, high U/Th, and low REE contents and results from zircon growth under metamorphic conditions (Ksienzyk et al., 2012). The other type of rim is CL-bright, with irregular re-crystallization fronts and relict zoning, and variable U/Th and REE concentrations that typically form during solid-state recrystallization of zircon (Hoskin and Black, 2000; Hoskin and Schaltegger, 2003; Xie et al., 2009; Ksienzyk et al., 2012). In these recrystallization rims, the U/Th ratio increases primarily due to Th loss, and the isotopic system is progressively reset due to expulsion of radiogenic lead (Hoskin and Black, 2000; Hoskin and Schaltegger, 2003). This project will use core-rim dating to determine metamorphic events in the source region, which likely included the Central Gneiss Complex of the Coast Plutonic Complex located to the south in British Columbia.
Figure 4: Two core-rim dated zircons from the Yakutat Group showing Precambrian cores and Cretaceous rims. [A] Precambrian core with oscillatory zoning, and CL-bright rim with possible recrystallization front and relict zoning. [B] Zoned Precambrian core, and complicated CL-dark high-uranium rims. Our earlier work using Raman (see below) suggested the radiation damage in these grains was Cretaceous, and this is confirmed by rim dating.
3. Damage dating (1 student). We are developing a technique to use zircon crystallinity to quantify the timing of metamorphism of Precambrian zircon. This new technique aims to quantify accumulated radiation in Precambrian zircon using degree of crystallinity measured by μ-Raman spectroscopy as indicated by the position of the ν3(SiO4) or the FWHM of the ν3(SiO4) (Marsellos and Garver, 2010; Garver and Davidson, 2015). In essence, this is radiation damage dating where the accumulated damage is used as a chronometer and can reveal the age of last metamorphism. We have made important advances in developing this technique so it can be used to solve tectonic problems with DZ data sets, and we have shown that disorder in Precambrian zircons in the CPW fall into two distinct arrays of radiation damage (Fig. 4). One cohort of Precambrian grains has been cool since the lower Paleozoic and another since the Cretaceous (Garver and Davidson, 2015). We are excited that this technique can be tremendously important for detrital studies, but more work is required to calibrate damage and Raman active modes. For this project, a student will use this technique to determine metamorphic histories of Precambrian zircon from the Orca and Valdez groups.
Figure 5: Raman shift for Precambrian zircon from the CPW (from Garver and Davidson, 2015).
4. Age and origin of mafic volcanic rocks in the Orca Group (1 student). Pillow lavas and sheet flows of mafic volcanic rocks are interbedded with the Orca Group; and on Glacier Island, pillow basalts, sheeted dikes, and gabbro are reported (Nelson et al., 1999). This project involves collecting major and trace element data from the mafic volcanic rocks throughout the area and to compare these data with previously published data from similar rocks to the south and west (Miner, 2012; Young, 2015), and the Resurrection and Knight Island ophiolites (Lytwyn et al., 1997; Miner, 2012).
5. Age and origin of the Eshamy Suite plutons (1 student). Our previous work in western Prince William Sound helped define the age and geochemistry of a unique series of 37-39 Ma gabbroic to granodiorite plutons that intrude the CPW accretionary wedge complex (Johnson, 2012). This project will work on two or three plutons near Valdez (Miner’s Bay, Cedar Pluton) that have been tentatively correlated to the Eshamy Suite based on major element geochemistry and K-Ar dates (Nelson et al., 1999). The goal of this project is to confirm the age of these plutons using U/Pb geochronology and to use whole rock major and trace elements, and Hf isotope data from zircon to help describe the petrogenesis of these rocks.
Analytical Work: Students will cut billets for thin sections at their home institution and thin sections will be made by the expert technician at Union College. Mineral separates will be done at Union or Carleton in late summer and early fall. Imaging zircon sample mounts on the SEM (BSE and CL) can be done at Carleton, Union, or the student’s home institution if they have the equipment.
U-Pb and Hf analyses will be done at the University of Arizona Laserchron Center in late November or early December, and this is an important time for the entire Keck team to reconvene and collect critical data. We will invite all students to participate in this excursion, whether they are using U/Pb data in their thesis or not. Data reduction is done onsite, so students will leave with their data set.
Keck students in the LaserChron center at the University of Arizona during a 3-5 day session in November.
