Evolution of Sediment Source of Sierra Nevada Cenozoic Paleochannels from Clustering in Detrital Zircon Age Spectra and Implications for Basin and Range Paleoelevation

Abstract:

Detrital zircon (DZ) age distributions from Cenozoic paleochannels in the Sierra Nevada (SN) offer a means to test fluvial connectivity, and thus relative paleoelevation, between the Basin and Range (BR) and SN. This test is independent from proxy records, inferred lapse rate, and assumptions of isotope behavior. Existing studies of DZ spectra from SN paleochannels have disagreed over such connectivity, due in part to differing sample coverage.

We present 11 new DZ spectra from the paleo-Yuba and American Rivers in the SN and BR, 5 of which result from large-n (300 grain) analyses (see Fig. A, B). Large-n analyses more accurately reveal the proportions of DZ age populations present in a given sample. Resampling experiments on a single, 4,117 grain DZ sample indicate that n = 300 analyses improve the accuracy of quantitative comparisons of multiple spectra as well as maximum depositional ages (MDA) from youngest DZ grains, relative to the smaller sample sizes used in many studies (n = 60, 100) (see Fig. C). Our analyses, along with 21 published samples, reveal 3 categories of DZ spectra in the paleo-Yuba and American Rivers, each with their own characteristic age peaks and MDA.

Type 1 samples (n = 12) occur in both the paleo-Yuba and American Rivers and are dominated by non-locally derived Cretaceous (K) grains, and have K MDA. Type 2 samples (n = 5) occur in the paleo-Yuba River, contain locally-derived Jurassic (J) and Devonian (D) grains, as well as extra-regional Eocene (Eo) grains, and have Eo MDA. Type 2 samples occur near J and D batholithic sources. Type 3 samples (n = 6) occur in the BR and eastern SN, contain Oligocene (O) grains, and have O MDA, suggesting derivation from O volcanics from the BR.

These results may reflect temporal evolution of SN paleochannel sediment sources, beginning with the erosion of K plutonic rocks to the east during K-Eo time (Type 1), followed by the aggradation of locally-derived material during Eo time (Type 2), then an influx of volcanic-rich sediments from the BR in O time (Type 3). These results suggest that paleochannels tapped regions east of the modern SN crest between K and Eo time, and again in the Oligocene. Expanded spatial and temporal sampling of paleochannel deposits, coupled with large-n analyses, may further elucidate the paleoelevation history of the BR throughout the early Cenozoic.