MULTI-STAGE CRETACEOUS BASIN DEVELOPMENT IN SOUTHWESTERN ALASKA: MARINE DEPOSITIONAL RESPONSES DURING SYN- TO EARLY POST-COLLISION (?) MARGIN EVOLUTION

Recent studies of the Kahiltna assemblage in the western Alaska Range demonstrate that west-directed basin-floor submarine-fan systems inboard of the Wrangellia composite island arc terrane (WCT) were fed from the subearial suture zone between the obliquely colliding allochthonous block and the pericratonic margin of North America. Two implications are that: (1) the composite island-arc terrane was in relatively close proximity to the North American margin by late Early Cretaceous time, and (2) the syn-collisional marine depocenter may have migrated southwestward in response to progressive suture-zone closure. Indeed, the Kuskokwim Group, located immediately west of the Alaska Range in the northern Kuskokwim Mountains, contains similar marine lithofacies that were derived from the east. Lithofacies distributions, paleocurrent trends, and provenance relationships, however, indicate a more complex, multi-stage history of Upper Cretaceous marine basin development that probably represents the final stages of marine deposition inboard of the WCT prior to incipient margin-parallel shearing.

The Kuskokwim Group contains at least three facies associations that collectively represent an upward-shallowing trend throughout Late Cretaceous time. Cenomanian-Coniacian deep-marine strata in the western-most study area represent the fringes of basin-plain distributary fans that were derived from the northwest (present coordinates). Coniacian-Campanian strata exposed in the central and eastern outcrop belt record westward deepening from deltaic to base-of-slope environments of deposition. Turonian-Campanian shoreface strata exposed north of the Iditarod-Nixon Fork fault represent a northeast-trending siliciclastic shelf along the northern basin margin. In contrast to the bi-modal (arc + continent) age affinities of grains from Kahiltna assemblage sandstones, Kuskokwim Group samples (n = 3) from the northern Kuskokwim Mountains yield grain occurrences in multiple Proterozoic, Paleozoic, and Mesozoic probability age bins. The resulting detrital “bar code” is insufficient to identify individual source regions, but, along with framework grain compositions, suggests that a well-established drainage network sampling multiple diverse tectono-stratigraphic terranes fed the Kuskokwim basin.