DIFFERENTIATING DIFFERENT SUBSIDENCE MECHANISMS IN THE CORDILLERAN FORELAND BASIN THROUGH THE LATE CRETACEOUS: EXAMPLES FROM THE PICEANCE AND DENVER BASINS

Although flexural subsidence has long been considered the dominant mechanism of the development of the Cordilleran foreland basin (CFB), dynamic subsidence related to mantle processes has been increasingly invoked to explain the subsidence and migration of depocenters in the CFB. To determine and distinguish different subsidence mechanisms responsible for the development of CFB, detailed stratigraphic analyses of the Upper Cretaceous strata in two Laramide basins—the Piceance and Denver basins—were conducted. The profusion of well logs and previous stratigraphic investigations allow for the high-resolution reconstruction of the geo-history in these two basins through the Late Cretaceous. Seventy-six well logs were used to generate short-term, high-resolution isopach maps using IHS Petra. The stratal thickness trend observed from successive isopach maps was used as a proxy to characterize the spatial distribution of tectonic subsidence in the study area through time. The 100-80 Ma isopach maps reveal the stratal thickness increases to the east in both basins, reflecting dominant flexural subsidence caused by the loading of the Sevier thrust belt. The 80-74 Ma isopach maps show increased sediment accumulation rates and thickening trends inconsistent with the predicted flexural subsidence profile, indicating the influence of dynamic subsidence in central Colorado since ~80 Ma. The change in isopach pattern can be linked to the migration of a hypothesized oceanic plateau that flattened Farallon Plate subduction, in front of which the mantle downwelling would cause dynamic subsidence in the Piceance and Denver basins. The 74-66 Ma isopach maps point to more complex interactions of different subsidence mechanisms. The subsidence in both Piceance and Denver basins likely was strongly influenced by flexural subsidence caused by the development of Laramide-style uplifts and a component of additional dynamic subsidence during this time. High-resolution results from this study provide better constraints of the timing and effects of different subsidence mechanisms in the Late Cretaceous CFB and more detailed insights into the role of dynamic subsidence on the development of retroarc foreland basins influenced by flat-slab subduction.