DEPOSITIONAL SYSTEMS AND PROVENANCE OF THE KUSKOKWIM GROUP: INSIGHTS INTO THE PALEOGEOGRAPHY OF THE LATE CRETACEOUS FORELAND BASIN SYSTEM OF SOUTHWESTERN ALASKA

The Kuskokwim Group, a thick (perhaps 12 km) Upper Cretaceous siliciclastic marine unit exposed across a 70,000 km²area in southwestern Alaska, provides an important link in our understanding of the paleogeography of the northern Cordillera. Investigations in this remote, relatively low-lying expanse of wilderness have been limited by logistical constraints and a relative paucity of bedrock exposure. Close collaborations with the U.S. Geological Survey and U.S. Bureau of Land Management, however, provided access to outcrops during three field seasons and, therefore, a foundation to continue building our understanding of the stratigraphy and tectonics of the region.

Analysis of stratigraphic assemblages, provenance trends, and age distributions of Kuskokwim Group strata highlights protracted Albian through Maastrictian marine sedimentation derived from at least three regional entry points. Siliciclastic shoreface and shelf strata comprise the northern basin margin, whereas strata exposed along the eastern and southern margins of the basin represent distributary mouth bar and delta front paleoenvironments. Transverse sediment transport systems along the margins of the basin transitioned to axial intraslope and base-of-slope distributary systems. Analysis of sandstone framework grain compositions and detrital zircon age distributions confirm that well-integrated drainage systems sampled diverse volcanic, metamorphic, and sedimentary provinces north and south of the Kuskokwim basin. Average sandstone framework grain populations include monocrystalline quartz (10%), polycrystalline quartz (13%), plagioclase (11 %), and lithic grains of volcanic (18%), sedimentary (23%), and metamorphic (21%) affinities. Detrital zircon samples from the Kuskokwim Group have age populations that span from the Late Cretaceous to Early Proterozoic. Some detrital zircon populations are consistent with signatures found in the Yukon-Tanana terrane, suggesting long-distance transport of sediment into the Kuskokwim basin. Other detrital zircon populations in the Kuskokwim Group are similar to populations in the age-equivalent Lower Cantwell Formation, suggesting the existence of a Late Cretaceous foreland basin system that extended through much of central and southwestern Alaska.