Geologists are intrigued by many aspects of the Guadalupes, including the gypsum sand of the Salt Basin Dunes. NPS Photo - Cookie Ballou

Geologic History

The Permian Period of geologic time occurred from 280 to 225 million years ago. The earth had already seen life diversify from simple, primitive forms such as algae and fungi to amphibians, fishes, and insects. The earth's surface had also been evolving and shifting. Thin plates of crust were constantly moving over the softer material below, steadily changing the pattern of the earth's surface. The supercontinent of Pangaea had not broken apart at this time and New Mexico and Texas occupied the western edge of this landmass nearer the equator. A vast Permian Ocean surrounded Pangaea. A narrow inlet, the Hovey Channel, connected the Permian Ocean with the Permian Basin. This inland sea covered parts of what is today northern Mexico and the southern United States. The Permian Basin had three shallow arms: the Marfa, Delaware, and Midland basins. The middle arm (the Delaware Basin) contained the Delaware Sea which was 150 miles long and 75 miles wide and was situated in what is now Western Texas and Southeastern New Mexico.

During the late Permian Period, a reef developed near the border of the Delaware Sea. This was the Capitan Reef, recognized as one of the premier fossil reefs of the world and best exposed in the Guadalupe Mountains. Growth of this massive reef ended near the close of the Permian Period. For several million years, the reef had expanded and thrived along the rim of the Delaware Basin, until events altered the environment critical to its growth. The outlet to the ocean was restricted and the Delaware Sea began to evaporate faster that it could be replenished. Minerals began to precipitate out of the vanishing waters and drift to the seafloor forming thin bands of sediments. Gradually, over thousands of years these thin bands entirely filled the basin and covered the reef.

About 26 million years ago, faulting occurred in this area uplifting this long-buried portion of the Capitan Reef nearly two miles from its original position. This uplifted block was then exposed to wind and rain causing the softer overlying sediments to be eroded until the resistant reef was uncovered. Today the reef towers above the desert floor as it once dominated the floor of the Delaware Sea 250 million years ago.

Capitan Reef Exposures

Rock exposures in Guadalupe Mountains National Park are composed of reef, back-reef, fore-reef, and basin sediments.

Reef

The reef, a submerged resistant mound or ridge formed by accumulation of plant and animal skeletons, is composed of the Capitan limestone. The Capitan is a massive, fine-grained fossiliferous limestone that formed by growth and accumulation of invertebrate skeletons of algae, sponges, and tiny colonial animals called bryozoans. These skeletons were stabilized by encrusting organisms that grew over and cemented the solid reef rock, unlike modern reefs built by mainly a rigid framework of corals. The Capitan limestone forms the thousand-foot high cliff of El Capitan, the most striking feature of Guadalupe Mountains National Park.

Back-reef

There was relatively low wave or current activity in the back-reef, an area between the reef and ancient shoreline. Only fine sediment was carried back into this area and the water was often stagnant, muddy, and had a high salinity. Sediment deposited in stagnant back-reef or lagoon waters often contained high amounts of magnesium, which combined with limestone to form the rock dolomite. Despite the high salinity, lifeforms were able to live in the back-reef. Brachiopods, crinoids and fusulinids are common fossils found in the back-reef sediments.

Fore-reef

Ocean currents and wave action battered the Capitan Reef, causing chunks to break off and slide down the reef front forming the fore-reef. The fore-reef is a debris slope that extended downward into the basin. In addition to debris, the fore-reef was also composed of fossils, such as trilobites, brachiopods, sea urchins, algae and bryozoans, lime muds, and calcium carbonate sediment, but the fore-reef did not become as highly cemented as the reef.

Basin

The basin in front of the reef sloped downward to depths of nearly half a mile. The sediments that washed into the basin during the building of the Capitan Reef later became thin black limestones separated by thicker beds of fine sandstones and occasional siltstones. The black limestone contains the organic-rich remains of the dead plants and animals that settled to the dark depths of the basin. Partial decomposition of the organic material in the stagnant depths used up all available oxygen, so most of the organic matter was slowly buried and preserved. Over millions of years, heat and pressure have changed the organic matter to oil and gas.

Ancient Life in the Delaware Sea

The Delaware Sea was host to a rich diversity of Permian life. The reef supported an abundance of organisms, including algae and sponges. Inhabitants of the rocky sea bottom were sea urchins, bivalve clams, and flower-like crinoids on long, slender stems. Horn corals were present, but relatively rare. There were also trilobites, a now extinct class of arthropods with segmented, three-lobed shells. Ammonoids and nautiloids, ancient cephalopods related to squid and octopi, propelled their chambered bodies through the sea in search of prey. Deeper on the reef, large clam-like brachiopods clustered together, each clinging to the seafloor by a pedicle, a single fleshy muscle. Tiny bryozoans clustered in colonies that resembled delicate, lacy fans. Most life forms could not survive in waters as salty as those of the back-reef, but fossils from those exposures tell us that some adapted well. Those lifeforms were blue-green algae, masses of small cigar-shaped fusulinids, and clam-like osracods.

The end of the Permian brought one of the greatest mass extinctions of all time. This event greatly affected life of the Delaware Sea. Horn corals and trilobites became extinct, along with certain groups of brachiopods, crinoids, bryozoans, ammonoids, and nautiloids. Sponges came near extinction, and many groups of algae died out, including most of the reef builders.

The Western Escarpment has played an important role in revealing the story of the Permian Period in North America. These exposures present one of the finest cross sections in the world of several transitions from shallow-water deposits to deep-water deposits. Abrupt changes in rock types are caused by the change in depth from the shallow submerged areas to the deep waters of the Permian Sea. Some two miles of Permian strata are exposed in and adjacent to the Guadalupe Mountains due to faulting which uplifted this section of the ancient fossil reef.

Faulting in this area began about twenty-six million years ago. Along a series of branching faults that run close to the base of the Western Escarpment, the western edge of the Guadalupe fault block has been lifted more than two miles from its original position below sea level. Fault zones that form the eastern border of the Salt Basin and the western edge of the Guadalupe Block are complex. They were formed by a series of branching faults that bend to the north-northwest from the southern end of the Delaware Mountains to the northern end of the Guadalupe Mountains. Most of the faults are nearly vertical and uplift ranges from 2,000 feet to a mile or more on individual faults.

The Western Escarpment extends from Bartlett Peak to El Capitan, with Shumard Peak and Guadalupe Peak, the highest peak in Texas at 8749 feet, in between. The massive rock face is composed of the Capitan limestone, or the reef complex. The slopes below the cliffs of Bartlett Peak and Shumard Peak consist of the "bank-ramp complex." The bank-ramp complex is made up of the Victorio Peak limestone, the Cuttoff Formation, and the Bone Spring limestone, which formed from unbound carbonate sediments deposited as broad banks. These banks stretched ten to twenty miles creating a gentle ramp dipping only one or two degrees toward the basin. These shallow carbonate ramps lack the binding organisms that are prominent components of the reef complex.

Below the cliffs of Guadalupe Peak and El Capitan are the Cherry Canyon siltstones and sandstones and the Brushy Canyon sandstones. These sandstones and siltstones were deposited as sediment filled in sub-marine channels in the basin.