Geologic History of Lake Mead National Recreation Area

Lake Mead National Recreation Area preserves one of the longest and most complete geologic records of any American national park, spanning almost 2 billion years of Earth history! The park is located along the boundary of two physiographic provinces-the Colorado Plateau (to the east) and the Basin and Range Province (to the west). The park encompasses many of the same rock formations exposed in the Grand Canyon, but in addition, the recreation area also contains younger sedimentary and volcanic rocks. The park preserves a complex landscape that records the history of many mountain-building episodes that affected that shaped the landscape of the western United States. Below are highlights of the geologic story behind the landscape.


Click here to see a geologic map of Lake Mead National Recreation Area and the surrounding region.

Click here to see a satellite image map of Lake Mead National Recreation Area and the surrounding region.

Precambrian Era

The Precambrian Era encompasses all of geologic time prior to 570 million years ago. The Earth is currently thought to have formed about 4.5 billion years ago in a stellar nebula. The oldest known rocks on the Earth's surface are about 3.8 billion years or older (Roach, 2008). Because of ongoing geologic forces change or recycle older material, and because younger deposits mask older, underlying deposits, understanding of Earth history of Precambrian time is relatively incomplete. The oldest rocks in Lake Mead National Recreation area are Middle to Early Proterozoic in age, (1.4 to 1.7 billion years or older and consist of rocks that have been locally heavily altered (metamorphosed) from older rocks (Beard and others, 2007). These rocks are assigned to the Vishnu Group, the collective name assigned to many of the Precambrian-age gneiss and schist in the Lake Mead and Grand Canyon region. These ancient rocks may have once been sediments deposited in an ocean basin or part of an ancient chain of volcanoes. However, they were deeply buried, highly deformed, and intruded by molten material that cooled to form granite and other igneous rocks. Forces associated with plate-tectonic motion attached these rocks to the growing North American continent. Over time measured in hundreds of millions of years, these rocks experienced multiple stages of mountain formation and additional alteration. Precambrian rocks are locally exposed in Lake Mead National Recreation Area along the shores of Lake Mead and Lake Mojave. However, the geologic story of Precambrian-age rocks and events are best known (and deciphered) from exposures in the Grand Canyon to the east of the park (Click here to see a stratigraphic column for the Grand Canyon).

In the Grand Canyon, the Vishnu Group are the oldest rocks interpreted to have crystallized about 1840 million years ago. Today, these rocks are part of the undifferentiated crystalline "basement" rocks that extend downward toward the bottom of the Earth's crust. These were buried by sediments and volcanic rocks before being metamorphosed about 1750 to 1730 million years ago, and later locally intruded by igneous material around 1400 million years ago. These rocks were later buried by a thick sequence of sedimentary and volcanic rocks between around 1200 million years ago and 740 million years ago. These younger sedimentary and volcanic rocks are assigned to the Grand Canyon Supergroup. However, these rocks are not exposed in the Lake Mead area. (Note: numeric ages on this website are provided by the National Park Service, Grand Canyon, after Mathis and Bowman, 2005.)

The Great Unconformity

Rocks between about 740 million and 525 million years old are not preserved or identified in the Grand Canyon or in Lake Mead National Recreation Area. Starting around 740 million years ago, the tectonic forces shaping in the region caused the land to steadily rise. Over the next 250 million years erosion slowly stripped away rock and sediments, wearing down the the landscape down to a near level plain. Mountain ranges that may have once existed in the region were gone by around 525 million years ago when the Early Cambrian sea gradually advanced across the region. Sediments of Early Cambrian age were deposited on the eroded surface of older Precambrian-age rocks. This "gap" is called the Great Unconformity and is recognized throughout western North America, but is perhaps best known from exposures in the Grand Canyon. An unconformity is defined as a gap or break in the geologic record representing a period of erosion or non deposition. The Great Unconformity is also exposed in the Frenchman Mountains west of Lake Mead near Las Vegas (Tingley and others, 2001).

Paleozoic Era

Paleozoic means "ancient life." Paleozoic refers to the time period beginning about 543 million years (the start of the Cambrian Period) to about 248 million years ago (the end of the Permian Period). Although evidence of life (living organisms) are known or inferred from some of the oldest rocks known on earth, it wasn't until the Cambrian Period that evidence of complex multicellar organisms and animals with shells appear preserved in abundance in the "fossil record." Fossils include body parts, casts, molds, impressions, tracks, and traces preserved between layers of sedimentary rock. Cambrian-age fossils have been found throughout the Western United State-in abundance is some locations; with trilobites being the most common variety.

Paleozoic-age rocks are well exposed in several areas in and around Lake Mead National Recreation Area. All Paleozoic rocks in the park are sedimentary rocks-shale, sandstone, and limestone. These rocks record a history of the gradual expansion and retreat of shallow seaways that flooded a flat continental margin and coastal plain many times over a period of more than 250 million years. Sediments were deposited when seas flooded across the region, and some of these sediments were stripped away when the seas retreated. Today we see evidence of this recorded in a sequence of sedimentary rock formations and unconformities that is about a mile thick. A rock formation is a body of rock strata that consists of a certain lithologic type or combination of types, and is typically of a definable age based on fossils and other materials they may contain, Formations may be subdivided into members, or individual units or beds of strata, or a lumped together into "groups." Formation and member names are assigned to any rock units that are mapable over a limited area or region.

