Evaporite Casts in Sandstone

I've been working, as part of the ol' PhD rigamarole, in the Wilkins Peak Member, which is part of the Eocene Green River Formation in southwest Wyoming.  Anyway, this part of the world during the Eocene was characterized by extremely high evaporation coupled with relatively low precipitation, which is a perfect setting for the formation of evaporites!  High evaporation rates drive off water, while leaving behind water soluable ions, resulting over time in highly concentrated brines from which evaporite minerals form. 

In previous posts, I've shown some modern examples of evaporite formation from Death Valley, where evaporites are precipitating on a playa surface, which is a pretty good analogue for parts of the Wilkins Peak.  This previous post actually shows some evaporite mineral casts from the purely lacustrine portion of the Wilkins Peak, probably some sort of Na-rich evaporite.

However, you don't have to be in the lake for evaporites to form!  These pictures below are evaporite mineral casts preserved in sandstones that formed as part of the floodplain/overbank setting of a fluvial system that fed into the lake. 

Here's a closer view, showing the bladed, fairly delicate looking crystalline features of the evaporites, preserved in the sands. 

I reckon these are Nahcolite casts, on account of their elongated crystal habit, but then again I'm no evaporite specialist.  Still, they're kind of nifty!

Sunday Sed Structures - Evaporite Casts!

Lookit here! Pictures of sedimentary structures on a sure-fire Sunday, rather than later. What a novel concept.

Anyway, these pictures below show some outcrop exposure of evaporite crystal casts from the Eocene Wilkins Peak in Southwest Wyoming. This first picture below shows a little crystal fan of some sort of evaporite.



These two pictures below show a bedding plane view of evaporite crystal-casts.





That's all...go eat some salt on your hard-boiled Easter Eggs to show some evaporite solidarity!

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