EVIDENCE OF THE SONOMA OROGENY IN THE SOUTHERN INYO MOUNTAINS

In the southern Inyo Mountains, the Triassic Union Wash Fm has been informally divided into conformable lower (UWl), middle (UWm) and upper (UWu) members. However, we present evidence for two distinct phases of deformation of the Union Wash Fm in the Cerro Gordo area marked by a pronounced unconformity between UWm and UWu. The age of UWl has been inferred to be ~250 Ma using condonts. To assess the age of UWm and UWu we sampled sandstones for detrital zircon analyses, which unfortunately did not yield young zircon populations that could be used to determine a maximum depositional age. A sandstone at the base of the adjacent Inyo Mountains Volcanic Complex (IMVC) was also dated and the analyses yielded a well-defined young population of zircons with a mean age of the 5 youngest grains of ~204 Ma. An ignimbrite at the top of the IMVC section yielded an age of ~170 Ma. The absence of zircons of this age range in the UWu leads us to infer that it is older than the IMVC. The deformation of the UWl and UWm is characterized by large isoclinal folds with well developed parasitic folds and recrystallization of the carbonates into marbles, similar to the deformation of the underlying Keeler Canyon Fm. In contrast, the deformation of the UWu is characterized by a phase of imbrication that predates a 140 Ma felsic dike and a phase of upright, tight folds with little recrystallization of the limestones that postdates it. Preliminary mapping of the IMVC suggests similar imbrication there. We propose that the deformation of the UWm and UWl was coeval with the deformation of the Keeler Canyon Fm, therefore making it early Triassic and possibly a continuation of the Sonoma Orogeny. After the burial and exhumation of these rocks, the UWu was deposited in a marine basin with a shallowing upward trend that culminates in continental facies. Given this difference, we propose that the UWu be separated into the Cerro Gordo Fm. This unit was imbricated with the IVMC, possibly during their structural juxtaposition. This phase must be post-170 Ma (age of IVMC) and pre-140 Ma (age of felsic dike). Finally, there is a phase of upright folding with no cleavage development that is post 140 Ma. These last two phases have been ascribed to the Eastern Sierra fold and thrust belt, but are also coincident with the Nevadan orogeny documented in the western Sierra foothills.