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Bio 28: Ecology of the Golden Gate National Recreational Area (GGNRA)
Professor Crima Pogge Geology of the GGNRA Larry Brasfield 16 September 2008 Fig 1
Overview
As you might expect from looking at a map ( see Appendix A - GGNRA Map, NPS ), the geology of the land that the GGNRA manages varies significantly from the areas north (such as the Bolinas Ridge and Muir Woods) to the areas centrally located (such as the Marin Headlands, The Presidio, Cliff House, Fort Funston and Ocean Beach) to the areas south (such as Milagra Ridge, Mori Point and Sweeney Ridge). As shown in Fig. 1 ( Graymer, Moring, Saucedo, Wentworth, Brabb, Knudsen - 2006 ), and with the exception of the areas centrally located (Beach and Dune Sand – Quaternary), the majority of the basement rock is considered Franciscan Complex (mostly Cretaceous and Jurassic periods).
Note: It is presumed the reader is familiar with basic geologic terms such as Epoch, Period, Era and Eon [ see Appendix B - Status of GSSPs in January 2002 (updated by James Ogg) ].
What
In general, geologists categorize rocks into three basic types: igneous (plutonic and volcanic), metamorphic (Serpentinite, the California state rock, is an example of metamorphic rock) and sedimentary and into various timeframes ( see Appendix B ). For the areas in the GGNRA under consideration – Table 1 Serpentinite Serpentinite is a metamorphic rock that normally forms at tectonic plate boundaries. In the area where we are, geologists believe it was formed when ocean water and rocks combined at great depth and high temperatures, to form serpentine-type minerals. Serpentine is made of silica tetrahedrons and sometimes also has striations consisting of other materials. Antigorite (light to dark green color) is the most common serpentine mineral. Being low in potassium (K) and calcium (Ca) and high in magnesium (Mg), nickel (Ni) and chromium (Cr), Serpentinite is not conducive to normal plant growth. North Central South
GGNRA
Bolinas Ridge Muir Woods Marin Headlands^ Presidio, Ocean Beach, Fort Funston Milagra Ridge, Mori Point, Sweeny Ridge Franciscan Complex Sedimentary (Cretaceous) Franciscan Complex Volcanic (Jurassic) Franciscan Complex Metamorphic (early Cretaceous and/or late Jurassic) Franciscan Complex Melange (Eocene, Paleocene and/ or late Cretaceous) Franciscan Complex Chert (early Cretaceous and/or late Jurassic) Franciscan Complex Volcanic (Jurassic) Beach and Dune Sand (Quaternary) Franciscan Complex Sedimentary (Cretaceous) Franciscan Complex Volcanic (Jurassic)
Geology
Beach and Dune Sand (Quaternary) Franciscan Complex Melange (Eocene, Paleocene and/ or late Cretaceous)
Going into the geology of the Marin Headlands in greater detail, notice (Figure 2) the separate areas of: Chert (50%), Basalt (20-25%), Sandstone, shale and conglomerate (25% ) and small percentages of Beach and Dune Sand, Melanage (mixture of rocks) and Diabase (igneous rock similar to volcanic basalt).
How
There are two major processes whereby the rock in the GGNRA “arrived”. Both are the results of tectonic plate movement: (1) subduction; and, (2) movement along fault boundaries. Subduction [ Please refer to Figure 1 ] Subduction occurs when one tectonic plate is forced under another. Frequently, rock from the plate moving underneath is scraped onto the one above. In our case, early on, rock from further west [ Great Valley (green)] was scraped onto the plate to the east. It is believed that, at one time, the west coast of the North American continent was hundreds of miles further to the east. In addition, later, subduction continued from south to north [ Franciscan Complex ( blue)]. It is believed much of the Franciscan Complex rock in the GGNRA originated as far south as the equator. Fault Boundaries [ Please refer to Figure 1 ] Again, when tectonic plates move relative to one another, rock is frequently scraped from one to the other. Rock [ Salinian (plum)] to the west of the San Andreas Fault has been scraped onto the rock east of the fault. The GGNRA is between two of the tectonic plates, the North American and Pacific Plates. This boundary is a transform fault - the plates are sliding past each other (Elder, 2001).
Now Figure 1 seems to make more sense.
Other sources ( Andersen, Sarna-Wojcicki, Sedlock, 2001 ) would add two more processes to this list: Sea Level Fluctuations These fluctuations are mainly due to the amount of water (or lack thereof) in the polar ice caps. Just imagine the difference in the amount of sedimentary rock due to additional areas either being under water or not.
Human Activity Although human activity probably had minimal impact on the geology of the GGNRA, forestry, agriculture, industrialization and population growth probably had some geologic effect and certainly changed the ecology.
When
As referenced in Table 1, most of the rock in the GGNRA “arrived” in the Late Jurassic or Early Cretaceous epochs (see Appendix B). The Franciscan Complex rock (blue) arrived between the Middle Jurassic [~176-161 million years ago (Ma)] and the Miocene (~23-5.3 Ma) epochs. The Great Valley rock (green) arrived during the Late Jurassic epoch (~150 Ma). The Salinian rock (plum) arrived during the Oligocene epoch (~34 Ma). The Beach and Dune Sand (rock) is from the Quaternary period (1.8-.02 Ma). The latest glacial maximum was ~20,000 year ago. At that time, sea level would have been as much as 425 feet lower than it is now. Just imagine San Francisco Bay as a fertile valley and being able to walk to what is now the Farallone Islands. Between 18,000 and 5,000 years ago, sea level rose to approximately its current level. Fig 3 ( Andersen, Sarna-Wojcicki, Sedlock, 2001 )
Appendix A
From: Class Handout Golden Gate National Recreational Area National Park Service U.S. Department of the Interior
Appendix B
