Although arid today, 250-300 million years ago this entire region was part of a vast inland sea. Read about the geology of Guadalupe Mountains National Park to learn how geoscientists know this was once an oceanic reef.
The Permian Period (299 million years ago to 252 million years ago) was the last period of the Paleozoic Era, the time on Earth when there was a vast explosion of complex animal life on Earth. During the Permian Period, reptile species diversified, the precursors to mammals evolved, and Earth’s land masses were part of one large supercontinent known as Pangaea. The end of the Permian is marked by the largest mass extinction in Earth’s history when ~96% of all species went extinct.
The Guadalupe Mountains are part of the geological area known as the Permian Basin, containing sedimentary rocks that are Permian in age. During the Permian Period, this area was located along the western edge of Pangaea, on the southern edge of the North American craton (the oldest part of the continental crust). The Permian Basin was an inland sea connected to the vast ocean surrounding Pangea by a narrow channel with three main arms: the Marfa, Delaware, and Midland Basins. The Delaware Basin contained the Delaware Sea, which covered the area of the present-day Guadalupe Mountains National Park and where the deposition of the Capitan Reef system formed 275 to 277 million years ago. The Capitan Reef complex makes up the Guadalupe Mountains and is one of the best-preserved Permian fossil reefs in the world.
The reef developed over several million years and is divided into four main parts: the fore reef, the reef, the back reef, and the basin, which represent the entire depositional structure. Each section of the reef is characterized by different types of rocks and fossilized reef fragments. During ancient times, waves would have battered the Capitan Reef causing chunks of the reef to fall off. These pieces accumulate at the front of the reef and form the fore-reef. The fore-reef is now marked by fragments of brachiopods, bryozoans, and echinoderms skeletons that have been fossilized through the process of silicification. Silicification is a process where the original pore spaces are filled with silicate minerals. The back-reef was the area between the reef and the land with stagnant, muddy, very salty water where fine sediments were deposited. along with sand banks, islands, lagoons, and sabkhas (salty mudflats). The basin was the area in front of the reef that sloped down to depths of over a mile. It is marked by carbon-rich, black limestones with abundant dead plant and animal material that washed down from the reef. In the stagnant depths of the basin, burial and decomposition of the dead organisms used up all the oxygen causing them to be preserved, and eventually with heat and pressure turned transformed the organic material into oil and gas
The low abundance of vegetation and soil over the formation, as well as series of perpendicular canyons that provide cross-sectional views of the reef make it easy to observe. Another amazing feature of the reef as a window to the past is that it was ‘pickled’ around 260 million years ago when the connection between the basin and the ocean was closed and the basin began to evaporate, leaving salt and other evaporite minerals behind and gradually covering the reef. Over time, mud and other sediments filled the basin and engulfed the reef. The reef system preserved in the Guadalupe Mountains shows remarkable similarities to modern-day reefs. The sections of the reef are somewhat analogous to those found in the Florida Keys today, but the climate in the Permian Basin would have been much warmer, more similar to the modern-day climate of the Persian Gulf.
The reef remained obscured until about 80 million years ago, when tectonic processes began to slowly uplift the entire region. Faulting between 20 and 30 million years ago uplifted a large, long-buried section of the reef. The uplift exposed the rocks to erosion, which wore away soft sediments and exposed the resistant reef of the Guadalupe Mountains that we see today. In 2018, the USGS identified the largest continuous oil and gas resource potential ever discovered within the Permian Basin. The thousands of wells drilled for oil exploration and extraction provide subsurface information about the region.