The Geology of Vacaville, CA

Vacaville is a city located in Solano County in northern California, nestled between the coastal ranges and the Sacramento Valley. With a population of around 100,000, it has grown rapidly in recent decades as part of the San Francisco Bay Area metro region. Though a thriving suburb today, Vacaville has a fascinating geological history stretching back millions of years. In this local guide, we’ll explore the geology that shaped this city and made it what it is today.

Regional Geologic Setting

Vacaville sits on the eastern edge of the California Coast Ranges, a series of northwest-southeast trending mountain ranges and valleys that run parallel to the San Andreas Fault system along the California coast. The Coast Ranges were formed due to tectonic compression between the North American and Pacific plates, causing folding and uplift of sedimentary rocks beginning around 30 million years ago in the Oligocene Epoch.

More locally, Vacaville lies on the flank of the Vaca Mountains, which are part of the Inner Coast Ranges. These mountains are composed mainly of Cretaceous marine sedimentary rocks around 65-145 million years old, including sandstones, mudstones, and shales. The Vaca Mountains form the border between California’s Sacramento Valley to the east and Suisun Bay, part of the San Francisco Bay estuary, to the south and west.

Bedrock Geology

Vacaville is underlain by a complex assemblage of sedimentary, volcanic, and metamorphic bedrock units spanning over 100 million years of geologic time. From oldest to youngest, these include:

• Great Valley Sequence – These Late Jurassic to Late Cretaceous marine rocks underlie the Sacramento Valley and form the core of the Vaca Mountains. They include sandstones, shales, and conglomerates deposited in a marine forearc basin.
• Great Valley Ophiolite – Remnants of oceanic crust and upper mantle rocks that formed during subduction off the continental margin around 160 million years ago. Includes serpentinite, gabbro, and pillow basalts.
• Marine Sedimentary Units – Additional Cretaceous marine sandstones, mudstones, and conglomerates that overlap the Great Valley Sequence. These record continued marine deposition.
• Volcanic Units – Erupted from nearby volcanoes between 35-20 million years ago during subduction along the continental margin. Include andesite and basalt lava flows as well as tuff and breccia deposits.
• Pliocene Sediments – Younger sandstones, mudstones, and conglomerates deposited between 5-2 million years ago as the Coast Ranges uplifted. Contain fossils of early horses, camels, and rhinos.

This complex bedrock geology records the long history of deposition, volcanism, metamorphism, uplift and erosion along California’s dynamic continental margin.

Local Geology

In the Vacaville area, the bedrock geology is overlain by thicker Quaternary sediments from the past 2.6 million years of the Pleistocene and Holocene epochs. These include:

• Alluvial Fans – Form where mountain streams exit into the valley, depositing eroded sediments in a fan shape. Composed of gravel, sand, silt, and clay. Underlie much of present-day Vacaville.
• Stream Terrace Deposits – Layers of sand and gravel left by ancestral streams that eroded the Compliance Alluvial Fans. Mark former floodplain levels above modern streams.
• Basin Deposits – Fine silts and clays deposited in low-lying parts of the Sacramento Valley floodplain, including areas along Alamo and Ulatis Creeks in Vacaville. Prone to flooding.
• Landslide Deposits – Rubble and displaced blocks of various bedrock units found on unstable slopes in the Vaca Mountains. Formed by gravity-driven landsliding.

This local geology provided the parent materials for the alluvial and residual soils found across Vacaville today.

Structural Geology

Several major faults cut through the Vacaville region and have shaped its landscape:

• Vacaville-Winters Fault Zone – This northwest-southeast trending right lateral strike-slip fault runs along the base of the Vaca Mountains through Vacaville. It has been active in the last 11,000 years.
• Green Valley Fault – Forms the abrupt western edge of the Vaca Mountains. This reverse fault has uplifted the mountains relative to the valleys to the west.
• Midland Fault – Another northwest-southeast trending strike-slip fault passing through the southern end of Vacaville. Part of the Concord-Green Valley Fault system.
• Unnamed Faults – Several smaller, concealed faults likely underlie parts of Vacaville as well based on offsets in geologic units. Overall, the city lies in a highly active fault zone.

