Agate Beach, Bolinas, Marin Co., California, USA
|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 ListThis is a list of exploitable or exploited mineral commodities recorded at this locality.
1 valid mineral.
Rock Types Recorded
Select Rock List TypeAlphabetical List Tree Diagram
Entries shown in red are rocks recorded for this region.
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
5.333 - 11.62 Ma
|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.