Blog: Whitewater Canyon

CabezonDinosaur02COPY
The mouth of the Whitewater River is seven miles east of the Jurassic fauna at Cabezon.

The Whitewater River is the principal drainage for the southeastern portion of the San Bernardino Mountains. It debouches into San Gorgonio Pass where it has constructed a large, side entry alluvial fan. Strong west winds blow down the pass across the fan, pick up sand and dust, and carry them down the trough into North Palm Springs, sand blasting automobiles making their way up the pass on I-10.

Take exit 114 off I-10 near Cabezon to explore Whitewater Canyon. At the mouth of the canyon an outlet sells decorative building stone obtained from two quarries upstream. The stone is gneiss and schist used for building facings, patios, walls, fireplaces, and other masonry work. Colorful decorative gravel, locally called Palm Springs Gold and obtained from another quarry, is a beautiful mix of white quartz, violet mica schist, pink piemontite, and various hydrothermally altered orange and reddish-brown rocks. These minerals and rocks are derived from metamorphic rocks farther upstream in the heart of the San Bernardino Mountains.

From the mouth of the wash proceed north and upstream along a two-lane road about 1 mile through sparse desert vegetation, until you abruptly encounter an oasis of lush vegetation in the bottom of the wash. The oasis marks the trace of the Coachella Valley–Banning fault, which strikes nearly east across the canyon, placing Cabazon fanglomerate of late Pleistocene age on the downstream side of the fault against gneiss and granite of Proterozoic age on the upstream side. The oasis is located where subsurface water percolating southward through canyon alluvium is dammed by impervious clay fault gouge along the fault. The water table is thus higher on the upstream side of the fault, so that springs flow at the surface and water the oasis.

The Whitewater fault lies within the east canyon wall, almost parallel to the canyon, and juxtaposes old alluvium against Coachella fanglomerate of late Miocene age. You can view a splendid exposure of the fanglomerate at the terminus of the road adjacent to a trout farm about 5 miles from the mouth of the canyon.

S007_WhitewaterCongCOPY
Coachella fanglomerate contains some unique clast types, including slightly metamorphosed granite with large crystals of potassium feldspar. The granite closely resembles that in the Cargo Muchacho Mountains near the Mexican border – a suggested correlation that requires 135 miles of right-slip along the San Andreas fault since deposition of the fanglomerate in Miocene time.            (33° 59.5N, 116° 39.4W)

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Southern California Geology in the News

20 May 2016: “World’s largest solar plant catches fire” – blog by Linda Kratachwill

10 May 2016: “Is the San Andreas fault ‘locked, loaded, and ready to go’?” – blog by Temblor.net

4 May 2016: “San Andreas fault “locked, loaded and ready to go with big earthquake” – originally published by Los Angeles Times

23 April 2016:  “Saving the Salton Sea”  –   originally broadcast by KCET

21 April 2016:   “Imperial seismic swarm lights up the southern tip of the San Jacinto fault in an area of extreme hazard”

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Blog: CA 58 – Bakersfield to Tehachapi

Posted 22 April 2016 by A. G. Sylvester ©

MaricopaBasin16D22
Generalized geologic map of southern San Joaquin Valley and Tehachapi Mountains, after Goodman and Malin (1992) with modifications.        Map prepared by Libby Gans ©

 

Southern San Joaquin Basin

The southern San Joaquin basin is a prolific oil-producing area that has had a complex tectonic and depositional history. It originated as the southern part of a large forearc basin in late Mesozoic time, was affected by both shortening and extensional tectonics in Paleogene time, and underwent a final episode of subsidence and infilling during Neogene time under the influence of strike-slip deformation associated with the North American-Pacific plate boundary. During Neogene time, thick diatomaceous oil-prone source rocks of upper Miocene Monterey Formation were deposited in the southern part of the basin. They underwent subsidence and diagenesis, and generated large volumes of hydrocarbons. Numerous and thick sandstone bodies are interbedded with the diatomaceous strata and form the main reservoirs. Shelfal clastic units surround the basin on the east, south, southwest, and north. Large submarine canyons were cut across the shelves, probably during low sea level periods, and funneled sand basinward, where thick, sand-rich, submarine fans accumulated.

During Pliocene time, shallow-marine and intertidal conditions prevailed throughout the basin, because it was gradually cut off from the Pacific Ocean. During Pleistocene time, thick nonmarine strata were deposited in the basin, dominated by fluvial, lacustrine, alluvial-fan, and fan-delta depositional systems of the Tulare Formation and related units. The basin continues to undergo subsidence in its axial portions, concurrently with uplift along the western, southern, and eastern margins.

