Geology Cafe

Geologic History of California

A generalized geologic history of the southwestern United States

Evidence for interpreting the geologic history are deciphered from clues provided by rocks, fossils, and landscape features. Not all rocks in the western region share a common history or formed at the same time or in their current location. The modern landscape reflect the cumulative history of the rocks observable in disparate outcropping along the western margin of the North American continent. The region has experienced, and continues to experience, mountain building, faulting (earthquakes), volcanism, weathering, erosion, and deposition. These events processes occur gradually or episodically, sometimes as catastrophic events. More recent activity obscures or removes evidence of earlier geologic events. However, through the study of rocks, fossils, and field mapping earth scientists over the decades have unraveled a remarkable story. The study of rocks in one area can be implied or inferred to other areas where rocks of different ages are not preserved or are buried by younger deposits. This is a short summary of current thought about the geologic origins of our western landscape, focus on California.
Plate Tectonics Model Model illustrating the origin of rocks exposed throughout the western margin of North America.

The story of the geologic origin of landscapes in the western United States is complex—involving processes associated the formation, migration, and destruction of oceanic and continental plates. Earth shaping events took place on scales of million to billions of years. Rocks exposed throughout the mountain ranges in California and elsewhere in the West formed in a variety of geologic or environmental settings. Through time, these materials were "assembled" to their current configuration by fault motion and other processes associated with continental accretion.

Perhaps the most important concepts to understanding the origin of rocks found anywhere are the concepts associated with:
1) the geologic time scale,
2) the "rock cycle,", and
3) plate tectonics.

Accretion is a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts or other igneous features, or blocks or pieces of continental crust split from other continental plates.This occurred in many stages through Earth history. The region that is now California slowly "assembled" starting well over a billion years ago from older materials recycled through the lithosphere (Earth's crust and mantle). The "basement" rocks in the crust beneath Pacific basin are both intrusive igneous and volcanic in origin, derived from material rising from the mantle. Ocean crust consists dominantly of basalt (and other rocks of mafic and ultramafic in composition), or metamorphic derivatives of these rocks. In most places, these crustal rocks are (or were) blanketed with younger sediments that accumulated in a variety of ways, depending on their location of origin. In the deep ocean basin, far from land, sediments accumulated slowly as mineral precipitates or derived from biological activity (mostly skeletal remains of plankton). These sediment may also contain content that was carried in as dust or volcanic ash from distant sources and extraterrestrial (meteoritic) matter settling from the atmosphere. Closer to continental margins or volcanic regions (island arcs like Japan or hotspot volcanoes like Hawaii) sediments are derived from erosion on land or by wave and current action along coastlines. These sediments are transported into the deep ocean by undersea landslides and turbidity currents. These currents are gravity driven by the increased density of sediment suspended in seawater that can flow as "undersea storms" across the seabed sometime for hundreds even thousands of miles away from continental margins before settling as layers of gravel, sand, and mud. In shallow coastal settings calcareous organic material can accumulate as great reefs, carbonate platforms, or atolls around volcanic islands.

Tectonic forces and volcanism build up the landscape, exposing rocks once submerged along coastal margins. Sediments eroded from the mountains and volcanic ranges are transported and deposited in the adjacent ocean basin gradually moving the shoreline oceanward. In this process, material moved by plate-tectonic motion are later reincorporated back into the earth before being uplifted, eroded, and recycled over again. Much of the continental crust that is now California was derived or recycled from crust that originally formed beneath the Pacific Ocean as ocean crust. These rocks and sediments were later subducted or accreted onto the margin of the North American continent.The short narrative below starts with what is known about the most ancient rocks in California and works progressively toward the present.

Evolution of Western North America Illustrated

Below are a series of generalized geologic cross sections following a transect between San Francisco (to the west) and Denver (to the east)—a distance of about 1,200 miles (1,900 kilometers). On the right are brief highlights in the chronology.

