Pre-Laramide salt tectonics in the Eagle Basin: a new paradigm for the tectonic evolution of central Colorado

Abstract

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.