12.2: Geology of the Great Valley

Geology of the Great Valley

The Great Valley is underlain by a thick sequence of sedimentary units often referred to as the Great Valley Sequence. This up to 20,000 meter (~66,000 feet) thick stratigraphic interval is Jurassic age or younger and is bound within an asymmetrical synclinal trough with a more gently dipping eastern limb. The push and pull of the tectonic plates, combined with a shallow interior seaway, caused much of California to be covered by ocean water—it’s crazy to think that ocean water came all the way up to the foot of the newborn Sierra Nevada as shown in Figure 12.2.1. This figure displays the paleogeography of the Great Valley during the Middle Miocene (~20-15 million years ago).The solid yellow line indicates subduction, while the dashed red line shows the strike-slip motion of the newly formed San Andreas Fault.

Figure \(\PageIndex{1}\): Paleogeography of the Great Valley during the Middle Miocene (~20-15 million years ago). "Paleogeography of the San Joaquin Valley Over Geologic Time" by Bartow and Nilsen is licensed under CC BY-NC 4.0. Access a detailed description.

As the seas withdrew in the early Eocene, terrestrial sediment deposition from the Sierra Nevada to the east increased. During the Eocene, there was tectonic uplift occurring at the boundaries of the valley which further caused the sea to recede, reducing deposition.

Sacramento Valley

The Sacramento Valley, located in Northern California, has a complex geologic history spanning millions of years. Its formation can be traced back to various tectonic activities, including the subduction of the Farallon Plate beneath the North American Plate, the uplifting of the Sierra Nevada Mountains, and the creation of the California Coast Ranges. Figure 13.2.2 is a simplified cross section of the Sacramento Valley that complements the geologic history outlined below. The left side of the figure is the westside of the valley and is punctuated by the presence of the Kirby Hills Fault. The right side of the figure shows the granitic basement of the Sierra Nevada. The yellow color represents major sandstone units, while the grey color represents fine-grained sediments, such as shale.

Figure \(\PageIndex{2}\): Cross section through the southern Sacramento Valley. "Simplified Cross Section though Sacramento Valley" by Cole Heap, P.G. and adapted from California Department of Conservation is in the public domain. Access a detailed description.

Precambrian Era

During the Precambrian era (approximately 4.6 billion to 541 million years ago), the Sacramento Valley did not exist as a distinct geographical feature. It was part of a larger landmass that underwent extensive geological changes due to tectonic activity, volcanic eruptions, and the gradual formation of sedimentary rocks.

Paleozoic Era

During the Paleozoic era (541 million to 252 million years ago), the Sacramento Valley region was submerged under a shallow sea. Sedimentary rocks, such as sandstone and shale, were deposited as the sea advanced and retreated. These sedimentary rocks are now found in the western part of the valley, forming a significant portion of the deeply buried strata that overlie the granitic basement.

Mesozoic Era

During the Mesozoic era (252 million to 66 million years ago), the Farallon Plate began to subduct beneath the North American Plate. This subduction resulted in intense tectonic activity and the uplift of the ancestral Sierra Nevada Mountains to the east of the Sacramento Valley. The deposition of sedimentary rocks continued in the valley as rivers carried eroded material from the rising mountains, forming sandstones, shales, and conglomerates.

Cenozoic Era

Around 66 million years ago, the Cenozoic era began with the Paleogene period. The tectonic forces continued to uplift the Sierra Nevada Mountains, and the Sacramento Valley underwent subsidence, resulting in the formation of a large basin. The valley was initially filled with marine and non-marine sediments, including sand, silt, and clay. Fossils of marine creatures, such as clams and gastropods, are found in these sediments, indicating the presence of a shallow sea.

During the Neogene period (23 million to 2.6 million years ago), the Sacramento Valley experienced significant volcanic activity as a result of the subduction of the Farallon Plate. Numerous volcanic eruptions occurred, depositing volcanic ash and lava flows across the region. These volcanic materials contributed to the fertile soils of the Sacramento Valley, making it an agriculturally productive region today.

