DETRITAL ZIRCON CONSTRAINTS ON ARCTIC ALASKA FORELAND BASIN EVOLUTION: BROOKIAN CHRONOSTRATIGRAPHY, CLASTIC PROGRADATION, PROVENANCE, AND OROGENIC EXHUMATION

A precise chronostratigraphic and paleogeographic framework across the Cretaceous Arctic Alaska foreland allows quantification of hinterland tectonic influences on basin evolution. The construction of this framework, however, has been limited by imprecise biostratigraphic and provenance data. We report new detrital zircon U/Pb geochronology and fission track thermochronology that provide absolute constraints on Brookian deposition and mountain-building events. First, determination of young detrital zircon U/Pb age populations in Brookian sandstone permits refined chronostratigraphy of 116-78 Ma strata. The young grains were likely derived from coeval volcanism in Russian Chukotka. Second, dating a suite of samples collected from proximal to distal settings in the voluminous Torok-Nanushuk sequence reveals that it prograded over 525 km from west to east between 116 and 104 Ma. The rapid progradation of a high-relief clinoform deposystem (~45 km/m.y.), sustained for 12 m.y., suggests a supply-dominated system. This deposystem filled relict Colville basin accommodation that had likely developed as a flexural response to earlier Brooks Range tectonic loading. Third, a major provenance change occurred following progradation of the Torok-Nanushuk sequence. Samples from Lower Cretaceous Torok-Nanushuk strata are dominated by zircons derived from Triassic flysch (360–200 Ma), clearly of Chukotkan provenance to the west. In contrast, samples from Upper Cretaceous strata (Tuluvak, Schrader Bluff, and Prince Creek Formations) lack the Triassic signature and instead are dominated by 700–360 Ma zircons, suggesting derivation from Paleozoic units within the Brooks Range to the south. Overall, the change in provenance at ca. 95 Ma suggests a shift from longitudinal, east-flowing sediment dispersal to transverse, north-flowing dispersal. This shift is consistent with post-95 Ma erosional unloading of the Brooks Range. Finally, detrital thermochronometers from basin fill samples quantify hinterland exhumation rates during deposition. Initial zircon fission track results suggest periods of orogenic growth (130–100 Ma), decay (100–60 Ma), and rebirth (60 Ma–present) that are consistent with geological constraints.