PROTEROZOIC RIFT SYSTEMS AND THEIR INFLUENCE ON THE FORMATION OF THE LATE CRETACEOUS-EARLY CENOZOIC WYOMING SALIENT

The salients and reentrants of basement-detached thrust belts often reflect antecedent rift architecture and subsequent thermal subsidence: Salients can be thought of as “downplunge projections” of basin fill, whereas reentrants mark regions of disrupted basin stratigraphy. The Wyoming salient of Utah, Wyoming, and Idaho is an unusually symmetric thrust system mostly detached on Cambrian Ophir/Gros Ventre shales. The southern boundary of the salient is the east-west trending Uinta Mountain Range, a contractional inversion of a Neoproterozoic rift basin. The northern boundary traces the southwestern margin of Mesoproterozoic Belt basin-equivalent rocks and the basement-involved Teton-Gros Ventre Ranges. These features give the impression that the Wyoming salient funneled detached shortening into the intervening low between basement-cored structures.

The Uinta Range exposes the Neoproterozoic Uinta Mountain Group (UMG) along its anticlinal axial trace. On the western limit of the range, Cambrian strata rest on the UMG, however through most of its length, the UMG is disconformably overlain by Devonian-Mississippian strata, suggesting that the Uinta Range had been structurally inverted prior to this time. This Ordovician–Devonian (?) inversion (Cortez-Tooele arch) removed the efficient Cambrian Ophir/Gros Ventre detachment and was likely responsible for the dramatic reentrant in the thrust belt.

Although the Teton- Gros Ventre range apparently marks the northern buttress of the Wyoming salient, the Pioneer and Copper Basin thrusts, located ~150 km inboard, strike parallel to the northern boundary of the salient. The strike of these thrusts coincides with the Lemhi arch, an antecedent Neoproterozoic to early Paleozoic feature in east-central Idaho that precluded deposition of muddy Cambrian facies, likely thwarting the thrust belt’s eastward translation. Beginning in the Santonian, the leading edge of the thrust belt was also constrained by emergent basement-involved structures (e.g. Blacktail-Snowcrest uplift).

The spatial correlation between the thickness/distribution of early Paleozoic stratigraphy and the trace of the Wyoming salient speak to their interdependence and architectural control. We believe that the Wyoming salient would have formed in the absence of Laramide deformation.