Quaternary Period

The Quaternary period (2.6 million years ago to the present) brought further changes to the Sacramento Valley. Glaciations occurred in the Sierra Nevada Mountains, resulting in the formation of large glaciers and the erosion of the mountain slopes. The eroded material, including sand, gravel, and boulders, was carried by rivers and deposited in the valley, forming alluvial fans and floodplains.

The Sacramento River and its tributaries played a crucial role in shaping the valley's landscape during this period. The river system transported sediments from the mountains and deposited them in the valley, creating fertile soils for agriculture. Human settlement and agriculture in the Sacramento Valley have been heavily influenced by these geological processes.

The geologic history of the Sacramento Valley involves the formation of sedimentary rocks during the Paleozoic and Mesozoic eras, the uplift of the Sierra Nevada Mountains, volcanic activity during the Neogene period, and the deposition of alluvial sediments during the Quaternary period. These seemingly simple, yet disparate geological processes have shaped the landscape, soils, and agricultural productivity of the Sacramento Valley.

Inset Box 12a: Sutter Buttes

The Sutter Buttes, located in Northern California, stand as a captivating geological wonder and hold significant historical and cultural importance. Rising abruptly from the surrounding flatlands of the Sacramento Valley, these majestic volcanic domes form the world's smallest mountain range. Revered as a sacred site by Native American tribes such as the Maidu and Yuba, the Sutter Buttes have been a focal point of indigenous myths and spiritual practices for centuries. Beyond their cultural significance, these Buttes are a haven for biodiversity, hosting a diverse array of plant and animal species amidst their rugged terrain. Hiking enthusiasts and nature lovers are drawn to the region, eager to explore the unique flora and fauna that thrive in this distinctive landscape. Preserving the Sutter Buttes not only conserves an ecological gem but also pays homage to the rich heritage of the Native American communities deeply intertwined with the land.

The scattered hills of Sutter Buttes rise ~2,000 feet (610 m) above the dead flat floor of the southern Sacramento Valley about 55 miles (~89 km) northwest of Sacramento. The Buttes are the defunct remains of a volcano that was active during Pleistocene time (~1.6 million years ago). The Buttes were created through a combination of volcanic eruptions and subsequent erosion processes. The core of the Buttes consists of volcanic and plutonic rocks, including andesite, dacite, and basalt, which solidified from lava flows and magma intrusions. As these volcanic materials accumulated, they gradually shaped the iconic dome-like structures that now define the landscape. Surrounding the core is an apron of fragmental material that was created by eruptions of the lava domes. This apron extends roughly 11 miles (18 km) east-to-west and 10 miles (16 km) north-to-south. Between the core and the debris apron are what appear as small valleys, referred to as the “moat.” This region of the Buttes was formed by erosion of older, exposed sedimentary rocks that underlie the volcanic rocks. Figure 12.2.3 was taken by the Expedition 32 crew. It has been cropped and enhanced to improve contrast, and lens artifacts have been removed. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth.

Figure \(\PageIndex{3}\): Astronaut photograph ISS032-E-10482 was acquired on July 29, 2012, with a Nikon D2Xs digital camera using a 180 mm lens, and is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center. Caption by William L. Stefanov, Jacobs/ESCG at NASA-JSC. "Sutter Buttes, California" by NASA Earth Observatory is in the public domain. Access a detailed description.

What is a dead volcano doing out in the middle the Sacramento Valley floor? While not all geologists agree, many would say that the volcano is the southern extension of the High Cascades. One can in fact draw a line from the Cascades and connect directly to the Sutter Buttes. The problem is that there is no indication as to why the Buttes exist so far away from volcanoes like Lassen, where swarms of andesite and rhyolite domes exist just like those that make up Sutter Buttes. Additionally, why are the Buttes so far south of the modern oceanic trench, which ends off of Cape Mendocino. Such a trench could not have extended so far south as the Buttes within the last 2 million years.

However, some geologists theorize that the Sutter Buttes are more closely related to volcanoes that formed the Coast Range and erupted as the Mendocino triple junction passed from onshore to offshore as it traveled northward. And why would just one volcano be left along a 177-mile (255 km) stretch? Such questions are not uncommon to the science of geology and lead to continued areas of active research.

