DETRITAL GEOCHRONOLOGY AND SEDIMENTOLOGY OF PALEOGENE SEDIMENTARY AND VOLCANIC ROCKS EXPOSED ON THE SOUTHWESTERN FLANK OF THE TALKEETNA MOUNTAINS, ALASKA: IMPLICATIONS FOR EXHUMATION AND BASIN DEVELOPMENT WITHIN A REMNANT FOREARC DEPOCENTER

Upper Paleocene–Eocene sedimentary and volcanic strata (Arkose Ridge Fm.) exposed in the southern Talkeetna Mountains record deposition between the Alaska Range-Talkeetna Mountains arc to the north and Chugach accretionary prism to the south. This is the first detailed study of the westernmost portion of the outcrop belt, which extends along the western flank of the Talkeetna Mountains and includes thick, well-exposed outcrops along Willow Creek.

Westernmost ARF strata unconformably overlie a diorite-granodiorite pluton that yields Late Cretaceous U-Pb zircon ages (69-79 Ma; 58 grains total from two samples). Four lithofacies associations characterize the ARF strata: poorly sorted, boulder-pebble conglomerate with minor channelized sandstone (FA1); poorly to moderately sorted, cobble-pebble conglomerate with imbricated conglomerate and channelized sandstone (FA2); channelized sandstone with scours, cross-stratification, and carbonaceous debris (FA3); and basaltic-andesitic lava flows with massive bases and vesicular tops (FA4). U-Pb ages from 189 detrital zircon grains in two sandstone samples reveal three main populations: Late Cretaceous (68-94 Ma; 93% of all grains); late Early Cretaceous (108-114 Ma; 4%) and Paleocene (58-59 Ma; 2%). Conglomerate detrital modes are dominated by plutonic clasts (diorite, granodiorite, gabbro – 31% of all clasts) and volcanic clasts (basalt, andesite – 60% of all clasts). Collectively, these new data indicate: (1) Sediment was deposited by debris flow and streamflow on high-gradient braided streams influenced by episodic volcanic eruptions. (2) Local Cretaceous plutons and Paleogene volcanic centers were important sediment sources. (3) Deposition took place after ca. 59-58 Ma, consistent with <59 Ma isotopic ages reported from ARF volcanic interbeds in other parts of the outcrop belt.

Exhumation of Late Cretaceous arc plutons followed by Paleogene sediment accumulation may have been induced by subduction of a spreading-ridge beneath south-central Alaska. Subduction of young oceanic crust beneath the Cretaceous magmatic arc would prompt increased compressive stress, rock uplift, and unconformity development in the upper plate followed by forearc subsidence and sediment accumulation during passage of a slab window and progressively older crust.