NORTHERN CALIFORNIA – The proposed Berryessa Snow Mountain National Monument region provides unparalleled access to geologic features associated with an ancient tectonic system where one plate descended beneath another.
The Coast Range Fault, noted on the map above, represents the ancient boundary between the upper North American plate and the descending lower plate.
Rocks of the upper plate include Great Valley sedimentary and volcanic rocks overlying serpentine, that is, remnants of ancient oceanic crust.
The upper plate represents part of the western edge of North America that formed 140 to 200 million years ago.
Lower plate rocks include the Franciscan complex – deformed and metamorphosed (recrystallized) sedimentary and volcanic rocks – that were scraped off the down-going plate and buried up to 12 to 20 miles beneath the North American edge as the plate went down and then uplifted to the surface by erosion.
The active San Andreas fault family was superimposed more recently on this earlier convergent plate situation.
The Great Valley sedimentary rocks themselves were deposited originally on top of oceanic crust, which had previously been incorporated into North American continental rocks.
Great Valley sediments were laid down in some 3,000 feet of marine waters at the edge of the North American continent.
Subsequent earth movements tilted these rocks from their original horizontal position to steeply inclined vertical layers.
These rocks are well exposed along the western side of the Great Valley, including in the southeast part of the Berryessa Snow Mountain region.
The serpentine and related rocks of the down-going plate represent remnants of oceanic crust formed at an oceanic spreading center and subsequently added to the North American continent.
Serpentine, scientifically called “serpentinite,” is a rock formed by combining water with rock that originally was part of the Earth's mantle, the layer beneath the Earth's crust.
Soils formed from serpentinite rocks lack certain elements required by most plants. Thus it is not surprising that the Berryessa Snow Mountain hosts unique plant species in a variety of landscapes and microclimates that have adapted to serpentine-derived soils.
Some rocks of both the Franciscan and Great Valley units constitute blocky landscapes formed by a chaotic mixture of diverse rock types.
Some of these rocks, often called “melange” after a French word meaning mixture, apparently formed as deposits of “mud volcanoes.”
Mud volcanoes are widespread in the Marianas trench area, where fluids derived from the down-going plate incorporate blocks of rock as they rise to the surface and spill over to form submarine earth flows some 20,000 feet or so on the ocean floor.
Some complex chaotic rocks found in the Great Valley and Franciscan units may have a similar origin. Other on-land exposures of similar mud volcano deposits may be present in Turkey, Iran, Afghanistan and southwest Pakistan, but none of these areas is as well-documented or as accessible as the Berryessa Snow Mountain area.
Similar tectonic processes are active today in various locations, such as off the Pacific Northwest and in the Marianas Trench National Monument.
Such modern geologic rock-forming processes lie below thousands of feet of water and are not directly observable.
To study such rocks, geologists employ deep-sea drilling and geophysical techniques of remote sensing, small deep-diving two to three-person submarines, or remotely operated submersible vehicles.
In contrast, one can walk across the preserved fossil boundary between the two former plates in the Berryessa Snow Mountain and see the rocks and geologic structures that formed during ancient plate interactions.
Snow Mountain itself represents a special feature. It contains submarine volcanic rocks, not more than 140 million years old, that look as if they were laid down only a few years ago.
However, minerals identified in the rocks indicate that they formed as an oceanic submarine volcano (seamount) far west of California, then migrated with the down-going plate to the continental edge, were buried 12 to 20 miles deep, and rose again to the Earth's surface.
Also, the Berryessa Snow Mountain area exhibits clusters of invertebrate fossils that apparently grew in deep water around chemical seeps.
Such clusters are widespread on some modern plate boundaries. The fauna live in the dark thousands of feet deep around cold to warm submarine springs that typically contain methane or hydrogen sulfide.
The animals thrive, however, using the chemicals as nutrients. About six such ancient sites lie in the Berryessa Snow Mountain area, enabling one to see such features closely and on land.
In summary, the geology of the Berryessa Snow Mountain region provides valuable instructive exposures of features and processes of a convergent tectonic plate margin. Nowhere else in the world are such features as well developed, preserved or accessible.
The proposed Berryessa Snow Mountain National Monument region is a unique region with world-class geology and biology. It is well worth national monument designation.
Eldridge Moores is a professor emeritus at the University of California, Davis. He is joined in writing this article by wife, Judith.
Tuleyome Tales: The remarkable geology of the Berryessa Snow Mountain region
- Eldridge and Judith Moores
- Posted On