Evolution of the western United States

A. Early Paleozoic: sediments eroded from older rocks exposed in the middle of the ancient North American continent progressively accumulated on an expanding continental shelf and stable (passive) ocean margin basin.
B. Middle Paleozoic: the Antler Orogeny (mountain building event) affected much of what is now the Nevada region. Shallow seas still covered much of western North America. What "is California" was mostly deep ocean basin.
C. Late Paleozoic: The North America continent slowly merges with supercontinent Pangaea. Ancestral Rocky Mountains rise and small land masses are accreted onto the western margin.
D. Early Mesozoic: the island arc (named Sonoma) develops and collides with the North American continent. Subduction changes from dipping eastward to dipping westward.
E. Late Mesozoic: Ongoing subduction results in the buildup of great batholiths beneath mountain ranges all along the western margin of North America. Sediments from these mountain ranges flood the interior of the continent flooded by the Western Interior Seaway.
F. Early Cenozoic: In the late Cretaceous Period, the North American Plate begins to "rapidly" override the Farallon Plate. In Early Cenozoic time, subduction extends far to the east resulting in massive volcanism throughout the Rocky Mountain region.
G. Late Cenozoic: North America overrides the spreading center between the Pacific Plate and the Farallon Plate. The spreading center migrates eastward causing massive crustal extension beneath what is now the Great Basin Region. North America overrides hotspots in the upper mantle resulting in extensive volcanism in the Yellowstone, Columbia River, and New Mexico regions, and cause the Sierras, Rocky Mountains, and Colorado Plateau to rise. Changes in plate boundary configurations result in the formation of the San Andreas and other regional fault systems.

Grand Tetons in northwestern Wyoming
Grand Tetons of northwestern Wyoming—ancient crystalline rocks (granites, gneisses) in the region of Achaean age range from about 2.5 to 3.4 billion years.

  Precambrian History

Achaean: Current theory suggests that the Earth formed in a stellar cloud around our newly formed sun in the range of about 4.5 billion years ago. Heat-driven mantle convection and asteroid collisions recycled or destroyed the oldest crustal material that may have formed during Earth's earliest history. The oldest rocks in North America are of Late Achaean age and are found in the Canadian Shield region and locally elsewhere in the Rocky Mountain region. The oldest known rocks of this age in the West crop out in core of mountain ranges in western Wyoming—some are estimated to be older than 3 billion years. Rocks as old as 2 billion years crop out in the deepest part of the Grand Canyon. No rocks of this age are known in California.

Early to Middle Proterozoic:
The oldest rocks preserved in California are metamorphic rocks in the Mojave Desert region and are Early Proterozoic age, ranging from about 1600 to 1800 million years old. These metamorphic rocks were derived from older sedimentary, igneous, and metamorphic rocks but were subjected to high heat, pressure, and partial melting and recrystallization during a period of mountain building associated with continental collisions of tectonic plates that existed long ago in Earth's history. Rocks of similar age and character are perhaps best know from the bottom of the Grand Canyon. These rocks could best be simply described as having formed long ago, far away, and having been altered from burial deep in the crust before finally coming to rest attached to the North American continent.

Late Proterozoic: Following the formation of the metamorphic and igneous basement rocks in Early to Middle Proterozoic time, the region that is now the Mojave Desert experiences extensive uplift and erosion that wore down ancient mountain ranges to what was probably a gentle regional plain or rolling pediment. By late Proterozoic time, about 1 billion years ago, the landscape had worn down enough for shallow seas to transgress across the region. Sedimentary deposits (mostly quartz sandstone and limestone) accumulated on a broad continental shelf or continental margin setting. The record of this is preserved as the Great Unconformity between underlying crystalline basement and overlying sedimentary rocks of Late Proterozoic Age (see "A" in Fig. 2). Over time, the seas advanced and retreated leaving behind thick beds of sandstone, limestone, dolomite, and shale (and their metamorphic equivalents-quartzite, marble, and slate). Today these rocks are locally exposed in the eastern Mojave and Death Valley regions. Passive-margin-style deposition continue along the California margin through the end of Proterozoic time into the following Cambrian Period of the Paleozoic Era.

