Current Field Forum:
Old or Young? The Topographic Evolution of the Sierra Nevada
Nevada and California, USA | 20–27 June 2022
Elizabeth Cassel, University of Idaho, Dept. of Geological Sciences, Moscow, Idaho
Chris Henry, University of Nevada Reno, Mackay School of Mines, Reno, Nevada
Craig Jones, University of Colorado, Dept. of Geological Sciences, Boulder, Colorado
John Wakabayashi, California State University, Dept. of Earth and Environmental
Sciences, Fresno, California
“Youth is the gift of nature, but age is a work of art.”
Stanislaw Jerzy Lec
Eocene Auriferous Gravels in the Malikoff Diggings State Park. Photo by Elizabeth Cassel.
Description and Objectives
After more than 150 years of geological investigation, the topographic history of the Sierra Nevada remains
contentious. Is the range the remains of a greater Sierra from the Cretaceous? Is the range a phoenix,
rising from the debris of an earlier range? These end-member conceptualizations have important implications
that extend well beyond the Sierra to the history of orogens in places like the Andes and the Tibetan
If the range is old, then erosion has been a minimal force through much of the Cenozoic. West-flowing rivers
deeply incised into bedrock below older Cenozoic rocks would reflect a changing climate with only minimal
removal of pre-Cenozoic material, an inference consistent with low post-Cretaceous exhumation and minimal
total unroofing recorded by thermochronology. Variations in the modern gradients of Eocene channels with
azimuth would be the product of bedrock anisotropy or a complex depositional history in a disequilibrium
system that would mean that river gradients are more complex than most geomorphic models assume. In this
case, geophysical observations of a relatively thin crust and buoyant mantle under the eastern half of the
range suggest that such changes since the Miocene have had a minimal topographic impact, indicating that an
older crustal root was effectively replaced by buoyant mantle with little net change in elevation.
If the range is young, we have a significant issue with our interpretations of several globally applied
paleoelevation proxies. The geometry of the elevated interior of the U.S. Cordillera would seem far
different than an Altiplano-like landscape if the western edge was lower than at present. The failure of the
mountains to rebound as erosion unloaded them would suggest some destruction of buoyancy through the early
Cenozoic. A range that recently increased its mean elevation would demand a mechanism only loosely tied to
modern plate interactions, either by straight thermal warming as a subducting slab was removed or by
physical removal of the old continental root through either lithospheric foundering or normal faulting.
This Field Forum will focus on disputed geologic features across much of the northern part of the Sierra
Nevada that comprise the observational basis for the range of uplift and elevation estimates. We will
consider observations and inferences from a broad range of specialties that have been employed to address
We will visit key locales that illustrate the following features:
- Early Tertiary rocks cropping out deep in modern canyons, suggesting that most Sierra erosion is mere
reoccupation of ancient canyons.
- The distribution of Paleogene relief and its relationship to post-Miocene incision.
- The nature and integration of the Eocene rivers that deposited the “Auriferous Gravels” of the ‘49er
Gold Rush, including channel gradients and sedimentary features.
- Depositional ages of the “Auriferous Gravels”: Do they represent many millions of years of accumulation,
or was deposition fairly short-lived?
- Evidence for and against tilting and how younger faulting might contaminate inferences.
- Evidence of relationships between weathering and erosion rates and the various controlling factors.
Additional discussions addressing observations not directly associated with outcrops will occur as relevant
in the field and in evening sessions.
This incredible seven-day Field Forum will originate in Reno, Nevada, USA, and then travel across the range
to visit locales in the northern Sierra for four days from a base in Grass Valley in the Sierra foothills.
The group will then tour outcrops to the south from a two-night stay in Modesto after which we will return
to Reno. Weather in June in this area is generally dry with temperatures from pleasant to warm or hot at
lower elevations during the days. Most outcrops will be near vehicles, with a few requiring a bit of
bushwhacking. Plans also include two optional hikes of up to one mile.
- Day 1 (June 20)
- Arrival (by 11 am) and introduction to early Cenozoic channels and paleocanyons and Eocene–Oligocene
sedimentary and igneous fill (Dogskin Mountain and Haskell Peak).
- Day 2 (June 21)
- Paleorelief and modern relief near the Sierra crest (Donner Summit, Royal Gorge area, Emigrant Gap).
- Day 3 (June 22)
- Examination of physical characteristics of the “Auriferous Gravels” and their channels (Malakoff Diggins
SHP, Alpha Diggins, and Paleo-Yuba River exposures).
- Day 4 (June 23)
- Geometry and age of channels associated with “Auriferous Gravels” (Paleo-South Fork of the Yuba, Red Dog
Diggins, and Chalk Bluff floral site).
- Day 5 (June 24)
- Post-Eocene relief or absence of relief (in situ and questioned exposures near the American and
- Day 6 (June 25)
- Evidence of uplift and erosional styles in the southern Sierra (San Joaquin Table Mountain and uplands
near Shaver Lake).
- Day 7 (June 26)
- The fluvial-ocean interface of the Eocene and a Miocene deformation marker (Ione Formation and
equivalents (?) near Oroville, Lovejoy Basalt).
Logistics and Attendees
The registration fee will cover six nights lodging, based on single occupancy, 20–27 June 2022. Breakfast,
lunch, snacks, and some group dinners along with trip materials and transportation during the field forum
will be included in the registration fee. Travel to and from Reno will be the responsibility of attendees.
Optional lodging prior to day one will be made available to attendees for a fee. If double occupancy rooms are allowed, registration fees may be further reduced. We currently estimate the cost for participants as between
US$100 and US$600 per person. Funds from cosponsoring GSA divisions, the GSA Cordilleran section, the National Science Foundation, and other generous donations allow us to greatly reduce the registration fee for all attendees. We also expect to subsidize most expenses (including travel to Reno) for junior scientists (students and recent PhDs). We encourage everyone to apply and the conveners are dedicated to ensuring the field forum is financially accessible to all participants.
GSA strongly encourages applications from low-income, underrepresented, first-generation, non-traditional, women,
veterans, LGBTQ+, students with disabilities, and others. Some financial resources are available for
students and early career scientists and those with financial need; please state such need in your
application. Attendees are expected to honor the GSA Code of
Applications and Registration
Application deadline: 7 Feb. 2022
Registration deadline: 28 Mar. 2022
Participants must commit to the full seven-day/six-night duration of the field conference. Group size is
limited to 40 participants. To apply, please contact the conveners through firstname.lastname@example.org with a letter of intent that includes a
statement of interests, the relevance of your recent work to the themes of the field conference, the subject
of a proposed presentation, and contact information. Please put “FF2022 Application” in the subject line of
your email. We also request submission of basic demographic information (race, gender, disability, and
ethnicity) and ask attendees whether they are willing to help drive one of the rented SUVs. Interested
graduate students, members of underrepresented groups, and early career faculty are strongly encouraged to
apply. Once you have been selected to participate, you will be sent registration information.
If you have any questions please feel free to contact Becky Sundeen (email@example.com).