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1. Zircon facies in the Paleocene-Eocene Orca Group indicate a provenance link to the Chugach Terrance, Prince William Sound, Alaska.

   Fisher, W.S. ( April 2019 , Proceedings of the Keck Geology Consortium.)

   Much of the southern Alaska continental margin is made up of marine sedimentary rocks and distinct terranes that have been deposited and accreted from the Cretaceous to the present (Plafker et al., 1994). The Upper Cretaceous to Eocene Chugach-Prince William (CPW) terrane is interpreted to be one of the thickest accretionary complexes in the world, and it is bounded to the north by the Border Ranges fault and Wrangellia composite terrane (Garver and Davidson, 2015). The CPW terrane is inferred to be the Mesozoic accretionary complex of southern Alaska (Amato et al., 2013), but alternate hypotheses suggest it originally formed far to the south (Cowan, 2003). The CPW consists of inboard mesomélange (the McHugh Complex & Potter Creek Assemblage) and stratigraphically younger outboard flysch facies (the Valdez & Orca groups) and associated volcanics (Plafker et al., 1989; Garver and Davidson, 2015; Amato et al., 2013). The blueschist to greenschist Potter Creek Assemblage formed in Cretaceous-Early Jurassic subduction (Amato et al., 2013). The McHugh Complex is made up of mélange and deformed conglomerates and sandstones and ages range from the Jurassic to mid Cretaceous (Amato et al., 2013). The majority of the CPW terrane (>90 %) is comprised of the outboardmore »flysch facies of the Late Cretaceous to Eocene Valdez and Orca groups juxtaposed along the Contact fault system (Garver and Davidson, 2015, Dumoulin, 1987; Fig. 1). The CPW terrane was intruded by the 61-50 Ma Sanak-Baranof belt (SBB) near-trench plutons that are diachronous (Bradley et al., 2003; Cowan, 2003). There are two predominant hypotheses concerning the intrusion of these plutons and the amalgamation and translation of the CPW terrane. The Baranof-Leech River hypothesis suggests the CPW terrane formed to the south and was then translated along the margin (Cowan, 2003). A more northern hypothesis where CPW terrane formed in situ and the Resurrection Plate subducted underneath it (Haeussler et al., 2003). These alternate hypotheses each require a different sediment provenance for the CPW terrane outboard flysch assemblages. The goal of this study is to determine the depositional age, provenance, and original tectonic setting of the flysch facies of CPW terrane, with an emphasis on the younger Orca Group. Using maximum depositional ages (MDA) and the KS test, we delineate four distinctive zircon facies: 1) Miners Bay (~61-59 Ma, n=2244 grains); 2) Sawmill (59-55 Ma, n=1340); 3) Hawkins (55-50 Ma, n=1914); and 4) Montague (52-31 Ma, n=1144) (Fig. 2). A major stratigraphic conundrum is that the oldest Orca is age-correlative and has a similar provenance to the youngest Valdez Group at 61-60 Ma, and the location of these rocks casts doubt of models that rely on the Contact fault system as a terrane-bounding fault.« less
2. Age and tectonic setting of the Paleocene Glacier Island volcanic sequence of the Orca Group in Prince William Sound, Alaska.

   https://doi.org/10.1130/abs/2019CD-329372  

   Noseworthy, C. ( April 2019 , Proceedings of the Keck Geology Consortium.)

