Overview of the Geology of White Sands National Monument Within the Tularosa Basin Lies a Unique Geological Creation

Overview of the Geology of White Sands National Monument Within the Tularosa Basin Lies a Unique Geological Creation

National Park Service White Sands Department of the Interior White Sands National Monument Overview of the Geology of White Sands National Monument Within the Tularosa Basin lies a unique geological creation. It is here that the world’s largest gypsum dunefield can be seen for hundreds of miles as a thin shimmering line across the base of the San Andres Mountains. This overview is intended to elaborate on some of the major geological factors set in place for such an exceptional place to exist. The Permian Sea The Laramide Uplift The unique geologic creation of the Approximately 70 million years ago, world’s largest gypsum dunefield toward the end of the Cretaceous has been a project in the making for Period, the same compressional over 250 million years. forces that developed the Rocky Mountains uplifted marine rocks During the Paleozoic Era (570-245 within what is now the Tularosa million years ago), a supercontinent Basin. Known as the Laramide known as Pangaea accounted for Orogeny, this process has thought to almost all of Earth’s landmass. have been primarily influenced by Major portions of North America, tectonic activity. predominantly across the western half, lay beneath a shallow body of Earth’s surface is comprised of seven water called the Permian Sea. continuously moving plates. Collision and diversion of these plates often Throughout the Permian period results in common geologic features (290-245 million years ago), sea including mountain ranges and rift levels rose and fell on numerous zones. occasions. It was at this point that prominent gypsum deposits began When a thin dense plate collides to occur. with a thicker but lighter plate, the thin plate will descend beneath the Gypsum is an evaporative mineral. thick plate through a process known During times of low sea levels and as subduction. (Figure 1) A well- higher rates of evaporation, calcium referenced example of the (Ca2+) and sulfate (SO42-) ions subduction process is the Andes inevitably became concentrated mountain range in South America. enough for gypsum rock Utah Colorado (CaSO4·2H2O) to develop. The Towards the end of the Cretaceous gypsum rock was then deposited on Period, the angle of subduction the sea floor which is now evident beneath the United States became within the Yeso Formation found in increasingly shallow. This in turn the surrounding mountains. transferred compressive forces from a westward direction into an Arizona New “Yeso” is Spanish for gypsum. The Mexico eastward one. Yeso Formation is a 1,500-foot thick deposit of gypsum rock that The eastward shift of compression is Ancient Landscapes of the Colorado Plateau expands throughout much of believed to be responsible for the the San Andres and Sacramento formation of the Rocky Mountains mountain ranges. This formation (Figure 2). The Laramide uplift © Ron Blakey, from can be seen in patches along the affected a very broad region, Extent of the Permian Sea 280 million years ago Sacramento Mountains. including the White Sands area. Formation of the Tularosa Basin The geologic development of the Tularosa Basin began approximately 30 million years ago with the formation of the Rio Grande Rift. The rift is a north-trending continental rift zone that extends from central Colorado in the north all the way to the state of Chihuahua, Mexico in the south. Figure 1: Normal subduction of an oceanic plate beneath a continental plate. Compression associated with subduction results in the uplift of a mountain belt near the plate boundary. The Rio Grande Rift formed during a time of geological stress caused by crustal extension. During this time, the San Andreas Fault began to form as continental plates moved away from each other. As the plates pulled apart, the Earth’s crust thinned until it finally separated. This separation of the crust caused enormous gushes of magma to push upwards from the Earth’s mantle. These upwellings stretched large areas of the southwestern United Figure 2: Shallow subduction beginning at the end of the Cretaceous Period. Change in the geometry of States into a massive formation the subducting plate resulted in the Laramide uplift in the interior of the United States. known as “Basin and Range.” The Basin and Range formation extends The Last Pleistocene Toward the end of the Pleistocene from southern Oregon down to Ice Age Ice Age, the climate began to northern Mexico, while a linear change once again. The cool branch of the province extends The most recent ice age took place wet environment began to turn from southern New Mexico into between 24,000 to 12,000 years ago. substantially more arid and Lake central Colorado. During this time the Tularosa Basin Otero dried up leaving