At least 40,000 feet of marine and terrestrial sedimentary rocks in the Maricopa sub basin are visible only in drill cuttings from the hundreds of boreholes that have punched the basin in the relentless search for oil and gas. The many pumpjacks along the route attest to the number and success of the drill holes. The drill cores may be studied at the California Well Sample Repository in Bakersfield.

The Maricopa sub basin lies upon metaigneous rocks that represent subsurface equivalents of ensimatic and ultramafic rocks that are exposed along the west side of the Sierra Nevada batholith and probably represent the floor of the batholith. The floor of the shallower Tejon embayment, by contrast, is crystalline granite, diorite or gneissic rock regarded as subsurface projections of exposed meta-tonalite and gneiss in the Tehachapi Mountains.

The oldest sedimentary rocks in the Maricopa sub basin are mid-Eocene shallow marine sandstone, siltstone, and shale beds that are overlain by Oligocene and Miocene volcanic rocks, coarse sandstone, and various conglomerates that interfinger with well-sorted, fine-medium grained marine sandstone and shale. The lower Miocene volcanic rocks have been dated at 22.7 million years and include a thick stacked sequence about 2,000 feet of basalt and dacite lava flows, lapilli and lithic tuff, volcaniclastic units, rhyodacite breccia, and dikes. The volcanic rocks are overlain by a very thick sequence of various shallow and deep water marine facies of sandstone, shale, and Monterey Shale of mid-Miocene age. Pliocene and Pleistocene deposits are represented by coarse alluvial fan and fluvial sediments, which interfinger with shallow marine shelf, brackish-water, and lacustrine deposits in the center of the basin. These basin center deposits indicate a gradual shoaling of the basin. The entire sedimentary succession contains major and minor unconformities, lateral and vertical facies changes, and is cut by various major and minor faults, all of which attest to the complex history of basin subsidence, deposition, and deformation of a huge pile of rocks that lie “under the rug” of the valley floor.

From Bakersfield to General Beale Road (Exit 127), the flatness of CA 58 belies the complex geology beneath Quaternary fluvial gravel and sand. North of the highway are dissected hills consisting of the Kern River Gravel, a non-marine, clastic, loosely consolidated, west-dipping alluvial fan sequence deposited not by the Kern River where it debouches into the valley, but instead by the now abandoned Caliente River (Saleeby et al, 2013). The deposits contain sand, pebble-sized gravel, rounded to sub-rounded cobbles and boulders, consisting of various igneous and metamorphic rocks, all derived from the core and west flank of the Sierra Nevada.

Tehachapi Mountains

Basinal marine strata interfinger with alluvial fan debris shed from the adjacent Tehachapi Mountains. The latest of these fans lies upon the valley floor close to the mountains. Some of them are dissected and are expressed by low hills before steep gradient of the highway begins.

BealvilleMap
Generalized geology map of the Bealville area, after Dibblee and Chesterman (1953) with modifications. Map prepared by Libby Gans ©

The Bealville Fanglomerate is one of the older alluvial fan deposits in the Bakersfield area and provides the earliest evidence of strong uplift of the area east or southeast of the south end of the present San Joaquin Valley. Its age is regarded as late Oligocene and early Miocene based on stratigraphic relations. The formation consists of a light-gray, loosely consolidated mass of unsorted granitic detritus, with semi-rounded boulders of quartz diorite up to three feet in diameter. It nonconformably overlies granitic and meta-tonalitic basement and is more than 7,000 feet thick at Bealville. Most of the roadcuts farther up the grade from Keene (Exit 139) to Tehachapi expose granitic rocks – rarely foliated hornblende-biotite quartz diorite with hornblendite and gabbro in small irregular and linear masses. In the 3-5 miles before Tehachapi, mica schist and marble lenses are in the quartz diorite.

CA 58 crosses the Edison fault, which separates quartz diorite and Bealville Fanglomerate about two miles east of General Beale Road (Exit 127). It also crosses the White Wolf fault, source of the 1952 Arvin-Tehachapi earthquake (M 7.7); the fault juxtaposes diorite against diorite near the Bealville turnoff. The Tehachapi Mountains are not only a wall between the south end of the Great Valley and the Mojave Desert, but they also form a lithologic and structural link from the Sierra Nevada to the Transverse Ranges. They are also an impediment to highway and rail traffic. Railroad engineers solved their problem by building the Tehachapi Loop.