   Southern Alaska has a long history of subduction, accretion, and coastwise transport of terranes (Coney et al., 1980; Monger et al., 1982; Plafker et al., 1994). The Chugach-Prince William (CPW) terrane is about 2200 km long and extends through much of southern Alaska (Plafker et al., 1994) (Fig. 1A). The inboard Chugach terrane can be divided into two parts, a mélange and sedimentary units that are Permian to Early Cretaceous in age and a turbidite sequence that is from the Upper Cretaceous (Plafker et al., 1994). In the Prince William Sound area, the outboard Prince William terrane is comprised of Paleocene to Eocene turbidites and associated basaltic rocks of the Orca Group (Davidson and Garver, 2017), and the turbidites of the inboard Chugach terrane are known as the Valdez Group. The turbidites are intruded by the Sanak-Baranof Belt (SBB), a group of 63-47 Ma plutons that are progressively younger to the east. The Border Ranges fault system marks the northern boundary of the CPW terrane, separating the Chugach terrane from the Wrangellia composite terrane and the Contact fault separates the Chugach and Prince William terrane (Fig. 1; Plafker et al., 1994). There are three ophiolite sequences in the Orca Group:more »Knight Island (KI), Resurrection Peninsula (RP), and Glacier Island (GI) (Fig. 1B). The KI ophiolite contains a sequence of massive pillow basalts, sheeted dikes, and a minor amount of ultramafic rocks (Tysdal et al, 1977; Nelson and Nelson, 1992; Crowe et al., 1992). The RP ophiolite is a typical ophiolite sequence and has interbedded Paleocene turbidites (Davidson and Garver, 2017). Paleomagnetic data gathered from the RP ophiolite indicated a mean depositional paleolatitude of 54° ± 7° which implies 13° ± 9° of poleward displacement (Bol et al., 1992). These data suggest that the RP ophiolite was translated northward to its current position after being formed in the Pacific Northwest, and thus the CPW terrane may have been originally located at 48-49° north and at 50 Ma was transferred 1100 km to the north by strike-slip faulting (Cowan, 2003). However, an opposing hypothesis suggests that the terrane has not experienced significant displacement and formed in Alaska due to a now-subducted Resurrection plate (Haeussler et al., 2003). KI and RP ophiolites have traditionally been assumed to be oceanic crust that was tectonically emplaced into the CPW terrane (Bol et al., 1992; Lytwyn et al., 1997). However, a more recent study suggests a hypothesis that the ophiolites originated in an upper plate setting and formed due to transtension (Davidson and Garver, 2017). Previous workers have used discriminant diagrams to identify the volcanic rocks of KI ophiolite and RP ophiolite as mid-ocean ridge basalts (Lytwyn et al., 1997; Miner, 2012). This project presents new geochemical and geochronological data from the GI ophiolite to determine its age and tectonic setting. The purpose of this study is to compare the data from GI with the data from KI and RP, and the comparison of the geochemical data will allow for a greater understanding of the tectonic setting of southern Alaska.« less
3. Age and provenance of the Upper Cretaceous to Paleocene Valdez Group of the Chugach Terrane from the Richardson Highway and Northern Prince William Sound, Alaska.

   https://doi.org/10.1130/abs/2019CD-329673  

   Gross Almonte, N. ( April 2019 , Proceedings of the Keck Geology Consortium)

   The age and provenance of the southern Alaskan Campanian to Paleocene Valdez Group of the Chugach terrane and its relationship with the younger outboard Paleocene to Eocene Orca Group of the Prince William terrane is poorly understood but an important component of the Cordilleran collage (Plafker et al., 1994). The Valdez and Orca Groups are both part of the Chugach-Prince William terrane (CPW), which is a thick accretionary complex that extends 2200 km along the southern Alaskan margin (Fig. 1; Cowan, 2003). The deep-water turbidites of these terranes are quartzofeldspathic and volcanic-lithic sandstones and basaltic rocks (Dumoulin, 1987; Plafker et al., 1994). The CPW is intruded by near-trench plutons of the Sanak-Baranof belt (Davidson and Garver, 2017) and are believed to be related to a slab window that formed during subduction of Kula-Farallon or Kula- Resurrection spreading ridges (Marshak and Karig, 1977; Delong et al., 1978; Moore et al., 1983; Kusky et al., 1997a; Bradley et al., 2003; Haeussler et al., 2003). There are two hypotheses for the formation of the CPW along the North American Cordilleran margin: 1) either the CPW terrane formed in situ by subduction of the Resurrection plate (Haeussler et al. 2003); or 2) the rocksmore »formed in the Pacific Northwest or California and were transported at least 2000 km along coastwise strike-slip fault systems (Cowan, 2003; Garver and Davidson, 2015). This study is an investigation into the age and provenance of the Valdez Group and its relationship with the Orca Group in the central Chugach Mountains using detrital zircon U-Pb dates. New detrital zircon U-Pb dates and their grain-age distributions from the Valdez and Orca Group turbidites are combined with dates from Kochelek et al. (2011), Amato et al. (2013), and Davidson and Garver (2017) and then synthesized to understand the difference in age between the units and provenance. New and existing U-Pb dates indicate maximum depositional ages (MDA) of the Valdez Group are concentrated in three groups: 84-78 Ma, 74-65 Ma, and 62-60 Ma. The youngest group of MDAs are age-correlative with the Orca Group but were collected from rocks in areas mapped as Valdez Group, indicating that either Orca Group rocks occur in the Valdez Group or the youngest Valdez Group rocks are stratigraphically equivalent to those of the oldest Orca Group. If the latter, the Valdez Group is not Campanian to Maastrichtian in age as has been traditionally viewed (Plafker et al., 1994) but is Upper Cretaceous to Paleocene and in part correlative to the lowest part of the Orca Group.« less
4. Crystallization ages and geochemistry of the Miners Bay and Cedar Bay Plutons, Prince William Sound, Alaska

   https://doi.org/10.1130/abs/2019CD-329495  

   Garcia, V.R. ( April 2019 , Proceedings of the Keck Geology Consortium)

