12: Great Valley
- Page ID
- 20346
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- Describe the Great Valley in terms of its rock types, age, and tectonic development.
- Assess the geologic hazards and risks associated with the Great Valley.
- Recall the major geologic units in this province as presented on a simplified map of California.
- Analyze the development and economic potential of natural resources within the Great Valley.
- Define how petroleum was formed and migrated to make the Great Valley’s oil and gas fields.
The geologic history of the Great Valley, California, is a fascinating story that spans millions of years. The valley's formation can be traced back to the tectonic forces that shaped the region. It all began with the subduction of the Farallon Plate beneath the North American Plate, which initiated the uplift of the Coast Ranges and the creation of a deep basin, known as the Great Valley. Over time, sedimentary rocks, eroded from the surrounding mountains, accumulated in the basin.
Around 20 million years ago, volcanic activity associated with the Cascades Range started to deposit ash and lava flows in the region. These volcanic materials, along with the continued sedimentation, contributed to the filling of the Great Valley. Subsequent tectonic activity, including faulting and folding, further shaped the valley's landscape.
Approximately 2.5 million years ago, the first of several glacial episodes occurred, affecting the region. The advancing and retreating glaciers modified the topography, carving out valleys and leaving behind moraines and other glacial deposits.
As the last Glacial Maximum ended 18,000 years ago, the valley experienced a period of increased volcanic activity, which resulted in the formation of volcanic cones and lava domes, and volcanic lava flows that came into the valley, oftentimes following existing riverbeds formed by glacial melt and tectonic uplift. This made for a true dance of fire and ice.
More recently, in the past several thousand years, the Great Valley has been shaped by the meandering of major rivers, such as the Sacramento and San Joaquin Rivers, which have deposited vast amounts of sediment and created fertile floodplain soils. Human activities, including agriculture and water management, have significantly influenced the landscape in the modern era.
Today, the Great Valley stands as a testament to the complex interplay of tectonic forces, volcanic activity, glaciation, and river processes. Its geologic history has played a crucial role in shaping the diverse ecosystems and agricultural productivity of this vast and dynamic region.
References
- 12.1: Geomorphology and General Overview of the Great Valley Province
- The Great Valley is characterized by a large northwest trending valley bounded by the Sierra Nevada province to the east, the Klamath Mountains on the north, the Cascade Range province on the northeast, and the Coast Ranges province to the west. Much of the Valley was formed through tectonic uplift and downwarping along the main axis of the valley. The Great Valley province is underlain by a thick sequence of sedimentary units known as the Great Valley Sequence which are Jurassic age or younger.
- 12.2: 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 thick stratigraphic interval is Jurassic age or younger and is bound within an asymmetrical synclinal trough. The push and pull of the tectonic plates, combined with a shallow interior seaway, caused much of California to be covered by ocean water and leave behind dynamic sedimentary deposits representing various depositional environments through time.
- 12.3: Soil, Streams and Agriculture
- Most of the surface of the Great Valley is covered by recent and Pleistocene-age alluvium that has been shed off the Sierra Nevada on the west side of the valley, as well as smaller volumes of sediment eroded from the Coastal and Temblor Ranges. Streams serve as pathways for sediment transportation from the mountains to the valley floor, where they are deposited. These sediments, characterized by distinct clay-rich mineralogy, contribute to the fertility of the agricultural land.
- 12.4: Natural Hazards of the Great Valley Province
- The Great Valley is prone to various natural hazards that pose significant challenges to its residents and infrastructure. From earthquakes and floods to droughts and land subsidence, the region experiences a wide range of hazards that demand careful planning, preparedness, and response.
- 12.5: Natural Resources of the Great Valley
- The Great Valley Province is known for its abundant geologic-based natural resources, with some of the main resources consisting of oil and natural gas, groundwater, and mineral deposits. The valley is home to significant reserves of oil and natural gas. Oil exploration and extraction have been a vital industry in the region for many years, contributing to California's overall energy production. The Great Valley's geologic history is responsible for its the many energy resources.
- 12.6: Chapter Summary and Key Terms
- A summary of highlights from the chapter.