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Agate Beach, Bolinas, Marin Co., California, USA

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Latitude & Longitude (WGS84): 37° 53' 48'' North , 122° 42' 35'' West
Latitude & Longitude (decimal): 37.89667,-122.71000
Köppen climate type:Csb : Warm-summer Mediterranean climate

Beach south-west of Bolinas. Well known for small speckled translucent chalcedony pebbles that contain petroleoum, so-called "oil agates".

Commodity List

This is a list of exploitable or exploited mineral commodities recorded at this locality.

Mineral List

1 valid mineral.

Rock Types Recorded

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Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org

Late Miocene
5.333 - 11.62 Ma

ID: 2529460
Santa Cruz Mudstone

Age: Miocene (5.333 - 11.62 Ma)

Description: Thin-to thick- bedded and faintly laminated olive-gray to pale-yellowish- brown siliceous mudstone contains thin elongate carbonate concretions. Silica content varies from 66.4 percent near Bolinas to 79.0 percent higher in the section (Clark and others, 1984, table 2). In the seacliffs southeast of Double Point, contains a few thin bituminous sandstone interbeds, whereas farther southeast bituminous sandstone dikes are locally common in the lower part of the mudstone section. As much as 1040 m of mudstone are exposed between Bolinas and Duxbury Points. With additional subsurface section, this formation may be as thick as 2000 m in the Bolinas Mesa area but thins to the northwest, where it pinches out east of Drakes Bay. A diatom flora from near Bolinas Point is assigned to the Nitzchia reinholdii zone of late Miocene age (John A. Barron, written commun., 1975, 1990). Calcareous benthic foraminifers are diagnostic of the Bolivina obliqua zone of late Miocene age (Clark and others, 1984, table 3).

Lithology: Marine siliceous mudstone

Reference: J. C. Clark and E. E. Brabb. Geology of Point Reyes National Seashore and Vicinity, California: A Digital Database. USGS Open-File Report 97-456. [83]

Data and map coding provided by Macrostrat.org, used under Creative Commons Attribution 4.0 License

This page contains all mineral locality references listed on mindat.org. This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in mindat.org without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.


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Mitchell, J.R. (1995) Gem Trails of Northern California.

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