 

Tehachapi Loop

CA 58 and the railroad line climb 28 miles out of San Joaquin Valley over Tehachapi Pass to connect the valley with the Mojave Desert. Look for trains passing through tunnels on the south side of the highway. In just two years between 1874 and 1876, up to 3,000 Chinese laborers cut the route through solid and decomposed granite using picks, shovels, horse drawn carts, and blasting powder.

About halfway upgrade to the pass, the world famous Tehachapi Loop spirals 0.73 miles around one of the granite hills. The track climbs a steady 2 percent grade within the loop and passes over itself, thereby lessening the grade. The loop gains 77 feet in elevation, and thus a train more than 4,000 feet long thus passes over itself going around the loop. The average grade is a steep 2.2 percent. It is such a remarkable engineering feat that in 1998 the Loop was named a National Historic Civil Engineering Landmark and is now California Historical Landmark #508.

 

References cited:

Dibblee, T.W., Jr., and Chesterman, C.W., 1953, Geology of the Breckenridge Mountain quadrangle, California: California Division of Mines and Geology Bulletin, no. 168, 56 p., (incl. geologic map, scale 1:62,500).

Goodman, E.D., and P.E. Malin, 1992. Evolution of the southern San Joaquin basin and mid-Tertiary “transitional” tectonics, central California. Tectonics 11, 478-498.

Saleeby, J., Z. Saleeby, and F. Souza, 2013, From deep to modern time along the western Sierra Nevada foothills of California, San Joaquin to Kern River drainages. Geological Society of America Field Guides 32, 37-62. doi: 10.1130/2013.0032(03)

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Blog: I-5 and CA 99 – Grapevine to Bakersfield

Posted 22 April 2016 by A.G. Sylvester ©

MaricopaBasin16D22
Generalized geologic map of the southern San Joaquin basin and the Tehachapi Mountains, after Goodman and Malin (1992). Prepared by Libby Gans ©

 

Southern San Joaquin Basin

The southern San Joaquin basin is a prolific oil-producing area that has had a complex tectonic and depositional history. It originated as the southern part of a large forearc basin in late Mesozoic time, was affected by both shortening and extensional tectonics in Paleogene time, and underwent a final episode of subsidence and infilling during Neogene time under the influence of strike-slip deformation associated with the North American-Pacific plate boundary. During Neogene time, thick diatomaceous oil-prone source rocks of upper Miocene Monterey Formation were deposited in the southern part of the basin. They underwent subsidence and diagenesis, and generated large volumes of hydrocarbons. Numerous and thick sandstone bodies are interbedded with the diatomaceous strata and form the main reservoirs. Shelfal clastic units surround the basin on the east, south, southwest, and north. Large submarine canyons were cut across the shelves, probably during low sea level periods, and funneled sand basinward, where thick, sand-rich, submarine fans accumulated.

During Pliocene time, shallow-marine and intertidal conditions prevailed throughout the basin, because it was gradually cut off from the Pacific Ocean. During Pleistocene time, thick nonmarine strata were deposited in the basin, dominated by fluvial, lacustrine, alluvial-fan, and fan-delta depositional systems of the Tulare Formation and related units. The basin continues to undergo subsidence in its axial portions, concurrently with uplift along the western, southern, and eastern margins.

At least 40,000 feet of marine and terrestrial sedimentary rocks in the Maricopa sub basin are visible only in drill cuttings from the hundreds of boreholes that have punched the basin in the relentless search for oil and gas. The many pumpjacks along the route attest to the number and success of the drill holes. The drill cores may be studied at the California Well Sample Repository in Bakersfield.

The Maricopa sub basin lies upon metaigneous rocks that represent subsurface equivalents of ensimatic and ultramafic rocks that are exposed along the west side of the Sierra Nevada batholith and probably represent the floor of the batholith. The floor of the shallower Tejon embayment, by contrast, is crystalline granite, diorite or gneissic rock regarded as subsurface projections of exposed meta-tonalite and gneiss in the Tehachapi Mountains.

