SEDIMENTOLOGIC AND ISOTOPIC CONSTRAINTS ON PALEOCENE PALEOELEVATIONS OF THE CENTRAL AND SOUTHERN SIERRA NEVADA

Recent work indicates the topographic evolution of the Sierra Nevada may be quite distinct for the southern, central, and northern sections of the range. Stable isotope and organic molecular proxies from the northern Sierra Nevada suggest Eocene range elevations comparable to modern Sierran topography in this part of the range, thus precluding major late Cenozoic surface uplift. Constraints from the southernmost portion of the range, however, are lacking. We use U-Pb dating of detrital zircons and δ¹⁸O values from Paleocene sediments deposited east of and on top of the Sierra Nevada batholith to constrain the Paleocene paleoelevation of the central and southern Sierra Nevada.

The Paleocene Goler Fm. outcrops in the El Paso Mountains to the east of the southern Sierra Nevada. Detrital zircon age spectra from the upper Goler Fm. are dominated by Middle and Late Jurassic ages, with a smaller peak of Late Triassic ages, and a few 1.7 Ga zircons. The White-Inyo Mountains in the eastern Sierra Nevada, where Jurassic plutonic and volcanic rocks, Late Triassic plutonic rocks, and Neoproterozoic sedimentary rocks all outcrop, are the most likely source. Paleontologic evidence suggests the Goler Fm. was deposited at or near sea level, however depleted δ¹⁸O values (-12.4‰ PDB) from micritic limestone within the Goler Fm. strongly suggest rainout over an orographic barrier ≥ 1 km in elevation. Taken together, these data imply at least modest elevations for the Paleocene Sierra Nevada at a modern latitude of ~37°Ν. Zircon age spectra from the Paleocene Witnet Fm., which lies atop the southern Sierra Nevada, are dominated by Early and Middle Triassic ages derived from the western Mojave Desert, indicating that the Witnet Fm. and the lower Goler Fm. share a common source. This requires the Witnet Fm. to have been deposited at paleoelevations less than that of the Goler Fm., constraining the Paleocene southern Sierra at ~35°N to be near sea level. This southward elevation gradient in the southern Sierra Nevada likely developed in response to Late Cretaceous lithospheric collapse beneath the Mojave Desert, and these results place new limits on the potential amount of mid to late Cenozoic elevation gain in this region resulting from the loss of a dense, eclogitic root.