%APope, M.D.%D2019%I %K %MOSTI ID: 10181886 %PMedium: X %TProvenance of sandstone clasts from conglomerate of the Paleocene- Eocene Orca Group in Prince William Sound, Alaska. %XThe thick flysch facies of the Cretaceous to Eocene Chugach-Prince William terrane (CPW) represents a thick accretionary complex that extends approximately 2200 km across southern Alaska, and in the central area is comprised mainly of the Valdez Group and the Orca Group (Fig. 1) (Garver and Davidson, 2015; Davidson and Garver, 2017). The Valdez Group is traditionally viewed as a Campanian to Maastrichtian turbidite deposit with mafic volcanic rocks that have experienced lower greenschist facies metamorphism (Dusel-Bacon, 1991; Gasser et al., 2012). The Orca Group is Paleocene to Eocene turbidite and volcanic deposit that, in most places, has undergone prehnitepumpellyite facies metamorphism (Dusel-Bacon, 1991; Wilson et al., 2012). The relationship between the Valdez Group and the Orca Group is poorly understood (Moffit, 1954). A common hypothesis suggested long ago is that they are stratigraphically related and are a continuous sequence (Capps and Johnson, 1915). Given recent zircon dating, the Valdez Group appears to have maximum depositional ages (MDA) of 75-65 Ma and the deposition of the Orca Group is between 60-50 Ma (Davidson and Garver, 2017). In this case, deformation of the Valdez Group may have occurred 65-60 Ma, just before the deposition of the oldest Orca Group turbidites began. Thus, the youngest strata of the Valdez Group must be older than the oldest strata of the Orca Group. An alternative hypothesis is that the Orca Group formed in a different location and was translated to its current position along strike slip faults after the deformation of the Valdez Group (cf. Plafker et al., 1994). This idea would mean that the ages of the two groups may overlap in age, and the time of juxtaposition of the Orca Group to the Valdez Group is unknown but important. After the deposition of the bulk of the Orca Group was completed, the CPW experienced plutonism by the near-trench Sanak- Baranof Belt (SBB) and the Eshmay plutons (Cowan, 2003; Davidson and Garver, 2017). If a pluton crosscuts two terranes then the age of that pluton is the minimum age that the two terranes were juxtaposed (Coney et al., 1980). The SBB plutons intruded the CPW from 63-47 Ma, with a distinct age progression from 63 Ma to the west to 50-47 Ma to the east (Davidson and Garver, 2017). In Prince William Sound the CPW terrane is also intruded by the Eshmay Suite Plutons (ESP) around 37-41 Ma (Fig. 1) (Johnson, 2012; Davidson and Garver, 2012; Garcia et al., 2019). The Eshamy suite plutons could be explained by high heat flow that melted Orca Group sediments and these melts then mixed in with mantlederived basalts (Johnson, 2012). The ESP stitch the two terranes, as they occur on both sides of the Contact Fault System (Fig. 1) (Davidson and Garver, 2017). A key link between the Orca and Valdez Groups may be conglomerates that occur in the Orca Group. There are five main localities of conglomerates in PWS, and some of the most significant exposures are in eastern and northern PWS. These conglomerates were described by Grant and Higgins (1910) as being near the bottom of the Orca Group stratigraphy, specifically at the basal unconformity. However, Capps and Johnson (1915) described the conglomerates as being at the top of the Orca Group, occurring after and interleaved with basaltic volcanic rocks (cf. Tysdal and Case, 1979). If the Valdez Group is the source of the Orca Group conglomerate clasts, then the two terranes were adjacent at a time earlier than previously known (38-39 Ma) (Davidson and Garver, 2017). Capps and Johnson (1915) proposed that the matrix of the conglomerates and the majority of the clasts were derived from the Valdez Group. They also suggest that a few clasts could be derived from the greenstones of the Orca Group. The provenance of the Orca Group conglomerates is important in our understanding of the relationship between the Valdez and Orca Groups as well as our overall understanding of the Cordilleran tectonics. This study will focus on understanding the Valdez Group and the Orca Group through the study of detrital zircons from sandstone clasts from the Orca Group Conglomerates and the host strata to those conglomerates. Country unknown/Code not availablehttps://doi.org/10.18277/AKRSG.2019.32.24OSTI-MSA