The oldest sedimentary rocks in the Maricopa sub basin are mid-Eocene shallow marine sandstone, siltstone, and shale beds that are overlain by Oligocene and Miocene volcanic rocks, coarse sandstone, and various conglomerates that interfinger with well-sorted, fine-medium grained marine sandstone and shale. The lower Miocene volcanic rocks have been dated at 22.7 million years and include a thick stacked sequence about 2,000 feet of basalt and dacite lava flows, lapilli and lithic tuff, volcaniclastic units, rhyodacite breccia, and dikes. The volcanic rocks are overlain by a very thick sequence of various shallow and deep water marine facies of sandstone, shale, and Monterey Shale of mid-Miocene age. Pliocene and Pleistocene deposits are represented by coarse alluvial fan and fluvial sediments, which interfinger with shallow marine shelf, brackish-water, and lacustrine deposits in the center of the basin. These basin center deposits indicate a gradual shoaling of the basin. The entire sedimentary succession contains major and minor unconformities, lateral and vertical facies changes, and is cut by various major and minor faults, all of which attest to the complex history of basin subsidence, deposition, and deformation of a huge pile of rocks that lie “under the rug” of the valley floor.

 

Wheeler Ridge

Wheeler Ridge, located west of I-5 where it diverges from CA 99, is a splendid example of an actively growing anticline. The fold is so recent that unconsolidated sand and gravel sediments are involved. It is an asymmetric anticline, steep on the north side, with a thrust fault along its northern margin. Several topographic saddles or wind gaps notch the crest of the fold. They were cut across the growing anticline by a stream that flowed north from the San Emigdio Range south of the fold. Eventually the fold grew eastward, like a mouse under a rug, faster than the stream could erode, and so the stream was forced to turn aside and drain progressively around the east end of the growing fold, leaving the older gaps high and dry.  Refer to Sharp and Glazner (1993) for additional information.

 DSC01089
View south of Wheeler Ridge anticline, which plunges from right to left. Four large aqueduct pipes occupy the large wind gap at the west and highest part of the anticline.

1952 Kern County earthquake

One of California’s largest historic earthquakes (M7.5) happened along the southeast edge of the San Joaquin Valley on July 21, 1952. Considerable damage was done to the town of Tehachapi located on the hanging wall block of the fault. It was the largest earthquake to strike California since the 1906 San Francisco earthquake and fire, and the first in California after researchers installed the first generation of seismographs in southern California. Known as the Kern County, or Arvin-Tehachapi earthquake, it occurred on the White Wolf fault, a previously mapped, but undistinguished fault that lies along the south edge of the San Joaquin Valley against the Tehachapi Mountains. Because little surface rupture occurred, it was the seismographic record that produced the most information about the fault, its earthquake, and aftershocks. It turned out that the displacement was reverse, where Bear Mountain in the Tehachapi Mountains was thrust upward about 5 feet and leftward about 4 feet, relative to the floor of the San Joaquin Valley or, more likely, the San Joaquin Valley subsided about 5 feet relative to the Tehachapi Mountains. Over the last 2 million years or so, the vertical displacement along the White Wolf fault has been about 2 to 3 miles; the horizontal separation has not been determined.

References cited:

Goodman, E.D., and P.E. Malin, 1992. Evolution of the southern San Joaquin basin and mid-Tertiary “transitional” tectonics, central California. Tectonics 11, 478-498.
Sharp, R.P., and A.F. Glazner, 1993. Geology Underfoot in Southern California. Mountain Press Publishing Company, Missoula, Montana, pp. 83-87.

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Blog: Salton Sea Sand Dunes

Posted 21 April 2016 by A.G. Sylvester ©

You may gain an up close and personal view of sand dunes near San Felipe Creek, about a mile down a poorly marked and unmaintained side road east of CA 86, 10 miles south of Salton City and about 4 miles north of the Agricultural Inspection Station at the junction of CA 86 and CA 78. The side road terminates at a small field of barchan dunes but it once continued to a dormant U.S. Navy testing base on the edge of the Salton Sea. The road is paved but do not drive onto the sand or your vehicle will get stuck when you park or turn around! The testing facility itself is off-limits to the public.

DSC00569
“Soft Sand by the Salton Sea” Original oil painting by Robin Gowen©

Barchan dunes are crescent-shaped with a long, gentle back slope and a steep frontal slope that is horn-shaped.  Individual barchans are 25 to 100 feet across and six to 30 feet high. In front of the dune is bare ground with some pebbles and vegetation that will be buried eventually as the dune advances. If you happen to be there when a gentle wind is blowing, you will see sand struggling up the rippled back slope and then cascading down the frontal slope. The horns advance almost imperceptibly. Look also for tracks in the sand made by lizards, stink bugs, and the occasional sidewinder.

