Eocene drainage evolution and erosion of the Sierra Nevada batholith across northern California and Nevada

Abstract

Detrital zircon analysis of Eocene to Oligocene fluvial sediments (“auriferous gravels”) in the Yuba and Feather River drainages, northern California, provides new constraints on the evolution of the paleo-drainage system, sediment provenance, and the timing of sediment deposition and erosion of the Sierra Nevada batholith. 1292 new detrital zircon U-Pb ages in 14 samples indicate a regional shift in sediment provenance, from local sources, predominantly the youngest phase of Sierra Nevada arc magmatism, to basement terranes and extra-regional sources. The majority of samples contain zircon from one of two distinct Mesozoic sources, as well as 5 to 30 percent Precambrian grains, reflecting derivation from both the Sierra Nevada batholith and basement metamorphic belts of the northern Sierra Nevada (for example, Shoo Fly Complex), as well as possible contributions from central Nevada (for example, Roberts Mountain Allochthon, western Nevada Triassic strata). Cretaceous–dominated samples, primarily located in the southwestern Yuba River drainage, were sourced predominantly from 85 to 120 Ma batholithic rocks, whereas Jurassic-dominated samples, primarily located further east, lack mid-Cretaceous zircon and instead reflect derivation from older Jurassic crystalline and metamorphic rocks. These Jurassic–dominated samples also contain sparse Eocene–early Oligocene zircon from 42.7 to 33.0 Ma that establish an upper bound for their depositional age and likely require some extra-regional sediment input to the fluvial system, consistent with previously published sedimentologic and stratigraphic data. Overall, sampled detrital zircon age populations show that the majority of sediment deposited within the fluvial system was supplied from local metamorphic basement and/or regional batholith sources within the ancestral Sierra Nevada, with a small component of extra-regional sediment. These results support the conclusion from stratigraphic and stable isotope paleoaltimetry studies that the Sierra Nevada had a relatively steep western gradient in the Eocene–Oligocene, allowing for significant paleovalley incision and regional transport of sediment from upstream. When compared with Upper Cretaceous Great Valley group sediments, samples from the southwestern Yuba River region exhibit significantly larger proportions of 85 to 100 Ma zircon, characteristic of the voluminous Tuolumne Intrusive Series of the Sierra Nevada batholith. The predominance of Late Cretaceous detritus indicates that early to middle Eocene deposition of fluvial sediments may have coincided with deep erosion of the Tuolumne Intrusive Series. Subsequently, sediment provenance shifted from Tuolumne sources in the Late Cretaceous–dominated sediments to local Jurassic basement and extra-basinal sources in late Eocene Jurassic–dominated sediments. This may reflect progressive canyon incision and extension of headwaters eastward, and bypass of Tuolumne Intrusive Series. The youngest single-grain detrital zircon ages are similar to the range of ages of Eocene volcanic rocks in central Nevada. The presence of these grains suggests that paleo-river headwaters extended into central Nevada by Late Eocene, and that the paleo-drainage divide must have been located in easternmost Nevada. This provides additional support for the existence of a region of high elevation across what is now Nevada by late Eocene time.