Influence of Pennsylvanian-Triassic salt tectonics on laramide shortening in central Colorado: a new tectonic model, The

Abstract

The Eagle Basin of central Colorado is a Pennsylvanian- to Permian-aged evaporite flexural basin formed in the foreland of adjacent Ancestral Rocky Mountain uplifts. Previous tectonic studies in this region focused on Laramide shortening and later deformational events, potentially overlooking a long period of Paleozoic to Mesozoic salt tectonics that significantly affected the pre-Laramide deformational template. Evidence for a pre-Laramide phase of salt tectonics includes halokinetic growth strata and vastly varying thicknesses of Pennsylvanian to Triassic aged units including the Eagle Valley/Gothic, Maroon, and State Bridge Formations, suggesting development of large, salt-floored minibasins, bounded by a polygonal system of salt walls. Structural and stratigraphic data collected in the Hardscrabble, Wolcott, and Bellyache minibasins through field mapping, stratigraphic logging and sampling are integrated into a series of new maps, stratigraphic correlations, structural cross sections, and sequential restorations. Newly identified hook halokinetic sequences have been identified along the flanks of the aforementioned minibasins, where Eagle Valley Formation strata are occasionally sub-vertical to overturned in close proximity to salt walls (within 200 m). These beds are truncated by more gently dipping strata of the Maroon Formation, which flatten up section and away from the salt walls. Additionally, dramatic thickness changes in the Maroon (100 m scale) and younger State Bridge Formations (1,000 m scale) are present across the Eagle Basin, indicating that the minibasins vary in age, constraining the timing and spatial distribution of salt evacuation and diapirism. These new interpretations highlight halokinetic deformation localized along the flanks of the minibasins, adjacent to salt walls that developed prior to regional shortening across the basin. Subsequent basement involved shortening during the Laramide orogeny was locally strongly influenced by the pre-existing salt walls and minibasins.