Thursday, April 21, 2011

Folding And Faulting In Kolob Canyons

Perhaps as early as 150 million years ago during the Jurassic period of geologic time, western North America began to experience “pushing” from the west as the denser Pacific plate began a relentless slide beneath a more buoyant North American plate.  Like wrinkles in a rug pushed up on a hardwood floor, the rocks of the Kolob Canyons section of Zion National Park were squeezed, gently folded, and broadly uplifted over a period of nearly 100 million years.   
FoldAndFault_ TaylorCreek_ Zion_NP
Folding and faulting near Taylor Creek, Zion NP

When compressional stress is applied gradually enough, rocks fold in on themselves and can yield like silly putty while not fracturing.  But when compressional forces are applied swiftly enough, deformation occurs with the development of brittle thrust faults in which rock layers fracture and are thrust over themselves.

I was able to take a look at some of this structural geology the other day as I drove south from Cedar City UT.  In particular, I wanted to see once again where the Moenave Formation was shoved over itself as part of the Taylor Creek Thrust Zone.  The Springdale Sandstone member of the Moenave is the prominent cliff-forming rock layer, while the Dinosaur Canyon member forms the bleached white layer beneath it.

017Springdale Sandstone ThrustFault
Springdale Sandstone thrust over itself

012ThrustFault_ Springdale Sandstone

During tectonic compression, the Kannara Fold was formed as a major crustal wrinkle in what would one day become the northwest corner of Zion National Park in southwestern Utah.  As the wrinkle became more intensely folded over time, brittle fractures developed on the eastern flank of the fold, allowing the rock to be thrust over on itself. 

The rocks I came to look at are on the east flank of the fold.  The upper part of the thrusted block has moved the Springdale Sandstone from the east toward the west over itself.

   
018DragFold_In_ AlbinoDino_Kolob Canyons
Drag fold in Dinosaur Canyon member
It was suggested to me that while I was in Kolob Canyons I should also take a look at the Albino Dino, the bleached white Dinosaur Canyon member found below the Springdale Sandstone.  “You can see a drag fold in the road cut!”
I’ll be the first to admit I had to stand there for a while, pondering a sense of movement in the rocks.  I considered the thrust faulting in the Springdale Sandstone I had been looking at, and its location just above the Dinosaur Canyon member, and realized these rocks were related to that thrusting.


019DragFold_ AlbinoDino_ KolobCanyons

I noticed  the fault area between the upper and lower arms of the fold.  Here was the drag fold, a minor fold in the rocks due to shear resulting from slippage on a fault.  The (faint!) red arrow in the image below indicates this area of slippage.

020DragFold_ AlbinoDino_ KolobCanyons
Drag fold along a small reverse fault












The drag folding along a reverse fault (which is what I believe we are looking at here) can be better visualized if the diagram is flipped so that the fault line dips in the other direction.  The rocks that are the recipients of the dragging would be the upward-curving rocks to the left.


1121DragFolds
Image courtesy of nasa.gov


022ThrustFault AndDragFold_ KolobCanyons
Thrust faulting and drag fold, Kolob Canyons, Zion NP


Reference:
Hamilton, W.L., 1984, The Sculpturing of Zion with road guide to the geology of Zion National Park, Zion Natural History Association.

4 comments:

  1. Very minor correction: the Springdale Sandstone has been moved to be the lowest member of the Kayenta Formation. It's no longer part of the Moenave Formation.

    So I guess that during the thrusting, the weak lacustrine Whitmore Point Member (which overlies the Dinosaur Canyon Member) of the Moenave Formation was entirely removed?

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  2. Dinogami - Good to have your input! Thanks for the clarification - I had forgotten about the Springdale Sandstone now considered to be in the Kayenta.

    In his description of the Moenave, Hamilton does talk about the Whitmore Point member as being between the Springdale Sandstone and Dinosaur Canyon. But in the chapter on compression, I didn't find any mention of what happened to it during the thrusting. It seems very likely, though, that the weaker Whitmore Point member may well have been removed at some time. Or perhaps other later investigators have found it in the field?

    This point is definitely worth further investigation. It's another good possibility for a field trip, or just a great opportunity to spend another day in Zion.

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  3. I can't wait to go hiking with you.

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  4. Gaelyn - I will be happily loaded down with geology field guides!

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