Analysis of Upper Cretaceous sedimentary and volcanic strata in the Wrangell Mountains of south-central Alaska provides an opportunity to study the tectonics, depositional systems, and provenance of a forearc basin that developed along an accretionary convergent plate boundary. New data from the 1150 m thick MacColl Ridge Formation indicate that deposition occurred during the Campanian on a coarse-grained submarine fan that was derived from an uplifted allochthonous terrane exposed in the hanging wall of a fault system that separated the forearc basin from the subduction complex. New age controls include palynoflora indicative of a late middle to late Campanian age, and compatible radiometric age determinations of volcanic vitric-crystal tuffs near the top of the formation which have 40Ar/39Ar isochron ages of 79.4 ± 0.7 and 77.9 ± 2.1 Ma. Sedimentological and paleontological data show that sedimentation occurred on the inner portions of a sand- and gravel-rich submarine fan system. Evidence for this interpretation includes dominance of channelized sediment gravity flow deposits, particularly turbidites and debris flows; microflora indicative of open-marine conditions; unidirectional paleocurrent indicators; and syndepositional slump features. The pyroclastic eruptions that formed the vitric-crystal tuffs of the MacColl Ridge Formation are interpreted as products of the Late Cretaceous Kluane magmatic arc that bordered the forearc basin to the north. Sandstone and conglomerate compositional data combined with northward-directed paleocurrent indicators suggest that detritus was derived mainly from igneous rocks of the allochthonous Wrangellia terrane located in the hanging wall of the Border Ranges fault system along the southern margin of the basin. From a regional perspective, deposition of the MacColl Ridge Formation was coeval with the early part of Campanian-Maastrichtian synorogenic sedimentation and contractile deformation documented throughout the northwestern Cordillera.

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