Current Field Forum:
Old or Young? The Topographic Evolution of the Sierra Nevada

Nevada and California, USA | 20–27 June 2022

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 Plateau.

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 this problem.

We will visit key locales that illustrate the following features:

1. Early Tertiary rocks cropping out deep in modern canyons, suggesting that most Sierra erosion is mere reoccupation of ancient canyons.
2. The distribution of Paleogene relief and its relationship to post-Miocene incision.
3. The nature and integration of the Eocene rivers that deposited the “Auriferous Gravels” of the ‘49er Gold Rush, including channel gradients and sedimentary features.
4. Depositional ages of the “Auriferous Gravels”: Do they represent many millions of years of accumulation, or was deposition fairly short-lived?
5. Evidence for and against tilting and how younger faulting might contaminate inferences.
6. 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.

Agenda

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 Stanislaus Rivers).

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 Conduct.

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 cjones@colorado.edu 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 (bsundeen@geosociety.org).