20 June 2014

Friday fold: the Castile Formation at the State line outcrop

Posted by Callan Bentley

IMG_2121

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.

IMG_2093

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:

IMG_2104

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:

IMG_2097

IMG_2096

IMG_2095

IMG_2098

IMG_2107

IMG_2105

IMG_2103

IMG_2101

IMG_2099

IMG_2115

IMG_2114

IMG_2113

IMG_2111

IMG_2110

IMG_2109

IMG_2108

IMG_2126

IMG_2124

IMG_2123

IMG_2119

IMG_2118

IMG_2130

IMG_2129

IMG_2127

And, why not, let’s finish this feast with a single fault example:

IMG_2116