Geology of the Lakes Basin and Haskell Peak
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 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 ½ day on Haskell Peak studying these and other rocks and the rest of the day (DAY 2) will be devoted to investigation of Miocene lahars (volcanic mudflows) near Chilcoot and Clio. A full day (DAY 3) will be devoted to a traverse of Haskell Peak, ending at the summit. On the way we will study two more of the ash-flow tuffs, a couple of conglomerates, a Miocene paleochannel containing andesitic lahar and gravel, and an even younger basaltic plug partially responsible for the peak itself. We also will see a Jurassic granodiorite - part of the Sierra Nevada batholith - upon which all of these Tertiary rocks rest.
The final two days of the workshop are set aside for study of the oldest, metamorphosed rocks in the area, those that comprise the Devonian island arc. On the first day, the Elwell Formation will be examined in the Packer Lake-Dugan Pond area, where wonderful exposures (“outcrops”) 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 adjoining 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 “dispersed” peperite in the Elwell Formation. Not to be neglected, we will see, in the oldest, Sierra Buttes Formation, pumiceous pyroclastic flows that were erupted onto the Devonian 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.
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.
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 last three days of the workshop. Evenings are your own.
- Monday: Marvel at glacial landforms and deposits (moraines), mostly along the Gold Lake Road. This will be an “in-and-out-of-the-van” day with only a short hikes 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 Tertiary (Miocene) lahars and a columnar-jointed dike. Return via Clio and Sattley to view another lahar and an Oligocene ignimbrite in the Mohawk Valley and its faulted equivalent above the valley. Finish on a shoulder of Haskell Peak to see another ash-flow tuff, the regionally important Nine Hill Tuff, that occupies a major paleovalley.
- Wednesday: Traverse of Haskell Peak, from base to summit (8107') to study the stack of Oligocene ash-flow tuffs and conglomerates that comprise most of the peak. Miocene lahar and even younger basaltic intrusions will also be encountered.
- Thursday: Rocks are getting much older (mostly Devonian), hence more complex. Hiking and a little bushwhacking to study subaqueous island-arc rocks near Packer Lake (morning) and Dugan Pond (afternoon). Mostly metamorphosed volcanic and sedimentary rocks in the Elwell Formation, some containing important fossils.
- Friday: More 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
- Good hiking boots
- 10X hand lens
- Day pack
- Plastic containers for lunch (sandwich, fruit, cookies…)
- Sun and insect protection
- Simple first-aid kit
- Rain gear
- Sleeping bag
- Alarm clock
- Bring your own tent or use tents with beds provided at the field campus
- Daily field guides will be provided, as will a number of other handouts. A geological dictionary will prove useful, but is not required (see your bookstore). 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 3½ 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.
- Special Publication 122, 2000, Brooks, E.R., P. 53-110 in Field guide to the geology and tectonics of the northern Sierra Nevada, E.R. Brooks and L.T. Dida, eds.
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
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