First-Day road log: From Washington Ranch to state line Castile Gypsum roadcut, Rader debris flow, Brushy Canyon submarine channel sand, Salt Basin, Capitan Reef Overlook, Chosa Draw evaporite karst features, and Parks Ranch Cave

Day 1 (Thursday, 10/5/2023): Regional geology of the Guadalupe Mountains, Delaware Basin, and evaporite karst of the Castile Formation.

The day will begin with exposures of the Upper Permian Castile gypsum at the famous state line roadcut on National Parks Highway. At this roadcut the section consists of couplets of white-to-gray gypsum laminae 1–5 mm in thickness, alternating with dark-brown laminae containing a mixture of calcite and organic matter. The gypsum- calcite couplets have been interpreted as seasonal varves, in part because of their remarkable lateral continuity. The next two stops will feature exposures of deep-water sands and limestones of the Delaware Mountain Group, including evidence of a submarine debris flow at the base of the Rader Member of the Bell Canyon Formation. Stop 3 provides a close-up view of cut-and-fill of a submarine channel sandstone of the Brushy Canyon Formation, displaying spectacular soft-sediment deformation features. Leaving the Permian section behind, the route west concludes with Quaternary deposits of the Salt Flat gypsum playa, the northernmost of ~40 gypsum playas of the greater Salt Basin. On the way back to New Mexico the route passes directly below El Capitan, a striking and much photographed promontory at the southern end of the Guadalupe Mountains, formed in rocks of the Capitan Reef and associated forereef talus. A lunch stop and photo-op are planned at a scenic turnout near the base of El Capitan. The remainder of the afternoon will be spent at Chosa Draw, an eerily beautiful landscape and a classic example of the Gypsum Plain/Chihuahua Desert ecosystem, where we will explore caves and sinkholes formed in the Castile gypsum, including Parks Ranch Cave, the second-longest gypsum cave in North America.