Oak Creek Canyon Overlook EarthCache
Hidden : 3/23/2015
Difficulty:
1.5 out of 5
Terrain:
1 out of 5

Size: Size: other (other)

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Geocache Description:


(Picture from Log of Geocacher hydnsek)

Oak Creek Canyon Overlook, in Coconino National Forest, is just off scenic route 89A. If you are driving the road from Sedona to Flagstaff, you probably will think that you are in a gorge that Oak Creek must have carved out of the Mogollon Rim. While this is certainly the case, and erosion indeed played a significant role in the forming of this canyon, Oak Creek Canyon is more than "just" a river canyon, it is also the location of a geological "fault" that runs roughly north-south along the river bed. A geological fault is a fracture in the Earth's crust, where portions of a body of rock have been displaced against each other. You might know of the famous "St Andreas Fault" over in California, where tectonic plates grind against each other and cause hefty earthquakes. The Oak Creek Fault right here is benign in comparison, this part of the Colorado Plateau is geologically stable, and locally triggered quakes are rare and very small.

The fault is most evident once you drive out of the canyon depths and up to the Oak Creek Overlook. From there, looking south, you will see that the left (east) wall looks distinctly different than the right (west) wall of the canyon. The rocks that you see are all mostly from Permian age (about 270 million years ago), but the layers are not contiguous across the valley, like they would be if this were a "simple" river gorge carved by erosion. The "fault" has shifted one side against the other, and there is about a 1000ft / 300m difference between the layers on both sides.

Starting from the bottom, the oldest rocks of the canyon are actually just below the surface of the road down in the canyon, and belong to the "Supai Group". Supai is a sedimentary rock that formed in an ancient river delta with sand dunes in between, resulting in sandstones and siltstones.

Above it is the Hermit Formation, barely poking above the surface of the canyon, and not very well visible. Hermit is a shale and conglomerate rock that is quite crumbly and weathers easily.

Next, we are getting into the layers that are visible from the viewpoint: The "Schnebly Hill" formation, again consisting of mainly sandstones. This formation is the layer responsible for the famous red rocks in Sedona, and also here, this layer is the most colorful in the cliff.

Further up on the right (west), we have Coconino Sandstone. More whiteish in color, this layer is the result of vast ancient sand dunes that covered enormous areas around the region. Due to the 1000ft displacement along the fault mentioned before, this is the first layer that is properly visible on the left (east).

The layers from here on up are the ones that are most prominently visible from Oak Creek Canyon Overlook, but the rocks on the left significantly differ from the ones on the right. Using the interpretive sign at GZ for help if needed, email me the answers to the following questions:

  1. What rock type is the top layer on the left (east) ?
  2. Where did it come from, and when was it deposited?
  3. Compare this left (east) rock against the layer at same elevation on the right (west). How does it differ in color and shape?
  4. What rock types are the two top layers on the right (west) ?
  5. Under which conditions was the topmost layer on the right (west) formed ?

    Bonus:
  6. Which of the layers is not also visible at the Grand Canyon, and why do you think this is the case?


 Pictures of the scenery are not required, but appreciated!

Caveat: Yes, this is Arizona with Dry Heat (tm), but the cache site is at around 7000ft altitude, and in most winters, there is ample snow fall, occasionally blocking the road. Also in summers, the canyon is sometimes closed due to wild fires. Plan accordingly!

Materials used
- interpretive sign on site
- A Guide to the Geology of Sedona and Oak Creek Canyon Area, Arizona (John Bezy)
- Roadside Geology of Arizona (Halka Chronic)

Additional Hints (Decrypt)

Na vagrecergvir fvta ng TM urycf jvgu zbfg bs gur nafjref. Sbe gur erznvaqre, guvax naq thrff :).

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)


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