Roadside Geology of Southern California

Posted 21 April 2016 by A.G. Sylvester ©

CA 166 lies across the south end of the Coast Ranges, north of the Transverse Ranges, and so should be considered in Roadside Geology of Central California. The route commences at its intersection with CA 1 in Guadalupe, 11 miles west of US 101. It follows the course of the Cuyama River, the boundary between Santa Barbara and Ventura counties, for most of its length.

The Cuyama River is longest river in Santa Barbara County; it heads in Ventura County, flows 100 miles to Santa Maria where it is renamed the Santa Maria River at US 101 and then runs 5 miles to the Pacific Ocean. The Santa Maria has surface water only after exceptional annual rainfall, because of dams and irrigation usage upstream. Most of Cuyama River is intermittent because of aridity of its headwaters. Wells drilled near the river mouth revealed a 230-foot-deep gorge in bedrock, carved during last Pleistocene glacial stage and then backfilled with the post-glacial rise in sea level.

Between Santa Maria and the west end of Cuyama Valley, the river has cut a narrow, winding gorge across the Coast Ranges. The rocks are Franciscan Formation ophiolite, Simmler Formation conglomerate, and Monterey Shale, capped here and there by river terrace deposits of sand and gravel.

 

US 101 to Cuyama Gorge

In the stretch of CA 166 from 0 to 4 miles east of US 101, look for isolated, light-colored, craggy outcrops on both sides of CA 166. They are resistant exposures of the Obispo Tuff, a volcaniclastic deposit thought to have erupted in Miocene time from the line of volcanoes between San Luis Obispo and Morro Bay.

Highway roadcuts expose white Monterey Shale between Bull Run Road and a turnout about 1.5 miles east of Suey Road overlooking Twitchell Reservoir, which was built mainly for Cuyama River flood control. Rarely does the reservoir have a significant volume of water.

Several big roadcuts expose well-layered, white Monterey Shale along the next 6 miles of highway from the Twitchell Reservoir turnout to a point just about where the highway begins to follow the Cuyama River, but about 2 miles before the highway’s junction with Tepusquet Road. Here the Monterey Shale consists of claystone, fine-grained sandstone, and siliceous shale. An especially good exposure is in a deep roadcut at Post Mile 20.

 

Cuyama River Gorge

In the 5 mile stretch east of Tepusquet Road, the Cuyama River has cut a narrow, winding gorge between the Sierra Madre (south) and La Panza (north) ranges of the California Coast Ranges to expose some of ranges’ oldest rocks – the Jurassic Franciscan Formation.

As you drive up the gorge, dark brown and dark gray, blocky outcrops of the Franciscan rocks are in canyon walls and roadcuts starting around Post Mile 21. A mile or so farther up the gorge, the rocks comprise bluish gray greywacke, bluish-green serpentinite, varicolored chert, and altered dark gray basalt and diabase. Franciscan Formation rocks

Blue-green serpentinite of the Jurassic-age Franciscan Formation in Cuyama River Gorge. (35° 02.4N, 120° 11.0W)

are commonly mixed up into a “mélange”, the French word for potpurri. The mixing happens when rocks of the upper part of a subducting oceanic plate are scraped off against the adjacent continental plate to form an accretionary wedge, analogous to the mixing and scraping leftovers off a dinner plate.

 

Simmler Formation

Prominent, well-lithified outcrops of conglomerate, exposed mostly in canyons on the north side of CA 166, are interbedded with terrigenous, pink-gray to red brown, micaceous, well-bedded sandstone and micaceous red-gray shale. One of the more

Inclined, red brown beds of sandstone and conglomerate  of Oligocene-age Simmler Formation. (35° 06.12N, 120°06.8W)

prominent exposures is a southwest-dipping ledge on the north side of the highway at Rock Front Ranch. The conglomerate is interpreted as alluvial fan deposits, whereas the sandstone facies of the Simmler Formation is interpreted as an alluvial plain complex formed by through-flowing, low-sinuosity streams.

