9.2: Ancient Seas Form the Oldest Rocks

What We Know From the Oldest Rocks

The Sierra Nevada's geological history is rich and complex, with the oldest rocks exposed in the range dating back to around 540 million years ago, during the Cambrian period. These ancient rocks provide a glimpse into the early stages of the Sierra Nevada's formation. However, even older rocks that offer more insights into the region's deep geological past are found in the nearby White and Inyo Mountains, located just to the east (see Basin and Range). These areas are part of the Basin and Range Province and contain rocks that predate the Sierra Nevada's exposed formations, helping geologists piece together the broader tectonic and sedimentary history of the western United States.

Geologists categorize the ancient rocks of the Sierra Nevada into two primary groups based on their ages: the Paleozoic rocks, which are older than 250 million years, and the Mesozoic rocks, which are older than 65 million years. The Paleozoic rocks predominantly consist of sedimentary sequences that were deposited in ancient marine environments, including limestone, shale, and sandstone. These rocks record a time when the region was submerged under a shallow sea, hosting diverse marine life. In contrast, the Mesozoic rocks are primarily igneous and metamorphic, formed during a period of significant tectonic activity associated with the subduction of the Farallon Plate beneath the North American Plate. This tectonic activity led to the formation of large magmatic bodies, regional metamorphism, and the accretion of various terranes, shaping the Sierra Nevada into its current form.

Sediments Accumulate in Ancient Seas

During the Paleozoic Era, more than 400 million years ago, the western margin of North America was a passive margin that ran from present day southeastern Idaho, through central Nevada, and into Southern California (see A Brief Geologic History of California). Offshore, where what is now the Sierra Nevada was under water, thick layers of sediments accumulated: 1 km (0.6 mi) in the south to 10 m (6 mi) in the north indicating that the ocean basin was deeper to the south.

For more than 500 million years, these sediments accumulated, creating a record of what the environment was like during that time. Most of these sediments are marine in origin indicating they formed in a shallow sea. The types of rocks and the few recognizable fossils that remain (most have been obliterated by metamorphism and deformation) indicate that the eastern side of the sierra was closer to shore.

Seaward, beyond the continent, were "island arcs" not unlike the Aleutian island arc that exists today (Figure \(\PageIndex{1}\)), an offshore omen of what would later become California and the Sierra Nevada.

Figure \(\PageIndex{1}\): An illustration of what western North America may have looked like during the Paleozoic era. The western edge was a passive margin where ocean sediments were depositing. Out at sea, subduction of oceanic crust beneath other oceanic crust was forming island arcs. This work by the USGS is in the public domain. Access a detailed description.

How these ancient seafloor sediments and offshore island arcs wound up metamorphosed and accreted onto the western margin of North America is an interesting and complex story and continues to be an area of active research for geologists.

Query \(\PageIndex{1}\)

Metamorphic Roof Pendants

Today, the oldest rocks of the Sierra Nevada no longer resemble ancient seafloor sediments. They have been metamorphosed by the intrusion of the Sierra Nevada batholith. Since the intrusion, weathering and erosion have exposed the batholith to the surface, leaving small remnants of the metasedimentary rocks interspersed throughout as "roof pendants". The name comes from the fact that the country rock is literally hanging down from the roof of the magma chamber like a pendant (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Features formed by the intrusion of plutonic igneous rock into preexisting country rock. Upon erosion, roof pendants form resembling "islands" of country rock surrounded by plutonic rock. This work by Jean-Pierre Berg is licensed under CC BY-NC 4.0. Access a detailed description.

More than one hundred of these roof pendants speckle the otherwise intrusive igneous rocks of the Sierra Nevada, concentrated in the central and southern parts of the province. The largest of these roof pendants include the Mt. Tom pendant, the Mt. Morrison pendant (Figure \(\PageIndex{3}\)), and the Saddlebag Lake pendant.

These "roof pendants" record moderate to high amounts of ductile deformation, resembling stretched taffy in some locations.

Figure \(\PageIndex{3}\): Mt. Morrison roof pendant visible from Convict Lake on the eastern side of the Sierra Nevada range. Dark-colored metamorphic country rock is in contact with the lighter colored plutonic rock. This work by Ralph Earlandson is licensed under CC BY 2.0. Access a detailed description.

