When my husband and I returned to our AirBnB, I did a little sleuthing on the internet and found a fantastic virtual field trip created by the late Dr. Francis Redfern, as physics professor. Thanks to his informative site I learned that these boulders fell from the canyon above and that the mysterious nodules were chert; possibly the boulders are from the Cretaceous Del Carmen Limestone. Redfern offered further ideas of why the chert appears both as nodules and veins in the limestone in Big Bend: Why it is a vein, I don't know (and neither, apparently, do geologists - there are at least a couple of ideas). Maybe there was a time when silica-shell builders were predominant and a silica ooze formed on the floor of the sea, or maybe it is the result of a diagenetic process, which is a change in the sediment after it was deposited, for example by an accumulation of chert over time from fluids rich in silica. The chert looks "concretionary" to me, so I favor the latter view. In any case, I was happy to have this new knowledge as I spotted more on a hike a few days later, although none were as glorious as the first one. If you're interested in more information on the geology of Big Bend National Park, check out the "Resources - Geology" tab on my site where I have direct links to Dr. Redfern's site as well as a few books that I found especially helpful. References: Redfern, Francis. "The Rio Grande's Main Squeeze: Santa Elena Canyon", https://prism-redfern.org/bbvirtualtrip/elena/elena.html  Teepee structures are some of the most unusual AND prolific sedimentary structures around Carlsbad and the Guadalupe Mountains. Once you see one, you begin seeing them everywhere. They are abundant in the Yates and Tansill Formations (Permian). If you ever travel to Carlsbad Caverns National Park, keep an eye open for them the entire way to the visitor’s center!
Teepee structures form when a sheet of evaporites grows but runs out of room. Instead of being able to continue to expand outward, the lack of space around the perimeter forces the layer to buckle upward. Interesting side note: During our field work measurements we noticed the rock layer where we found teepees was much more reactive to HCl, indicating a higher presence of calcium carbonate compared to the more dolomitic layers both immediately above and below. Without the presence of teepees, visually the layers would appear the same, but we chose to identify layer with the teepees as limestone due to how notably different it reacted to the acid test. 
These pictures show a sort of cross-section of that collapse on a roadcut on Hwy 90 between Alpine and Marfa. This area would have been inside the volcano when it was active, but once the magma chamber finally emptied itself, it could no longer support the weight of the earth above. Boulders and geologic debris fell as the volcano caved in on itself; these rocks and boulders were then re-cemented to create the "collapse breccia" you can see in the images above.
References: Speaking of Geoscience GSA Guest Blog, "Big Bend Snapshot of History Project's Geology of the Paisano Pass Volcano", https://speakingofgeoscience.org/2012/04/12/big-bend-snapshot-of-history-projects-geology-of-the-paisano-pass-volcano/ |
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