Detrital zircon U-Pb ages and provenance of Paleogene paleochannel strata, Sierra Nevada and western Nevada: Implications for paleotopographic evolution
Authors: Tye Alexander , Niemi Nathan A. 
Published: August 2024

The Paleogene paleotopography of the western United States, which may grant insight into Farallon subduction dynamics and North American Cordillera orogenic processes, can be constrained by reconstructing drainage networks through sediment provenance studies. In the northern Sierra Nevada, the provenance of SW-directed, Paleogene paleochannel deposits is controversial; prior studies favor either sources proximal to the deposits or distal source areas in central Nevada. These different hypothesized source areas would have contrasting implications for the paleotopography of the Sierra Nevada and western Basin and Range. We conducted a new provenance analysis of paleochannel deposits in the Sierra Nevada and western Nevada using new and compiled detrital zircon U-Pb age data from across the preserved paleochannel network, together with compiled bedrock geochronologic ages of potential sediment source areas for the paleochannel deposits. The geochronology compilation of potential source areas reveals systematic longitudinal variations in bedrock ages within the study area. Jurassic ages are present primarily in the western Sierra Nevada batholith, Cretaceous ages are dominant in the eastern Sierra Nevada batholith, and Eocene ages are present only in north-central Nevada. The distribution of potential source ages allows confident inference of sediment provenance from detrital zircon U-Pb ages.

The distributions of detrital zircon U-Pb ages in the paleochannel deposits can be categorized into three distinct types. The first type, found in the vicinity of Malakoff Diggins State Historic Park (SHP) and further northwest, is dominated by Jurassic, Paleozoic, and Precambrian ages reflecting derivation from local sources in the western Sierra Nevada, and also contains scattered Eocene ages. The lack of Late Cretaceous ages in these samples, despite the presence of Late Cretaceous plutons in close proximity to the east, suggests small sediment source areas with fluvial transport of zircon grains no farther than 50 km. The few Eocene ages in these samples likely reflect volcanic air fall, consistent with an overlying tuff at one sample site that is interpreted to have its volcanic source to the north, outside the paleochannel network. The second zircon age distribution type, from samples south of Malakoff Diggins SHP, includes Cretaceous and Jurassic ages representative of exposed bedrock across the entire width of the Sierra Nevada batholith but no Eocene ages. This combination of ages suggests a sediment source area that encompassed the entire batholith but did not include north-central Nevada. The third zircon age distribution type, from samples to the northeast of Malakoff Diggins SHP, contains Jurassic, Cretaceous, and Eocene ages consistent with fluvial derivation of sediment from the entire width of the Sierran batholith as well as from north-central Nevada. This third type only occurs in fluvial deposits interbedded with Oligocene ignimbrite tuffs, whereas samples older than the Oligocene tuffs belong to zircon age distribution types 1 or 2. Thus, prior to the emplacement of Oligocene ignimbrites, there is no evidence of fluvial transport of sand-sized sediment from north-central Nevada sources to Sierran paleochannel deposits. The lack of pre-Oligocene fluvial transport across the Sierra Nevada may reflect either a Paleogene drainage divide that separated the Sierra Nevada from north-central Nevada or a large-scale knick zone with a low-gradient upstream reach that trapped sand and larger sediment. The first arrival of fluvially transported Eocene zircon grains following Oligocene ignimbrite emplacement suggests that ignimbrite volcanism, and related hinterland uplift, established or promoted sediment transport from north-central Nevada across the Sierra Nevada by driving drainage reorganization or steepening existing channels.

Our provenance analysis broadly confirms previous paleochannel network reconstructions, especially for the Ancestral Yuba River, and thus validates the use of paleochannel deposits as a datum by which to infer post-Paleogene tilting of the northern Sierra Nevada. Azimuthal trends in Ancestral Yuba River paleochannel gradient suggest ~0.6° SW-directed, post-Paleogene tilting of the northern Sierra Nevada, which would have resulted in ~1 km of uplift of the range crest. Such a magnitude is consistent with Eocene–Oligocene stable isotope paleoelevation estimates.