Paleozoic History

A massive sequence of Paleozoic-age sedimentary formations are preserved in California's mountain ranges in the eastern Mojave Desert and the Death Valley region. These rocks, consisting mostly of limestone and dolomite, preserve evidence of the evolution of marine life forms. The oldest sedimentary rocks of the Cambrian Period preserve an abundance of fossil algae (stromatolites) and locally contain an abundance of early invertebrate fauna, including trilobites. Changing sea-level conditions caused the seas to advance and retreat numerous times across the region. By middle Paleozoic time (Devonian and Mississippian time), the rocks in the Death Valley region preserve evidence of the formation of carbonate reefs along the margin of an ancient tropical marine shelf. These reef deposits preserve an abundance of fossil corals, brachiopods, and other invertebrates.

The seas continued to advance and retreat across the region through the Late Paleozoic Era, resulting in the formation of more fossiliferous limestones of Pennsylvanian and Permian age. Farther to the west, deeper water conditions persisted. Marine shale and sandstones of Paleozoic age accumulated in deep water settings, but these rocks were mostly altered by metamorphism, destroyed, or displaced by plate-tectonic subduction, igneous intrusion, or uplift and erosion in the following Mesozoic and Cenozoic eras. Large portions of the ancestral California continental margin bearing rocks of Paleozoic age were ripped away and carried northward along the Pacific margin and metamorphose, buried, subducted, and accumulated as accreted terranes in the Pacific Northwest, possibly even into the Alaska region. However, great fault-bounded blocks of Paleozoic age rocks remain in the Mojave Desert region and are particularly well exposed in Death Valley National Park and and around the Mojave National Preserve.

Much of the western margin of the North American continent was subjected to mountain building (orogenesis) in the middle of the Paleozoic Era. Evidence of the Antler Orogeny is preserved as complex fault and structural features preserved in sedimentary and metamorphic rocks of Devonian age throughout what is now Nevada. This was followed by the Sonoma Orogeny throughout the western region during the Permian and Triassic periods. During these orogenies, small land masses (terranes) were accreted onto the Cordilleran margin, with the largest accretionary event involving the the addition of the Sonoma island arc (landmass) mostly into what is now Nevada. Current thought is that prior to the Sonoma Orogeny subduction was occurring in the eastern Pacific in a westward-dipping direction (forming Sonoma and other ancient island arcs). Once oceanic crust beneath a moderately shallow seaway basin was consumed by subduction, the Sonoma landmass collided with western North America. What followed was a change to eastward dipping subduction on the western side of the newly accreted Sonoma terrane. This new configuration of subduction lead to the formation of the great series of island arc that developed in the Mesozoic era in the Cordilleran region.
Gneiss boulder in Death Valley
Oldest rocks in California are ancient metamorphic rocks (gneiss and schist) range in age from about 1.4 to 1.7 billion years. This example is from the Funeral Range in Death Valley.
Proterozoic and Cambrian age sedimentary rocks exposed in mountain ranges in Death Valley
A sequence of sedimentary rocks of Late Proterozoic to Early Cambrian age (~1.2 billion to 550 million years) crops out in the mountain ranges in the Mojave region. This view shows a nearly 10,000 feet thick section exposed along Titus Canyon in Death Valley National Park.
Paleozoic age sedimentary rocks crop out on the west flank of the Panamint Range in Death Valley National Park
Sedimentary rocks of Middle to Late Paleozoic age crop out in portions of the Mojave Desert region. Nearly 8,000 feet of fossil-bearing rocks of these ages crop out in the Panamint Range in western Death Valley National Park.
Mesozoic History

