Paleo-caves in Death Valley
You have probably heard of "caves", or a natural void formed in rock. Caves are formed in a number of ways, but the most familiar are probably those formed in limestone - called karst or karstic. Karst caves are formed by the steady dissolving of limestone rock by naturally acidic groundwater percolating through the ground, often along faults or other natural weak areas.
Death Valley in California is not a hotbed of limestone caves, though evidence of them can be seen in a number of places! Ancient caves, often tens or hundreds of millions of years old, sometimes collapse or fill in with silt/minerals/debris. An ancient cave formation is called "paleokarst", and there are a few areas in Death Valley where you can find it. The easiest to access is in Titus Canyon, a major canyon who's lowest reach carves its way through limestone bedrock, leaving behind towering polished rock walls and revealing the internal structure of the rock.
Titus Canyon's narrow section is carved through the Bonanza King Formation, a Cambrian-age (538 - 485 million years ago) marine sedimentary rock package several kilometers thick. It's found in many areas of Death Valley and the surrounding region. Across its area and thickness, it alternates from dolomite to limestone (dolomite minerals versus calcite minerals). Dolomite doesn't usually dissolve and form caves, but limestone does - and the many fractures in this rock appear to have allowed numerous tiny caves to form somewhere in the distant past. Most of these have since been filled in with calcite minerals, making them paleokarst.
In this image, you can see the void of the cave lined with thinly-layered calcite. It has a flat floor, and these features are characteristic of having formed in a cave. In Titus Canyon, we can find these from a few inches to a few meters across, and as the canyon has been carved into the rock, these voids are exposed. This one below tells an exceptionally clear story:
You don't need to be a geologist to see how this differs from the first example. There's a weird pendant-shaped protrusion hanging from the top, there are layers in the minerals, the bottom half is red… there's a lot going on. Let's get some lines on there and figure out what we can see:
That's better. OK - I've separated out the parts of this outcrop which from my desk look to be distinct. Here's a close-up of part of the left-center area to show how it appears different up close.
OK, so we have the blue-gray limestone bedrock on the right, top, and lower left, then a gray-red area with layers and what looks like gravel or other angular pieces of the limestone. Above that is a finely-layered calcite deposit (remember? that's cave-like!), and above that is a layer of calcite which is rather porous. The top surface of this area is sort of scalloped and curved, which is also characteristic of limestone caves.
Here's the fully interpreted version. Let's go through the sequence of events. First, we have the limestone bedrock of the Bonanza King Formation (BKL). At some time in the distant past, a portion of it dissolved away and formed a cave. Over time, silt and debris filled in the bottom of this cave. Because it is ancient, we call it paleo-fill (OPF). This might have happened at a time when the cave was dry (not containing water), since if it was wet we would expect some amount of calcite to be deposited around its perimeter as is typical in limestone caves.
Above that is a layer of thinly-layered calcite, the cave mineral. This layer contains some reddish silt and the occasional fragment of the limestone, so the cave was probably happily active during the time that was deposited! This is the oldest cave mineralization phase I can discern in this outcrop, so I call it Old Cave Calcite (OCC).
Next, we have a slightly porous, light-colored calcite deposit. The porosity suggests to me that it was deposited faster, but I am not an expert on cave deposits so will not comment further. This unit fills the cave to its top in most places! I call this the Middle-age cave calcite (MAC).
Finally, the youngest unit is another cave-deposited calcite (YCC). As in the first cave picture above, it is a coarser-grained pale yellow color with layers that parallel the sides of the void. Most of the paleo-caves you can find in Titus Canyon are filled with this sort of calcite, and it may be geologically recent (last million years?). It filled the final remnant of this ancient paleo-cave, and is contrasting with the material around it. This visual and textural contrast is what inspired me to write this post in the first place - the best example I've seen in person of paleo-karst and the evolution of a miniature cave system, right in the heart of Death Valley - my favorite place in the world.