behind experienced a significantly cooler enormous deposits of gypsum. As the crust continued to pull and wetter climate. In fact, the basin apart in the Basin and Range area, was predominantly filled with an Though now long past, the Ice Age numerous fault zones developed, enormous body of water called was essential in the development uplifting the yeso formation. Large Lake Otero. of dunefield today. It was also blocks of crust also dropped critical in flushing large quantities of thousands of feet along these faults, Higher levels of rainfall throughout gypsum from the mountains into the forming basins in between fault- the Pleistocene Ice Age extracted Tularosa Basin. The relatively slow bounded mountain ranges. the gypsum that had been solidified rate of evaporation, combined with within the ancient marine rocks high These processes resulted in the in the mountains. This gypsum laden formation of several basins and water then collected in Lake Otero. valleys, including the Tularosa Basin. The Tularosa Basin is one of four The high gypsum content in water basins that make up the Rio Grande flowing from the San Andres and Rift and lies at its southern end. Sacramento mountain ranges Geologists believe that much of the inevitably led to Lake Otero basin formation associated with the becoming saturated with dissolved gypsum. Rio Grande Rift occurred within the Lake Lucero and the Alkali Flat are all that’s left last 10 million years. of Lake Otero Expansion and contraction caused Many scientists find this baffling by large temperature fluctuations due to the fact that almost all of the and periodic freezing will break up basin lay beneath Lake Otero during selenite crystals along these planes. this time. It is possible though that When crystal grains become small there was a local evaporative period enough to be transported by wind, roughly 16,000 years ago, allowing a further breakdown will occur. small dunefield to form. Gypsum is one of the softest Fossil dunes can be found northeast of Lake Lucero and have Selenite crystal blades at Lake Lucero minerals. Unlike hard quartz sand, gypsum sand will be ground down been dated back to approximately saturated, muddy conditions along into smaller and smaller grains 6,500 years ago. This date has the edges of the lake, allowed the when moved by the wind. This shown to be much more typical dissolved gypsum to crystallize as grain size reduction can be seen of the dunes that had developed selenite “discs” and bladed crystals. when comparing coarse crystal during the early, large scale grains near Lake Lucero to very fine evaporation of Lake Otero.The The evaporation of Lake Otero sand near the eastern edge of dunes we see today are significantly left behind two playas, the only the dunefield. younger; most of them are only remainders of what was once an hundreds of years old and sit on top enormous body of water. These of over twenty-five feet of packed ephemeral lakes are now referred to gypsum from years before. as Lake Lucero and the Alkali Flat and together make up the source of The process of dune formation the gypsum sand. is still taking place today. As old crystals emerge from the protective Neither playa holds standing water mud surface of Lake Lucero and very often, but when they do it can new ones form due to evaporation, often be contributed to the ever the wind and weather gradually constant, shallow water table that break them down into sand which sits directly below the surface. At is then blown into the dunefield. At Lake Lucero, as the water table rises, the same time, however, the sand the ground often becomes marsh- is being ground down into silt size like, while only a few inches of water particles that can be blown out of covers the crystals. the Tularosa Basin. This endless cycle of creation and destruction The crystals at Lake Lucero feeds the dunefield at White Sands continue to form today, although the National Monument and helps to majority of the selenite developed ensure that future generations will as Lake Otero dried. Following the get to enjoy one of nature’s greatest evaporation of this enormous lake, treasures. large stretches of the Tularosa Basin must have been littered with selenite crystals as Lake Lucero is today. A map of Lake Otero and the current dunefield. Dune Formation Most of the gypsum sand in the Geologically speaking, the dunefield dunefield was formed by the has only existed for such a short breakdown of selenite crystals. amount of time and is a relatively These crystals occur in a variety of young geologic formation. Exact shapes and sizes but common to all dates regarding the dunefield’s selenite crystals from the monument original development remain to be is that they are comprised of paper- determined but research suggest thin layers similar to those found in that dunes were forming as far back as 18,000 years ago.

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