Posted 19 April 2016 by A.G. Sylvester ©
CA 118 from Moorpark through the Simi Valley passes through the Sespe Formation (late Oligocene, early Miocene) exposed in several canyons on the south flank of Big Mountain north of the highway, especially in the vicinity of Madera Canyon Road (exit 22).
At the east end of Simi Valley and the west end of San Fernando Valley, between Kuhner Drive (exit 30) and De Soto Avenue (exit 35), are splendid bold outcrops and road cuts of hard, light brown sandstone. These thick-bedded sedimentary rocks are assigned to the Chatsworth Formation of Upper Cretaceous age (about 70 million years). Total thickness of the formation, whose base is not exposed, is about 6,000 feet, comprising thousands of individual turbidite flows. It is broadly folded into a gently west-plunging synform. Hill slope exposures are tan because of surface weathering of iron-bearing mica and clay minerals over time, whereas road cuts are gray because they have been exposed to weathering only a relatively short period of time – too short for oxidation to occur. You may study these rocks more closely along old Santa Susanna Pass Road by exiting south on Kuhner Drive (exit 30), proceeding eastward, and then returning to CA 118 via Topanga Canyon Boulevard (exit 34, CA 27). Good views of the formation may also be had at Stoney Point Park near the end of Topanga Canyon Boulevard.
|Each sandstone bed in the Chatsworth Formation formed as a massive, deepwater marine flow or turbidite deposit, which flowed tens of miles down submarine canyons and was deposited on the ocean floor at depths of 4,000 to 5,000 feet. Fossils are sparse in the formation, because they are usually battered and ground up during turbulent transport over such distances. The line across the center of the image is CA 118. (34° 16.5N, 118° 39.0W)|
The upscale community of Porter Ranch lies north of CA 118 between De Soto and Zelzah Avenues. A utility company stored natural gas in a subsurface reservoir of a depleted oilfield near the community. One of the aging wells into the reservoir developed a massive leak of methane and ethane in 2015 that wasn’t controlled and capped until early 2016. Residents were obliged to leave their homes for several months during the remedial work. An estimated 100,000 tons of gas was released, making it the largest such leakage in US history in terms of environmental impact.