Over the millennia, extensive erosion from wind, water, and other natural forces sculpted the once larger volcanic complex into the distinct set of buttes seen today. The surrounding flatlands of the Sacramento Valley were once part of the larger volcanic terrain, but the Sutter Buttes now stand as solitary sentinels amid the vast plains. Their unique geologic history and relative isolation have contributed to their exceptional biodiversity, fostering the growth of diverse plant communities, and supporting a variety of animal species that have adapted to this distinct environment. Exploring the geology of the Sutter Buttes offers not only a glimpse into the dynamic geological past of California but also provides valuable insights into the processes that have helped to shape the Great Valley.

San Joaquin Valley

The San Joaquin Valley, located in central California, has a rich and complex geologic history that spans millions of years. This region has been shaped by various geological processes, including tectonic activity, sedimentation, and erosion. Similar to the Sacramento Valley, the San Joaquin Valley contains old sediments beneath Cenozoic aged sediments. Figure 12.2.4 exhibits a simplified cross section through the San Joaquin Valley. The left side of the figure is the westside of the valley and is punctuated by the presence of the San Andreas Fault. The right side of the figure shows the granitic basement of the Sierra Nevada. The yellow color represents major sandstone units, while the blue color represents fine-grained sediments, such as shales. The non-shaded portion labeled “Tulare” represents fluvial-deltaic sandstones and shales where the majority of the San Joaquin Valley’s groundwater is extracted from. The cross section extends from Paso Robles to the west of the valley, to the Round Mountain area, east of Bakersfield. The colors represent sedimentary strata thar dominantly comprised of silts and shales in grey and sandstones, in yellow. The pink color represents the Jurassic age granitic and pre-Cretaceous meta-sediments that formed through intrusive igneous and regional metamorphic processes. As noted in Figure 12.3.4, the westside of this sedimentary basin is deeper and contains close to 20,000 meters (66,000 ft) of sedimentary rock. Major formations are also labeled on the cross section. These formations have significance to the valley as they help to tell the history of the southern portion of the Great Valley. Much of which has been pieced together through oil and gas exploration efforts; in fact, many of these formations contain petroleum.

Figure \(\PageIndex{4}\): Cross section through the southern San Joaquin Valley. "Simplified Cross Section though San Joaquin Valley" by Cole Heap, P.G. and adapted from California Department of Conservation is in the public domain. Access a detailed description.

Tied to thick sedimentary stratigraphy is the geologic history of the San Joaquin Valley. The following is a high-level geologic history of the San Joaquin Valley that complements the simplified cross section displayed in Figure 12.2.4.

Paleozoic Era

During the Paleozoic Era (540-250 million years ago), the San Joaquin Valley area was covered by a shallow sea. Sediments, primarily composed of sand, silt, and clay, accumulated on the seafloor. Over time, these sediments were compacted and lithified, forming dominant sedimentary rock sequences of sandstone and shale.

Mesozoic Era

During the Mesozoic Era (250-65 million years ago), the San Joaquin Valley region experienced significant tectonic activity. The area was part of a large subduction zone, where an oceanic plate was being forced beneath the North American Plate. This subduction resulted in the formation of a volcanic arc along the western edge of the valley.

Volcanic activity in the region led to the emplacement of intrusive rocks, such as granites, as well as the extrusion of volcanic flows and ash deposits. These volcanic rocks can be found in the western foothills of the valley.

Cenozoic Era

Starting with the Paleogene Period (65-23 million years ago), the early part of the Cenozoic Era, the San Joaquin Valley area was uplifted and subjected to intense tectonic forces. This uplift led to the erosion of the existing rocks and the deposition of sediments in the valley.

In a combination of tectonic uplift and down drop, the San Joaquin Valley became a basin accumulating thick layers of sediment derived from the surrounding mountains. The sediments consisted of sand, silt, and clay, which were deposited by rivers and streams flowing from the adjacent Sierra Nevada and Coast Ranges. These sediments eventually compacted and formed the geologic formations known as the Great Valley Sequence.