Accreted Terranes

While roof pendants contain the oldest rocks of the Sierra Nevada, a majority of the metamorphic rocks are preserved as roughly north-south trending metamorphic belts. Sediments stopped being deposited around 180 to 160 Ma. During this time, subduction of the Farallon plate beneath the North American plate initiated, slowly shoving the oceanic crust beneath the continental crust. As oceanic crust continued to subduct, it brought along with it slivers of volcanic arcs and microcontinents which were too buoyant to subduct and were thus accreted or attached to the western edge of North America. The earliest known of these orogenic or accretion events in western North America is known as the Antler Orogeny.

These sequential accretions are known as terranes or exotic terranes because they are pieces of crust that were transported from elsewhere. Today, each terrane is comprised of numerous identifiable rock units and each terrane has a distinct geologic history. What separates these terranes are faults and shear zones, sometimes called sutures or suture zones. Not coincidentally, major gold deposits were discovered along these suture zones.

Figure \(\PageIndex{4}\): An exotic terrane attached to subducting oceanic crust approaching the continent. Beneath the continent, flux melting is taking place, forming a continental volcanic arc within older accreted terrane. This work by the National Park Service is in the public domain. Access a detailed description.

The Sierra Nevada terranes resemble a series of books that have fallen down on their book shelf: dipping down toward the east, with rocks within each terrane getting younger toward the east. The foothills terranes are incredibly complex and difficult to parse, thus the exact number of terranes remains an area of active research and debate among geologists. Most recent observations suggest that there are five distinct exotic terranes: the Northern Sierra terrane, Feather River terrane, Calaveras Complex, Jura-Triassic arc belt, and Middle—Late Jurassic arc sequence (Figure 9.2.5).

Figure \(\PageIndex{5}\): Major exotic terranes of the northern Sierra Nevada province. The five terranes: Northern Sierra Terrane (dark gray), Feather River (dark blue), Calaveras Complex (brown), Jura-Tirassic arc belt (green), and Middle-late Jurassic Arc Sequence (tan) are separated by faults and shear zones (white lines). The Melones Fault and Bear Mountain Fault are two major boundaries. "Terrane map of the western Sierra Nevada Foothills metamorphic belt with location of gold districts" by Arizona Geological Society in the public domain. Access a detailed description.

Northern Sierra Terrane

The Northern Sierra Terrane (Figure \(\PageIndex{5}\) indicated by dark gray) is the easternmost of the five terranes in the Sierra Nevada. More of the terrane is exposed in the north than in the south. It contains a number of different rock units, but its most notable unit is the Shoo Fly Complex. The Shoo Fly Complex consists primarily of:

• Mylonitic gneisses
• Schist and calcareous rocks
• Rare amphibolite
• Phyllite
• Metabasite and metaultramafic blocks

The protolithic sediments of the Northern Sierra terrane were initially deposited during the early Paleozoic, before the Antler orogeny. They were accreted to western North America during the Antler orogeny.

Shoo Fly Complex

The Shoo Fly Complex is a vital component of the Northern Sierra Terrane, comprising Paleozoic metasedimentary and metavolcanic rocks that have undergone extensive deformation and metamorphism. This complex provides critical insights into the tectonic processes that shaped the Sierra Nevada. Its primary rock types include mylonitic gneisses, schist, calcareous rocks, rare amphibolite, phyllite, and blocks of metabasite and metaultramafic rocks. These varied rock types reflect the diverse depositional environments and tectonic settings during the early Paleozoic era. Mylonitic gneisses, characterized by their foliated texture, formed under intense shear stress conditions, indicating significant deformation typically associated with major fault zones. Schist (Figure 9.2.6), known for its pronounced foliation and abundant mica minerals, and calcareous rocks, including marble and calc-silicate rocks, suggest ancient carbonate sediment presence. Rare amphibolite, formed from basaltic rock metamorphism and rich in hornblende, indicates volcanic activity and high-pressure metamorphism. Phyllite, a low- to medium-grade metamorphic rock with a foliated texture finer than schist but coarser than slate, forms from the metamorphism of mudstones and shales. Metabasite and metaultramafic blocks, remnants of ancient oceanic crust and mantle materials, suggest the incorporation of oceanic lithosphere into the continental margin during subduction.

Figure \(\PageIndex{6}\): Chlorite schist like that found in the Shoo Fly Complex. This work by by Siim is licensed under CC BY-SA 3.0. Access a detailed description.

The Shoo Fly Complex records a complex tectonic history beginning in the early Paleozoic, prior to the Antler Orogeny. Initially deposited in a marine environment, the protolithic sediments of the Shoo Fly Complex were accreted onto the western margin of North America during the Devonian period due to the Antler Orogeny. This significant tectonic event involved the collision and accretion of island arcs and other terranes onto the North American continent, causing extensive deformation and metamorphism in the Shoo Fly Complex, evidenced by mylonitic textures and high-grade metamorphic minerals. The subsequent subduction of the Farallon Plate during the Mesozoic further influenced the complex, leading to additional metamorphic and deformational events.

