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San Diego Formation

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San Diego Formation
Stratigraphic range: Pliocene
TypeGeologic formation
Lithology
PrimaryConglomerate
OtherSandstone
Location
RegionSan Diego County, California,
Baja California
CountryUnited States,
Mexico
Type section
Named forSan Diego, California

The San Diego Formation is a geological formation in southwestern San Diego County, California, United States, and northwestern Baja California, Mexico.

Geology

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It is a coastal transitional marine and non-marine pebble and cobble conglomerate deposit and marine sandstone rock with marine fossils, from a former bay, deposited during the Middle Pliocene to Late Pliocene ages (2–3 million years ago), of the Pliocene period during the Cenozoic Era.

This formation is found from the south side of Mount Soledad in San Diego County to Rosarito Beach in northern Baja California, including Tijuana, Mexico, and the southwestern corner of San Diego County from San Ysidro to Pacific Beach.

San Diego Formation deposits were formed in a large, open, crescent-shaped bay similar in size to Monterey Bay that existed on the coast in Pliocene times.[1][2]

Aquifer

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The formation contains the San Diego Formation Basin, a large aquifer under Imperial Beach, Chula Vista, National City, and southern portions of the city of San Diego. The San Diego Formation Basin is a confined shallow aquifer. It has a basin ground surface area of 79,724 acres (32,263 ha) and an estimated groundwater storage capacity of 960,000 acre-feet. The depth to groundwater is about 100 feet (30 m).[3] The groundwater in the San Diego Formation is brackish, and its quality is considered to be fair to poor.[3] Due to its proximity to the ocean, the risk of seawater intrusion is a primary concern regarding water quality. To avoid intrusion, the Sweetwater Authority constantly monitors the basin water levels, which have remained stable since the 1980s. A factor that contributes to limit the groundwater pumping is the importation of Colorado River water. However, to reduce demands for imported water, desalination plants have been installed. The Richard A. Reynolds Groundwater Desalination Facility, in Chula Vista, was completed in 1999 and holds a production capacity of 4 million gallons of drinking water per day.[3] The Claude "Bud" Lewis Carlsbad Desalination Plant in Carlsbad, California, was completed in 2015 and produces 50 million gallons of water per day.[4] In this way, these plants contribute to reducing vulnerability and secure water supply, especially during drought periods. The use of groundwater contributes to a regional effort to reduce demands for imported water.[5]

Fossil content

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Besides those of clams and other mollusks, quite a few bird fossils (which are generally rare) have been found in this geological formation. Among them is a possible ancestor of Cassin's auklet (Ptychoramphus aleuticus) and the loons Gavia concinna and G. howardae. The former was quite likely a close relative or even ancestor of the large black-headed[6] loons; the latter was perhaps a small relative of the large grey-headed[7] loons.[8][9]

Fauna

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Cetaceans

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Mysticetes

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Mysticetes reported from the San Diego Formation
Genus Species Stratigraphic position Notes Images

Balaenoptera

B. davidsonii[10]

An extinct rorqual belonging to Balaenoptera.

"Balaenoptera"

"B." portisi

Formerly assigned to Balaenoptera, probably a member of Cetotheriophanes.

Herpetocetus

H. morrowi

A cetotheriid.

Norrisanima

N. miocaena

A stem-balaenopteroid, previously considered to be a relative of humpback whale.

Odontocetes

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Odontocetes reported from the San Diego Formation
Genus Species Stratigraphic position Notes Images

Parapontoporia

P. sternbergi[11]

A close relative of the baiji.

Semirostrum

S. ceruttii

A porpoise with a lower jaw extended few inches beyond the upper jaw (similar to a halfbeak).[12]

Pinnipeds

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Pinnipeds reported from the San Diego Formation
Genus Species Stratigraphic position Notes Images

Callorhinus

C. gilmorei[13]

A relative of the northern fur seal.

Dusignathus

D. seftoni[14]

A walrus.

Valenictus

V. chulavistensis[14]

A walrus.

Sirenians

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Sirenians reported from the San Diego Formation
Genus Species Stratigraphic position Notes Images

Hydrodamalis

H. cuestae

A close relative of the Steller's sea cow.

See also

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References

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  1. ^ California Geological Survey, Regional Geologic Map No. 3, 1:100,000 scale Compiled by: Michael P. Kennedy and Siang S. Tan, 2008.
  2. ^ Scott Rugh, Clams of Champions: The San Diego Formation, from sdnhm.org, the San Diego Natural History Museum website accessed November 26, 2013.
  3. ^ a b c "Groundwater Assessment Study" (PDF). Metropolitan Water District of Southern California. 2007.
  4. ^ "Seawater Desalination".
  5. ^ City of San Diego.gov: San Diego Formation Basin, with map.
  6. ^ Common loon (G. immer) and yellow-billed loon (G. adamsii)
  7. ^ Pacific loon (G. pacifica) and Arctic loon (G. arctica)
  8. ^ Brodkorb, Pierce (1953). "A Review of the Pliocene Loons" (PDF). Condor. 55 (4): 211–214. doi:10.2307/1364769. JSTOR 1364769.
  9. ^ Brodkorb, Pierce (1963). "Catalogue of fossil birds. Part 1 (Archaeopterygiformes through Ardeiformes)". Bulletin of the Florida State Museum, Biological Sciences. 7 (4): 179–293 (224). Archived from the original on 2007-03-11. Retrieved 2009-07-29.
  10. ^ T. Demere. 1986. The fossil whale, Balaenoptera davidsonii (Cope 1872), with a review of other Neogene species of Balaenoptera (Cetacea: Mysticeti). Marine Mammal Science 2(4):277–298
  11. ^ W. K. Gregory and R. Kellogg. 1927. A fossil porpoise from California. American Museum Novitates 269:1–7
  12. ^ Rachel A. Racicot, Thomas A. Deméré, Brian L. Beatty, Robert W. Boessenecker. Unique Feeding Morphology in a New Prognathous Extinct Porpoise from the Pliocene of California. Current Biology, 13 March 2014 DOI: 10.1016/j.cub.2014.02.031
  13. ^ A. Berta and T. A. Demere. 1986. Callorhinus gilmorei n. sp., (Carnivora: Otariidae) from the San Diego Formation (Blancan) and its impliations for otariid phylogeny. Transactions of the San Diego Society of Natural History 21(7):111–126
  14. ^ a b T. A. Demere. 1994. Two new species of fossil walruses (Pinnipedia: Odobenidae) from the Upper Pliocene San Diego Formation, California. Proceedings of the San Diego Society of Natural History 29:77–98

Further reading

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