Saturday, May 08, 2010

Evapor-art from the Permian Castile Formation, west Texas


The Late Permian Castile Formation is a ~500 m thick accumulation of evaporites in west Texas and south-eastern New Mexico. Its most striking feature is the vast number of alternating thin layers of lighter- and darker-colored deposits, layers that seem to be continuous across most of the Delaware Basin. The white laminae are mostly gypsum and anhydrite; the darker layers consist of calcite and organic matter.


Most experts agree that these laminations reflect seasonal changes; that is, a pair of white and dark layers corresponds to one year. The thicker gypsum layer was deposited during the dry season; the thinner calcite layer with the organic material formed during the humid season when algae were more abundant and only carbonates could precipitate from the lower-salinity water [for more details, see this paper].


The image above gives an idea how laterally persistent these laminations can be; the two photographs come from cores that are 24 km (~15 miles) apart (source: Kirkland, D.W., 2003, An explanation for the varves of the Castile evaporites (Upper Permian), Texas and New Mexico, USA, Sedimentology 50, p. 899-920).

These evaporites are often affected by small-scale faulting and folding; the resulting patterns are quite variable and aesthetically pleasing (well, at least according to me). I shot these photos on a recent geological trip to Guadalupe Mountains National Park, in a roadcut near highway 180; more pictures here.





4 comments:

Jessica Ball (AKA Tuff Cookie) said...

Beautiful photos! I have a chunk of one of those outcrops, collected on one of the first stops of my first field course. I always loved the folding in it.

Silver Fox said...

Oh, they are beautiful! Thanks for posting them.

CJR said...

Very nice! Thanks for sharing. Is the folding and faulting thought to be tectonic in origin? Or is it due to diagenetic volume changes from dissolution/reprecipitation?

zs said...

CJR - the answer is yes :). As this outcrop description suggests, part of the deformation is due to volume changes that accompany hydration and dehydration reactions (between anhydrite and gypsum). On the other hand, predominant fold axis orientations suggest the influence of Tertiary faulting that occurs in the area.

 
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