DETRITAL ZIRCON U-PB AGES FROM PALEOGENE RIVER GRAVELS OF THE NORTHERN SIERRA NEVADA AND WESTERN NEVADA: IMPLICATIONS FOR PALEOGENE DRAINAGE EVOLUTION

Deposits of a Paleogene paleochannel network in the northern Sierra Nevada (SN) may record the topographic and geomorphic evolution of the North American Cordillera during rollback and/or removal of the Farallon slab. An Oligocene drainage divide east of the SN is inferred from ignimbrites in the paleochannels that originated from calderas in central Nevada. However, the age and provenance of paleochannel deposits underlying these ignimbrites is uncertain, obscuring the implications of these deposits for understanding the evolution of the Cordilleran margin.

We present new large-n detrital zircon (DZ) age data that constrain the ages and source areas of the paleochannel network of the northern SN and Nevada. Samples were collected from deposits in the western Sierran foothills, near the modern Sierran crest, and in western Nevada. All of the sample localities have been interpreted to be part of the paleo-Yuba river system.

Sample maximum depositional ages (MDAs) range from pre-Cenozoic to Oligocene (though all samples are thought to be Paleogene), and distinct DZ age patterns emerge from samples with Oligocene MDAs versus pre-Oligocene MDAs. Samples with pre-Oligocene MDAs display inter-sample variation in age distributions. In the western SN, such samples either are dominated by ages consistent with local sources in the Jurassic western SN batholith and Paleozoic metamorphic terranes or are dominated by Cretaceous ages consistent with derivation from the eastern SN batholith. Pre-Oligocene MDA samples near the SN crest and in Nevada contain ages consistent with local derivation (Cretaceous and Early Mesozoic ages, respectively). In contrast, all samples with Oligocene MDAs either are dominated by Cretaceous age peaks or entirely lack pre-Cenozoic ages.

These observations suggest that sediment sourcing within the paleo-Yuba river channels became more homogenized from latest Eocene to Oligocene time, potentially reflecting network integration and propagation. The dominance of Cretaceous and Oligocene ages in samples with Oligocene MDAs implies sourcing from the eastern SN batholith and ignimbrite deposits, and delineates a drainage divide as far east as central Nevada. The observations support substantial drainage network evolution and divide migration in late Eocene to early Oligocene time.