Provenance and Paleoflora of the Cretaceous Caribou Pass Formation, South-Central Alaska: Development of a Collisional Foreland in the Final Stages of Island Arc Accretion

Newly-documented occurrences of Cretaceous, fossil-leaf bearing strata along the inboard margin of the Wrangellia composite terrane reflect a transition from marine sedimentation in a closing ocean basin to nonmarine sedimentation in basins associated with crustal shortening and development of the Alaska Range suture zone. Previous provenance studies of Upper Jurassic–Lower Cretaceous marine strata in this region document exhumation associated with island arc accretion involving detrital contributions from Mesozoic island-arc sources (Mz-46%) and Paleozoic–Precambrian sources (Pz-Pc – 16%-38%) associated with the continental margin. New U-Pb SHRIMP detrital zircon ages from Cretaceous nonmarine strata in the northwestern Talkeetna Mountains (informally referred to as the Caribou Pass formation) reveal Mesozoic sources (Mz-88%) with minor contributions from Paleozoic–Precambrian sources (Pz-Pc – 5%-7%). A maximum depositional age of Late Cretaceous is inferred from a peak probability age of 81.8 Ma and TuffZirc age of 79.9 +/- 1.3 Ma. These ages corresponds with Cretaceous palynomorph age ranges documented from this unit.

Preliminary collections of a leaf megafossil impression flora reveal the occurrence of ferns (Cladophlebis), conifers (Metasequoia), and dicots (including Dicotylophyllum, at least one member of the Menispermaceae, platanoid forms, and others). These remains may indicate temperate climate conditions in mid- to high-latitude regions of Alaska during the Cretaceous and provide a biostratigraphic link to similar paleofloras that have been documented from Upper Cretaceous nonmarine strata elsewhere in southern Alaska.

The Caribou Pass formation and age-equivalent strata throughout the Alaska Range suture zone record the first occurrence of nonmarine sedimentation associated with the final stages of island arc accretion. These strata may represent exhumation of primarily Mesozoic source areas of the Wrangellia composite terrane in a collisional foreland basin setting. Further study is needed to compare trends in provenance and paleoflora occurrences from nonmarine Cretaceous strata throughout the Alaska Range suture zone.