Structural features of the Shoo Fly Complex include numerous folds, faults, and shear zones that record the intense tectonic forces shaping the region. Major faults, such as the Melones Fault Zone, define the boundaries of the Shoo Fly Complex and its juxtaposition with adjacent terranes. The complex's folds are typically tight and asymmetrical, reflecting compressional tectonic regimes, while prevalent shear zones, characterized by mylonitic fabrics, indicate significant ductile deformation. These structural features are crucial for understanding the tectonic evolution of the Northern Sierra Terrane and the broader Sierra Nevada region.

Feather River Terrane

The Feather River Terrane (Figure 9.2.5 indicated in dark blue) is a geologically significant and distinct component of the Northern Sierra Nevada, primarily exposed in the northern region of the range. This terrane, indicated by dark blue on geological maps, is a narrow zone dominated by Devonian-age ultramafic rocks, particularly serpentinized peridotite. These rocks are remnants of ancient oceanic crust and upper mantle that were accreted to the North American continental margin during the late Paleozoic to early Mesozoic periods. The Feather River Terrane is especially notable for including the Permian Devil’s Gate ophiolite, which likely formed atop the older peridotite. This ophiolite complex, composed of gabbro, sheeted dikes, and basaltic pillow lavas, represents a well-preserved slice of oceanic lithosphere that has undergone varying degrees of metamorphism since its emplacement.

Structurally, the Feather River Terrane is bounded by major fault zones that delineate its relationships with neighboring geological units. To the east, it is separated from the Northern Sierra Terrane by the Downieville Fault, while to the west, it is bordered by the Calaveras Complex, with the Goodyears Creek and Rich Bar faults marking this boundary. These faults are part of the larger Melones Fault Zone, a critical structural feature that played a central role in the accretion and deformation processes affecting the Feather River Terrane. The absence of the Feather River Terrane south of Interstate 80, where the Shoo Fly Complex of the Northern Sierra Terrane comes into direct contact with the Calaveras Complex, underscores the localized nature of this terrane and its unique tectonic history.

The Feather River Terrane provides valuable insights into the tectonic evolution of the Sierra Nevada, particularly in understanding the processes of subduction and accretion that shaped the region. The complex interplay between these ultramafic rocks, their overlying ophiolitic sequences, and the surrounding fault systems reveals a dynamic geological past, marked by significant crustal movements and metamorphism.

In the southern Sierra Nevada, south of Interstate 80, the Feather River Terrane is not present and the Shoo Fly complex of the Northern Sierra Terrane is in direct contact with the Calaveras Complex.

Calaveras Complex

The Calaveras Complex, a significant geological unit within the Northern Sierra Nevada Terrane, is characterized by its complex and challenging assemblage of rocks. Following the Antler Orogeny, additional sedimentary rocks accumulated along the newly formed continental margin. As the North American plate converged with the subducting Farallon plate, these sediments were progressively shoved into the accretionary wedge. This tectonic activity metamorphosed the sediments into a variety of metamorphic rocks, including slates, phyllites, and serpentinite, creating the intricate and diverse rock assemblages seen in the Calaveras Complex today.

The ongoing subduction and associated flux melting resulted in the intrusion of granitic magma into these metamorphosed sediments. These granitic intrusions further complicated the geological interpretation of the Calaveras Complex, making it a challenging area for even experienced geologists to decipher. The region south of Interstate 80 highlights the complexity of this terrane, where the Shoo Fly Complex of the Northern Sierra Terrane comes into direct contact with the Calaveras Complex along a significant thrust fault. This contact zone provides valuable insights into the tectonic processes that have shaped the Northern Sierra Nevada, reflecting the dynamic interactions between sedimentary deposition, subduction, metamorphism, and magmatic intrusion.

As previously mentioned, south of Interstate 80, the Shoo Fly Complex of the Northern Sierra Terrane and the Calaveras Complex are in direct contact. This particular contact is a thrust fault.

Jurassic-Triassic Arc Belt (Central Belt)

The Jurassic-Triassic Arc Belt, also know as the Central Belt, is the next terrane to the west. Like the Calaveras Complex, it is a complicated mess of rocks. At its most basic, the Jurassic-Triassic Arc Belt is Paleozoic ocean crust, overlain by a volcanic arc that existed approximately 200 to 150 million years ago. The Central Belt is composed of a diverse assemblage of metamorphosed volcanic and sedimentary rocks, including basaltic flows, volcanic breccias, tuffs, cherts, and argillites. These rocks were originally deposited in a marine environment, forming part of a volcanic island arc that developed above a subduction zone where oceanic crust was being consumed beneath the North American continental margin.