Many of the rock formations that are well known in the Grand Canyon are also preserved in the Lake Mead area. The Tapeats Sandstone is the oldest Paleozoic-age rock formation above the Great Unconformity. The Tapeats Sandstone consists of sand, gravel, and fine materials that were deposited in a shallow ocean setting as coastal erosion gradually carved a shoreline steadily eastward across the continental margin beginning about 525 million years ago in the Cambrian Period. As the shoreline progressed farther eastward, the sediments deposited gradually changed to finer sand, silt, and clay, to lime mud deposited in shallow, warm, clearwater settings. The lime mud was formed by algae, both as planktonic forms or attached to the seabed, that incorporated calcium carbonate into their cell structure. This calcium carbonate from algae became lime mud after the algae died and decay or were eaten by other organisms and recycled into the sediment. The lime mud became limestone or was altered to become dolomite (calcium-magnesium carbonate).

After the first major marine transgression of the Cambrian Period, the seas continued to advance and retreat across the region. Sediments were likely deposited on in shallow marine shelf environments only to be stripped away by erosion when the seas retreated. A major unconformity separates rocks of Cambrian age and Devonian age leaving no trace of evidence of seas that may have existed in the region during two intervening geologic periods-the Ordovician (490 to 443 million years ago) and the Silurian (443 to 417 million years ago). This gap in the sedimentary record is an unconformity beneath rocks of Middle Devonian age (about 385 million years old).

Beginning in the Middle Devonian time the shallow, warm seas advance again across the region, and sediments bearing marine invertebrate fossils were deposited. The result is a massive layers of limestone we see in the upper walls in the Grand Canyon and Grand Wash Cliffs, and in many of the resistant ridges in the Virgin and Frenchman Mountains. Shells of brachiopods, cephalopods, and corals can be locally seen preserved in the rock.
Middle- to Late-Devonian-age limestone is assigned to the Sultan Formation in the Lake Mead area and the Temple Butte Formation in the Grand Canyon region. Perhaps the most recognizable rock formation in the Paleozoic section is the massive Redwall Limestone of Early- to Middle-Mississippian age (around 240 million years old). This massive, erosionally-resistant limestone rock formation forms high cliffs in the canyons and ridgelines in the mountains throughout the region.

Marine and coastal sedimentation continued intermittently across the region through Pennsylvanian into Early Permian time. The stratigraphic record for the Late Paleozoic is preserved mostly as shale and sandstone rock formations. Fossils and traces found in these formations include plant material, marine invertebrates, and rare fish and early tetrapod vertebrate tracks. The names Hermit, Esplanade, Toroweap, and Kaibab are used for rock formations in both the Grand Canyon and in the Lake Mead region. The Toroweap Formation preserves cross-bedded sandstones and trace fossils that suggest they were accumulated as coastal beach, sandy barrier island, and coastal dunes environments. The Kaibab Formation consists of fossiliferous limestone and marl that locally bears an abundance of marine invertebrate fossils, mostly brachiopods. The Kaibab Formation crops out along the rim of the Grand Canyon and the Grand Wash Cliffs along the eastern end of Lake Mead National Recreation Area.

The end of the Paleozoic Era coincides with a record of a great mass extinction about 248 million years ago. The cause and character of this mass extinction is still being investigated and debated. What is known is that about 98 percent of the life forms known from the previous era vanished before the beginning of the Triassic Period when a variety of new life forms began to appear in the fossil record.

Early Mesozoic Era

Large-scale changes began to occur throughout western North America beginning in the Triassic Period-the first period of the Mesozoic Era. Life survived the mass extinction at the end of the Paleozoic, and new life forms evolved including dinosaurs and swimming reptiles, birds, early mammals, and flowering plants. At the beginning of the Triassic Period, a great ancient supercontinent called Pangaea was slowly being split apart by forces associated with plate tectonics. Whereas the modern Atlantic Ocean basin was developing along the eastern margin of the North American continent, in the West volcanic mountain ranges (island arcs like modern Japan) began to from along the Pacific margin of North America. Sediments derived from these volcanic areas and from ancient mountains in the Rocky Mountain region were transported by and nearshore shallow marine currents, streams, and wind, and deposited across the Lake Mead region.

The transition from marine bay environments to desert dune environments took place over a period of about 50 million years. In the Early to Middle Triassic (248 to 227 million years ago), the regional landscape changed from being dominated by shallow coastal bays, to broad coastal floodplains with forested swamps and migrating stream channels. These layers are assigned to the Chinle Formation.

