Geology of the Lakes Basin: Architecture of an Island Arc
General Description:
This part of the “Lost Sierra” offers extremely varied geology, from Pleistocene glacial moraines-- not even rocks yet! --to twice-, even thrice-metamorphosed volcanic and sedimentary rocks formed in an ancient (Devonian) island arc, probably not far from the North American continental margin at that time. Rocks of intermediate, Tertiary age also are present, and include the Oligocene “ignimbrites” (ash-flow tuffs) and conglomerates on Haskell Peak. The tuffs are now well-dated and are correlated with tuffs in western Nevada that originated in calderas still farther east; we will spend 1 ½ days on Haskell Peak studying these and other rocks. One-half day will be devoted to investigation of Miocene “lahars” (volcanic mudflows) near Chilcoot and Clio.
The final two days of the workshop are set aside for study of the oldest, metamorphosed rocks in the area, those which comprise the Devonian island arc. The Elwell Formation will be examined in the Packer Lake-Dugan Pond area, where wonderful exposures reveal the secrets of formation of the rare rock type “peperite”, found in association with phosphatic chert and andesitic sills. At Dugan Pond, we will search for ammonoids that have been baked by contact metamorphism about a gabbro stock. The ammonoids are extremely important in arriving at the late Devonian age for the island arc. The last day will be spent circumnavigating beautiful Upper Salmon Lake, looking at pillow lavas, debris flows and turbidites in the Taylor Formation, and more chert, sills, and peperite in the Elwell Formation. A sill emplaced just below the Devonian sea floor has a columnar-jointed base and a pillowed top. Finally, not to be neglected, we will see, in the oldest, Sierra Buttes Formation, pumiceous pyroclastic flows that were erupted onto the sea floor.
Should many of these rock names be foreign to you, it is because the rocks are indeed rarely encountered and thus seldom described in the literature. Not to worry-- we will have fun looking at them and trying to determine how they formed anyway. All levels of experience are welcome, from none to the graduate level.
Instructor
Elwood (Woody) Brooks is Emeritus Professor of Geology at California State University, East Bay, and currently is a Research Associate in the Geology Department at the University of California, Davis. His Ph.D. in geology was earned at the University of Wisconsin, Madison. He taught CSU, Hayward’s summer field geology course for 25 years, starting in 1970, in the Lakes Basin and on Haskell Peak.
Class Schedule
Plan to arrive at the Sierra Nevada Field Campus on Sunday. We will get acquainted after dinner that evening, and an introduction to the class will follow. All instruction will be in the field, “on the outcrop”. Be prepared for moderately strenuous hikes, on and off the trails, on the third, fourth, and last day of the workshop. Evenings are your own, for relaxation.
- Monday: Marvel at glacial deposits and landforms, mostly along the Gold Lake Road; this will be an “in-and-out-of-the-van” day with only a short hike and brief bushwhack so that you can become acclimatized. Note that we begin with the youngest (Pleistocene) deposits, ones that you likely are most familiar with.
- Tuesday: Even more driving, all the way to Frenchman Lake, mostly to see magnificent Miocene lahars and a columnar-jointed dike. Return via Portola, Clio, and Sattley to view another lahar and an Oligocene ignimbrite in the Mohawk Valley. Finish on Haskell Peak to see the Jurassic Haypress Creek granodiorite, basement for the overlying Tertiary rocks, and another ignimbrite.
- Wednesday: Hike to the top of Haskell Peak (8107’) to study some of the 9 ignimbrites and 6 conglomerates mapped there. Also present are basalt plugs and sills, and a Miocene stream gravel (at 8000’!).
- Thursday: Rocks are getting more complex: Hiking and some bushwhacking to study Devonian island-arc rocks near Packer Lake (morning) and Dugan Pond (afternoon). Mostly metamorphosed volcanic and sedimentary rocks in the Elwell Formation.
- Friday: Hiking and bushwhacking to study additional handsome island-arc rocks, mostly in the Taylor and Elwell Formations, in the Upper Salmon Lake-Horse Lake-Deer Lake area.
Supplies and Other Useful Items
Field Equipment
- Good hiking boots
- 10X hand lens
- Day pack
- Water, lunch
- Sun and insect protection
- Simple first-aid kit
- Camera
- Notebook
- Rain gear
Camping gear
- Sleeping bag
- Flashlight
- Alarm clock
- Bring your own tent or use tents with beds provided at the field campus
Optional Textbooks
- Daily field guides will be provided, as will a number of other handouts.
- A geological dictionary will prove useful
- a topographic map that covers the areas visited on Days 1, 4, and 5 is available from the U.S. Forest Service (Lakes Basin, Sierra Buttes and Plumas Eureka State Park, Recreation Guide, 1999).
- For the really gung-ho, the following two publications are available from the California Geological Survey, 345 Middlefield Road, MS 520, Menlo Park, CA 94025; phone (650) 688-6327. They provide geologic maps and detailed descriptions of the rocks seen on the last 3 ½ days of the workshop:
- Map Sheet 55A, 2008, Brooks, E.R., Henry, C.D., and Faulds, J.E., Age and character of silicic ash-flow tuffs at Haskell Peak, Sierra County, California: Part of a major Eocene(?)-Oligocene paleovalley spanning the Sierra Nevada-Basin and Range boundary.
- Special Publication 114, 1992, Brooks, E.R., p. 54-78 in Field guide to the geology and metamorphism of the Franciscan Complex and Western Metamorphic Belt of northern California, P. Schiffman and D.L. Wagner, eds.
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Please direct all questions concerning registration procedures and fees to:
J.R. Blair, Sierra Nevada Field Camp Director,
Department of Biology, San Francisco State University,
1600 Holloway Ave., San Francisco, CA 94132
email: jrblair@sfsu.edu
