Alamitos Formation is a geologic formation exposed in the southern part of the Los Angeles Basin and adjacent coastal areas of Southern California, United States. It comprises predominantly marine sedimentary rocks that were deposited during the late Miocene epoch, with some authors extending its age into the early Pliocene.
Lithology
The formation consists mainly of fine‑ to medium‑grained sandstones, siltstones, and shales. The sandstones are generally well‑sorted, quartz‑rich, and display cross‑bedding and ripple marks indicative of shallow marine to offshore depositional environments. Interbedded shales contain occasional glauconite and are often fossiliferous.
Thickness and Extent
Measured thickness of the Alamitos Formation ranges from about 100 m to over 300 m, depending on location. Outcrops and subsurface exposures are documented in the western Los Angeles Basin, the Palos Verdes Peninsula, and portions of the Santa Monica Mountains. The formation underlies the younger Puente Formation and overlies older units such as the Fernando Formation and the earlier marine deposits of the Repetto Formation.
Age and Stratigraphic Relationships
Biostratigraphic and radiometric data place the deposition of the Alamitos Formation primarily in the late Miocene (approximately 11 to 7 million years ago). Some sections, particularly in the western basin, contain fauna that suggest an early Pliocene (≈5 million years ago) continuation. The formation is part of the larger Miocene‑Pliocene marine succession that records a progressive deepening of the basin during the Neogene.
Paleontology
Fossils recovered from the Alamitos Formation include marine mollusks (bivalves and gastropods), echinoids, and foraminifera, which collectively indicate a warm, shallow marine setting. The assemblages are useful for regional biostratigraphic correlation and for reconstructing Neogene paleoceanographic conditions along the California coast.
History of Investigation
The unit was first described in detail by geologists mapping the Los Angeles Basin in the mid‑20th century. Subsequent studies refined its stratigraphic position, age, and sedimentary characteristics through field mapping, core analysis, and paleontological work. The formation is referenced in U.S. Geological Survey publications and in regional geological atlases.
Significance
The Alamitos Formation provides insight into the tectonic and sedimentary evolution of the Los Angeles Basin during a period of active crustal deformation and sea‑level change. Its well‑preserved marine sediments serve as a reference horizon for hydrocarbon exploration, groundwater studies, and paleoenvironmental research in Southern California.
References
- United States Geological Survey (USGS) publications on the geology of the Los Angeles Basin.
- Regional geologic maps and stratigraphic lexicons issued by the California Geological Survey.
- Peer‑reviewed articles on Neogene marine stratigraphy and paleontology of Southern California (e.g., works by Kelley, T. H.; R. M. Nash).
Note: The above information reflects the consensus of available geological literature as of the knowledge cutoff date.