|
| Western Earth Surface Processes Team |
| Geologic History of Southern California |
 |
Highlight of SoCal Geologic History This website provides a general overview of the geologic history of Southern California. More detailed summaries are listed below in the References and Links. Evidence of past history are deciphered from the rocks and landscape features in the region. Not all rocks in the 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 now present in the region along the western margin of the North American continental plate. The region has experienced, and continues to experience, mountain builting, faulting, erosion, deposition, and volcanic activity. These events processes occur gradually or episodic, potentially catastrophic, events. More recent activity obscures or removes evidence of earlier geologic events. However, field mapping and 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. The region that is now Southern California slowly "assembled" over a billion years from older materials recycled through the lithosphere (Earth's crust and mantle) or accumulated from precipitation and biological activity in the oceans, or carried in as ash and dust in the atmosphere. Tectonic forces and volcanism built up the landscape, and sediments eroded and deposited along the margin of the North American continent, later to be uplifted and recycled over again. Much of the continental crust that is now California was derived or recycled from crust that formed beneath the Pacific Ocean region and later subducted or accreted onto the margin of the North American continent. This occurred in many stages through Earth History. The short narrative below starts with what is known about the most ancient rocks in California and works progressively toward the future. |
|
| < Geologic time scale. Time subdivisions and geologic ages in millions of years (Ma) are after the Geological Society of America 1999 Geologic Time Scale (image from Stoffer, 2006). | ||
 |
Precambrian History Archean: 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 Archean age and are found in the Canadian Shield region and locally elsewhere in the Rocky Mountain region. 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. |
|
Generalized geologic history of the Southen California and greater Mojave region (from Stoffer, 2004). This diagram illustrated the general character of the greater Southern California region over long stages of its geologic history. Figure A represents the early Paleozoic Era before land that might be considered California did not yet exist. Figure B shows the landscape during the middle Mesozoic Era when the Sierra-Nevada volcanic arc was forming. Figure C shows the California region in the late Cenozoic Era after the modern fault systems and the Great Basin extensional structure had already developed.
|
| 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 numberous 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 intrustion, or uplift and erosion in the following Mesozoic and Cenozoic eras. |
 |
|
| A nearly 3,000 meter-thick section of Paleozoic-age sedimentary rocks are exposed in the mountain ranges of the eastern Mojave Desert region. This view shows Paleozoic-age rocks exposed on the eastern flank of the Panamint Mountains in Death Valley National Park. |
| Mesozoic History Triassic-Jurassic-Cretaceous Periods: The Mesozoic Era was a turmoltuous 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 Mesozic age dominate the bedrock exposed in the Peninsular Ranges, western Transverse Range, the southern Sierra Nevada, and the greater Mojave region. |
 |
 |
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 interfingered 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. | |
| 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. |
| 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 responisible 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 cyles 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 intermittant periods of ice ages and warm periods. During cool, wet periods, alpine glaciers carved canyons in the Sierra Nevada and great lakes flooded intermontane 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 occured 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. 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. |
 |