Field of Science

The Basal Chinle Formation in Canyon DeChelly National Monument

The base of the Chinle Formation is well exposed in Canyon DeChelly National Monument just outside of Chinle, Arizona.  Here the Chinle fills paleovalleys incised into Permian-age rocks, in this case the aeolian DeChelly Sandstone.  The only trail accessible in the monument is the White House Trail, which leads down to abandoned ancient puebloan indian sites on the valley floor and in the canyon walls.  Thus, this trail is very popular with visitors to the monument; however, I'll bet that the majority of them miss this feature, a 'u'-shaped valey carved in the DeChelly Sandstone and filled with gravels of the Shinarump Member of the Chinle Formation.

The capping darker sandstone is the Triassic Shinarump Member of the Chinle Formation, and the buff sandstone is the Permian DeChelly. In the direct center of the photo is a dark patch in the buff.  This is a 50 foot deep gravel filled paleovalley cut into the DeChelly.  You can also trace it to the left and right in the photo.  Note the figure on the trail just to the left of the valley, and a figure under the tree just to the right.

Something looks very wrong with the photo above. This is the contact between the Triassic Shinarump (on the left and the Permian DeChelly (on the right). The contact between the two is vertical along the sides of the paleovalley. This is relatively flat lying strata. Pretty rare to see this.

One very cool aspect of a vertical contact (and unconformity) like this is that it is pretty obvious that the older unit (in this case the cross bedded sandstone) must have been fully lithified before the younger unit was deposited.  In this case the was a slot canyon in the DeChelly that was filled in in the Late Triassic.  Actually Triassic topography is visible here.  You can stand on the end and look inot where the canyon would have been about 220 million years ago.

4 comments:

  1. That is a pretty spectacular contact.

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  2. Hey, Bill. I just got back from Denver so this is my first chance to catch up on reading your blog since your visit. The pictures on these posts look good. It was a lot of fun to hang out and watch you and Jeff work; hopefully you all can come up and visit again soon. You are welcome anytime.

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  3. Hi Bill! What is the source on that info about the paleovalley fill? (If you ever read this!)

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  4. Hi J,

    The source is that you can see the superposition for yourself :). Actually the infilling of the Shinarump is well documented in the Chinle "bible", Stewart et al. 1972.

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