There is a great pull out at this location, a nice view of Downieville, and several informational signs about the history of the hamlet below. I do some “fair use” quotes below because those who have studied this much more than me have summarized it much better.
The best place to view the rocks is in the sharp “road” cut below the canon. This provides a great exposure of what Deierdorfer and Day (1992) describe as “disharmonically folded, graphic metasedimentary rocks” of the Calaveras Complex. That is:
- Irregularly (disharmonically) folded are when the rock foliation/layers are folded but not in a regular, repeated pattern.
- Graphic is when the layers, foliation, or minerals are at regular angles to each other, like graph paper. These regular angles aren’t necessarily at 90°.
- Metasedimentary means sedimentary rocks that have undergone metamorphosis.
As with all EarthCaches you will need to send the Cache Owner (me) some answers to the questions below. Go ahead and log you cache immediately, but send me the answers in a timely manner, or I’ll be forced to delete your log. I honor all good faith attempts regardless of the accuracy of the answer.
- Name of the EC and the GC code
- Describe the color or colors you see in the rock. Are the colors regular, mottled, or some other pattern? Describe.
- Discuss the layering or other physical patterns in the rock. Patterns of any observable minerals, layers, or foliation.
Concise Background on the Sierra Geology - Not required for the cache answers
This segment is (or will be) repeated in several of the Geotour Hwy 49 (GT49 caches). Going west to east the Sierra is a series of terranes (yes the correct spelling) that are juxtaposed. That is each of these terranes are a series of units that my vary widely in rock type but originated for a common geological process.
First a bit of background if you were to do a line of section west to east from the Central Valley to core of the Sierra (highway 49 starts about center of this line. You encounter sequences as follows:
- Jurassic Foothill Belt (JFB): The JFB is unconformably (there is a erosion and time gap between the two) adjacent to Jurassic-Triassic Arc Belt. The JFB is a 165-155 million year old volcanic arc metamorphosed to greenstone, and sand and silts metamorphosed to greywacke and slate, respectively. Within JFB but more along Highway 20 and 70 is the ocean crustal segment known as the Smartville Complex.
- Jurassic-Triassic Arc Belt (JTA): This is a complex jumble of rocks from about 200 million years ago. This an volcanic arc that formed over subducting ocean crust was shoved under the continental rocks. The Calaveras Complex are the oceanic materials shoved into the the subduction trench. As this ocean crust slid under the continent and melted, volcanoes formed above them.
- Calaveras Complex (CC): The Calaveras Complex are the oceanic crust and overlying sediment that were shoved into the subduction trench when the JTA was erupting inland from the melting subducted materials. In the process of subduction these different rocks (sedimentary, volcanic, and ocean crust) became a jumbled mess. As Alt & Hyndman (2016) described it, “The best that can be said of the Calaveras Complex rocks is that they are a unholy mess, difficult to decipher in the field and impossible to describe adequately on maps or in words. The rocks include a scrambled assortment of pieces of old oceanic crust and seiments tat were deposited on them, all torn up along faults and jammed into tight folds, and recrystalized into metamorphic rocks in the heat of the deep trench.”
- Feather River Peridotite Belt (FRPB): A narrow belt of Devonian-age (~420 to ~360 million years ago) ocean crust metamorphosed to serpentine rock, and is roughly time synchronous with the Shoo Fly Complex.
- Shoo Fly Complex (SFC): These set of rocks originated from sediments deposed about 800 to 700 million years ago, and among the oldest sedimentary rocks in California. These rocks were shoved into North America along a subduction zone during the Antler mountain building event about 300 million years ago. Found in the complex are metamorphosed sandstones, shales, schists, limestone, and chert.
- Sierra Nevada Batholith and associated overlying volcanic (SNB): From about 225 to about 80 million years ago, large blobs of magma welled up during the subduction that helped build the modern coast ranges. Over these blobs of magma were volcanoes that in many cases have been eroded away, especially in the high Sierra near Lake Tahoe and into the southern Sierra. Individual "blobs" magma that cooled to granitic rocks are plutons. The mass of all the consolidated pultons on a batholith.
- Outlier Plutons are individual plutons associated with the granitic rocks along the crest of the Sierran Batholith, but that emplaced in the rocks outside the batholith. See GC9WG31 - GT49 Yuba Rivers Pluton.
References:
Alt, D & Hyndman, D (2016). Roadside Geology of Northern and Central California. Mountain Press Publishing Company.
Deierdorfer, R.E. and Day H.D (1992) Metamorphic Features of the Smartville Complex, Northern Sierra Nevada, California (see stop 11 page 53) In Field Guide to Geology and Metamorphism of the Franciscan Complex and Wester Metamorphic Belt of Northern California by Schiffman, P and Wagner D.L.. California Divisions of Mines and Geology. Special Publication 114.
Durrell, Cordell (1987). Geological History of the Feather River Country, California. University of California Press.
Turner, H.W. 1969. Economic Geology Sheet of Downieville Quadrangle, California. US Geological Survey.