Geology of the Pt. Reyes Peninsula

The Geologic Setting

The portion of California's Marin County shown here is called the Pt. Reyes Peninsula. While it is connected to North America here, it is an immigrant, geologically speaking. Imported (kicking and protesting?) by the Earth's plate tectonic motions that created the infamous San Andreas Fault, there is convincing evidence that the Peninsula has been moved from the Monterey Bay area in the last 15 million years.  It apparently sat adjacent to Monterey from approximately 60 to 15 million years ago and then began it's travel up the coast. The Peninsula's earlier history, while superficially similar to that of the Sierra Nevada, contains indications that it may not have been part of the Sierra Nevada Batholith. Those indications give rise to a hypothesis that it may have been docked some time before 60 million years ago with North America as an exotic "terrane."  It is important that  the reader know that the names of the rock units used here are those designated in work done in the 1960's by Alan Galloway (a Marin County oral history of the Marin County Free Library) and published by the California Division of Mines and Geology in 1977. 

It is important to note that major revisions were proposed by Blake, et. al. (1974) and ultimately resulted in a very important USGS online resource on the geology of the Pt. Reyes Peninsula that was first posted to the web in 1997 and the links are at the bottom of this page.
  (a personal note on revision)

A Geological History of the Pt. Reyes Peninsula, Marin County, California, USA

The following goes from oldest to youngest and violates the law of superposition on this page! Don't be surprised if there is very little in each of these areas since this is a work in progress and almost everything is text based at the moment.
1. A sub-sea history recorded in the Pre-Cretaceous metamorphic rocks (pKm)

2. A period of granitic rock (gr) formation, with intrusion and metamorphism of the pre-existing rocks.

3. Uplift and erosion of the land exposing the granitic rocks unconformity 1 (nonconformity)

4. Submergence of the land and deposition of the Pt. Reyes Conglomerate (TPr)

5. A period of emergence of the land and erosion of the Pt. Reyes Conglomerate and other preexisting rocks. unconformity #2

6. Submergence and deposition of the Laird Formation (Tl)

7. Further submergence and deposition of the Monterey Formation. (Tm)

8. Deformation, uplift and erosion of the Monterey and earlier rocks Unconformity #3 (angular).

9. Submergence below the sea and deposition of Drakes Bay glauconitic sands (Tdbs).

10. Further submergence and deposition of the Drakes Bay Siltstones (Tdbc)

11. A period of emergence, deformation and erosion of the Drakes Bay Fm and earlier rocks. Unconformity #4

12. Ice age and modern depositional systems including beaches, dunes, bay, and alluvial deposits.The current geological conditions.


Misc. Resources

Information on the Point Reyes ecosystem (authors: Marti Brown, Barbara Holzman), referred by Doug Campbell (dougcampbell@ggbts.edu)

REFERENCES

Reference List

Blake, Jr., M.C., Bartow, J.A., Frizzell, Jr, V.A., Schlocker, J., Sorg, D., Wentworth, C.M., and Wright, R.H., 1974, Preliminary Geologic Map of Marin and San Francisco Counties and Parts of Alameda, Contra Costa and Sonoma Counties, California, U.S.G.S, Misc. Field Studies Map, MF 574.

Clark, J.C. and Brabb, E. B, 1997, Geology of Point Reyes National Seashore and Vicinity, California: A Digital Database", an online USGS open-file resource listed below.

Galloway, A., 1977, Geology of the Point Reyes Peninsula, Marin County, California, Bulletin 202, California Div. of Mines and Geology, Sacramento, California.



Personal observations on the USGS revisions by Jim Locke:

The discovery of a Sirenian (a seacow rather like the Manatee) fossil by one of our students and its subsequent excavation led to an interesting revision in the stratigraphy of the Peninsula and revisions in the geological map. The story begins with Galloway identifying the rocks at Bolinas as Monterey Formation. However, when a Berkeley graduate student, Darrell Domning (who was doing his research on the Sirenians of California) came over to look at the fossil, he looked at it and told me that it was "impossible" that this fossil could have come out of rocks that were Monterey age (Mohnian). He was certain, given his understanding of the evolutionary sequence of this group that the rocks were misrepresented as Monterey (since they were representative of Delmontian, upper Miocene to Pliocene) and suggested in several forums that the Bolinas area should be remapped with this in consideration.

Clark. et. al. of the USGS  did come in to look at the area. Their conclusion was that the Monterey rocks of Bolinas were recognized to be lithologically very similar to Monterey rocks but were much younger (an interesting problem that geologists have had in the past by assuming that it looks like the "xxx", therefore it must be "xxx").  Domning and the USGS interpretations indicate that they were formed 5-10 million years apart. In this interpretation, having been formed in the same type of physical environment (bathyal or greater depths, in an oxygen minimum zone?) they ended up having the similar appearance.

When the USGS came in, they compared the rocks to the rocks on the San Mateo Peninsula and the Santa Cruz coast that they had been studying in detail for many years.What followed was a recommendation to rename some of the units entirely. The Laird Formation and Monterey stayed the same, but the overlying units were suggested to be changed from the Drakes Bay Sandstone(Tdbs) and the overlying Drakes Bay Shale (Tdbc) of Galloway into three units like those on the San Mateo coast. They became the Santa Margarita Sandstone (equivalent to the Tdbs of Galloway, glauconitic, etc), the Santa Cruz Mudstone (thin to thick-bedded and faintly laminated olive-grey to pale yellowish-brown siliceous mudstone with thin elongate orangish carbonate concretions) and the Purisima Formation (the bioturbated, concretionary beds found at Drakes Beach). Their stratigraphic columns suggest that at Drakes Bay all three are found but are only about 2000 feet thick. They interpret the sequence at Bolinas as having no Purissma in it and a thickness in excess of 6000 feet thick.

With these correlations they created a strong case for the horizontal transport of the Pt Reyes block from the Monterey area to Marin, picking up each of the units over the Monterey along the coast as it moved north.

The USGS has made available via the web:

"Geology of Point Reyes National Seashore and Vicinity, California: A Digital Database"
By: Joseph C. Clark and Earl E. Brabb

Resources include:

Page author: Jim Locke, College of Marin (started Dec. 1994)