The tectonic history of the Jurassic-Triassic Arc Belt is marked by intense deformation and metamorphism, reflecting the dynamic processes at play during its formation. The rocks within the Central Belt have been significantly folded, faulted, and sheared, indicating multiple phases of tectonic activity. This deformation is closely associated with the accretion of the arc complex onto the western edge of North America, as well as subsequent tectonic events that shaped the Sierra Nevada. The Central Belt is also intruded by granitic plutons of the Sierra Nevada Batholith, particularly during the Late Jurassic to Early Cretaceous periods, which further influenced the structural and metamorphic characteristics of the belt. The interaction between these intrusions and the arc rocks has led to contact metamorphism, creating complex mineral assemblages and adding to the geological diversity of the region. The Jurassic-Triassic Arc Belt is essential for understanding the tectonic and magmatic evolution of the Sierra Nevada and provides a window into the processes that operated along the western margin of North America during the Mesozoic era.

Middle-Late Jurassic Arc Sequence/Jurassic Foothills Belt

The Middle-Late Jurassic Arc Sequence, often referred to as the Jurassic Foothills Belt, is a prominent geological feature within the Northern Sierra Nevada Terrane. This sequence represents the remnants of a volcanic arc that was active approximately 160 to 150 million years ago during the Jurassic period. The Jurassic Foothills Belt is composed of a complex assemblage of metavolcanic and metasedimentary rocks, including volcanic flows, tuffs, graywackes, and shales, all of which have been subjected to varying degrees of metamorphism. These rocks were originally deposited in a marine environment, likely as part of a volcanic island arc system associated with the subduction of oceanic crust beneath the western margin of North America.

The tectonic history of the Middle-Late Jurassic Arc Sequence is marked by significant deformation events, which have imparted complex structural features to the belt. The rocks within the Jurassic Foothills Belt exhibit folding, faulting, and shearing, indicative of the intense tectonic forces that shaped the region during the Jurassic period. These structural complexities are often associated with the emplacement of granitic plutons from the Sierra Nevada Batholith, which intruded into the arc sequence during the Late Jurassic to Early Cretaceous periods. The interaction between these plutons and the surrounding metamorphic rocks has resulted in contact metamorphism, further altering the original characteristics of the arc sequence. The Jurassic Foothills Belt is a key component in understanding the evolution of the Sierra Nevada and the broader tectonic framework of the western United States during the Mesozoic era.

Due to its nature and formation, yet another ophiolite appears here in the Middle-Late Jurassic Arc Sequence: the Smartville Complex ophiolite.

Smartville Complex Ophiolite

The Smartville Complex is composed of a range of ultramafic and mafic rocks, including serpentinized peridotite, gabbro, sheeted dikes, and basaltic pillow lavas, which collectively represent the different layers of oceanic lithosphere. These rocks were originally formed at a mid-ocean ridge or back-arc basin and were later thrust onto the North American Plate as the Farallon Plate subducted beneath it. The accretion of the Smartville Complex provides critical insights into the processes of subduction, obduction, and the assembly of the Sierra Nevada terranes during the Jurassic. This ophiolite is also significant for its association with gold-bearing quartz veins, making it an area of historical mining interest, much like many of the terranes found in the Northern Sierra Terranes (Figure 9.2.7).

Figure \(\PageIndex{7}\): Map showing plutons and accreted terranes of the Sierra Nevada, California by the USGS is in the public domain. Access a detailed description.

Some of the oceanic crust, rather than being subducted, was instead obducted up onto the continent where it has been preserved for geologists to study. This process of obduction is further described in the Klamaths and Coast Ranges chapters.

Southern Sierra Terrane

The Sonoma Orogeny brought additional terranes to the North American continent including the Kings River and Kaweah Terranes in the Southern Sierra Nevada. The Southern Sierra Nevada region is characterized by a series of metamorphic terranes that provide key insights into the tectonic history and geological evolution of the area. These terranes, which include the Kings River, Mineral King, and Kaweah terranes, represent ancient volcanic and sedimentary sequences that have been significantly altered by metamorphism and deformation. Each terrane has a unique geological history and composition, contributing to the overall complexity of the Southern Sierra Nevada.