Triassic-Jurassic-Cretaceous Periods: The Mesozoic Era was a tumultuous time in California's geologic past. In early Triassic time, an extensive volcanic-arc system began to develop along the western margin of the North American continent. In Southern California, this volcanic arc would develop throughout the Mesozoic Era to become the geologic regions known as the Sierra Nevada Batholith, the Southern California Batholith (in the Peninsular Ranges), and other plutonic and volcanic centers throughout the greater Mojave region. These massive belts of plutonic (intrusive) and volcanic (extrusive) regional belts and isolated centers developed as plate convergence and subduction took place farther west along the western continental margin. These igneous provinces shed vast quantities of sediment both eastward into the Western Interior Seaway and westward into Pacific margin basin. At the same time, older sedimentary materials and rocks were subjected to regional metamorphism throughout much of Southern California. In the region today, granitic rocks of Mesozoic age dominate the bedrock exposed in the Peninsular Ranges, western Transverse Range, the southern Sierra Nevada, and the greater Mojave region.
  Map of the Western Interior Seaway and Cordilleran Range
Pluton in Joshua Tree National Park
Erosion in the Joshua Tree National Park area exposed this igneous pluton of late Mesozoic age. The bedrock in the surrounding landscape consists of older Mesozoic-age metamorphic rocks. Plutons like this can be seen throughout the Sierra Nevada Range with exceptional examples exposed in Yosemite National Park (Half Dome is a classic example). The granitic bedrock of the Sierra Nevada Range and the Peninsular Ranges in Southern California are remnants of vast volcanic arcs that formed in Mesozoic time.
  Although igneous activity in the Southern California region was ongoing throughout the Mesozoic Era, the peak of the plutonism in the southern Sierra Nevada region was in the Late Cretaceous, about 100 to 80 million years ago. In addition, thick sequences (accumulations) of Mesozoic-age sedimentary rocks-mostly marine shales and sandstone of Jurassic and Cretaceous age -are locally preserved along the western side of the Peninsular Ranges and throughout the western Transverse Ranges in parts of the Santa Monica Mountains and mountainous Los Padres National Forest region north of Santa Barbara. A thick sequence of terrestrial sedimentary rocks are also preserved in the McCoy Mountains region near Blythe, California. Except for the sedimentary rocks mentioned above, most of the Mesozoic-age rocks in preserved in Southern California display intermediate to high grades of metamorphism, typical of material that may have been buried to mid-crustal depths (probably in a range of 10 to 20 kilometers). Although the region was no doubt extensively covered with terrestrial and marine sedimentary deposits that inter fingered volcanic deposits from the volcanic centers, most of this material was stripped away by erosion following regional uplift that continued into the following Cenozoic Era.

Rocks of Mesozoic age crop out extensively through the coast ranges of Central and northern California. In the San Francisco Bay region, rocks assigned to the Franciscan Formation consist of rocks in the Pacific Ocean basin and along the western continental margin. The Franciscan Formation, sometimes referred to as an "assemblage" or "basement" complex consist of rock sequences derived from throughout the Pacific Basin region and carried by plate-tectonic motion as fault-bounded terranes that were accreted onto the continental margin over time. Portions of the Southern California batholith were ripped off and carried northward along the California margin, this process has been ongoing into the present via the extensive greater California fault system (which includes the more modern San Andreas Fault).
The map series (below) illustrates the changing landscape and coastline of the coastal region over the past 100 million years based on geologic interpretation of the rocks preserved in the region (modified from USGS Professional Paper 1501).
Click on maps for a larger view.
Cenozoic History (Tertiary and Quaternary Periods)

Paleocene-Eocene-Oligocene: During the early Tertiary Period, subduction continued along the western North American continental margin. However, volcanic-arc style igneous activity that had occurred earlier (during the Mesozoic Era) in the Sierra Nevada and Peninsular Ranges region had diminished or ceased. Instead, volcanic activity occurred in region much farther east (in the Rocky Mountains region). Much of the region that is now Southern California underwent extensive uplift and erosion, and over time the landscape wore down and probably became an extensive pediment (rolling lowlands with a few mountains) bordered on the west by a coastal plain, shallow embayments, and coastal uplands. Rivers and stream carried vast quantities of sediments and deposited them in offshore basins along the continental margin. Great fault systems that predate the modern San Andreas Fault system carried great blocks of crystalline basement rocks and their overburden of sedimentary materials derived from the Southern California northward into the Central Coast region and beyond.

