20 June 2014
One of the real treats of the spring break field course in west Texas was visiting a road cut of the Castile Formation, a Permian evaporite formation south of the Guadalupe Mountains. It’s on the border between Texas and New Mexico, and I’d been wanting to see it for years after seeing a photo of the folds there in a structural geology paper about folding.
The strata of the Castile Formation are gypsum/anhydrite (the white layers) and organic-rich limestone (the dark layers). They are thought to represent changes in the salinity of the basin in which the strata were deposited (the Delaware Basin). The regular periodicity of the alternation of sediment type suggests some sort of rhythmic influence on the salinity of the basin, and many workers interpreted theses black and white layers as varves – seasonal deposits.
But it’s probably a more complicated story than just “seasons only” – as there are anomalously thick layers interspersed throughout:
But we didn’t come here for the layers themselves. We came to see their high-contrast outcrop pattern when folded. Though this post runs the risk of overwhelming you with gorgeous fold imagery, I’m going to just let loose the avalanche:
And, why not, let’s finish this feast with a single fault example: