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Old or Young? The Topographic

Evolution of the Sierra Nevada

Reno, Nevada, USA | 20–27 June 2022

LEADERS of the old continental root through either lithospheric foundering
Elizabeth Cassel, University of Idaho, Dept. of Geological or normal faulting.
Sciences, Moscow, Idaho, USA This Field Forum will focus on disputed geologic features
Chris Henry, University of Nevada Reno, Mackay School of across much of the northern part of the Sierra Nevada that com-
Mines, Reno, Nevada, USA prise the observational basis for the range of uplift and elevation
Craig Jones, University of Colorado, Dept. of Geological estimates. We will consider observations and inferences from a
Sciences, Boulder, Colorado, USA broad range of specialties that have been employed to address
John Wakabayaski, California State University, Dept. of Earth this problem.
and Environmental Sciences, Fresno, California, USA We will visit key locales that illustrate the following features:
1. Early Tertiary rocks cropping out deep in modern canyons,
“Youth is the gift of nature, but age is a work of art.” —Stanislaw Jerzy Lec suggesting that most Sierra erosion is mere reoccupation of
ancient canyons.
DESCRIPTION AND OBJECTIVES 2. The distribution of Paleogene relief and its relationship to post-
After more than 150 years of geological investigation, the topo- Miocene incision.
graphic history of the Sierra Nevada remains contentious. Is the 3. The nature and integration of the Eocene rivers that deposited
range the remains of a greater Sierra from the Cretaceous? Is the the “Auriferous Gravels” of the ‘49er Gold Rush, including
range a phoenix, rising from the debris of an earlier range? These channel gradients and sedimentary features.
end-member conceptualizations have important implications that 4. Depositional ages of the “Auriferous Gravels”: Do they repre-
extend well beyond the Sierra to the history of orogens in places sent many millions of years of accumulation, or was deposition
like the Andes and the Tibetan Plateau. fairly short-lived?
If the range is old, then erosion has been a minimal force through 5. Evidence for and against tilting and how younger faulting might
much of the Cenozoic. West-flowing rivers deeply incised into bed- contaminate inferences.
rock below older Cenozoic rocks would reflect a changing climate 6. Evidence of relationships between weathering and erosion rates
with only minimal removal of pre-Cenozoic material, an inference and the various controlling factors.
consistent with low post-Cretaceous exhumation and minimal total Additional discussions addressing observations not directly
unroofing recorded by thermochronology. Variations in the modern associated with outcrops will occur as relevant in the field and in
gradients of Eocene channels with azimuth would be the product of evening sessions.
bedrock anisotropy or a complex depositional history in a disequi-
librium system that would mean that river gradients are more com- AGENDA
plex than most geomorphic models assume. In this case, geophysi- This incredible seven-day Field Forum will originate in Reno,
cal observations of a relatively thin crust and buoyant mantle under Nevada, USA, and then travel across the range to visit locales in
the eastern half of the range suggest that such changes since the the northern Sierra for four days from a base in Grass Valley in
Miocene have had a minimal topographic impact, indicating that an the Sierra foothills. The group will then tour outcrops to the south
older crustal root was effectively replaced by buoyant mantle with from a two-night stay in Modesto after which we will return to
little net change in elevation. Reno. Weather in June in this area is generally dry with tempera-
If the range is young, we have a significant issue with our inter- tures from pleasant to warm or hot at lower elevations during the
pretations of several globally applied paleoelevation proxies. The days. Most outcrops will be near vehicles, with a few requiring a
geometry of the elevated interior of the U.S. Cordillera would bit of bushwhacking. Plans also include two optional hikes of up
seem far different than an Altiplano-like landscape if the western to one mile.
edge was lower than at present. The failure of the mountains to
rebound as erosion unloaded them would suggest some destruc- Day 1 (20 June): Arrival (by 11 a.m.) and introduction to early
tion of buoyancy through the early Cenozoic. A range that recently Cenozoic channels and paleocanyons and Eocene–Oligocene sed-
increased its mean elevation would demand a mechanism only imentary and igneous fill (Dogskin Mountain and Haskell Peak).
loosely tied to modern plate interactions, either by straight thermal Day 2 (21 June): Paleorelief and modern relief near the Sierra crest
warming as a subducting slab was removed or by physical removal (Donner Summit, Royal Gorge area, Emigrant Gap).

22 GSA Today | December 2021