The horns of barchan dunes point and migrate in the direction of the prevailing wind. The crescents of these dunes point east, indicating a prevailing west wind. They move eastward about 50 to 80 feet per year to their ultimate demise in the Salton Sea. The mineralogy of the sand indicates that the bulk of the sand comes from the poorly consolidated and easily weathered Cenozoic sedimentary strata in the hills west of CA 86. Lesser amounts come from nearby stream washes.

 S013_BarchanDuneCOPY The Salton barchan dunes are located below the high-water level of old Lake Cahuilla, so the dune field is probably less than 400 years old. (33° 10.9N, 115° 51.1W)

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Blog: CA 1 – Lompoc to Guadalupe

Posted 21 April 2016 by A.G. Sylvester ©

CA 1 proceeds west and then north through Lompoc on the Santa Ynez River flood plain. CA 1 crosses the Santa Ynez River about two miles north of the center of town, then climbs upon the broad, chaparral-covered Burton Mesa and heads west to the main gate of Vandenberg Air Force Base. The mesa consists of the Orcutt Sand, a deposit of tan to rusty brown, friable to locally indurated wind-blown sand interbedded with gravel covering a large area between Lompoc and Santa Maria. Gullies notch the plain and expose outcrops of diatomaceous mudstone of the Pliocene Sisquoc Formation and of siliceous shale and porcelanite of the Monterey Shale.

Northward from the Vandenberg AFB gate (PM 28.6), the highway passes across 4 miles of locally deformed Orcutt Sand, being tilted as much as 12 degrees on the flanks of anticlines. The maximum outcrop thickness of the Orcutt Sand is between 50 and 100 feet throughout the Santa Maria area. It is eroded into badlands at the intersection with Firefighter Road (RM 29.9). CA 1 descends the long Harris Grade into San Antonio Valley and Barka Slough (PM 37.1) to Pliocene Sisquoc Formation at the base of the grade.

The low rolling hills on both sides of the highway between Barka Slough and Guadalupe are parts of an extensive series of stacked longitudinal sand dunes of Late Pleistocene to Recent age. Prevailing northwest winds carried the sand inland from coastal beaches.

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Blog: CA 27 – Topanga Canyon

Posted 21 April 2016 by A.G. Sylvester ©

This route transects the Santa Monica Mountains from south to north between Pacific Coast Highway (PCH) and US 101 in Woodland Hills. The geology is complicated by faults and much of it is shrouded by vegetation. Rocks in the roadcuts within the first half mile of the canyon are strongly fractured and shattered from deformation within the Malibu Coast fault zone.

 

Cretaceous (?) and Paleocene Sandstone and Conglomerate

South-dipping beds of brown sandstone of Cretaceous age and Paleocene conglomerate are exposed in low roadcuts here and there on both sides of the canyon in the first two miles of the canyon. Between PM 1.3 and 2.0, bold stream cuts expose impressive outcrops of upper Cretaceous cobble conglomerate, consisting mostly of granitic, metavolcanic, and quartzitic detritus. Such rocks are widely distributed in southern California and reflect the erosional stripping of the rocks of, and above, the Sierra Nevada and Peninsular Ranges batholith during their tectonic uplift in late Cretaceous and early Cenozoic time.

 

Topanga and Modelo Formations

From PM 2.7 to PM 4.3, CA 27 goes through light gray to brown sandstone of the lower Topanga Formation of early Miocene age. Beds are thick and dips are mostly north. Conejo Volcanics, consisting of olive gray and black dikes and sills of diabase and which are rather featureless in roadcuts, such as that at PM 3.0, have extensively intruded and overlie the Topanga Formation.

At the intersection of CA 27 and Old Topanga Road (Pm 4.3), CA 27 enters north-dipping upper Topanga Formation and continues through it for about three and a half miles (PM 7.8). It was deposited in marine or brackish water and consists of light brown to tan, thick-bedded sandstone.. A rather featureless intrusion of Conejo Volcanics diabase is in roadcut at PM 7.5.

The Topanga Formation is overlain unconformably by north-dipping Monterey Formation strata is exposed from PM 8.0, through all the switchbacks, to the intersection of CA 27 with Mulholland Drive at PM 11.0. Here in the Santa Monica Mountains, the Monterey Formation consists of moderately hard, white-weathering, platy, siliceous shale. The fresh rock is dark brown.