Faulting and seismicity has produced the rugged, folded topography of the Vaca Mountains bordering Vacaville and shaped the straight, linear stream valleys that characterize the local drainage patterns.

Economic Geology

The geology of Vacaville has supported mining activity in the past, though little goes on today. Resources that have been extracted include:

• Mercury – Cinnabar (HgS) deposits in the Great Valley Sequence were mined in the Sulphur Creek District southeast of Vacaville in the late 1800s. Pollution remains from these abandoned mines.
• Sand and Gravel – Several aggregate mines operate along Pleasants Valley Road west of Vacaville, extracting sand and gravel from the Alluvial Fan deposits for construction.
• Crushed Stone – The Legett Quarry southwest of Vacaville mines limestone and claystone in the Great Valley Sequence for crushed stone aggregate and cement manufacturing.
• Natural Gas – Small amounts of natural gas are extracted from wells drilled into Great Valley Sequence sandstones near Elmira.

While mining has tapered off, these geologic resources supported early settlement and growth in Vacaville.

Geologic Hazards

The city faces several geologic hazards stemming from its setting adjacent to active faults amidst weak, saturated sediments:

Earthquakes – Numerous active faults can generate damaging earthquakes. Large events occur on average every 50-200 years. Ground shaking can liquify sediments.

Landslides – Steep, unstable slopes in the Vaca Mountains periodically fail, especially during wet winters. Can block roads and damage properties at the mountain front.

Floods – Low-lying areas of Vacaville along creeks are prone to flooding during heavy rains, especially along the Delta and Basin Deposits. Levees help mitigate this.

Soil Liquefaction – Saturated sandy soils may temporarily liquefy during intense shaking, causing damage to structures. Most susceptible along Alamo and Ulatis Creeks.

Subsidence – Compaction of fine-grained Basin Deposits causes gradual land subsidence. Can impact infrastructure over time.

Through zoning and geotechnical standards, the city works to minimize these hazards and prevent damage.

Unique Geologic Features

Some particularly interesting and accessible geologic features to check out in and around Vacaville include:

• Sweets Ravine – A canyon along a tributary of Alamo Creek with exposures of Pliocene sediments containing fossils of early horses and rhinos.
• Vaca Mountains Geologic Walk – An interpretive trail explaining the complex bedrock geology of the Vaca Mountains, including oceanic ophiolite sequences.
• Lake Frey – A small recreational lake occupying a natural sag pond along the Vaca Fault zone southeast of the city. Formed by fault-created depressions.
• Nelson Slough – Shows an example of subsidence caused by compaction of Basin Deposits. This inland bay of Suisun Marsh used to be above sea level but subsided below it.
• Lagoon Valley – A scenic alluvial valley created by deposition from streams draining the Vaca Mountains into Pleasant Valley. Underlain by Alluvial Fan deposits.

Vacaville’s diverse geology provides a classroom for understanding the forces that created much of California’s dramatic landscape. Learning about the regional setting, bedrock formations, faults, and landforms gives insight into how this city came to be. So next time you’re out exploring Vacaville, think about the deep geologic origins under your feet!

Conclusion

In this local guide, we’ve unpacked the eventful geologic history that shaped Vacaville, California. From its beginnings as marine sediments and volcanics along an active tectonic margin, to uplift of the Coast Ranges, and finally stream erosion and deposition forming the present landscape, the regional and local geology provided the foundation that made Vacaville possible.

While earthquakes, flooding, landslides and other hazards stemming from this setting pose challenges for the city, an understanding of the underlying geology informs effective planning and development.

Vacaville’s distinctive blend of mountains, valleys, faults and rocks reveal stories extending back millions of years for those curious enough to explore its fascinating geology.