STRATIGRAPHY AND EVOLUTION OF THE ANCESTRAL RIVER CHANNELS (PALEOVALLEYS), NORTHERN SIERRA NEVADA, USA

The channels/paleovalleys of the Sierra Nevada (SN) – Lindgren’s (1911) ancestral rivers – and their contained sedimentary-volcanic deposits are critical in understanding the SN’s Cenozoic tectonic and climate evolution, especially its controversial uplift history. The essential stratigraphy of pre-volcanic gravels, Oligocene-Miocene rhyolite ignimbrites, and middle Miocene and younger intermediate lavas and volcaniclastic rocks has long been known but poorly dated. Published and new detrital U-Pb zircon and ⁴⁰Ar/³⁹Ar Kspar dates from gravels and precise sanidine ⁴⁰Ar/³⁹Ar dates of Oligocene-Miocene ignimbrites resolve some questions. The ancestral channels probably existed by 80 Ma but were mostly eroding so deposition was restricted to the western marine boundary. Major gravel deposition in paleovalleys is mostly ≤43 Ma, and mapped “pre-volcanic” gravel commonly is as young as 25 Ma. The oldest, undated and undatable? kaolinitic gravel is older than ≤43 Ma, but little was preserved by ongoing erosion in paleovalleys. Gravel younger than ~31.5 Ma, the oldest ignimbrite in the SN, is temporally Valley Springs Formation. The famed Chalk Bluff and other paleoflora, long considered to be ~50 Ma, are mostly, maybe entirely ≤40 Ma. Climate changed considerably 50-40 Ma, so the new dating complicates climate vs paleoelevation effects on flora and implications for uplift.

Precise sanidine dating reveals 15 major ignimbrites that span 31.5-22.95 Ma erupted from calderas in central Nevada and flowed in paleovalleys into the SN, many to the western foothills. SN ignimbrites generally young southward because magmatism in source areas migrated southwestward. Combined gravel and ignimbrite/tuff deposition mostly filled the paleovalleys. Middle Miocene and younger rocks filled shallow remnants and spread extensively over adjacent interfluves. The modern deep canyons mostly postdate the youngest of these, ~3 Ma.

Three major questions remain enigmatic and controversial: When and why erosion changed to major deposition; Location of the pre-late Eocene-early Oligocene paleodivide. Detrital dates demonstrate a paleodivide at least as far east as western Nevada, over the Late Cretaceous batholith, but do not preclude it being farther east; Relative contributions of Late Cretaceous and late Cenozoic uplift.