THE IMPORTANCE OF STRATIGRAPHIC ARCHITECTURE IN TRACKING MIGRATION OF DYNAMIC SUBSIDENCE IN THE CORDILLERAN FORELAND BASIN

Recent 1D backstripping along the UT-CO transect of the Cordilleran Foreland Basin (CFB) highlighted the importance of dynamic subsidence in a foreland basin that has traditionally been attributed to flexure. The transect of backstripped well-data showed migration and the geometry of long-wavelength, mantle-driven subsidence. The dynamic basin was interpreted to be 600-800km wide and <600m deep (Liu et al., 2014). Although thickness and gross stratal geometries were helpful in identifying the basin position through time, the detailed stratigraphic architecture and sedimentary process within the sequences provide critical insight into basin formation and fill. Using detailed sedimentology and regional outcrop-to-subsurface correlation, this study delineates high-resolution stratigraphic architecture within the Uinta-Piceance-Denver basins showing a northeast-directed migration of flat-to-falling sequence sets compatible with dynamic subsidence migration. Flat-to-falling sequence sets that span >400 km and have anomalously fast progradation (i.e., LAR wedges of Aschoff and Steel, 2011) may mark the passage of the dynamic basin-axis, and suggest that migration of dynamic subsidence may be faster than previously thought (~3-5 My). Progressive removal of the dynamic component of subsidence as the basin-axis migrates causes long-transit transgressions and regressions that lead to rapid, and anomalously fast and far progradation as seen in the LAR sequence sets in the CFB. Flat-to-falling sequence sets are succeeded by expansive (>1000’s km) unconformities and thin conglomerate sheets marking complete removal of dynamic subsidence. In the CFB, the oldest LAR sequence includes the Sego-Neslen interval spanning the Uinta basin (~76-75 Ma). Next, upper Williams Fork to Ohio Creek formations (~73-72 Ma) form a LAR sequence set extending from the Piceance to Denver basins. Finally, the Fox Hills S.s. (~69-66 Ma) forms a LAR sequence set that spans the Denver basin northward to the Dakotas. Each LAR succession is capped by an extensive unconformity and conglomerate sheet marking the complete passage of the dynamic basin. Paleocurrents and sequence progradation direction suggest that migration of dynamic subsidence was northeast-directed, and compatible with the Shatsky Rise trajectory.