DETRITAL ZIRCON GEOCHRONOLOGY OF THE UPPER CRETACEOUS KUSKOKWIM GROUP, SOUTHWESTERN ALASKA

The Upper Cretaceous Kuskokwim Group (ca. 95-77 Ma) is the most extensive rock unit in southwestern Alaska covering an area of 90,000 sq km. This turbidite basin formed between previously amalgamated terranes that shed detritus from Mesozoic arc and oceanic rocks, Paleozoic continental margin deposits, and Precambrian basement. For this study, detrital zircons from 16 sandstone samples were analyzed by LA-ICP-MS at Univ. of British Columbia and Washington State Univ. (approx. 100 grains/sample).

Zircon populations from 11 samples of bona fide Kuskokwim Gp. are broadly similar and provide the first detrital zircon barcode for this unit. A composite of these 11 samples shows two main peaks at 93 and 185 Ma and lesser peaks at 134, 278, 350, 391, 515, 1122, and 1822 Ma. In addition to these peaks, nearly every 50-m.y. interval as far back as 3000 Ma is occupied by dated zircons. The youngest grains in each of the 11 samples are as follows: 72±6, 77±5, 78±2, 80±2, 81±2, 85±3, 89±1, 91±1, 92±3, 100±2, and 105±4 Ma. These results are consistent with the previously established age of the Kuskokwim Gp., but the abundance of post ~90 Ma zircons is surprising given the overwhelmingly common Turonian (93.5-89 Ma) fossil ages for the unit. The Kuskokwim Gp. barcode is easily distinguished from detrital zircon barcodes of flanking Cretaceous sandstones (the restricted Gemuk Group to the west and the Koksetna River sequence to the east), both of which lack significant Precambrian populations.

Using our composite Kuskokwim Gp. template, we evaluated detrital zircon populations of 5 other samples whose identities were uncertain based on map position and petrography. Three are excellent matches for Kuskokwim Gp. and two are passable matches. The excellent matches extend the map distribution of the Kuskokwim Gp. and establish two important geologic interpretations: 1) Kuskokwim Gp. was involved in thrusting of older terranes on both western and northeastern basin margins, and 2) an anomalous metamorphic belt along the Mulchatna fault includes Kuskokwim Gp. protoliths. The two passable matches lack a 93-Ma peak, possibly suggesting they are older. However, these two coarser than normal, chert-rich samples are petrographically similar to classic Kuskokwim Gp.; hence their variation from the composite barcode is more likely due to local effects.