The straight and relatively flat part of CA 27 through the residential part of Woodland Hills lies on gently dipping Modelo Formation claystone and siltstone that overlie the Monterey Formation along the south edge of San Fernando Valley between Mulholland Drive to US 101.

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Blog: San Diego County Road S 2 – Ocotillo to Warner Springs

Posted 21 April 2016 by A.G. Sylvester ©

AKA: The Great Southern Overland Stage Route

SD S2

Geology along County Road S2 between Ocotillo and Warner Springs

 Map prepared 2015 by Libby Gans©

County Road S2 begins in the arid Yuha Desert at Ocotillo in the southwest corner of the Salton Trough and gradually ascends the east edge of the Peninsular Ranges into oak and pine meadowland near Warner Springs. Much of the route faithfully follows the historic southern Overland Trail that was later followed by the equally historic Butterfield Overland Mail stagecoaches (1858–1861). Some of the route skirts the west edge of Anza-Borrego Desert State Park, a designated and protected state wilderness area and one of the largest state parks in North America with an area of 916 square miles, almost the size of the Rhode Island. S2 offers several vistas across the park and access to a few unimproved dirt roads that penetrate it. You would be well advised to have a proper off-road vehicle before attempting any of those roads, and note that all natural and cultural features in the park are fully protected. Excellent guidebooks by Paul Remeika and Lowell Lindsay (1992) and Lowell Lindsay and Diana Lindsay (2006) provide directions to the park and its roads and sites. Distances along S2 are indicated by green mile markers (MM) and increase from north to south.

The south half of S2 follows the trace of the Elsinore fault along the straight southwest edge of the Coyote Mountains from Ocotillo to Agua Caliente Springs and then along the northeast edge of the Tierra Blanca Mountains escarpment. Subparallel fault strands in a 200-foot-wide zone mark the fault trace along the bases of straight, uplifted mountain fronts where crystalline basement and sedimentary rocks are juxtaposed. Aligned vegetation lineaments, springs and seeps, benches, scarps, and aligned saddles mark some of the individual fault traces.

The Coyote Mountains have a core of crystalline basement rocks overlain by early Miocene Alverson Formation volcanic flows, mid-Miocene to Pliocene Imperial Formation, and Pliocene to Pleistocene fanglomerate. The Imperial Formation, deposited on the floor of an early version of the Gulf of California, contains a rich fauna of fossil molluscs having an affinity to Gulf Coast taxa, suggesting the Gulf of California was once connected to the Caribbean Sea. The Pliocene beach sand deposits contain seashore-loving sand dollars (Dendraster sp.), sea biscuits (Clypeaster sp.), and oysters shells (Ostrea sp.) that look very much like what you would collect along any San Diego region beach or backwater bay today. Fossil collecting is not permissible within Anza-Borrego Desert State Park.

 

Volcanic Hills

Enter Anza-Borrego Desert State Park at MM 56 and pass through a water gap eroded through a basaltic lava flow at MM 55. These volcanic rocks, part of the Alverson Formation, form the hummocky terrane west of the highway and are similar in type and age to other dark-colored olivine basalt and lighter-colored andesite units that crop out in several places in the Anza-Borrego region. One of these units is the Jacumba basalt near Table Mountain, about 15 miles to the south and 3,000 feet higher. The elevation difference of these rocks indicates the amount of vertical movement along this section of the Elsinore fault zone. These and similar volcanic rocks extend southward along the fault for 30 miles across the border into Mexico. Their extrusion is thought to have coincided with the initial rifting of Baja California from the Mexican mainland about 23 to 15 million years ago. The Alverson andesite has been dated at about 17 million years.

 

Carrizo Badlands

A short, dirt side road along the highway at MM 51.5 leads to a sweeping view of the Carrizo Badlands and the entire Vallecito–Fish Creek drainage basin. These badlands formed in interbedded lake and alluvial sediments, such as marl, claystone, sandstone, and conglomerate, which were deposited in the southwest part of the Salton Trough in Pliocene and Pleistocene time.