During the Neogene Period (23-2.6 million years ago), the San Joaquin Valley underwent further tectonic activity, resulting in the formation of the San Andreas Fault system. This fault system is a major right-lateral strike-slip fault, with the main trace located along the western edge of the valley. The San Andreas Fault has played a significant role in shaping the valley's geology. It has accommodated the horizontal displacement of the Pacific Plate relative to the North American Plate, resulting in the formation of the Transverse Ranges and the uplift of the Coast Ranges. During this time, movement of large granitic blocks in the Temblor Range along the San Andreas Fault caused large, periodic loads of sediment to be deposited on the west side of the valley. When combined with movement along the San Andreas Fault, one particularly large granitic block, called the Salinian Block, accounted for large pulses of sediment to be deposited from the south to the north as shown in Figure 12.2.5. There are numerous sand bodies shown in this diagram that filled the basin to form portions of the Great Valley Sequence. Note how this diagram represents active deposition west of Buena Vista Hills during Late Miocene time. The yellow color in the figure exhibits sediment transport off of the Salinian highlands (i.e. the Temblor Range) on the westside of the valley and deposition into the Central Valley.

Figure \(\PageIndex{5}\): Diorama of deposition from the moving Salinian Grantic Block. "Spill and Fill" by Web and California Department of Conservation is in the public domain. Access a detailed description.

These deposits were sorted by energy and grain size resulting in rhythmic deposits of sandstone-shale-sandstone, called turbidites (Figure 12.2.6). This figure is a great example of a turbidite deposit that resulted from pulses of sand being transported by gravity from a high elevation into water. The shale-sandstone layering not only shows times of high-energy, but also times of low-energy, which allow silts and clays, that once compacted and lithified, form the sedimentary rock known as shale. These turbidite deposits account for a large portion of petroleum reserves on the westside.

Figure \(\PageIndex{6}\): From the bottom of the image to the top: Rhythmic layering of shale, sandstone, gypsum, shale, sandstone, and shale. "Panoche Hills Turbidites" by Cole Heap, P.G. is licensed under CC BY-NC 4.0. Access a detailed description.

As the shallow inland sea remained at this time, diatoms and other plankton thrived in it, and when these organisms died, they accumulated on the basin floor to create organic-rich shales that include the Eocene Kreyenhagen, and Miocene Monterey Formations. Figure 12.2.7 is an image of compacted diatom frustules (the hard and porous cell wall or external layer of diatoms) capture with a scanning electron microscope. Diatomite forms in marine environments, as well as some lake environments. Note how the average thickness of the diatoms are 100 microns. The average thickness of human hair is 70 microns. The integrated effects of heat and time then acted on the buried organic matter within these shales to create oil, and the detritus eroded from the Coast Ranges and the Sierra Nevada provided reservoir rocks where the oil could accumulate.

Figure \(\PageIndex{7}\): Scanning electron microscope image of diatomite, a bioclastic sedimentary rock made of opaline frustules of diatoms. "SEM Diatomite" by California Geologic Energy Management Division and is in the public domain. Access a detailed description.

Evidence of the shallow inland sea is shown in Figure 12.2.8 the Santa Margarita Formation near the Coalinga Oil Field. The light-colored beds contain arcida, or oysters, that once dwelled in a nearshore environment of the shallow inland sea that filled the southern portion of the San Joaquin Valley. Figure 12.2.9 shows an oyster from the same strata shown in Figure 12.2.8.

Figure \(\PageIndex{8}\): Oyster beds in the Santa Margarita Formation near Coalinga Oil Field. "Sandy Oysters" by Cole Heap, P.G. is licensed under CC BY-NC 4.0. Access a detailed description.

Figure \(\PageIndex{9}\): Arcida (oyster) from the Santa Margarita Formation near Coalinga Oil Field. "Santa Margarita Oyster" by Cole Heap, P.G. is licensed under CC BY-NC 4.0. Access a detailed description.

Further evidence of this paleo inland sea may be found in rock samples collected, as drill core, hundreds to thousands of feet below ground surface. Drill core is very much like taking a straw and sticking it into layered cake. When the straw is removed, the cake layers may be observed. This is a great example of how drill core is collected and interpreted, allowing geologists to understand when sea levels were high, when they were low, and when the Great Valley was an inland sea.

