RAPID PROGRADATION AND 400 K.Y. PACING OF THE WORLD’S LARGEST FORELAND CLINOTHEM, CRETACEOUS ARCTIC ALASKA

An Aptian–Cenomanian depositional sequence stretches 1000 km across Arctic Alaska and represents the world’s most voluminous (1.2 million km³) and highest relief (>1 km thick) foreland basin clinothem. Three recent, huge oil discoveries in the Torok-Nanushuk clinothem attest to its petroleum potential. However, quantitative understanding of clinothem dynamics has been hindered by imprecise biostratigraphy with uncertainties >5–10 m.y. We present zircon U/Pb geochronology that quantifies the provenance and timing of clinothem deposition from 20 sites. Youngest detrital zircon (DZ) age populations match tephra ages and thus reliably approximate depositional ages (2σ ≈ 1 m.y.). We integrate our chrono- and sequence stratigraphy to quantify clinothem dynamics. DZ ages and clinoform dip directions indicate a mainly Russian Chukotka provenance and axial sediment routing in the Colville foreland basin. The DZ data reveal a progradational surge between 115 and 105 Ma when the shelf margin prograded more than 450 km in a supply-dominated system. Progradation slowed after 105 Ma and shelf-margin trajectories became more complex, with significant retrogradational and aggradational segments. Rates of progradation and sediment flux decreased three-fold, and the system prograded only another 125 km eastward to an ultimate shelf margin at 98 Ma. A prominent lowstand systems tract ensued, and deposition did not renew on the relict shelf until 95.5 Ma. The overlying depositional sequence displays a change from Chukotkan to Brooks Range provenance and significant reductions in sediment flux and clinoform relief. We estimate ~400 k.y. cyclicity for the Aptian–Cenomanian clinothem sequences, which is equivalent to the Milankovitch long eccentricity cycle, and suggest that deposition was paced orbitally via insolation-driven fluctuations in sea-level and perhaps hydrology. By integrating geochronology with seismic stratigraphy, our study demonstrates how changes in depositional rates impact clinothem architecture. We interpret tectonic forcing of 3^rd order sequences (e.g., flux, progradation rate) and climatic pacing of 4^th order sequences (e.g., shelf margin periodicity). Overall, our findings have implications for clinothem dynamics, Cretaceous greenhouse climate, and Arctic hydrocarbon resources.