Miocene: In early Miocene time, subduction along the western margin of the North American Continent slowly shifted to transform faulting. This gradually occurred as the ancient Farallon Plate disappeared into the subduction zone and the North American Plate came in contact with the Pacific Plate (click here for more detailed information about the formation of the San Andreas Fault). By late Miocene time, the shoreline geometry of southern California changed significantly as mountain ranges began to rise along the coastline, exposing marine sedimentary deposits that had accumulated in offshore continental shelf and marine basin environments. These sedimentary basins filled with sediments derived from the land and from organic remains deposited in the sea. These organic materials would become the major source of California's modern petroleum resources (oil and gas). Volcanism, tectonic uplift and crustal extension in the eastern Mojave and Great Basin regions are responsible for the formation of the basin-and-range landscape features and formation of the arid, isolated interior-draining basins. In Late Miocene time the Baja Peninsula began to separate from mainland Mexico and begin its gradual migration northward. The rift valley between the Mexican mainland and Baja California eventually flooded with marine-water conditions extending northward in the the Salton Trough region.

Pliocene: Climate cycles associated with ice ages began to affect the region. Uplift in the Peninsular, Transverse, and Coastal Ranges, and the Sierra Nevada and eastern Mojave region, start to shape the height and extent of mountain ranges visible today. With the cooling of the climate, great lakes started to form in the interior basins of the Mojave, Death Valley, and Great Basin regions.

Pleistocene: Climate fluctuation caused intermittent periods of ice ages and warm periods. During cool, wet periods, alpine glaciers carved canyons in the Sierra Nevada and great lakes flooded intermountain valleys. Sea level rose and fell with each glaciation cycle. During low sea level periods streams carved downward into their valleys, and during high sea level periods coastal valley became flooded and back-filled with sediments. During high-standing seas, ocean embayments covered the coastal plains and lowland basins along the coast. During low-standing seas, the shoreline extended to the margin of the modern continental shelf and many of the islands now offshore were connected via peninsulas to the mainland. Ongoing tectonic forces (faulting and folding) helped shape the uplifts and basins visible on the landscape as they appear today. Uplift and subsidence was particularly active in the Coast Ranges, Transverse Ranges, and Peninsular Ranges west of the San Andreas Fault. Volcanism occurred along the eastern Sierra Nevada region and in the Death Valley and Mojave regions. During the Pleistocene Period, modern river systems of the region (including the Mojave and Colorado Rivers) developed at the expense of older drainage systems the were captured or diverted by tectonic uplift, faulting, volcanism, or as interior basins filled and spilled over into adjacent valleys.

Late Mesozoic Paleocene Early Eocene
Cretaceous ~100Ma Paleocene ~59Ma Early Eocene ~52Ma
Middle Eocene Early Miocene Oligocene
Middle Eocene ~45Ma Oligocene ~30Ma Early Miocene ~21Ma
Middle Miocene Late Miocene Pliocene
Middle Miocene ~16Ma Late Miocene ~10Ma Pliocene ~4-5Ma
pliocene Late Holocene Satellite Mosaic
Pleistocene ~0.7Ma Late Holocene (today) Satellite mosaic
San Francisco Holocene: Sea level continued to rise following the peak of continental glaciation during the last ice age (Wisconsin age, about 15,000 years ago) when sea level was as much as 350 to 400 feet (~120 meters) lower than present levels. Early human populations migrated into the California region starting about 10,000 years ago (possibly earlier). Many large mammalian species that lived in the region became extinct at the beginning of the Holocene Epoch. The California Gold Rush beginning in 1849 initiated one of the greatest human migrations in modern history. The "Anthropocene" begins—human activities (mining, agriculture, and construction) surpasses the the effects of natural erosion, deposition, and other processes in increasing areas on the landscape.