NEW INSIGHTS ON THE STRUCTURAL AND BASIN EVOLUTION OF THE SALT LAKE SALIENT AND WASATCH FAULT ZONE NEAR SALT LAKE CITY, UTAH

New geologic mapping of the Salt Lake salient (SLS) and geochronology of the SLS’s Paleogene to Neogene sedimentary and volcaniclastic rocks provide a revised stratigraphic framework and structural evolution of the SLS and the faults that bound it. The SLS is northeast of Salt Lake City, Utah, and is bounded by the inactive Rudys Flat fault (RFF) on the east and parts of the active Wasatch fault zone (WFZ) on all other sides. Paleozoic strata are overlain by Paleogene to Neogene strata that include a lower conglomerate (Tc₁), middle volcaniclastic units (Tt and Tv), an upper conglomerate (Tc₂), and a capping alluvial deposit (QTaf). A new detrital zircon (DZ) age from the top of Tc₁ indicates a maximum depositional age (MDA) of early Eocene, which may correlate with the late Paleocene – early Eocene Wasatch Formation exposed to the east. New DZ and ⁴⁰Ar/³⁹Ar data from the Tt and Tv units corroborate previous studies that yielded middle Eocene ages. New DZ data show that Tc₂ contains sparse Cenozoic zircons, but potentially indicate a MDA of early Oligocene. The lack of Miocene zircon grains in Tc₂ (which are abundant in the middle Miocene to Pliocene Salt Lake Formation in the surrounding area) and dips up to ~30° suggest that Tc₂ is likely early Oligocene to early Miocene.

Eocene to early Miocene ages for the rocks on the SLS suggest they were deposited in an orogenic collapse basin prior to onset of Miocene extension ~ 17 Ma. The RFF bounds the Paleogene to Neogene rocks on the east and may have initiated during deposition of Tc₂. The RFF likely continued to be active during basin and range extension and was potentially part of the Weber segment of the WFZ. We assume that QTaf deposits now stranded ~ 600 m above the valley floor began to be uplifted and incised when the active movement of the WFZ stepped westward from the RFF to the Warm Springs fault (WSF). Using a vertical-slip, footwall uplift, and denudation rate of 0.2 mm/yr, we conclude that activation of the WSF, formation of the SLS, and full abandonment of the RFF started ~3 Ma. We do not see evidence of Quaternary movement or fault scarps along the trace of the RFF or any map-based evidence for a direct link between the RFF and the East Bench fault.