Sunday, February 10, 2008

Thrusting Shoo Fly

On YouTube, at

is an animation of a flight up the American River Canyon, using the USGS Digital Elevation Model 30-meter data set, and merging a couple dozen DEM quadrangles to build a landscape spanning Colfax on the west, the Sierra Crest on the east, the San Juan Ridge and Grouse Ridge to the north, and the Middle Fork of the American on the south. The virtual camera follows an almost due east heading from west of Rollins Lake, crossing over Lovers Leap and Green Valley, and flying on up the canyon into the Royal Gorge. The animation finishes with the virtual camera making an orbit of 360 degrees around Snow Mountain.

Thrusting Shoo Fly: on my iMac I can set the screensaver to loop through any folder of images in my iPhoto library. It so happens that right now it loops through a folder of some of my favorite photographs in the North Fork. Here is Giant Gap, from the west, and now from the east; or the 500-foot waterfall in New York Canyon, or Big Valley Bluff at dawn, as seen from the North Fork, a couple miles up the canyon.

And so on. It's not hard to take beautiful photographs in such a beautiful place.

It happens that one of these special photographs shows what I call Bluff Camp, an old mining camp immediately adjacent to the river, set on a cliff-bounded strath terrace bearing a fine grove of Canyon Live Oaks. From the North Fork American River Trail, connecting Sailor Canyon to Mumford Bar, a side trail leads one down a hundred yards, or so, to Bluff Camp. I have camped there many a time. It is half a mile or so east of Tadpole Canyon, and directly below Big Valley Bluff, rising in ragged cliffs all of 3500', across the river to the north.

And the photograph was taken from a point upstream from Bluff Camp; so one sees a part of the encircling cliffs, and a flat area--the strath terrace--perhaps thirty feet above river level. (A "strath terrace" is a glacio-fluvial landform associated with glacial outwash sediments which once occupied the terrace itself; and it was these very sediments which planed down the bedrock, to make the terrace).

It caught my eye, the other day, the Bluff Camp photo, as it filled the screen; I could see an abrupt change in the bedrock, right at the upstream end of the strath terrace. Slowly, dimly, I realized I was seeing a thrust fault. Two disparate bodies of rock had been juxtaposed by faulting.

The bedrock for miles up and down the canyon is composed of metasediments of the early-Paleozoic "Shoo Fly Complex," the oldest rocks in all the Sierra. I have a wonderfully precise geologic map of this part of the North Fork canyon, made by David S. Harwood et. al. of the USGS, in the early 1990s. Harwood shows many thrust faults in the Shoo Fly Complex near Big Valley Bluff, Sugar Pine Point, and New York Canyon. The faults sometimes bring big blocks of chert, hundreds of yards in extent, or more, into contact with slates and other types of rock in the Shoo Fly Complex.

By the way, it is called a "Complex" because it is composed of many distinct formations, spanning many millions of years in time, but all very old. Harwood describes and names four such formations in this particular area. His map does not show the Bluff Camp Thrust, which is probably a sensible choice, for it is likely not very long or large as thrust faults go, and if he were to put every such minor thrust fault on his map, well, there would be room for precious little else.

It has long been considered that the great mashing-together, the epochal juxtaposition of the disparate Sierran metamorphic rocks alongside one another, took place around 145 million years ago, in what was named the "Nevadan Orogeny" (an "orogeny" is a mountain-building). It was this Nevadan Orogeny which acted to rotate all these disparate bodies of metamorphic rock almost 90 degrees to the east, so that what were once flat-lying beds are now almost vertical, or even slightly overturned. And it is considered that the "penetrative fabric" of these disparate metamorphic rocks is mainly due to the Nevadan Orogeny. The compressive and shearing forces which imparted the fabric were fairly well parallel with the current, almost-vertical orientation of the beds. Very likely it all had to do with continental accretion, at a time when Pacific ocean floor was being actively subducted beneath the continental margin, moving from west to east, but also plunging steeply down.

However, in many of these different metamorphic rock formations, whether they be down by Auburn or up at Big Valley Bluff, an experienced eye can detect at least two different episodes of deformation, each leaving its footprint, or imposing its fabric, upon the rocks. There is the later Nevadan Orogeny; and at Bluff Camp, there is a thrust fault vastly older than the Nevadan Orogeny. That is to say, the Shoo Fly was already well-deformed, well-sliced and diced by thrust faults, long before the Nevadan Orogeny.

And Harwood discusses all this in the twelve-page essay which accompanies his map. There are a couple of typographical errors in this essay which play the very devil in understanding the thing.

It is not at all easy to learn to recognize these different rock types. That this is chert, and that is quartzite, may not be discernible except under a microscope. To develop a simple portrait of the bedrock geology, one can read what was written about it a century and more ago. At that time the focus was upon the broad outlines, not the higgley-piggley details. And for a time, the following usage had currency, for instance, in the articles by C.J. Brown of Dutch Flat, published in the Mining & Scientific Press, in 1875.

Brown divides the metamorphic rocks as follows: the Western Slate, the Middle Slate, and the Eastern Slate. Between the Middle Slate and the Eastern Slate, he identifies the long narrow serpentine belt we now name for its associated Melones Fault Zone.

Hence his Eastern Slate corresponds to the Shoo Fly Complex, and those other Paleozoic and Mesozoic rocks which lie on top of the Shoo Fly, and therefore, to the east (the whole shebang, be it remembered, rotating 90 degrees to the east during the Nevadan Orogeny).

Brown's Middle Slate corresponds to the Calaveras Complex, another complex of formations, but late-Paleozoic in age, and he correctly identifies the rock of Giant Gap as metavolcanic--in fact, Brown declares it to be metabasalt; and his Western Slate corresponds to all those metamorphic rocks west of Cape Horn, in which there are several distinct formations, often dominated by metavolcanic rock, but containing some metasediments, too.

So, if we wish to blur our focus and appreciate the broader outlines of local bedrock geology, we might give C.J. Brown's Western/Middle/Serpentine/Eastern model a try.