Davidson and Garver have been doing research in the northern Pacific Rim for over 30 years (each) with primary field areas in Kamchatka, Alaska, British Columbia, and Washington. Thus we are familiar with safety issues primarily those related to Bears and Boats. There is little question that there are a host of inherent risks in this proposed work. Here we briefly address the most common concerns.
Bears. The primary issue is Black and Brown Bears. We will spend the bulk of our time in coastal waters as a group, hence our exposure is minimal. We note that our field season coincides with the Sockeye run, so almost all bears (black or brown) tend to be pre-occupied with fish. We train all participants in the use of bear deterrents. We will have bear bangers (a small pen-sized explosive charge) and Bear Spray for everyone.
Boats and Communication. We will be using two 15 ft Zodiac inflatable boats with aluminum floors and 30 hp 4-stroke engines. All participants will be instructed in safe boating practices including protocol for VHF radio use and will be required to wear life jackets. We also have a satellite phone for emergencies.
All students are required to complete a 4-6 page paper (short contribution) that will be published in the Proceedings of the Keck Geology Consortium 2019 Volume (see examples from previous years). The first draft of this paper will be reviewed by your research advisor at your home institution sometime in late February, 2019, with revised version sent to the project directors by March 1. Final versions of your paper and figures will be submitted to the Keck Office in Mid-March.
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Bol, A.J. and Gibbons, H., 1992, Tectonic implications of out-of-sequence faults in an accretionary prism, Prince William Sound, Alaska: Tectonics, v.1, p. 1288-1300.
Bol, A.J., and Roeske, S.M., 1993, Strike-slip faulting and block rotation along the contact fault system, eastern Prince William Sound, Alaska. Tectonics 12, 49–62.
Bradley, D. C.; Parrish, R.; Clendenen, W.; Lux, D.; Layer, P.; Heizler, M.; and Donley, D. T., 2000, New geochronological evidence for the timing of early Tertiary ridge subduction in southern Alaska: US Geological Survey Professional Paper, 1615:5-21.
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Davidson, C., and Garver, J.I., 2015, Tectonic evolution of the Prince William terrane in Resurrection Bay and eastern Prince William Sound, Alaska: Short Contributions, Keck Geology Consortium 28th Annual Symposium Volume, Union College, NY.
Davidson, C. and Garver, J.I., 2017, Age and origin of the Resurrection Ophiolite and associated turbidites of the Chugach-Prince William terrane, Kenai Peninsula, Alaska. Journal of Geology, in press: doi.org/10.1086/693926.
Garver, J.I., and Davidson, C., 2012, Tectonic evolution of the Chugach-Prince William terrane in Prince William Sound and Kodiak Island, Alaska, Proceedings from the 25th Keck Geology Consortium Undergraduate Research Symposium, Amherst, p.1-7.
Garver, J. I., and Davidson, C., 2015, Southwestern Laurentian zircons in Upper Cretaceous flysch of the Chugach-Prince William terrane in Alaska: American Journal of Science, 315:537-556.
Gasser, D., Bruand, E., Stüwe, K., Foster, D.A., Schuster, R., Fügenschuh, B., and Pavlis, T., 2011, Formation of a metamorphic complex along an obliquely convergent margin: Structural and thermochronological evolution of the Chugach metamorphic complex, southern Alaska: Tectonics, v. 30, p. TC2012, doi:10.1029/2010TC002776.
Gehrels, G.E., Rusmore, M., Woodsworth, G., Crawford, M., Andronicos, C., Hollister, L., Patchett, J., Ducea, M., Butler, R., Klepeis, K, Davidson, C., Mahoney, B., Friedman, R., Haggard, J, Crawford, W., Pearson, D., Girardi, J., 2009, U-Th-Pb geochronology of the Coast Mountains Batholith in north-coastal British Columbia: constraints on age, petrogenesis, and tectonic evolution. Bulletin of the Geological Society of America, v. 121, p. 1341-1361.
Haeussler, P.J., and Nelson, S.W., 1993, Structural evolution of the Chugach-Prince William terrane at the hinge of the orocline in Prince William Sound and implications for ore deposits, in Dusel-Bacon, Cynthia, and Till, A.B., eds., Geologic Studies in Alaska by the U.S. Geological Survey, 1992: U.S. Geological Survey Bulletin 2068, p. 130-142.
Haeussler, P.J., Gehrels, G.E., and Karl, S., 2005, Constraints on the age and provenance of the Chugach terrane accretionary complex from detrital zircons in the Sitka Greywacke, near Sitka, Alaska: in Haeussler, Peter J., and Galloway, John, eds., Studies by the U.S. Geological Survey in Alaska, 2004: U.S. Geological Survey Professional Paper 1709-F, p. 1- 24.
