Corriganville Park - The Chatsworth Formation
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Enter Corriganville Park and park at the eastern end of the parking lot (N 34° 15.800 W 118° 39.340). It is a short, not too difficult, and beautiful hike to the GZ.
The Chatsworth Formation
The Chatsworth Formation is the name given to the massive sandstone rock outcropping, at the eastern end of Simi Valley, and along the uppermost portion of the Simi Hills. The Chatsworth Formation was the backdrop for many hundreds of western movies and was the home of the cowboys and Indians and the late-night T.V. movies. The formation is Upper Cretaceous in age, so they were deposited more than 65.8 million years ago. The northwestern extent marks the end of the age of the dinosaurs. When the Chatsworth Formation was deposited, dinosaurs ruled the Earth! However, while some people have imagined that they have seen dinosaur tracks or have "heard" of some, remember that the sand was deposited in the deep ocean. Few fossils are present because not much survived the turbidity currents which deposited these turbidites.
The rock was derived from a volcanic and metasedimentary terrain. The sand was deposited originally on the continental shelf off of Central America, or perhaps as far north as off of the Peninsular Ranges of Baja or California. Massive turbidity currents (These are under water gravity slides), often times tens of miles in length and a half mile of more in width, dumped into marine canyons, resulting in the deposition of the sand in the abyssal of the ocean at a depth of 4,000 to 5,000 feet. The massive flows are called turbidity currents and the resulting deposits are called turbidites. If you look at the Chatsworth Formation from either the north or the south, so that a cross section can be observed, you can see that some of the deposits, or strata, are a few feet in thickness, while others are 20 or 30 feet thick. The thickness of each stratum reflects the thickness of each submarine landslide where each came to rest.
In-between these catastrophic events, there were quiet periods without turbidity currents. During these periods, silt and clay particles rained down from the surface of the ocean. These "fines" were carried into the ocean by rivers and streams during periods of heavy rains and resulted in relatively thin strata of siltstone. Siltstone is more subject to erosion than are the sandstones. The erodibility of the siltstone has helped accentuate the space between the individual turbidite deposits. Interbeds of pebble-to cobble-conglomerates also, but more rarely, occur.
The Chatsworth Formation is dominated by a light gray, fine to medium grained sandstone with abundance of mica and kaolinite (a clay). When exposed to the air, the exposed rocks turn golden as a result of oxidation of iron rich minerals in the sandstone. This process takes time. You will note that the bedrock cuts in the Chatsworth Formation along the 118 Freeway in the Santa Susana Pass area are still gray in color. The total thickness of the formation in the Simi Valley/Chatsworth area, as measured by Orrin Sage, is 6,080 feet.
The formation is part of the North Pacific Plate, which is moving to the northwest at a current rate of about 2.5 inches per year. (That is about the rate that your fingernails grow.) As the plate boundary, the San Andreas Fault turns to the west in the area just north of Palm Springs. Since plate motion is continuous, the result is a shortening of the Earth's crust and the formation of mountains, specifically the Transverse Ranges, of which the Simi Hills and the Santa Susana Mountains are a part. Consequently, the Chatsworth Formation has been raised above sea level and has been tilted to the northwest and west in the Simi Valley area. South of the Susana Knolls, the beds dip 20-30 degrees to the northwest; at Santa Susana Pass they dip 17 degrees to the northwest; and north of the pass, they dip 20-40 degrees to the northwest, west and southwest.
The formation is bounded on the north and west by the Santa Susana Formation, which is also made up primarily of turbidites. However, before the Santa Susana Formation was formed, the Chatsworth Formation was above sea level for a long enough periods for the youngest material to be eroded away. The contact between the Chatsworth and the Santa Susana formations exhibits such features as stream valleys.
Few fossils occur in the Chatsworth Formation in the Simi Valley area, because anything that was deposited with the sands on the continental shelf was ground up during its long and turbulent journey into the deep ocean. Sharks are surface feeders, which lose their teeth when feeding. Consequently, an occasional sharks tooth is found, because it has drifted down from the surface of the ocean after the sands had collapsed into the depths.
The Chatsworth Formation usually abuts the younger Santa Susana Formation north of the freeway. At Kuehner Drive the cobble deposits due north of the freeway interchange are the "Simi Conglomerates" of the Santa Susana Formation. The best places to see the Chatsworth Formation close up in the Simi Valley area are from the Rocky Peak, Hummingbird and Stagecoach trails and in Corriganville Park.
If you're part of a group, each member must provide their own unique answers to these questions.
Please send me email (not a message) with :
1) The text "Corriganville Park - The Chatsworth Formation" on the first line.
2) The number of people in your group.
3) Take a look at the rocks, up and down the trail, near the GZ and in the distance to the east. Describe their color. Are the colors the same or different? Explain why you believe they are the same or different.
4) Why are there so few fossils in the area?
5) What are turbidites?
6) Optional: Post a picture of yourself, or your group, at the GZ.
This Earthcache was placed in support of the Simi Valley Wildlife Corridor CITO Event.
Many thanks to Mike Kuhn, for his sage advice, encyclopedic knowledge, and extreme patience. Preparing this cache has been a treasured learning experience.
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Coordinates are in the WGS84 datum