San Diego Formation

The San Diego Formation is a geologic formation in southwestern San Diego County in southern California (United States), and northwestern Baja California (México).

San Diego Formation Stratigraphic range: Pliocene
Type	Geologic formation
Lithology
Primary	Conglomerate
Other	Sandstone
Location
Region	San Diego County, California, Baja California
Country	United States, Mexico
Type section
Named for	San Diego, California

Geology

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

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 AF. 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, there have been installed desalination plants. 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 San Diego, 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

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

Mysticetes

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

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

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

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

• Geology of San Diego County, California
• Pisco Formation
• Urumaco

References

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. Metropolitan Water District of Southern California (2007). Groundwater Assessment Study. Retrieved from http://edmsidm.mwdh2o.com/idmweb/cache/MWD%20EDMS/003697466-1.pdf
4. http://www.sdcwa.org/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.
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).
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. 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

• Ellis, Arthur Jackson; Lee, Charles H. (1919). Geology and ground waters of the western part of San Diego County, California. Department of the Interior, United States Geological Survey.
• Kennedy, Michael P. (1975). Geology of the San Diego metropolitan area, California. California Division of Mines and Geology.
• "General Plan Final Program EIR: 3.11 Paleontological Resources" (PDF). City of San Diego.