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Evidence for Permo-Triassic Salt Tectonics in the Eagle Basin, Colorado, USA


Combined field mapping, measured stratigraphic sections, and balanced cross-sections of the

Pennsylvanian-aged Eagle Valley Evaporite and overlying Late Pennsylvanian- to Jurassic-aged

strata indicate a long-lived phase of salt tectonics in the Eagle basin, central Colorado. Diapiric

salt structures exposed at the surface represent a series of formerly connected, polygonal salt

walls flanked by deep, elongate minibasins in the southern part of the basin. Previous work in

this area has interpreted these structures to be the result of the Laramide Orogeny and

younger tectonism; however, this phase of salt-influenced deformation in the Eagle Basin has a

similar history as the Paradox Basin to the southwest, and suggests a new paradigm for the

tectonic and stratigraphic evolution of this region.

Structural and stratigraphic analysis of four key regions in the Roaring Fork Valley of the Eagle

basin offers compelling evidence for long-term deformation compatible with salt tectonics prior

to the Laramide Orogeny. This work suggests the following interpretation of a prolonged salt

tectonic history subsequently overprinted by tectonic shortening: (i) over-thickened,

unconformity-bound strata contain abundant growth structures and represent Permo-Triassic

minibasins, which subsided into mobilized Eagle Valley Evaporites; (ii) north-northwest trending

linear structures cored by evaporites represent former diapiric salt walls that grew during

Pennsylvanian through Triassic time between minibasins; and (iii) Laramide-age shortening

resulted in basinal contraction and welded salt walls that were reactivated as thrust structures.

This work has significant implications for the timing and magnitude of both Ancestral Rocky

Mountain uplifts and Laramide-age shortening. Additionally, diagnostic structural and

stratigraphic features present in the Eagle basin are analogous to salt-dominated regions that

have undergone shortening of pre-existing diapirs and minibasins such as the Sivas Basin of

Turkey, the Western Gulf of Mexico, and the Pyrenees, Zagros, and Flinders ranges. This study is

critical to understanding the evolution of salt basins that have been subjected to basement-

involved shortening.