The region was an extensive depositional basin, now surrounded by mountains on all sides, where considerable subsidence and sedimentation occurred in recent geologic time. The 9,000-foot-thick sedimentary section is famous for its rich assortment of mega- and microvertebrate fossils, especially in the Pliocene and Pleistocene Imperial and Palm Spring Formations. Two to four million years ago the Carrizo Badlands were covered with grasslands, lakes, streams, and scattered forests that supported large herds of mastodons, llamas, camels, horses, and tapirs. Since that time, the remains of those animals became fossilized, and the basin has been folded, faulted, and uplifted. Hiking or driving through this huge area is the only way to gain a proper appreciation for its vastness and unparalleled solitude.

 DSC01573CarrizoCOPY
View northeast across Carrizo Badlands. Canis Latrans Creek is in the foreground. (32°50N, 116°10W)

At the crest of Sweeney Pass (MM 51.3) you stand on a mesa covered with a thin veneer of desert pavement on light-colored sedimentary rocks deposited in lake and savanna environments in Pliocene and Pleistocene time. If you were to walk about 10 miles cross-country in a northeast direction, you would go from these richly fossiliferous sediments at your feet down section into late Miocene alluvial fan deposits in Split Mountain Gorge.

North of Sweeney Pass, County Road S2 goes down through bouldery alluvial fan deposits in Sweeney Canyon to Carrizo Creek (MM 49). The creek drains 1,200 square miles of watershed on Jacumba and In-Ko-Pah Mountains and flows to San Sebastian Marsh below sea level near the Salton Sea.

 

{B} Faults at Mountain Palm Springs and Canebrake

About four hundred native California fan palms (Washingtonia filifera) abound in six main palm groves easily reached on a good, half-mile gravel road (turnoff at MM 47) that traverses a cholla forest west of County Road S2. Typically these palm trees flourish in areas of near-surface water, commonly where groundwater is dammed behind a subsurface zone of impermeable rocks, such as gouge along a fault zone. Here the palms are at the juncture of the Elsinore and Tierra Blanca faults.

 PalmOasisCOPY
Native palm oasis at Mountain Palm Springs Camp located at the juncture of the Elsinore and Tierra Blanca faults. The bedrock here is a pretty, white, brecciated, biotite-hornblende granite of the great La Posta pluton. (32°51.7N, 116°13.3W)

Several Elsinore fault strands exhibit evidence of recent earthquake activity along County Road S2 between Sweeney Pass and Canebrake Ranger Station. The highway lies along the northeast edge of the Tierra Blanca Mountains, whose mountain front is a youthful scarp on the west side of County Road S2 opposite the Well of Eight Echoes. Right-slip along this fault strand is at least 4 miles. Prominent alluvial fans extend from the mountain front toward the highway from two narrow canyons. Such canyons, with a narrow stem at the mountain front that widens into a goblet shape farther into the mountains, are called wineglass canyons and are evidence of recent uplift. Small, aligned scarps, about 3 feet high, can be traced across the modern fans between Canebrake (MM 45) and Agua Caliente Springs (MM 40). The scarps look youthful enough to have formed during the 1892 Laguna Salada earthquake. The rough, corrugated surface of the Canebrake fan consists of granitic rock brought down from the Tierra Blanca Mountains.

Vallecito Creek (MM 43) leads to an extensive area of serrated brown hills and winding washes in the Vallecito Badlands east of County Road S2. See the earlier discussion about the Carrizo Badlands.

 

{B} Agua Caliente Springs to Rancho Vallecito

Historically, hot (167° to 185°F) groundwater percolated up from depth at Agua Caliente Springs (MM 38) along the Elsinore fault zone. The hot water not only altered strongly fractured granitic rocks to light-colored, easily weathered clay and claystone, but it also provided warm water for mineral baths and bathing pools. In the mid-twentieth century, an earthquake reorganized the subterranean plumbing, shifting the hot water discharge away from the old rock Indian pools to a nearby site now developed as a state park. Since the earthquake, the old pool has filled with cold spring water.

Northeast of Aqua Caliente Springs, County Road S2 proceeds a couple of miles through a broad area of arroyos, roadcuts, and intervening hillsides, all carved in grayish tan, poorly bedded, pebbly to bouldery conglomerate, and mantled with cactus. These sediments, deposited on the margin of the basin, are assigned to the Pliocene and Pleistocene Canebrake Conglomerate, and are mainly granitic boulders and cobbles derived from the Pinyon-Vallecito highlands to the north and northeast. This conglomerate grades laterally into lake and river deposits of the Palm Spring Formation of late Miocene and Pliocene age.

