# Monterey Formation Canonical page: https://lithomap.app/resources/geologic-formations/united-states/monterey-formation Last reviewed: September 17, 2026 Page purpose: Comprehensive United States geologic formation reference for human readers, search systems, and language models. Major region: Pacific Coast Regional setting: Coastal California basins and ranges Age: Miocene Age detail: Principally middle to late Miocene; local sections extend beyond that interval Depositional setting: Productive marine basins along the evolving California margin Rock types: Siliceous shale, Chert, Porcelanite, Diatomite, Dolostone States in recognized usage: California ## Summary The Monterey Formation is a varied marine succession of siliceous, organic-rich, and carbonate rocks that records exceptionally productive Miocene seas along coastal California. ## Distribution Monterey rocks occur in separate coastal and offshore basins from southern through central California. They are especially prominent in the Coast Ranges and around the Santa Barbara, Santa Maria, Ventura, and Los Angeles basins, but thickness, age range, and member names differ sharply among those areas. Faulting and folding have tilted many outcrops, and similar-looking siliceous units may belong to different local members. A regional formation label therefore needs to be checked against a detailed geologic map before it is applied to a particular exposure. ## Geological history Diatoms and other microscopic organisms supplied silica and organic matter to offshore basins, while clay, volcanic material, and carbonate entered in changing proportions. Burial converted biogenic opal to progressively more ordered silica, producing diatomite, porcelanite, and chert as temperature increased. Tectonic subsidence, restricted circulation, high biological productivity, and later compression created a complex mosaic rather than one uniform layer. Fractures and folds later helped make parts of the formation important petroleum reservoirs and source rocks. ## Fossil record Diatoms, foraminifera, radiolarians, fish remains, marine mammals, and mollusks occur in suitable beds. Microscopic fossils are especially valuable for correlating sections and reconstructing ocean productivity and water conditions. Fossils are not evenly visible: hard chert may show little without laboratory work, while softer diatomaceous or calcareous intervals can preserve abundant microfossils and occasional larger remains. ## Minerals and geological resources Silica occurs as opal, cristobalite-tridymite, and quartz depending on burial history. Clay minerals, calcite, dolomite, feldspar, and pyrite also occur, and phosphatic material is locally present. Oil and natural gas are geologic resources rather than minerals. Organic-rich beds, fractures, and silica alteration have major economic importance, but weathered coastal exposures can also be weak, landslide-prone, and unsafe to climb. ## Field context - Thin beds may be tightly folded, faulted, and deceptively unstable along sea cliffs and roadcuts. - Beach access, tide, and cliff-fall hazards can change rapidly; observe closures and property boundaries. - Use a local member name and basin framework when comparing ages or rock character. ## Connected field guides - Chert (rock; Named rock type) — https://lithomap.app/resources/rock-identification/chert - Dolostone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/dolostone - Shale (rock; Named rock type) — https://lithomap.app/resources/rock-identification/shale - Calcite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/calcite - Dolomite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/dolomite - Feldspar (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/feldspar Mapped location and formation context support an identification but cannot establish one. Compare several physical properties and use laboratory analysis when the material remains ambiguous. ## Connected state geology guides - No directly overlapping state guide is currently published. ## Authoritative sources - Geology of the Palos Verdes Peninsula — U.S. Geological Survey: https://pubs.usgs.gov/of/2004/1050/Geology.htm - Geolex Monterey references — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/UnitRefs/MontereyRefs_2830.html ## Formation photograph - Image: https://lithomap.app/_delivery/formations/monterey-formation.webp - Description: Thinly layered Monterey Formation exposed at Gaviota State Park, California - Creator: Antandrus - License: CC BY-SA 3.0 — https://creativecommons.org/licenses/by-sa/3.0/ - Source record: https://commons.wikimedia.org/wiki/File:MontereyFm.jpg Formation-level information does not prove that a fossil or mineral occurs at a specific coordinate. It also does not establish ownership, access, collecting permission, present conditions, or specimen identity.