The Capistrano Formation is a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

in coastal southern Orange County,

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

. It preserves

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

dating back to the late

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

to early

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...

are recognized from the Capistrano Formation alongside a diverse array of marine mammals including up to five species of walrus.

Geography

The Capistrano Formation, named for the town of

San Juan Capistrano San Juan Capistrano (also known colloquially as San Juan or SJC) is a city in southern Orange County, California, United States. The population was 35,253 at the 2020 Census. Named for Saint John of Capistrano, San Juan Capistrano was founded ...

, is located in southern

, specifically the northern extent of the

Peninsular Ranges The Peninsular Ranges (also called the Lower California province) are a group of mountain ranges that stretch from Southern California to the southern tip of the Baja California peninsula; they are part of the North American Pacific Coast Range ...

, which stretch from the

Los Angeles Basin The Los Angeles Basin is a sedimentary Structural basin, basin located in Southern California, in a region known as the Peninsular Ranges. The basin is also connected to an wikt:anomalous, anomalous group of east–west trending chains of mountai ...

Baja California Baja California, officially the Free and Sovereign State of Baja California, is a state in Mexico. It is the northwesternmost of the 32 federal entities of Mexico. Before becoming a state in 1952, the area was known as the North Territory of B ...

. It crops out along the coast from

Dana Point Dana Point () is a city located in southern Orange County, California, United States. The population was 33,107 at the 2020 census. It has one of the few harbors along the Orange County coast; with ready access via State Route 1, it is a popu ...

San Clemente San Clemente (; Spanish for " St. Clement" ) is a coastal city in southern Orange County, California, United States. It was named in 1925 after the Spanish colonial island (which was named after a Pope from the first century). Located in the O ...

, and inland for seven miles.

Geology and Stratigraphy

The Capistrano Formation is a heterogenous marine formation that can be differentiated into two distinct but adjacent units. One of these units is the Oso Member, which is composed of arkosic sandstone and preserves a nearshore environment. This unit shows outcrops across Orange County, which are recognizable as medium to coarse grained, white to tan rock. The second member primarily consists of

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

, preserves an environment that would have been located further out at sea in deeper waters and has not yet been named. At the southwestern border of the Oso Member the two units connect. Depending on the locality, the Oso Member of the Capistrano Formation either conformably overlies the Puente Formation (specifically Soquel Member), while in other areas it unconformably overlies the

Monterey Formation The Monterey Formation is an extensive Miocene oil-rich geology, geological sedimentary formation in California, with outcrops of the formation in parts of the California Coast Ranges, Peninsular Ranges, and on some of California's off-shor ...

and is succeeded by the

Niguel Formation The Niguel Formation is a geologic formation in California. It preserves fossils dating back to the Neogene period. See also * List of fossiliferous stratigraphic units in California * Paleontology in California Paleontology in California re ...

. The Oso Member has been dated to the early late

Hemphillian The Hemphillian North American Stage on the geologic timescale is a North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 10,300,000 to 4,900,000 years BP. It is usually considered t ...

(6.6 to 5.8 Ma) based on the presence of ''Dinohippus interpolatus'' and absence of older or younger taxa, but the formation as a whole extends into the Pliocene, rendering the Oso Member coeval with only the lower strata of the unnamed siltstone member.

Foraminifers Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...

and microfossils have also previously been used to date the formation, which indicate an early Pliocene age (5.6 to 4.9 Ma) for its upper boundary.

Paleoenvironment

The two units of the Capistrano Formation preserve environments of difference distance from the shore. The Oso Member preserves a near-shore environment, thought to be submarine delta deposits situated in a shallow embayment of the Pacific Ocean. The presence of marlin fossils could indicate that parts of the formation deposited at bathyal depths, between 200 and 2.000 meters deep, and Fierstine argues that the preservation of the material suggests that it was not subject to extensive postmortem transportation. He also argues that the presence of this fish suggests warm water temperatures during the Miocene, with Marlins typically preferring average surface temperatures of 24 °C. Barboza and colleagues meanwhile argue that this alone is not sufficient evidence for depth, citing the preservation of marlin fossils in much shallower waters including one specimen found in deposits of the

supralittoral zone The supralittoral zone, also known as the splash zone, spray zone or the supratidal zone, sometimes also referred to as the white zone, is the area above the spring high tide line, on coastlines and estuaries, that is regularly splashed, but not s ...

. The proximity to the shore is apparent due to the presence of terrestrial fauna.

Paleofauna

Chondrichthyes

Osteichthyes

Reptilia

Aves

Mammalia

Afrotheria

Artiodactyla

Carnivora

Lagomorpha

Perissodactyla

References

* {{Neogene Footer Neogene California Geology of Orange County, California Siltstone formations of the United States Mudstone formations of the United States