|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.
|Springdale Sandstone thrust over itself|
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.
|Drag fold in Dinosaur Canyon member|
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.
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.
|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.
|Image courtesy of nasa.gov|
|Thrust faulting and drag fold, Kolob Canyons, Zion NP|
Hamilton, W.L., 1984, The Sculpturing of Zion with road guide to the geology of Zion National Park, Zion Natural History Association.