   The Late Eocene Eshamy Suite plutons (ESP) intrude turbidites of the Chugach-Prince William (CPW) terrane in western Prince William Sound. The Prince William Sound region of South-Central Alaska consists of the Chugach-Prince William Terrance (CPW), containing the Valdez and Orca Group separated by the Contact fault. The Valdez Group is Late Cretaceous in age and is characterized by a thick sequence of interbedded siltstone, greywacke, and pebble conglomerate likely deposited as turbidites on submarine fans (Tysdal and Plafker, 1978). South of the Valdez is the Orca Group of Paleocene to Eocene age, characterized by folded and faulted rocks that have been intruded by younger plutons (Davidson and Garver, 2017). The CPW has been traditionally interpreted as a Late Cretaceous to Paleocene accretionary wedge complex that was either formed in situ or was deposited farther south and subsequently transported a significant distance along the continental margin (cf. Cowan, 2003; Haeussler et al., 2003). The ESP is a bimodal suite of granites dominated by biotite granites and leucogranites with subordinate gabbro. Questions for the ESP include the crystallization ages for the plutons, their relationship to igneous rocks found farther inboard, and the nature of the tectonic setting. For this study, we examinedmore »a set of granites from Miners Bay and Cedar Bay to compare with the rocks from the Nellie Juan and Eshamy Bay plutons to the southwest (Fig. 1), and the Caribou Creek volcanics (CCV) that occur 200 km inland. Our results show that the Miners and Cedar Bay plutons fall within the age range of the CCV and are marginally older than the Nellie Juan (NJP) and Eshamy Bay plutons (EBP) (Cole et al., 2006; Johnson, 2012). However, preliminary geochemical data suggest that the ESP may not be directly related to the CCV and therefore may have formed in a different tectonic setting.« less
5. U-Pb dating of detrital zircon from turbidites of the Chugach and Prince William terranes, Alaska: Implications for the significance of the Contact fault system as a terrane boundary.

   https://doi.org/10.1130/abs/2019CD-329396  

   Malik, A. ( April 2019 , Proceedings of the Keck Geology Consortium.)

   The Gulf of Alaska is rimmed by a Mesozoic- Cenozoic accretionary wedge complex comprised of the Chugach and the Prince William terranes. This study focuses on understanding the history and relationship between turbidites of the Chugach and Prince William terranes in the Prince William Sound area: the Campanian-Maastrichtian Valdez Group and the Paleocene-Eocene Orca Group. Critical in understanding this system is the Contact fault system, a poorly understood collection of fault strands that has traditionally been viewed as the terrane boundary between the two (Winkler and Plafker 1975; Plafker et al. 1977; Tysdal and Case 1979; Dumoulin 1988; Bol and Gibbons 1992; Bol and Roeske 1993; Arkle et al. 2013). It was thought to be an original accretionary fault that separates terranes (Plafker et al. 1977; Nokleberg et al., 1986; Dumoulin, 1988); however, more recent work has characterized it as a sequence of faults that may be related to internal deformation of the accretionary complex (Bol and Gibbons, 1992; Bol and Roeske, 1993). The main goal of this study is to determine whether the Contact fault functions as a terrane boundary across Prince William Sound by presenting new detrital zircon U-Pb ages from either side of the fault system (Fig.more »1). First, we specifically focus on understanding the age relationships between the Valdez Group and the Orca Group on either side of the Jack Bay fault and the Landlock fault in Valdez Arm, northeastern Prince William Sound (Fig. 2). Second, we focus on strands of the Contact fault system in three other locations: Unakwik Inlet, Kings Bay, and Seward (Fig. 3). This second goal of this study is to determine the nature of the Landlock block, an area currently defined by the convergence of the Jack Bay and Landlock faults (Fig. 2). Early maps portrayed the Landlock block as Valdez Group (Moffit, 1954; Winkler and Plafker, 1975), but the current location of the Contact fault in Jack Bay includes the block as part of the Orca Group (Dumoulin, 1998; Bol and Roeske, 1993). Therefore, an important question is the affinity of the rocks of the Landlock block, whether they are Orca Group or Valdez Group.« less