Rancho Vallecito is on the north edge of the Sawtooth Mountains Wilderness. From there you have a fine view of the impressive escarpment of the Laguna Mountains, 5 miles to the southwest, surmounted by Garnet Mountain (elevation 5,679 feet) to the southwest and Oriflamme Mountain (elevation 4,813 feet). About 5 miles northeast of Rancho Vallecito is massive, dark gray Whale Peak (elevation 5,349 feet) in the Vallecito Mountains.

 

{B} Campbell Grade

Campbell Grade (MM 30) crosses a bedrock spur that connects the Vallecito Mountains north of County Road S2 with the Sawtooth Mountains to the south. Vallecito Creek has cut a gorge across the spur. The main trace of the Elsinore fault cuts through here, too, but the fault is obscured in this area by large rock slides that came off the east side of the Peninsular Ranges. Minor faults are clearly exposed in roadcuts along County Road S2 where it lies in granite and older rocks intruded by the granite. From a viewpoint at the top of the grade, look southeast and see the trace of the 1858–1861 Butterfield Overland Mail stagecoach route.

 

{B} Box Canyon

County Road S2 follows the Elsinore fault from Campbell Grade through Mason Valley and then turns abruptly eastward into Box Canyon at the north end of Mason Valley along the south side of Granite Mountain (elevation 5,633 feet). Miners worked several prospects in pegmatite intrusions in Julian Schist in the Box Canyon area, presumably for tourmaline and related gem minerals. Dikes and layered metasedimentary rocks give a striped appearance to hillsides on both sides of County Road S2 around Granite Mountain. The east side of Granite Mountain is a large Quaternary landslide.

 DSC01580JulianSchistCOPY
Gray and brown Julian Schist and quartzite in roadcut exposures in Box Canyon are intruded by light-colored pegmatite dikes associated with intrusion of the 94-million-year-old La Posta pluton of the Peninsular Ranges Batholith. (33°00.8N, 116°27.0W)

The road in Box Canyon was cut by the Mormon Battalion with hand tools in 1847 and was the first road into southern California. It was widened in 1858 to permit passage of the famed Butterfield Overland Mail stagecoaches. Here you will also see a fine display of vegetation that characterizes this part of the Colorado Desert, including barrel cactus, ocotillo, agave, and the tallest cholla specimens in the Anza-Borrego region.

 

Earthquake Valley

County Road S2, CA 78, and the Pacific Crest Trail intersect at Scissors Crossing (MM 17) below the south face of Grapevine Mountain (elevation 3,955 feet). County Road S2 jogs briefly onto CA 78, and then continues north of CA 78 for 12 miles along the Earthquake Valley fault in Earthquake Valley. Local lore says that Earthquake Valley once held a natural lake and that an earthquake in the early 1900s caused it to drain into San Felipe Creek, thence through Sentenac Canyon to the Salton Sea.

ElsinoreFaultSplays

Geologic map of Elsinore fault strands between Earthquake Valley and Temecula.            Map prepared 2015 by Libby Gans©

The Elsinore fault is more or less continuous from I-8 as far as Lake Elsinore. In Earthquake Valley, however, several major subsidiary fault strands leave the Pinyon and Vallecito Mountains and head northwestward, parallel to the main fault. County Road S2 follows the Earthquake Valley fault to Warner Valley. The Agua Tibia fault strikes from the northeast side of Lake Henshaw to the southwest side of the Palomar Mountain Range and the Palomar astronomical observatory. CA 79 mostly follows the Agua Caliente fault zone from Warner Springs to Aguanga. Numerous subparallel faults slice the 10-mile-wide zone of granitic crystalline rocks and the metasedimentary rocks they intruded between the main Elsinore fault and the Agua Caliente fault zone. All of these faults are considered to be active, although none has generated an earthquake much greater than magnitude 4.5 in historic time.

The Elsinore fault strikes along the west edge of Warner Valley, giving rise to such features as the linear Mesa Grande ridge and Lake Henshaw, a sag pond where water accumulated along the fault zone because drainage was impounded. Warners Ranch (PM 0.7) was a stop on the southern overland route into California. The first Butterfield stagecoach stopped here in 1858 on its 2,600 mile, twenty-four-day trip from Tipton, Missouri, to San Francisco.

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Blog: CA 166: Santa Maria to Cuyama

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

 DSC00045SerpentiniteCOPY 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

SimmlerFmCOPY  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

 DSC00055_ChalkMtnCOPY 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

 DSC00056CalienteMtnCOPY 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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