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

The North American Cordillera is one of the longest mountain belts in the world with important mineralogical resources and it serves as a modern analogue for global tectonic environments. The Chugach-Prince William terrane, in the northern Cordillera, is interpreted to be one of the thickest accretionary complexes in the world. The Paleocene-Eocene (~62-45 Ma) Orca Group comprises the outboard segment of the deep-water sedimentary rocks of the Prince William terrane along the southern Alaska margin. The Orca Group is inferred to be faulted against the adjacent and older more metamorphosed Valdez Group across the Contact fault system, but early stratigraphic models suggested an unconformable contact. The Orca Group is dominated by turbidites, deep-water gravity flow deposits, but it includes conglomerates and ophiolites (oceanic crust) 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 zircons (DZ) from 38 sandstone samples were U-Pb dated by LA-ICPMS at LaserChron in Arizona. Using maximum depositional ages (MDA) and the KS test, we delineate four grain-age populations or facies with unique characteristics: 1) Miners Bay ; 2) Sawmill; 3) Hawkins; and 4) Montague. To understand the significance of the sediment contribution from the adjacent Chugach terrane, the Valdez Group, McHugh Complex, and Potter Creek assemblage, the three primary elements of the Chugach terrane, were modeled to estimate the maximum sediment contribution of the Chugach terrane to the adjacent Orca basin. 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% and this result implies that significant recycling of the Valdez Group into the Orca 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 fault. A metaplutonic arc and adjacent accretionary complex (Valdez) was a likely source. Constraining the provenance and timing of deposition of the Chugach terrane creates a better tectonic framework for how the Cordillera developed over time.