Chert Nodules in Limestone

On a hike in Big Bend National Park one December this unusual boulder sitting nearly in the middle of the path immediately caught my attention, and not just because we had to walk around it. Its freakish blobs weren't like anything that had appeared on the hike at that point, although I would later see them again along the Santa Elena Canyon trail. Not having a resource for the geology with me at the time, I simply took photos and made some guesses.

​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