The old quarry at this location provides an excellent view of the ribbon chert common to the Marin Headlands. Watch out for the prolific poison oak. All the rocks are protected so there is no collecting. Please no digging as it will erode the cliff faces.
In the ocean, billions of tiny organisms called radiolarians grow skeletons of silicon dioxide as they float near the surface. As these organisms die, their tiny shells fall down to the ocean floor accumulating thick sediments. Over time the skeletons may recrystallize as chert. Sedimentary rocks derived mainly from biologic activity are considered biogenic sedimentary rock. Chert is quite resistant to erosion, so it often supports the tops of ridge lines.
Between the layers of chert are thin layers of shale. The layering could be the result of undersea landslides originating from the mid-ocean ridge or slight changes in the material raining down from the ocean surface based on Earth's orbital cycles. While somewhat similar in composition, shale contains less silca and is quite a bit softer than the chert.
In this chert, not all of the skeletons recrystallized. Tiny skeletons, about the size of sand grains, are still visible and identifiable. The oldest fossils fossils found here are species that lived near the equator some 200 million years ago, where nutrient-rich waters supported many organisms. As the chert layers get younger, they get thinner suggesting that the area was moving out of the productive equatorial area toward the north. The youngest chert layers are about 100 million year old. This 100 million year sequence of chert is one of the longest in the world. It is also thought that these layers of chert formed far out at sea, at least 600 miles because they lack sediments derived from rivers. The little clay that is present probably derived from dust blown off the continents.
The cherts are typically red as a result of oxidized iron contained in them. Though light green or white sections are present in the younger part of the sequence. In the oldest part of the chert layers, near its contact with the older pillow basalts ( The Pillows of Point Bonita) manganese from the hydrothermal activity of the spreading ridge colors the chert black or silvery gray.
These deep sea sediments were shoved up to the surface by the convergence of the Farallon Plate and the North American plate. Blocks of the chert were ripped off the Farallon Plate and smashed up against the North American Plate as part of the Franciscan Melange while the rest of the plate subducted. This process resulted in many faults and folds throughout the area.
This exposure also has examples of the folding. Folding from regional tectonics such as subduction would likely result wide scale folding that affects entire sequences of rock. However, some of the folding in the chert layers affects only a few layers while nearby layers are relatively undeformed. It is thought that these folds are the result of slumping of the sediments soon after they were deposited while they were still on the the slopes of the mid-ocean ridge.
Logging questions:
- The text "GC3BPET Rodeo Lagoon Ribbon Chert" on the first line
- The number of people in your group.
- What are the average thickness of the chert layers?
- Based on the coloration of the chert layers are these from the older, middle, or younger part of the sequence?
- Do you agree with the conclusion of the type of folding at this location? Why or why not?
- If anyone finds fossil radiolarians that would be really cool to hear about and see some pictures. However the rocks are protected so there is no collecting.
The above information was compiled from the following sources:
- http://geology.about.com/od/rocks/ig/sedrockindex/rocpicchert.htm
- http://geology.com/rocks/chert.shtml
- http://pubs.usgs.gov/bul/b2188/b2188ch3.pdf.