U/PB DETRITAL ZIRCON STUDY OF THE UPPER CRETACEOUS TO EOCENE STRATA OF KODIAK ISLAND, ALASKA

Detrital zircon samples collected on Kodiak Island are used to track the progression and evolution of source rocks for the Cretaceous-Tertiary Chugach-Prince William terrane in south-central Alaska. The stratigraphic units sampled include the Kodiak Formation, Ghost Rocks Formation, Sitkalidak Formation, and the Narrow Cape Formation. As expected for detrital suites, each sample yields detrital zircon ages that are wide ranging, and in this case from about 55 Ma to 2306 Ma. The youngest population of grains in the Kodiak Formation (Camp-Maastrictian) yields a peak age of 63 Ma, with other prominent peaks ranging from 70 to 371 Ma. The Kodiak Formation includes Precambrian peaks ranging from 1199 Ma to 2306 Ma. The tectonically adjacent but more outboard Ghost Rocks Formation (probably Paleocene) contains grains primarily in the Cretaceous to early Triassic, but with a prominent youngest population at 66 Ma in one sample, and 67 Ma in another; Precambrian grains in this unit yield peaks ranging from 1038 Ma to 1517 Ma. The more outboard Sitkalidak Formation (Eocene) has young populations of 55 Ma and 59 Ma but this unit also yields older component ages of 69 Ma to 235 Ma, and 11 Precambrian grains with peaks at 1013 Ma, 1058 Ma, and 1989 Ma. The Narrow Cape Formation (Miocene) lies unconformably above the Sitkalidak Formation and is the youngest of the rocks collected in the section. This formation produced age populations in the Paleocene, mid-Cretaceous, mid-Jurassic, and early Triassic: the youngest component is 62 Ma, with the oldest population at 221 Ma. These data show that in most cases a maximum age deposition for each of these formations can be constrained through detrital zircon dating. A key distinctive attribute of these data is that older formations (Kodiak) contain a significant number of Precambrian grains, which may mean they were derived from a broader source area, or perhaps a different source altogether.