Kings River Terrane

The Kings River Terrane is a significant metamorphic terrane in the southern Sierra Nevada, primarily composed of Paleozoic and Mesozoic rocks. This terrane is characterized by a complex assemblage of metamorphic rocks, including schist, gneiss, quartzite, and marble. These rocks were originally sedimentary and volcanic in origin but have undergone extensive metamorphism due to tectonic processes, such as subduction and continental collision.

The Kings River Terrane is notable for its high-grade metamorphic rocks, which indicate deep burial and high-temperature conditions during metamorphism. This terrane is also associated with several significant faults and shear zones, which have played a crucial role in its geological evolution. The presence of these structural features suggests that the Kings River Terrane has experienced multiple episodes of deformation, contributing to its complex geological history.

Kings River Ophiolite

The Kings River ophiolite, located in the southwestern Sierra Nevada foothills of California, is a significant geological formation that offers insights into the region's complex tectonic history. This ophiolite sequence represents remnants of ancient oceanic crust and upper mantle that were thrust onto the continental margin during the Late Paleozoic to early Mesozoic era. The Kings River ophiolite is composed of a variety of rock types, including serpentinized peridotite, gabbro, basalt, and chert, which collectively provide a snapshot of oceanic lithosphere and the processes associated with its formation and emplacement. The presence of these rocks in the Sierra Nevada foothills (Figure 9.2.8) indicates a history of subduction and accretion, where pieces of oceanic crust were scraped off and added to the edge of the North American plate.

Figure \(\PageIndex{8}\): Generalized map showing the Kings River ophiolite, Kaweah serpentinite melange, and metasedimentary rock pedants aloing the eastern margin of the ophiolite belt by J. Saleeby and Caltech Tectonics Observatory is licensed under CC BY 2.0. Access a detailed description.

The structural complexity of the Kings River ophiolite is evident in its faulted and folded nature, reflecting the intense tectonic forces that have acted upon it. This ophiolite is crucial for understanding the tectonic evolution of the southwestern Sierra Nevada, as it marks the location of a former subduction zone where oceanic and continental plates interacted. Studies of the Kings River ophiolite have revealed valuable information about the processes of subduction, accretion, and the dynamics of ancient oceanic crust (Figure 9.2.9). The ophiolite also hosts various mineral deposits, including chromite and asbestos, which have been of economic interest in the past. Ongoing research continues to unravel the complex geological history recorded in this portion of the Sierra Nevada.

Figure \(\PageIndex{9}\): A zoomed in and generalized map showing the Kings River ophiolite by J. Saleeby and Caltech Tectonics Observatory is licensed under CC BY 2.0. Access a detailed description.

Mineral King Terrane

The Mineral King Terrane is another important metamorphic terrane in the southern Sierra Nevada, known for its diverse rock types and significant mineralization. This terrane primarily consists of metavolcanic and metasedimentary rocks, including amphibolite, schist, and phyllite. The Mineral King Terrane is also characterized by the presence of serpentinized ultramafic rocks, which are remnants of ancient oceanic crust that have been metamorphosed and altered by tectonic processes.

Metamorphism in the Mineral King Terrane occurred under conditions of medium to high pressure and temperature, resulting in the formation of distinctive mineral assemblages. The terrane is also notable for its mineral deposits, particularly those containing copper, zinc, and silver. These mineral deposits are associated with hydrothermal activity along fault zones and shear zones, which provided pathways for mineral-rich fluids to circulate and precipitate ore minerals.

Kaweah Terrane

The Kaweah Terrane is a key metamorphic terrane in the southern Sierra Nevada, characterized by its complex lithology and tectonic history. This terrane includes a wide variety of metamorphic rocks, such as schist, gneiss, quartzite, and amphibolite. The Kaweah Terrane also contains significant amounts of metavolcanic and metasedimentary rocks, which were originally part of an ancient volcanic arc and associated sedimentary basin.

Metamorphism in the Kaweah Terrane occurred under a range of pressure and temperature conditions, leading to the development of diverse mineral assemblages and metamorphic textures. This terrane is also associated with major fault zones, such as the Kern Canyon Fault, which have played a significant role in its tectonic evolution. The presence of these fault zones indicates that the Kaweah Terrane has undergone significant deformation, contributing to its complex geological structure.

The Kaweah Terrane is also notable for its mineral deposits, particularly those containing gold and other precious metals. These deposits are often found in association with quartz veins and shear zones, which provided conduits for hydrothermal fluids to circulate and precipitate ore minerals. The economic importance of these mineral deposits has historically made the Kaweah Terrane a focus of mining activity in the southern Sierra Nevada.

References

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