Tuesday, April 15, 2008

Death Valley Evaporites

I realized, looking through my pictures, that I had a bunch of thematically similar pictures from a scattering of day; specifically, a bunch of playa lake pictures (evaporative lakes are a favorite topic of one of the profs on the trip) and a bunch of alluvial fans (a favorite of mine). So, I thought I’d just glom them all into individual posts. An alluvial fan post will be coming up soon, so today I submit pictures of playas for your viewing pleasure.

Playa lakes are bodies of water that are intermittently wet, often shallow, and commonly sites of evaporite deposition, due to the high evaporation/precipitation ratio.

As I typed that sentence above, I had a great idea for a t-shirt or bumper sticker: “Don’t hate the playa, hate the evaporation/precipitation regime”. It’s so pure, I might just cry.

Anyway, Death Valley and its neighboring valleys are hot and dry, so it’s a good place to go playa spotting. The famous playa in Death Valley proper is that Badwater playa, which is filthy with tourists. If you are looking for a more picturesque spot away from the crowds, I would suggest heading west toward the Panamint valley, and wandering along the road heading south of Ballarat towards all the BLM land. The picture below shows one of these evaporative lakes:

Walking out onto the playa, you can come across some really slick evaporative minerals; note the round little balls and the whispy, thread-like crystals in the picture below (snagged from another of my fellow field-trippers, with the very edge of a camera lens cover for scale). There was a major component of halite in this lake setting (it tasted like salt).

The growth of these evaporites under intermittent conditions leads to the development of large scale shrink-swell features, such as seen in the picture below (with someone’s great tromping footprints for scale). Evaporites can be seen filling in the cracks.

The action of waves and moving water is still important in these shallow lakes. In the picture below, symmetrical wave-ripples that parallel the shoreline (which is off-camera) can be seen, already encrusted with evaporites. These speak to periods of time when the water level in the lake was higher; this rapid and often dramatic fluctuation of lake level is an important characteristic of modern playa lakes. The stratigraphic implications of these geomorphic systems can be a little off-putting; what is a Waltherian succession within lacustrine strata?

Water conduits (and even a few channels) were obvious along the lake margins; these areas were muddy, devoid of plants, and often had little evaporite deposition within them.
The picture below shows some interesting bedform features indicative of fairly vigorous flow conditions. I interpreted these as being muddy parting lineations, with some subordinate mud-rip up features and mud-ball formation. Given time and compaction, a resultant mudrock succession would look fairly uniform and rather boring, leading us to erroneously assume fall-out suspension as the primary depositional feature of fine-grained sediment in this setting.

Next time: Modern Alluvial Fans

1 comment:

Callan Bentley said...

One of the coolest experiences I ever had was finding a hypersaline pool near the Devil's Golf Course (north of Badwater in D.V.) and sticking my arm into it. As I pulled out my arm, almost instantly, the salt began to crystallize all over my arm hair and skin. It was astonishing to me how quickly it happened once I exposed my arm to the air!