El Capitan!
El Capitan is a peak in Culberson County, Texas, located within Guadalupe Mountains National Park. The 10th-highest peak in Texas at 8,085 feet, El Capitan is part of the Guadalupe Mountains, an exposed portion of a Permian period reef uplifted and exposed by tectonic activity during the late Cretaceous period. The southern terminus of the Guadalupe Mountains, El Capitan looms over U.S. 62/180, where its imposing height and stark outline have made it one of the iconic images of the Trans-Pecos to generations of travelers.
El Capitan is the southernmost peak of the Guadalupe escarpment, an ancient limestone reef that forms the present-day Guadalupe Mountains. These mountains are an exposed portion of the Capitan Reef Barrier, a 350-mile long reef constructed primarily from calcareous sponges, encrusting algae, such as stromatolites, and lime-rich mud directly from the ocean. This reef surrounded much of the Delaware Sea, an inland ocean that covered parts of modern southern New Mexico and Trans-Pecos Texas in the Permian period (about 290 million years ago). Near the end of the late Permian period, in the Ochoan epoch, the outlet that allowed sea water to enter the inland waters began to silt over, occasionally closing the inland sea from its source. Mineral-rich and cut off from replenishment, the inland sea began to evaporate into layers of alternating gray anhydrite/gypsum, brown calcite, and halite, which formed the Castile Formation. As salt concentrations increased, laminated halite, anhydrite, sylvite, and polyhalite formed the Salado Formation, which eventually covered and grew beyond the lower Castile Formation.
By the end of the Ochoan, these deposits had filled the roughly 1,800-ft-deep basin and covered the reef with dry land. Red silt and sand deposited by rivers crossing these new lands eventually formed the dolomitic Rustler Formation, and the Dewey Lake Formation, burying the reef even deeper. The Capitan Reef stayed buried for over 150 million years, until the late Cretaceous period of the Mesozoic era (80 million years ago), when tectonic activity associated with the Laramide Orogeny caused a significant uplift in the area and created a major fault line, the Border Fault, in the area west of the Delaware Basin. Exposing the Guadalupe Mountains area of the reef, the impact of this tectonic event can be clearly seen in the difference in height between the towering El Capitan and the adjacent salt flat graben, vertically driven 1,000 feet apart by the uplift. Once exposed, the natural forces of wind and rain slowly stripped away the softer sediments, further uncovering the ancient reef and revealing the sheer limestone walls of the Guadalupe Mountains.
Development
During the middle part of the Permian Period a reef developed along the margin of the Delaware Sea. This was the Capitan Reef, now recognized as one of the most well-preserved fossil reefs in the world. For several million years the Capitan Reef expanded and thrived along the rim of the Delaware Basin until events altered the environment critical to its growth approximately 260 million years ago. The outlet connecting the Permian Basin to the ocean became restricted and the Delaware Sea began to evaporate faster than it could be replenished. Minerals began to precipitate out of the vanishing waters and drift to the sea floor, forming thin, alternating bands of mineral salts and mud. Gradually, over hundreds of thousands of years these thin bands completely filled the basin and covered the reef.
About 80 million years ago tectonic compression along the western margin of North America caused the region encompassing west Texas and southern New Mexico to be slowly uplifted. A transition in tectonic events 20-30 million years ago initiated the formation of steep faults along the western side of the Delaware Basin. Movement on these faults over the last 20 million years caused a long-buried portion of the Capitan Reef to rise several thousand feet above its original position. This uplifted block was then exposed to wind and rain causing the softer overlying sediments to erode, uncovering the more resistant fossil reef and forming the modern Guadalupe Mountains. Today the reef towers above the desert floor as it once loomed over the floor of the Delaware Sea 260 to 265 million years ago.
Questions and Tasks
1. This exposed portion of the Capitan Reef Barrier was constructed primarily of what?
2. What kind of deposits from rivers crossing these lands eventually caused this formation?
3. What kind of activity and during what period exposed El Capitan?
4. To prove you were at this location, please post a picture of yourself and/or your group with El Capitan in the background.