Late Mesozoic Era

By Late Jurassic time, volcanism and tectonism associated with mountain building was shaping the great Cordilleran mountain chain along the western margin of the North American continent. This mountain building period that lasted throughout much of the Cretaceous Period is called the Sevier Orogeny and impacted the region mostly to the north and west of the Lake Mead area. The buildup of the ancient Cordilleran mountain ranges that included ancestral mountain ranges extending from the Sierra Nevada region to western Utah and affected the entire region extending from Alaska to Mexico. The Sevier Orogeny coincided with the development of a great seaway that flooded across the interior of North America, extending from the Gulf of Mexico northward to the Arctic Ocean, and from the Mississippi River region westward to Nevada. The Western Interior Seaway began forming about 150 million years ago in the Late Jurassic, and persisted through the Cretaceous Period that ended about 65 million years ago. The region around what is now Lake Mead was located along the western margin of this seaway. Sediments derived from volcanic eruptions and erosion of mountain ranges to the west flooded into this shallow seaway basin. Today, sediments that were deposited in the seaway are now exposed throughout the Rocky Mountain region, but in the Lake Mead area they have been completely stripped away by erosion during the following expanse of time leading to the present.

Cenozoic Era

The Cenozoic Era began with a mass extinction caused by at least one massive asteroid collision with Earth about 65 million years ago. Many life forms vanished at the end of the Cretaceous Period including all the dinosaurs on the land and the ammonites in the sea. But once again, some species survived and continued to evolve into the multitude of life forms we see today.

Beginning in the Late Cretaceous and continuing through the early part of the Tertiary Period, the western United State experienced an extensive period of mountain building. This mountain-building interval, called the Laramide Orogeny, produced many of the massive geologic structures (faults and folds) and mountain ranges and sediment-filled basins in the Rocky Mountain region. At the beginning of the Tertiary Period, the landscape across the western United States was near sea level, but over time the land began to steadily rise. By the Late Eocene Epoch (about 40 million years ago) most of the Rocky Mountains had formed (but they were not as high as they are today). Erosional processes probably dominated the landscape in the Lake Mead region during the Early Tertiary Period (Paleocene, Eocene, and Oligocene Epochs) because no trace of rocks of those ages are preserved in the park region. During that time interval, river systems drained north and eastward across the region away from upland areas formed by earlier mountain-building periods during the Sevier and Laramide orogenies. However, deposition again resumed in the region in the Early Miocene Epoch.

Sedimentary rocks of Early- to Middle-Miocene age, about 26 to 13 million years old, in the Lake Mead region are assigned to the Horse Spring Formation. The Horse Spring Formation is exposed around the flanks of mountains that have been uplifted later in time. The presence of the Horse Spring Formation show that the Basin and Range structure was already forming in the region west of the Colorado Plateau-the Grand Wash Cliffs along the eastern end of Lake Mead National Recreation Area generally define the western boundary of the Colorado Plateau. The sediments of the Horse Spring Formation accumulated in a structurally low basin, called the Grand Wash Trough, that was bounded by uplifts and volcanic centers in the surrounding region.

However, large-scale events started taking place throughout the Lake Mead region during Middle Miocene time beginning about 17 million years ago. The plate-tectonic regime of the West began to change. Crustal forces associated with the opening of the Gulf of California extended northward into the Nevada region. The Earth's crust throughout the western United States was being pulled apart as much of what is now California began a slow migration to the north and west. The crust began to split apart and spread along a series of north-trending faults, and great crustal blocks began to rotate and move along fault plains that extended downward to the bottom of the crust. Some of these fault and fracture zones became pathways for magma to migrate toward the surface. In some cases, the magma did reach the surface to form volcanoes and extensive lava flows that flooded the valleys.

Quaternary Period

Starting in late Pliocene time, about 3.8 million years ago, the global climate system change. Continental glaciers began to form and sea level began to rise and fall through a series of ice ages. The Quaternary Period began about 1.8 million years ago when the first of a series of great ice ages produced continental continental glaciers that blanketed much of northern Europe, Siberia, and North America. In North America, major continental glaciers extended as far south as Ohio and Missouri Rivers and formed alpine glaciers throughout the Rocky Mountains and Sierra Nevada Range. During each ice age, the climate in the west became cooler and much wetter, and great lakes flooded internally drained basins throughout the region. Once the Colorado River system became established, streams began to carve downward into floodplains, and deep canyons were carved through narrow passages through mountain ranges along the paths of the major drainages. During the wettest periods, catastrophic floods must have discharged through the canyons. As the canyons got deeper, smaller streams carved canyons upstream into their headwater regions. The result is the rugged, high relief we see on the landscape today.

Our Modern World

Because of the semiarid to arid conditions in the region, evidence of prehistoric human activity is well preserved in the region. Humans have used the Colorado River corridor extended back in time to near the end of the last ice age, more than 11,000 years. However, early populations in the region were scant relative to what has happened to the region in the starting after the Civil War with the discovery of gold and other mineral resources along the navigable lower Colorado River. Las Vegas valley was first described by Spanish traders exploring for a route to Los Angeles in the early 1700s. A scout named Rafael Rivera was the first person of European ancestry to visit and describe the grass-covered valley. Las Vegas is Spanish "The Meadows." The town of Las Vegas was established in the 1880s as a water and refueling stop along the railway linking Salt Lake City to Los Angeles.

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