Saturday, April 5, 2008

Death Valley Day 4: Ubehebe Crater and Race Track Playa

Heading towards exposures of the Devonian and Ordovician in the north end of the park, we stopped off at some of the more tourist-y stops in Death Valley, including Ubehebe Crater and Race Track Playa. There were a fair amount of gawkers, of course, but the geology was pretty slick nonetheless.

Ubehebe crater is part of a chain of 13-16 craters recording Holocene (~6000 years, I think) phreatic volcanism in the region. In addition to the craters proper, base surge deposits can be found in relatively close proximity to the craters, and preserve some phenomenal flow structures.

The picture below was taken from a lookout along the rim of the crater, and shows the fine-grained lacustrine and coarser conglomerate deposits through which the volcano punched. The darker, banded unit that forms the rim proper (and shows an angular discordance with the underlying sediment) are tuffaceous beds with lots of pumice and basalt shards.

To the south of the main crater is Little Hebe, shown in the picture below. It is one of the younger craters, and shows evidence for effusive basalt flows (the rim of this crater is basalt, rather than the base surge deposits seen in the main crater).

If you head south of little hebe, off the beaten path, you will come across some phenomenal sedimentary structures, formed by the high concentration surge deposits as they flowed away from volcano. The picture below shows antidune cross-stratification, a fairly rare sedimentary structure produced by upper flow regime conditions and rapid deposition.

After Ubehebe, we sped northwards towards Racetrack Playa, probably one of the more famous spots in the park. Playa lakes, of course, are known for being FLAT ephemeral ponds, and are often evaporitic in nature; what is unique about Racetrack Playa, however, is that it is not a terminal playa. Rather than being found at the low point in the valley, Racetrack is actually 3000+ meters up in the mountains. There are no evaporite minerals (or casts) to be found in this playa; just a lot of (potentially thixotropic) mud and silt.

That, and the famous moving rocks.

There is still some debate as to what process exactly is responsible for moving these stones. Wind is a favorite explanation, but I would point out that if you look at the picture below, you’ll see a preferred orientation for many of the tracks, as well as a marked similarity in the angle of the changes in their direction. Also, the vast majority of these rocks are sourced from a north-facing prominence of basement rock exposed on the edge of the playa. Given the north-oriented face, 3000+ meter elevation, and the uniform nature of the tracks seen on the edge, we wondered if ice might not play an important role in the entrainment of these blocks.

Coming up next…more Paleozoic Carbonates!


Mel said...

First, I'm really, really jealous ;-)
Second, on the moving rocks. I agree that it's probably not the wind, because then the track marks should get disturbed as well. But, what do you mean by ice? I was thinking of the frost/snow line. If these rocks are on a boundary where it commonly freezes and thaws, that might be why they are moving.

Eric said...

Ice was suggested as a mechanism on the basis of a couple of observations, some of which I didn't actually mention in the post (cause I'm lazy).

First of all, some of the tracks themselves don't actually have any rocks in them; barring touristoturbation having mucked things up, the explanation for having a trail produced without a terminal rock might be that an ice-locked block bounced and scrapped along the bottom, only to be carried away after the trail was produced.

Similarly, some of the trails had smaller fragments of the rock in the trail, or chunks of rock that were resting in a depression but without an attached trail; might these not be...DROPSTONES!?!?!!?!?!

Unfortunetly, we didn't see any evidence for chatter marks, etc, which would be much stronger support for ice influence on the playa. An alternative idea might be the thixotropic mud; if there is a slight regional dip in the playa (which is possible, given how high up we are), maybe the wetted mud would allow the rocks to slide with relative ease in a preferred direction?

Regardless, they are pretty weird.