Massive set of concentric ribs (arrest lines)?
My MSSE advisor John Graves (previously mentioned here) went on a float down the Green River in Utah last weekend.
This appears to be a huge set of concentric ribs (a.k.a. "arrest lines") on the face of a big joint in massive quartz-rich sandstone. Bedding runs ~horizontally across the image, though not to be confused with the perfectly horizontal "bathtub ring" waterstains from the river. John says, "My best guess from the guide book is that it's Entrada Sandstone, Carmel Formation & Navajo Sandstone top to bottom." The fracture appears to have started in the middle of the cliff and propagated downward and outward. Note how the ribs "flare" out at the far edge. I guess an alternative hypothesis is that this is some weird kind of dune cross-bedding in the Navajo Sandstone: the inside of a barchan dune, perhaps? (though barchans wouldn't form in the "sand sea" situation in which the Navajo was deposited)
Anyone else want to offer another interpretation for this? I think that's what it is.
3 Comments:
Nice photo. Actually, I think that the cliff is Wingate Sandstone, topped by Kayenta and Navajo. The Wingate, another huge aeolian deposit, often fractures with that conchoidal pattern. I cannot say how they formed however.
Doesn't look like any sort of cross-bedding I've ever seen; I can buy the fractures.
Definitely a conchoidal fracture and agree with David, Wingate!
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