Thrust Fault

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The law of superimposition says that stuff found on top is younger than stuff found lower down, in a geological or archaeological column. This is generally true, but there are exceptions, mostly trivial and easily understood. If a cave forms in a rock formation, the stuff that later ends up in that cave is younger in depositional age than the rock underneath which it rests (the rock in the roof of the cave, and above).

One of the coolest examples of what seems to be (but really is not) a violation of this Law of Geology is a thrust fault. A thrust fault is essentially a horizontal fault (as opposed to the more common vertical fault) in which rock from one area slides completely over another area. When this happens, the rock at the base of the upper unit (the one that slid over the other rock) is older than the rock on which it rests.

The fault isn’t really horizontal. but it’s horizontal enough for this to happen. And, in fact, the whole thrust-faulting thing is actually fairly complicated, and there are different processes that cause a similar effect. But in the end, you get an older layer sitting on top of a younger layer.

The reason this is not really a violation of superimposition is this: The older rock was actually deposited on top of the younger rock later in time than the formation of the younger rock. In a way, this is not much different than an ancient mountain made of ancient stuff eroding and generating sand that flows downstream and covers some pre-existing sediment. The fact that the grains of sand were formed a long time ago does not make that recently formed sand deposit old. It is young. But it is a young deposit made of old stuff. A thrust fault is the same thing but instead of there being a zillion tiny grains of sand deposited on some earlier sediment, it’s all one big giant piece!

Here’s a couple of photographs of the Keystone Thrust fault. Tell me if it looks familiar to you:



(Click either image to see a full sized version. Photos copyright Greg Laden.)

The dark grayish rock is the older rock thrusted upon the red and yellow rock.

If you go to Las Vegas and gaze westwards you’ll see a ridge in the distance. Some people say that part of the ridge resembles a man lying on his back. That man’s outline is the upper edge of this thrust fault, which is exposed here in Red Rock Canyon reserve, a BLM property where you can go and hike, climb rocks, and observe interesting geology and wildlife.

Technically,the Keystone is a Reverse Fault with a shallow dip. The gray rock is Cambrian limestone (Bonanza King Formation) and it rests on top of the Jurassic Aztec Sandstone that gives Red Rock Canyon its name.

People do argue that the thing in this photograph is actually part of different thrust fault system related to the Keystone. I’m not sure if that argument is settled yet. Either way, it’s a good example of a thrust fault.

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14 thoughts on “Thrust Fault

  1. Wow, those are really big: I can just barely make out the geological hammer! 😉

    Those are nice photos. I wonder if those Red Rocks are the same as which form Red Rocks Amphitheater in Colorado and make red mud in Oklahoma.

  2. It is my conjecture that there is a Red Rock(s) Something Something (county/state/national park/preserve/conservancy etc.) in every state in the US. Perhaps only in every state west of the Rockies. I’m still compiling the list.

  3. True story: At GSA a number of years ago, someone from UNLV seriously proposed that the term ‘thrust’ be replaced, as the sexual implication might make female students uncomfortable.

    I would guess mostly in the Rockies; during the Triassic, large amounts of sediments were deposited there above sea level and consequently oxidized. Any similar sediments in the east were either eroded away or are buried under Cenozoic rocks in the Coastal Plain.

  4. It’s remarkable how little deformed those cambrian strata are. Imagine the geometry of the force vectors that did the thrusting!

  5. When I first saw those pictures, I thought, “Oh, that looks like Red Rock, probably from somewhere near the scenic loop.” So, you made it to Las Vegas, eh, Greg? For TAM, probably. Everyone knows there’s not really anything else to interest you here. Hoover Dam, Lake Mead, and Mt. Charleston (peak = 11,900′) are farther.

  6. Mal Adapted @ 5:

    It’s remarkable how little deformed those cambrian strata are.

    It’s a bit deceptive to take a long distance view and imagine there is little deformation. Get up close and personal with the fault itself and the rock will be sheared in the extreme, sometimes becoming a glass.

  7. Greg Laden,

    I’m sure there are dancing girls in MN, also. They may not have the same costumes or lavish musical accompaniments, but hey, dancing girls are dancing girls almost anywhere you go. Now, the sports betting may be slightly more out in the open here, but how many office pools are there during March Madness and how many friendly wagers between buddies are made during the Super Bowl in places that have nothing to do with a casino, tribal or otherwise, hmmm?

    Anyway, glad to hear you can travel to see family. Have fun next time you make it over here, and remind us about John Ensign and Sharon Angle every time someone over here brings up Michele Bachmann.

  8. Held for moderation with no links, profanity, and more than 20 hours since the last post? Man, you’re really cracking the whip over here.

  9. Shawn, I have no idea why most comments that get held get held. Oh wait, there is a profanity! Check out the last two words of your comment……

  10. Cool post. Brings up a whole host of side topics caused by thrust faulting like allochthons or completely over turned beds. In some areas you can even have thrusts on top of older thrusts.

  11. Oh wait, there is a profanity! Check out the last two words of your comment……

    hahahahaha. I would have thought that the other two names were more profane, but it’s probably because of the proximity, not the ability to affect society.

    OT: Angle is going to run for the now open House seat in a free-for-all. If there is only one Democratic candidate, that would be the only chance for the NV-2 district to not go Republican.

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