Tuesday, December 6, 2011

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!

1 comment:

Hollis said...

those are neat! ... quite beautiful.