Stratigraphy, geochronology, geometry, and evolution of the Cenozoic paleovalleys (“auriferous gravel” river valleys), northern Sierra Nevada, western United States

The paleovalleys of the northern Sierra Nevada, which contain the ancestral river channels, their sedimentary and volcanic deposits, and hosted flora, are critical to understanding the controversial uplift and climatic history of this mountain range. The paleovalleys and their deposits have been used as tilt indicators and as records of climate. However, the paleovalleys evolved over many tens of millions of years during which tectonics, climate, sediment supply, discharge, topography, and sea level varied tremendously. The stratigraphy of Cenozoic prevolcanic (“auriferous”) gravels, Oligocene−Miocene ignimbrites (Valley Springs Formation), and middle Miocene−Pliocene intermediate lavas and volcaniclastic rocks (Mehrten Formation) has long been recognized but poorly and partly incorrectly dated. New geologic mapping, stratigraphic and mineralogic analyses, new and published detrital zircon U-Pb and K-feldspar 40Ar/39Ar dates from gravels, and abundant, high-precision sanidine 40Ar/39Ar dates of ignimbrites significantly change the interpreted ages of gravel and contained flora, resolve many aspects of paleodrainage evolution and geometry, and provide some constraints but do not resolve the long-standing questions about uplift history. The ancestral rivers and valleys probably formed by 80 Ma during erosion of a Late Cretaceous batholith highland. The rivers and valleys initially were eroding, and deposition only occurred at the western marine boundary at the edge of today’s Great Valley. Erosion changed to major deposition in the paleovalleys, although precisely when and why are not resolved. Prevolcanic deposits consist of lower, thin, kaolinite-rich, coarse gravel and sand and upper, thick sand, fine gravel, and mudstone in which the proportion of smectite increases up section. Detrital zircons in Eocene−Oligocene upper gravel (no lower gravel is dated) throughout the northern Sierra Nevada are mostly Cretaceous and lesser Jurassic grains derived from the batholith. Sparse Cenozoic (43−25 Ma) zircons are present in most samples of the upper gravels. The scarcity of Cenozoic grains reflects great distances to Cenozoic igneous sources, which, at 43−40 Ma, were no closer than northeastern Nevada and east of all proposed paleodivides relative to rivers that drained across the Sierra Nevada. The few Cenozoic grains demonstrate that most upper gravel is younger than or ca. 43 Ma, and some gravel mapped as prevolcanic is the temporally intervolcanic Valley Springs Formation. The famed Chalk Bluff flora, long considered to be ca. 50 Ma and used to assign an age to the prevolcanic gravels and interpret paleoelevation and climate, is restricted to the upper, more smectitic gravels and is mostly, maybe entirely, younger than or ca. 40 Ma. Precise sanidine dating reveals that 18 major ignimbrites erupted from calderas in central Nevada between 31.5 Ma and 23.0 Ma and were channelized into paleovalleys that traversed the northern Sierra Nevada. By 31.5 Ma, rivers drained from the Nevadaplano in the Great Basin across the northern Sierra Nevada; the Sierra Nevada−Basin and Range structural and topographic boundary did not exist until after 23.0 Ma. With some overlap, ignimbrites become younger southward through the Sierra Nevada because magmatism in source areas migrated southwestward. Despite repeated incision between ignimbrites, thick tuffaceous deposits largely filled the paleovalleys and induced two major course changes to intervolcanic channels. Middle Miocene and younger lavas and volcaniclastic rocks filled shallow paleovalley remnants and spread extensively over low-relief interfluves until ca. 4−3 Ma. Sierra Nevada and Great Basin paleovalleys were much wider than deep, ∼6−8 × ≤1 km, which suggests similar evolution. Modern, deep, V-shaped canyons of the northern Sierra Nevada generally postdate the 4 Ma rocks, cut across paleovalleys, and did not reexcavate them. Ignimbrite distributions and detrital dates confirm many of the previously described paleovalleys but also reveal significant inconsistencies. Gravel in a major paleovalley that can be tracked from western Nevada based on correlative ignimbrite sections has the common Cretaceous > Jurassic detrital pattern, but dated sediments in the two closest possible downstream continuations lack Cretaceous zircons. The most likely explanation for these discrepancies is that some generally accepted ancestral river courses are wrong, and we suggest several alternative courses. The distributions of ignimbrites younger than or ca. 31.5 Ma in the northern Sierra Nevada and Great Basin document a north-south paleodivide to east-draining rivers through central Nevada. Ignimbrite distributions and published stable isotope data suggest this divide existed by ca. 45 Ma. In contrast, several studies interpret a Late Cretaceous paleodivide that followed the Late Cretaceous batholith north-northwest through the Sierra Nevada and then northeast through northwestern Nevada. Abundant 100−85 Ma detrital zircons in all samples north of ∼39°N, where batholithic rocks with U-Pb zircon ages younger than 100 Ma are within 10 km of the Nevada border or farther east, require a paleodivide at least as far east as northwestern Nevada during all upper gravel deposition. Although not resolving the relative contributions of Eocene or older and latest Cenozoic uplift, our data indicate uplift of the northern Sierra Nevada between ca. 43−40 Ma and 4 Ma is unlikely. Proposed late, younger than or ca. 5 Ma, uplift of the northern Sierra Nevada would have been coeval with both the latest down-to-the-east faulting/extension that generated the eastern structural/topographic margin of the range and the interpreted ca. 3.5 Ma, delamination-induced uplift in the southern Sierra Nevada. However, the magnitude of extension younger than or ca. 5 Ma probably was insufficient to generate the proposed 1−1.5 km of footwall uplift, and the northern Sierra Nevada lacks the distinctive potassic, basaltic magmatism that accompanied delamination. The enigma of apparent high elevation in the Eocene and late uplift remains.