When geologists observe sequences of rock in drill core that consist of limestone, and shale that is capped with sandstone, they interpret a decrease in sea level. The sandstone makes for near shore, or beach-like, environment. It is near and in the water line that oysters tend to make their homes. Thus, the presence of oyster beds and this sea level decrease noted in the rock record help geologists to understand when an inland sea was present and when it began to disappear (this is referred to as sea regression). The opposite is also true. When drill core shows a sequence of sandstone and shale that is capped with limestone, sea level rise may be inferred (this is referred to as sea transgression).

Query \(\PageIndex{1}\)

Quaternary Period

In the Quaternary Period (2.6 million years ago - present), the San Joaquin Valley experienced continued tectonic activity and climate fluctuations. The San Andreas Fault remained active, producing numerous earthquakes throughout the region.

Climate changes during this period resulted in cycles of erosion and deposition. During wetter periods, rivers and streams flowing from the mountains transported large amounts of sediment into the valley. In drier periods, these waterways cut down into their own deposits, creating terraces and alluvial fans.

In modern times, human activities, such as agriculture and water extraction, have also significantly impacted the San Joaquin Valley's geology and hydrology.

Overall, the geologic history of the San Joaquin Valley is a testament to the dynamic nature of Earth's processes, including tectonic forces, erosion, sedimentation, and climate fluctuations. It has resulted in the formation of diverse rock formations and landscapes that contribute to the unique character of the region both on the surface and in the subsurface.

Inset Box 12b: Sharklook Hill - More Evidence of a Shallow Interior Seaway

Sharktooth Hill, located in Kern County, California, is a renowned geological site that has captured the interest of paleontologists and fossil enthusiasts alike. The area is famous for its rich and diverse fossil deposits, dating back to the Miocene epoch, approximately 15 to 16 million years ago. The region was once a shallow marine environment, part of an ancient oceanic system known as the Pacific Coast Seaway.

The geological formations at Sharktooth Hill consist primarily of sedimentary rocks, such as sandstone and shale, which have perfectly preserved a plethora of marine fossils. Among the most notable findings are the abundant fossilized shark teeth, giving the site its name. These teeth belong to various species of prehistoric sharks, including the fearsome Megalodon, an ancient giant shark that ruled the oceans millions of years ago. Figure 12.2.10 shows an upper tooth from an extinct white shark speices (middle Miocene in age). In addition to shark teeth, researchers have unearthed numerous other marine fossils, including bony fish, marine mammals, rays, and invertebrates, offering valuable insights into the paleoenvironment and the ancient marine ecosystem that once thrived in this area. Today, Sharktooth Hill remains an important site for scientific research and a popular destination for fossil collectors and tourists eager to explore the geological wonders of California's ancient past.

Figure \(\PageIndex{10}\): Carcharodon hastalis (shark) tooth found at Sharktooth Hill, CA. by N. Pearson (2022) is licensed under CC BY-NC 4.0. Access a detailed description.

References

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3. McPherson, J. G., & Miller, D. D. (1990). Depositional Settings and Reservoir Characteristics of the Plio-Pleistocene Tulare Formation, South Belridge Field, San Joaquin Valley, California. Structure, Stratigraphy and Hydrocarbon Occurrences of the San Joaquin Basin, California, The Pacific Section American Association of Petroleum Geologists.doi: 10.32375/1990- GB65.17
4. Page, B. M. (1966). Geology of the Coast Range of California. Geology of Northern California, California Division of Mines (Bulletin 190), 255-276.
5. Page, R. W. (1983). Data on Depths to the Upper Mya Zone of the San Joaquin Formation in the Kettleman City Area, San Joaquin Valley, California. US Geological Survey.doi:10.3133/ofr81699
6. Tulare Basin | USGS California Water Science Center. (n.d.). California Water Science Center. Retrieved July 10, 2023, from https://ca.water.usgs.gov/projects/c...are-basin.html
7. Webb, G. W. (1983). Stevens and earlier Miocene turbidite sandstones, southern San Joaquin Valley, California. American Association Petroleum of Geologists, Bulletin 65, 438-465.