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This web feature will explore the geology and paleontology of the Fourth Bore.
I. Geologic and tectonic history of the East Bay Hills
The continental shelf retreats eastward and what is now the East Bay becomes an ocean basin between 1500 and 1600 meters deep. Between 17 and 14 million years ago, clays, sands, the frustules (hard, porous cell walls) of diatoms, and the tests of foraminifers accumulate on the sea floor (the Claremont Formation).
As the San Andreas Fault System draws nearer, the sea floor begins to rise. By 12 million years ago, the sea has receded, the sea floor is exposed, and terrestrial sediments begin to accumulate. A high area forms in the vicinity of the current San Francisco Bay and silt, sand, gravel, and cobbles are shed eastward towards a shallow marine embayment east of what is now Moraga. A thick layer of this material accumulates as an alluvial fan (the Orinda Formation) over a two-million-year period as the land continues to rise. By 10 million years ago, the marine embayment starts to recede.
With the change from subduction to a strike-slip fault boundary, a "slab window" forms (see graphic below left), allowing upwelling mantle to fill the space once occupied by the subducting Farallon slab. Lava erupts from what is now Round Top in the Sibley Volcanic Regional Preserve about 9.5 million years ago. This heralds in a period of volcanism lasting almost two million years; a series of lava flows (the Moraga Formation or Moraga Volcanics) buries the previously deposited sediments. The high land to the west continues to shed sediments eastward; periodically, sediments cover the lava flows.
The volcanism ceases about 8.4 million years ago and sediments begin to accumulate in a lake that extends northwest from what is now the region of Moraga. After another couple million years, deposition in the area is replaced by a long period of erosion.
Then, about 3.5 million years ago, there is a small change in the angle at which the Pacific and North American plates collide. Instead of sliding by "smoothly," a compressional component is introduced. At 2.5 million years ago, the rocks laid down over the past several million years are faulted and folded; the East Bay Hills begin their rise. A series of thrust faults cause the emergence of Mt. Diablo.
Today, both the relatively young (geologically speaking) East Bay Hills and Mt. Diablo continue to rise at the respectable rate of one to two millimeters a year; this translates to ten to twenty meters after a mere 10,000 years (assuming no change in the rate of erosion). Earthquakes are a constant reminder that the Pacific Plate continues to move northwest relative to the North American Plate (see graphic below right). Generally, lateral movement along the two plates averaging just over 4 cm per year is either a slow creep or a sudden slip. If it's a big slip, resulting in the release of many years of stored energy, then watch out!
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