ZIRCON FACIES IN THE PALEOCENE-EOCENE ORCA GROUP INDICATE A PROVENANCE LINK TO THE CHUGACH TERRANE, PRINCE WILLIAM SOUND, ALASKA

The Paleocene-Eocene Orca Group comprises the outboard segment of the turbidite facies of the Chugach-Prince William terrane along the southern Alaska margin. The Orca Group is inferred to be faulted against the inboard and older Valdez Group across the Contact fault system, but early stratigraphic models suggested an unconformable contact. The Orca Group is dominated by turbidites, but it includes conglomerates and ophiolities with an uncertain stratigraphic position. This study focuses on samples from the Orca Group in Prince William Sound (PWS) to determine their age, stratigraphic setting, and provenance. Detrital zircon (DZ) from 38 sandstone samples were U-Pb dated by LA-ICPMS at LaserChron. Using maximum depositional ages (MDA) and the KS test, we delineate four distinctive zircon facies: 1) Miners Bay (~61-59 Ma, n=1347 grains); 2) Sawmill (59-55 Ma, n=1232); 3) Hawkins (55-52 Ma, n=1265); and 4) Montague (52-31 Ma, n=1237). To understand the significance of the sediment contribution from the adjacent Chugach terrane, a synthetic DZ signal was made from the Valdez Group (n = 2762), McHugh Complex (n= 916), and Potter Creek assemblage (n = 874), the three primary elements of the Chugach terrane. These composites were used in a three-component model to estimate the maximum sediment contribution (MSC) of the Chugach terrane to the Orca. The Miners Bay facies (primary populations at 70 and 108 Ma) has a MSC of ~38% and the Sawmill facies (59 and 108 Ma) has a MSC of ~48%. The Hawkins and Montague facies have a primary population of 71 Ma characterized by a MSC of ~64-70%. Analysis of the DZ facies show that the provenance changed over time and all facies have a variable but significant fraction of young grains (65-55 Ma) that undoubtedly represent active volcanism. Our modeling suggests that Chugach-supplied sediment in the Orca Group may be as high as 75%; this result implies that significant erosional exhumation of the Valdez Group occurred from 62-52 Ma. The Miners Bay facies and the inboard Valdez Group show DZ and MDA overlap casting doubt on models that rely on the Contact fault as a terrane-bounding structure.