Process and Products of Long-Transit Forced Regression Driven by Dynamic Subsidence Migration in the Cordilleran Foreland Basin: Insight From Regional Analysis of the Fox Hills Sandstone From Denver, Colorado to Ft. Peck, Montana

Abstract

Evidence for mantle-driven dynamic topography and associated sedimentary system reorganization has grown in recent years thanks to numerous studies in modern systems, and several landmark studies in the Cordilleran Foreland Basin (CFB) that emphasize the importance of dynamic subsidence. Several authors proposed that low-aspect-ratio, long-transit (>100's km) forced regressive sequences (i.e., LAR wedges) in the CFB were driven by the N-NE migration of dynamic subsidence with subduction of the Shatsky Rise (Aschoff and Steel, 2011; Liu et al., 2014). The oldest LAR wedge includes the Sego-Neslen interval of the Uinta basin (∼76-75 Ma), followed by the upper Williams Fork to Ohio Creek Fm. (∼73-72 Ma) spanning the Piceance and Denver basins. Finally, the Fox Hills S.s. (∼72-68 Ma) forms the terminal LAR sequence extending from Denver northward to Alberta (e.g., Eastend and Horeshoe Canyon fms.). The Fox Hills S.s. is the most striking long-transit regressive shoreline, traversing >1300 km in 4 My during the final retreat of the Western Interior Seaway. The Fox Hills S.s. has been studied in isolated areas throughout the CFB (e.g., Washakie Basin), yet the regional framework, sedimentary processes and products, and external drivers for long-transit progradation are not well known. Preliminary subsurface to outcrop correlation and integration of geochronologic data delineates three, sharp-based, strongly offlapping sequences within the Fox Hills S.s. from Denver to Ft Peck. At least two incised valley systems are present: 1) an older system in East-Central WY at the contact between the Lewis Sh. and Fox Hills S.s., and 2) a younger system marked by the Colgate S.s. that rests atop the Fox Hills S.s. near Ft. Peck, MT. The north- to northeast-younging valley systems fed associated north- to northeast-directed, offlapping, regressive shorelines. Analysis of the Fox Hills S.s. near Casper, WY defines 3 constituent high-frequency sequences, and a mixed-energy, forced-regressive deltaic system constituting part of the long-transit forced-regressive sequence set. As in other LAR wedges in the CFB, the Fox Hills S.s. is predominantly composed of sandy, forced-regressive deposits that alternate with muddy, transgressive valley-fills. These new data and preliminary correlations provide insight into the architecture, sedimentary process and products of long-transit (100's km) forced regression driven by the migration of dynamic subsidence in the foreland basin.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016