The Simmler Formation is Oligocene in age (34-23 million years ago) and both temporally and lithologically similar to the more widespread Sespe Formation in Santa Barbara and Ventura counties. These two formations represent a major interlude of regional uplift and nonmarine deposition in the otherwise uninterrupted history of marine deposition over the Transverse and Peninsular parts of southern California from late Cretaceous through Pliocene time. Some geologists postulate that uplift was caused by a change in subduction style from a steep down-going oceanic slab to a shallow slab. That event is also thought to be responsible for triggering the silicic volcanism that prevailed at that time over so much of Nevada, western Arizona, and southeastern California.

 

Cuyama Valley

CA 166 leaves Cuyama Gorge and enters the broad, upper Cuyama Valley on which the Cuyama River has traced a meandering course and left sets of broad river terraces. Here and there you may see good examples of the meander bends in the river from the highway. Some of the low roadcuts expose river terrace gravel.

Cuyama Valley is a desert because it lies in the winter rain shadow of the California Coast Ranges. It is a long, fault bounded, intermontane valley south of the barren hills of the Caliente Range to the north. The chaparral-covered San Rafael Mountains to the south consist of 35,000 feet of Paleocene and Eocene sedimentary beds of sandstone, conglomerate and shale that have been thrust 6,000 feet northeastward over the south edge of the valley along the South Cuyama fault. The valley floor is covered by fluvial sand and gravel deposited by the Cuyama River and alluvial fans shed from the bounding mountains.

The Caliente Range, an integral part of the California Coast Ranges, consists of 3,000 to 10,000 feet of Miocene marine sedimentary rocks that have been shoved upward and southward over Cuyama Valley along the Morales thrust fault. Thus, Cuyama Valley is caught in a vise between two mountain masses, the San Rafael and Caliente ranges, which are being shoved toward one another. The consequent shortening of the valley block has caused it to be warped into a deep syncline having a structural relief of 10,000 to 15,000 feet. Several oil fields were discovered along the upturned edges of the syncline. The highway passes one of them, the Russell Ranch field near Call Box 166 567, and although most of them are pretty well depleted now, sporadic exploration still continues in the valley.

 

Chalk Mountain

The aptly named Chalk Mountain (elev. 2,402 feet), the little white peak near the highway and its neighboring ridge on the west (PHOTO 6-TR074), lies in the core of a big

View north of Chalk Mountain, Cuyama Valley. (35° 01.0N, 119° 49.6W)

syncline on the west flank of an even larger anticline developed in the Miocene Vaqueros and Monterey formations at the edge of the Caliente Range (Photo 6-TR075). In  the hills

View north of Caliente Mountain anticline. (35° 02N, 119° 49.6W)

farther east and north of Cuyama and New Cuyama, the Simmler Formation is exposed in a highly faulted complex with the Vaqueros Formation. Morales Formation and older alluvium underlie the long slope.

 

Side Trip: San Andreas fault

About 10 miles east of the town of “old” Cuyama, CA 166 intersects CA 33. Continue another 4.5 miles up Grocer Grade to the highway summit at Reyes Station (aka Camp Dix), which lies at the intersection of Cerro Noroeste and Soda Lake roads. The San Andreas fault strikes along both of these roads and through the intersection of both roads with CA 33. Do not expect to see a gapping chasm; instead you’ll see scarps and sags, typical of the San Andreas fault zone along much of its 700 mile-long length.

Numerous low linear ridges along the San Andreas fault may be encountered in just a 3-mile drive southeastward along Cerro Noroeste Road. They represent slivers of resistant rocks between fault strands.

Drive or walk up the Soda Lake Road about 100 feet past a cattle guard to where the road drops down across a recent scarp. Most of its height is probably due to the 1857 Fort Tejon earthquake. Other well-developed sags and sag-ponds may be encountered within a 3-mile stretch of Soda Lake Road.

Soda Lake Road continues northwestward along the Elkhorn Hills and into the Carrizo Plain where classic photos may be taken of stream courses that have been right-laterally displaced along the San Andreas fault. But that is another topic for another guide book.

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