MID-CRETACEOUS VOLCANISM AND FLUVIAL SEDIMENTATION IN THE ALASKA RANGE SUTURE ZONE: IMPLICATIONS FOR THE ACCRETIONARY HISTORY OF THE WRANGELLIA COMPOSITE TERRANE

Cretaceous volcanic and sedimentary strata in eastern Alaska record the evolution of surface processes within the collisional suture zone separating the oceanic Wrangellia terrane to the south from inboard continental terranes to the north. In the Nutzotin Mountains, unnamed nonmarine sedimentary strata (Ks unit of Richter, 1976) depositionally overlie marine volcanic and sedimentary rocks (Chisana Formation and Nutzotin Mtns. sequence) and older rocks of Wrangellia. Measured stratigraphic sections at Beaver Lake and Eucher Mtn. demonstrate that the Ks unit consists mostly of pebble-cobble conglomerate, lithic sandstone, and carbonaceous mudstone/shale with preserved thicknesses >400 m. Deposition occurred in anastomosing to braided channel-bar complexes and vegetated overbank areas with standing water judging from lithofacies data, the terrestrial nature of recovered kerogen and spore assemblages, and delicately preserved terrestrial plant fossils (ferns, sequoia, horsetails). Sediment was eroded mainly from local igneous sources judging from Early Cretaceous-Late Jurassic ages of detrital zircon and amphibole together with the abundance of volcanic and plutonic clasts in conglomerate. Deposition of the Beaver Lake section occurred ~117-116 Ma given that underlying lavas yield ~119-117 Ma ⁴⁰Ar/³⁹Ar ages, the youngest U/Pb detrital zircon ages are ~118-117 Ma, intrusions in lowermost fluvial strata yield ~117 Ma ⁴⁰Ar/³⁹Ar ages, and the strata yield a sparse assemblage of spores indicative of the Late Valanginian to Early Cenomanian. Strata exposed at Eucher Mtn. are younger given that the youngest U/Pb detrital zircon ages are ~119-118 Ma but the youngest ⁴⁰Ar/³⁹Ar detrital amphibole ages are ~111 Ma.

Our results document subaerial exposure of the formerly marine suture zone by ~117 Ma, consistent with previously reported ~117-105 Ma intrusions that crosscut deformed marine strata and faults >40 km to the west (Suslota Pass and Antler Creek plutons). Deposition of Ks strata just prior to slightly younger (~113-94 Ma) fluvial strata (Caribou Pass Fm.) >350 km to the west supports models of diachronous east to west closure of the suture zone. Our results also constrain the age of previously reported dinosaur footprints from the Ks unit that represent the only dinosaur remains known from eastern Alaska.