CRETACEOUS TIME-TRANSGRESSIVE OBLIQUE ISLAND-ARC COLLISION IN SOUTHWESTERN ALASKA: STRATIGRAPHIC EVIDENCE FROM THE KAHILTNA ASSEMBLAGE AND KUSKOKWIM GROUP

New stratigraphic and compositional data from Cretaceous strata in the Alaska Range and Kuskokwim Mountains of southwestern Alaska record dominantly west-directed axial sediment transport in response to diachronous collision of the Wrangellia composite terrane. The Kahiltna assemblage exposed in the eastern McGrath and western Talkeetna quadrangles consists of >5.56 km of variably channelized, mixed sand-mud bathyal submarine-fan strata. Sandstone (Q₂₃F₉L₆₈) and conglomerate compositions and diagnostic detrital zircon assemblages indicate sediment contributions from both the island-arc complex and the Mesozoic continental margin. We advance a model for collision in which the Kahiltna basin was fed by sediment recycled from uplifted parts of the suture zone to the northeast. The lower part of the Kahiltna assemblage in southwestern Alaska contains 110 Ma detrital zircons; the overlying 4.32 km Kahiltna assemblage strata therefore represents Albian or younger basin development.

Upper Cretaceous submarine-fan strata of the Kuskokwim Group are exposed in the Sleetmute and Iditarod quadrangles immediately west of the Kahiltna assemblage. We recognize multiple discreet episodes of basin development in the Kuskokwim Group on the basis of lithofacies distributions, sandstone provenance, and paleocurrent estimations along with limited age determinations taken from the literature. Cenomanian to Campanian strata exposed south of the Iditarod-Nixon Fork fault (INFF), along the Kuskokwim River corridor, record westward progradation of intraslope and deltaic environments presumably along the axis of the suture zone. Sandstones from this part of the basin have a complex provenance that includes contributions from arc and continental sources (Q₂₉F₁₇L₅₄). Modal concentrations contrast with Campanian strata exposed northwest of the INFF. These strata contain abundant mono- and polycrystalline quartz (Q₆₆F₇L₂₇) and developed on a siliciclastic shelf that formed the northern basin margin during the waning stages of marine sedimentation.

We interpret the Kahiltna assemblage and Kuskokwim Group to collectively represent time-transgressive axial migration of the marine depocenter in response to progressive suture-zone closure.