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

Detrital zircon U-Pb ages from turbidites of the Upper Cretaceous Valdez Group of the Chugach terrane in Prince William Sound, Alaska help demonstrate the complex relationship with the outboard turbidites of the Paleocene-Eocene Orca Group. The Valdez and Orca Groups, while similar in composition, historically have been divided by metamorphic grade, age, and geography. Current mapping separates the older Valdez Group of the Chugach terrane from the younger Orca Group of the Prince William terrane along the Contact fault system. Turbidites of the Valdez Group are typically more deformed and record greenschist facies metamorphic conditions, while the younger Orca Group rocks record prehnite-pumpellyite conditions. Eight sandstone samples from the Valdez Group and nineteen from the Orca Group were collected from northern Prince William Sound and along the Richardson Highway. Detrital zircons were U-Pb dated at the Arizona Laserchron Center by LA-ICP-MS. Maximum depositional ages (MDA) of the Valdez Group range from 84 to 60 Ma and are concentrated in three clusters: 85-84 Ma, 70-68 Ma, and 62-60 Ma. The oldest cluster includes a previously published sample from Mount Magnificent near Anchorage and one from our study along the Copper River near Chitina that have similar Late Cretaceous populations between 84 and 97 Ma and in the Early Jurassic (186 and 196 Ma). Six samples with MDAs between 70-68 Ma have similar grain-age distributions with the majority of grains forming pronounced Late Cretaceous populations between 73 and 71 Ma. Three samples collected from rocks mapped as the Valdez Group along the Richardson Highway have MDAs between 62-60 Ma, and these have similar MDAs and grain-age distributions to rocks of the lower Orca Group in Prince William Sound. These samples are dominated by zircons with Late Cretaceous populations between 72-70 Ma and Early Cretaceous between 109-107 Ma. The young samples are Paleocene and are younger than what is traditionally viewed as the age range of the Valdez Group, and thus they present a dilemma. There are several options for their occurrence, including: 1) Orca Group rocks occur in the Valdez Group as structural slices or are depositional and subsequently folded into the Valdez Group; or 2) the youngest Valdez Group rocks are stratigraphically equivalent to those of the oldest Orca Group.