The oldest rocks in the El Paso-Juarez area were deposited along the shoreline of a shallow tropical sea. Oldest of these is the Castner Formation which is exposed in a number of places along the eastern slopes on the east side of the Franklin Mountains. These rocks record deposition of lime-rich muds. Later, they were metamorphosed into marble.
Overlying the Castner Marble is a thin basalt flow known as the Mundy Breccia. It is, in turn, overlain by a thick sequence of quartz sands which have been metamorphosed to the Lanoria quartzite. Many features of the Lanoria are similar to those seen in modern beach systems such as the Texas Gulf near Padre Island. This quiet scene ended abruptly about 1.1 BY ago by an extended period of geologic disruption and violence. Chief of the destructive processes was magmatism -- intrusion of great volumes of molten rock. Magmas were silicon-aluminum rich and produced six varieties of granitic rocks, collectively called the Red Bluff Granite. Magmas intruded into the Castner, Mundy, and Lanoria formations, heated them, and sometimes ripped chunks of these rocks from the magma chamber walls. These chunks formed structures called xenoliths within the solid granitic rocks. At some time, some of the granitic magma breached the earth's surface to initiate a series of volcanic eruptions. These eruptions included pyroclastic ash-flow tuffs (the sort of eruption that destroyed the ancient city of Pompeii in 79 AD) as well as numerous lava flows. Collectively, these volcanic units are called theThunderbird Group.
Following the Red Bluff/Thunderbird events, things quieted down and erosion of the igneous rocks began. This erosion lasted until about 500 MY (million years) ago when the rising sea level gradually flooded the El Paso-Juarez region. The first of the marine sediments deposited over the erosion surface was sandy material that was lithified to form the Bliss Sandstone. For the next 250 MY, this region was part of the North American continental shelf, a low-lying region very close to sea level. As sea level fluctuated, the area alternatively was inundated by the sea and marine sediments were deposited. Later, as sea level fell, the area stood above sea level as dry land. During the submerged episodes, remains of a variety of marine organisms were buried in the sediment and formed the fossils characteristic of these Paleozoic rocks. Many examples of these fossils are on display in the Strain hall of Paleontology in the El Paso Centennial Museum.
At the end of the Paleozoic Era, the American southwest was uplifted above sea level, a position it occupied during most of the succeeding geologic interval, the Mesozoic Era. Rocks were being eroded and little deposition to form Mesozoic sedimentary rocks occurred around El Paso until the middle of the Cretaceous Period, in late Mesozoic time. In the Cretaceous, The El Paso-Ju rez area was near the head of an arm of the Gulf of Mexico called the Chihuahuan Embayment. Shallow water marine sediment was again locally deposited. Today, these sedimentary rocks are visible along the base of the of the western slope of the Franklin Mountain, around the flanks of Cerro de Cristo Rey, and throughout the Sierra de Ju rez. Numerous fossils of marine invertebrates are present, but no remains of terrestrial dinosaurs have been found in them. Examples are on display in the Strain hall of Paleontology.
Sea level fell (or the continent rose) as the Cretaceous period ended 65 MY ago, and a new chapter in El Paso's geologic history began. The Cenozoic Era, as the interval from 65 MY ago to present is called, was a time of major change in this region. Early in the Cenozoic, mountain-building forces again were in action and bodies of molten magma moved into the crust. At this time, however, none broke through to form volcanoes; rather, the magma bodies cooled in the crust forming andesite plutons. Some have been exposed by later erosion -- the Campus Pluton, the Cristo Rey Pluton, and a number of other, smaller ones. Emplacement of the plutons took place 45 to 50 MY ago. Shortly before plutonism occurred, immense compressive force developed to the southwest and as a result, great masses of Cretaceous limestone were thrust from the southwest to the northeast as a result. These masses constitute the rocks of the Sierra de Juarez. Some geologists believe that initial uplift of the ancestral Franklin Mountains took place at about the same time while others believe that the Franklins are a younger feature which formed only a few million years ago.
In any case, the compressive mountain-building forces waned and the region was geologically quiet until about 35 MY ago when a new system of stresses began. These were extensional, or "pull-apart" stresses and they generated many of the local features. Perhaps the most obvious is the Rio Grande Rift, a feature produced as the continent is being pulled part in this region. The rift begins in Colorado, near Leadville, and extends southerly through New Mexico to El Paso and on into Mexico where it seems to die out. One product of the tensional forces and rifting has been the formation of numerous elongate basins, separated from one another by mountains. Local examples include the Hueco Mountains, the Hueco Bolson (or basin), the Franklin Mountains, and the Mesilla Bolson. As they were uplifted, the mountains began to accumulate in the basins, a process that continues to this day. In addition to these sediments, other sediment was carried into this area by the ancestral Rio Grande which, for a few million years, emptied into Lake Cabesa de Vaca, a large, intermittent lake that occupied the sites of the present-day basins. River sediment and lake sediments are also part of the basin fill. Desert sand, blown into the area from the southwest, is another constituent of the loose, unconsolidated sediment o the basins. These sediments are called the Fort Hancock Formation. The basins are deep structures -- more than 9,000 feet of sediment underlies the El Paso International Airport and 12,000 feet of basin fill sediment rests on Cretaceous rock under the Mesilla Bolson surface.
Associated with continental rifting, basaltic volcanic activity took place. The basaltic magma arose deep in the crust or at the top of the mantle, rose to the surface, and erupted to form the cinder cone volcanoes and lava flows that dot the Mesilla Bolson surface.
During deposition of the Fort Hancock sediments, a diverse mammalian community lived here. The skeletons (fossils) of creatures such as wooly mammoth, giant ground sloths, horses, camels, and smaller animals occur in the sediments. They are not common, but examples are on display in the Centennial Museum, as part of a photographic exhibit of excavation of mammoth bones from a sand quarry near the present site of Vista Hills Hospital.