EVIDENCE OF EASTWARD MIGRATION OF DYNAMIC SUBSIDENCE DURING THE SEVIER-LARAMIDE TRANSITION: A STRATIGRAPHIC COMPARISON OF 3 LARAMIDE SUB-BASINS IN A WEST-EAST TRANSECT ACROSS THE ROCKIES

Dynamic subsidence (mega-regional subsidence), and development of dynamic topography (regional uplift) are commonly overlooked drivers of accommodation that affect stratigraphic architecture in foreland basins. Such regional (100’s- 1000’s km) processes are linked to viscous corner-flow of the asthenosphere above a subducting slab. The long-wavelength subsidence affects a broad area of foreland basins, and can be augmented by basement-cored uplifts. In the Cordilleran Foreland basin, the dynamic component of subsidence initiated during the foreland basin stage and continued well into the "punctuated" foreland basin, or "Laramide", stage of evolution. Numerous authors link flat-slab subduction to the switch from thin- to thick-skinned deformation. Numerical models predict that flat-slab subduction is more conducive to higher dynamic subsidence rates that can be >100 m-m.y.^-1, with arguably > 600 m. sediment accumulation. Additionally, a systematic, eastward migration in dynamic (or, “background”) subsidence is expected from one Laramide basin to another as subduction continues to flatten. However, evidence for the migration of dynamic subsidence is tentative at best, and it is not clear how quickly this migration happens. Stratigraphic architecture provides a critical marker for the migration of dynamic subsidence out one sub-basin and into another. This study presents regional correlations and analysis of Campanian-Paleocene strata that document the relative onset of widespread, reduced accommodation in 3 "Laramide" sub-basins within a southwest to northwest transect across the Rockies. The basins include the Uinta, Piceance and Denver basins where the timing of basin-bounding structures is relatively well known, and sufficient radiometric dates are published. In these 3 basins, the characteristic, wide-spread, low-accommodation zones get progressively younger in an easterly direction, supporting the interpretation that dynamic subsidence progressively migrated eastward from the Uinta to Piceance and Denver basins over about 2 million years.