Provenance of sandstone conglomerate clasts of the Orca Group in Prince William Sound, Alaska

The Campanian to Paleocene Valdez Group of the Chugach terrane and the Paleocene to Eocene Orca Group of the Prince William terrane are adjacent units dominated by turbidites and thought to be separated by the Contact Fault. The relationship of these two terranes is unclear, but a key link between them may be clasts in conglomerates that occur in the Orca Group. The stratigraphic position of the conglomerates is uncertain but is believed to be at the top or bottom of the Orca Group. This study focuses on four conglomerate localities in Prince William Sound: Miners Bay, Simpson Bay, Galena Bay, and Outpost Island. Detrital zircons from sandstone clasts and matrix were U/Pb dated by LA-ICPMS, and the Maximum Depositional Age (MDA) for each sample was resolved by determining the youngest zircon population. At Miners Bay the conglomerate (matrix MDA ~59 Ma) is poorly sorted, clast-supported, and dominated by coarse-grained sandstone and volcanic clasts. Two sandstone clasts from this site yield MDAs of ~60 and ~62 Ma with major age populations at 61 Ma and 63 Ma, respectively. At Simpson Bay the conglomerate (matrix MDA ~57 Ma) is clast-supported, disorganized, and dominated by coarse-grained sandstone clasts but has volcanic and plutonic clasts. Four sandstone clasts from this site yield MDAs of ~62 Ma, ~60 Ma, ~58 Ma, and ~66 Ma, with major age populations at 69 Ma, 67 Ma,  61 Ma, and 74 Ma, respectively. The clasts that yield an MDA of ~62 Ma and ~66 Ma have distinct veins with a mineral assemblage consisting of epidote, quartz, and potassium-rich feldspar. The Galena Bay conglomerate (matrix MDA ~52 Ma) is massive, clast-supported, and dominated by sandstone and volcanic clasts. Two dated sandstone clasts from this site have MDAs of ~56 Ma and major age populations at 58 Ma and 64 Ma. The Outpost Island conglomerate (matrix MDA ~40 Ma) is dominated by clasts of volcanic rocks and sandstone. A sandstone clast from this site has an MDA of ~60 Ma, and a major age population at 73 Ma. The MDAs of the conglomerates indicate that they occur throughout the Orca Group stratigraphy. The MDA and probability density functions for each clasts can be matched with dated bedrock samples to determine their source. This data for the majority of clasts indicates that they originated from the cannibalization of the Orca Group. Two epidote-bearing clasts from Simpson Bay have a Valdez Group affinity, and may provide the earliest link between these two terranes at ~57 Ma.