Day 2 : Stop 1

DAY 2 (9/27/2025) - STOP 1

"Trail Canyon Road Cut - Castille Formation"

COORDINATES: 32.03289° N, 104.84396° W

 

            Fig. 6 Bended Layers                

Fig. 7 Human Scale of Bended Layers

   Our second day brought us to our first site, surface exposures of the Castile Formation. This stop put us at the top of the Delaware Basin’s sedimentary record. After the Capitan Reef formed and the basin slowly grew shallower, turning the entire region into an enclosed lagoon. In this environment, evaporation began to surpass freshwater input, and the basin began to accumulate a larger concentration of evaporites.

    The Castile Formation clearly records this transition. At the outcrop, the first thing that stood out was the significant banding with alternating brownish layers of calcite and brighter white layers of gypsum as well (fig. 6 & 7). These laminations are extremely thin and continuous, which are moderately to poorly sorted, with some small contortions, microfaults, and chicken-wire textures where gypsum crystals pushed against each other during growth. Also evident is the bending of the layers, which reflects the shallow water conditions in which the sediments accumulated, where shifts in water depth, energy, and salinity caused variations in how each lamina was laid down, leaving behind curved bands which recorded these fluctuations (fig. 8 & 9). 

    The mineral alternations are key to understanding the environment in which these deposits formed. Calcite and gypsum do not precipitate from seawater under the same conditions. Calcite forms when the basin becomes relatively water-rich, such as during floods or prolonged wet periods. Gypsum, however, only precipitates when evaporation events increase and brines reach higher concentrations. The Castile Formation, therefore, represents thousands of repeated cycles of drying and wetting periods when the Delaware Basin shifted between hypersaline conditions
and intervals of increased freshwater input. These fluctuations were probably driven by climate patterns, sea-level changes, and variations in basin hydrology.

    Because the Castile Formation sits above the reef and slope deposits, it marks the end of open-marine sedimentation in the basin. After the Capitan Reef grew and local uplift continued, the basin became progressively more isolated until it behaved like a giant evaporative lagoon. The gypsum–calcite layers at this site are the direct record of that shift. They show the basin at its final stage of filling, just before the region was uplifted, dissolved, and eventually eroded to reveal both the reef and the evaporites resting on top of it.

Fig. 8 Calcite (brown) and Gypsum Layers (white)

    There were no ripple marks, cross-beds, and very little evidence of current activity, signifying a relatively quiet environment during deposition. The Castile Formation is one of the few places in the world where an evaporite sequence of this scale is exposed at the surface. It’s a long record of water levels rising and falling, preserved layer by extremely thin layers.

Fig. 9 Calcite (brown) and Gypsum Layers (white)