Briones Formation
Stratigraphic range: Miocene
TypeGeologic formation
UnderliesSan Pablo Formation
Location
RegionContra Costa County,
California
CountryUnited States
Type section
Named forBriones Hills

The Briones Formation is a Late/Upper Miocene epoch geologic formation of the East Bay region in the San Francisco Bay Area, California.[1]

It is found in western Contra Costa County.[1]

Geology

The formation preserves fossils dating back to the Late/Upper Miocene epoch of the Neogene period.[2] Common fossils in the area include Spisula, Calyptraeidae, Clinocardium, and S. falcata. [3] The presence of shell beds and mollusk fossils suggests the area was once under a shallow bay, with relatively uniform levels of salinity and sand porosity. [4] The shell beds form a conglomerate in a calcerous sandstone matrix; today, this conglomerate is a major component of notable geological features in the Bay Area, including Mission Peak. [5]

The base of the formation includes sandstone and siltstone in distinct, parallel layers. [5] The sandstone is unique in its relative coarseness and light coloration. Some areas also include a pebble conglomerate. [3]

The Tularcitos syncline is a significant geological feature in the southern part of the formation. The high level of deformation, compared to the northern area, suggests that there was significant metamorphic activity, enabling the formation of laumontite.[4]

The vegetation present in the area is primarily annual grasses and forbs. The soil, which is primarily composed of sand, is considered "excessively drained" because of its high permeability and high levels of runoff. Today, the land is used primarily for grazing. [6]

The presence of rocks of the Briones formation near the surface creates conditions conducive for the development of Chaparral as opposed to open grasslands or forest, though many different vegetation types have been documented occurring on the Briones Formation.

See also

References

  1. 1 2 Google Books: "Names and Definitions of the Geologic Units of California", Issues 825-830, pg 73.
  2. ↑ Various Contributors to the Paleobiology Database. "Fossilworks: Gateway to the Paleobiology Database". Retrieved 17 December 2021.
  3. 1 2 Robinson, G. D. "Geology of the Hayward quadrangle, California". ngmdb.usgs.gov. Retrieved 2022-10-22.
  4. 1 2 Murata, K. J.; Whiteley, K. R. (1973). "Zeolites in the Miocene Briones Sandstone and related formations of the central Coast Ranges, Calif" (PDF). Journal of Research of the U.S. Geological Survey. 1 (3): 255–267.
  5. 1 2 "Mission Creek - Geology". www.msnucleus.org. Retrieved 2022-10-16.
  6. ↑ "Official Series Description - BRIONES Series". soilseries.sc.egov.usda.gov. Retrieved 2022-10-22.


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