Geology of the Chugach-Prince William terrane in northern Prince William Sound, Alaska

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.

Turbidites on the SW shore of Hinchinbrook Island, Prince William Sound, Alaska

When: June 16-July 9, 2018

Where: Southern Alaska: [1] Gathering and field trips in Anchorage and staying at the University of Alaska-Anchorage, [2] stay in Valdez with road and boat work in Prince William Sound, [3] Put in a remote camp by boat in northern Prince William Sound.

Who: Six students and project Leaders: John I. Garver (Union College) and Cam Davidson (Carleton College)

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.

Expectations and Obligations:
1. Participation in field work during the summer (June 16-July 9, 2018)
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).
4. Write a short contribution (4-6 pages text + figures) to be published in the Proceedings of the Keck Geology Consortium 2019 Volume (first draft due Mid-February).
5. Expected but not required: Use this work for the completion of a senior thesis (or equivalent) at your home institution.

PROJECT DESCRIPTION

Geologic Overview

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.

Study Area

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).

Goals and Significance of the Project

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.

Student Projects

  1. 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.
  2. 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.

PROJECT LOGISTICS

Field Work: Our tentative dates are June 16 – July 9, 2018.  We will meet in Anchorage and spend a few days introducing everyone to the CPW geology near Anchorage, and meet with geologists at the USGS.  Then we travel to Valdez (Fig 3) where we will spend part of the time working out of Valdez to access the eastern transect (Fig 3, 18A), and the rest of the time (about a week) in a remote camp near Glacier Island, southwest of Valdez (Fig 3, 18B)

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.

ALASKA SAFETY ISSUES

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.

PROFESSIONAL DEVELOPMENT

We plan to take all participants to the GSA Cordilleran section meeting in Portland, Oregon (15–17 May 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.

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.

References

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