Hoskin P.W.O., Black L.P., 2000, Metamorphic zircon formation by solid-state recrystallization of protolith igneous zircon. Journal of Metamorphic Geology 18, 423–439.
Hoskin, P.W.O., and Schaltegger, U., 2003, The Composition of Zircon and Igneous and Metamorphic Petrogenesis: Reviews in Mineralogy and Geochemistry, v. 53, no. 1, p. 27-62.
Johnson, E., 2012, Origin of Late Eocene granitiods in western Prince William Sound, Alaska; Proceedings from the 25th Keck Geology Consortium Undergraduate Research Symposium, Amherst MA, p. 33-39.
Ksienzyk, A.K., Jacobs, J., Boger, S.D., Kosler, J., Sircombe, K.N., Whitehouse, M.J., 2012, U–Pb ages of metamorphic monazite and detrital zircon from the Northampton Complex: evidence of two orogenic cycles in Western Australia. Precambrian Res. 198–199, 37–50.
Kusky, T.M., Bradley, D.C., Donely, D.T., Rowley, D. & Haeussler, P.J. 2003, Controls on intrusion of near-trench magmas of the Sanak-Baranof Belt, Alaska, during Paleogene ridge subduction, and consequences for forearc evolution; Geology of a transpressional orogen developed during ridge-trench interaction along the North Pacific margin, Special Paper – Geological Society of America, vol. 371, pp. 269-292.
Lytwyn, J.N., J.F. Casey, S. Gilbert, and T.M. Kusky, 1997, Arc-like midocean ridge basalt formed seaward of a trench-forearc system just prior to ridge subduction: An example from subaccreted ophiolites in southern Alaska, J. Geophys. Res., 102, 10,225-10,243.
Marsellos, A.E., and Garver, J.I., 2010, Radiation damage and uranium concentration in zircon as assessed by Raman spectroscopy and neutron irradiation; Am. Min., v. 95, p. 1192–1201.
Miner, L., 2012, Geochemical analysis of Eocene Orca Group volcanics, Paleocene Knight Island Ophiolite, and Chenega Island volcanics in Prince William Sound, Alaska; Proceedings from the 25th Keck Geology Consortium Undergraduate Research Symposium, Amherst MA, p. 40-49.
Nelson, S. W., Miller, M.L., Haeussler, P.J., Snee, L. W., Phillips, P.J., and Huber, C., 1999, Preliminary geologic map of the Chugach National Forest Special Study Area, Alaska: U.S. Geological Survey Open-File Report 99-362, scale I :63,000.
Rick, B.J., Frett, B.K., Davidson, C.M., and Garver, J.I., 2014, U/Pb dating of detrital zircon from Seward and Baranof Island provides depositional links across the Chugach-Prince William terrane and southeastern Alaska. Cordilleran Tectonics Workshop, University of British Columbia – Okanagon, Abstracts with program, p. 35-36.
Roeske, S.M., Snee, L.W. & Pavlis, T.L. 2003, Dextral-slip reactivation of an arc-forearc boundary during Late Cretaceous-early Eocene oblique convergence in the northern Cordillera; Geology of a transpressional orogen developed during ridge-trench interaction along the North Pacific margin, Special Paper – Geological Society of America, vol. 371, pp. 141-169.
Sample, J.C. & Reid, M.R. 2003, Large-scale, latest Cretaceous uplift along the Northeast Pacific Rim; evidence from sediment volume, sandstone petrography, and Nd isotope signatures of the Kodiak Formation, Kodiak Islands, Alaska; Geology of a transpressional orogen developed during ridge-trench interaction along the North Pacific margin, Special Paper – Geological Society of America, vol. 371, pp. 51-70.
Xie, Q.-X., Zheng, Y.-F., Yuan, H.-L. Wu, F.-Y., 2009, Contrasting Lu-Hf and U-Th-Pb isotope systematics between metamorphic growth and recrystallization of zircon from eclogite-facies metagranites in the Dabie orogen, China: Lithos, 112, pp. 477–496.
Young, E., 2015, Geochemistry of the Orca Group volcanic rocks in eastern Prince William Sound, Alaska: Short Contributions, Keck Geology Consortium 28th Annual Symposium Volume, Union College, NY.

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