Stop 1:  Santa Monica Slate at Sepulveda Pass

We will start our virtual tour of the Santa Monica Mountains at the most visited location -- Sepulveda Pass, where the I-405 freeway crosses between Westwood and San Fernando Valley.  In the long canyon south of the crest, the underlying mud-colored rock is named the Santa Monica Slate, visible at a few exposed cliffs in the canyon. The rock was derived from mud deposited in an offshore marine basin, which was later mildly metamorphosed by burial, heating and deformation.

The Santa Monica Slate is the oldest rock unit in the Santa Monica Mountains, formed during the Jurassic Period, about 220 to 250 million years ago, during the age of dinosaurs.  In contrast, the oldest rock underlying most of North America is 5 to 10 times older.  For example, in the Grand Canyon, about 200 miles east, the underlying rock unit is the Vishnu Schist, dated radiometrically at about 1.7 billion years; and the overlying horizontal sedimentary layers are 100 to 400 million years older than the Santa Monica Slate.

Concurrently during the Jurassic period:

• To the east, the current Colorado Plateau of Arizona and Utah was elevated above sea level and covered by extensive sand dunes similar to the Sahara Desert.  These massive sand dune deposits are now exposed in Zion Canyon, Monument Valley and Arches National Park.
• To the north, much of northern California was formed by accretion to North America of micro-continents and volcanic island chains, as oceanic crust was subducted back into the Earth’s mantle.  Numerous serpentine areas of northern California represent fragments of oceanic crust (with chemistry high in iron, low in potassium) that were incorporated into North America by these micro-continent collisions.

Although serpentine is rare in southern California, the formation of the Santa Monica Slate and its accretion to North America was similar to the formation of northern California in Jurassic time.

The original continental margin of western North America was located in central Nevada and was formed about 800 million years ago, when a supercontinent named Rodinia became fragmented and the current continents of Australia and Antarctica were separated from North America by a slowly widening ocean basin.  For nearly 400 million years, during much of the Paleozoic Era, the western continental margin subsided slowly and quietly, like the Atlantic margin now, and accumulated a wedge of carbonate and clastic sediments, similar in age and origin to layers exposed in the Grand Canyon.

After accretion of micro-continents to North America during Jurassic time, in Cretaceous time the plate tectonic geometry switched to that of western South America, where the Andes volcanic chain is generated by subduction, or descent, of oceanic crust beneath the continent – to be discussed at Stop 2.

Stop 2: Granite at Griffith Park

You might be surprised to learn that granite rock is present in the Santa Monica Mountains, at the east end in Griffith Park, because the granite exposures are much less prominent than in Yosemite Valley and throughout the Sierra Nevada.  Granitic rock is widespread in California – from the Sierra Nevada to the Peninsular Ranges of San Diego County and into Baja California.  The age and origin of granite outcrops between Griffith Park and Encino Reservoir are similar to granitic rock elsewhere in California.

During Cretaceous time, about 120 to 80 million years ago – California resembled the Andes Mountains of South America or the Cascade Range of Oregon and Washington, with a line of volcanoes located about 100 miles inland from the continental margin. The granitic rock of California formed as the solidified roots of these volcanoes.  Below active volcanoes, molten rock or magma that was not erupted to the surface cooled and crystallized slowly, which allowed large crystals to form of feldspar, quartz and mica.

Although the present-day volcanoes of the Andes and Cascade Range are spaced about 50 miles apart, during millions of years in Cretaceous time, the volcanic vents moved about, so that eventually, granitic rock continuously covered hundreds of square miles.

The igneous activity that produced Cretaceous volcanoes and granitoid rock resulted from subduction, or descent, of an oceanic plate under North America.  The down-going oceanic plate carried lots of water entrained in clay minerals, which lowered the melting point of the oceanic crust upon reaching depths of about 60 miles, thereby producing the line of inland volcanoes parallel to the subduction zone at the continental margin.

Sandstone sediments of late Cretaceous age are present in the Simi Hills and foothills of the Santa Ana Mountains of Orange County; but because Cretaceous sediments are not prevalent in the Santa Monica Mountains, we’ll continue on to the unit -- the reddish, river-deposited conglomerate and sandstone of the Sespe Formation.

Stop 3: Sespe Formation

The Sespe Formation adds color to the Santa Monica Mountains, as cliff-forming layers of red, pink and tan sandstone and conglomerate, which is sandstone containing abundant rounded pebbles and cobbles. The unit’s geologic age is mostly Oligocene – from 40 to 20 million years ago, about 30 million years after dinosaurs disappeared and mammals became dominant.

Judging from its sedimentary character, the Sespe sandstone was deposited as sand bars in a braided flood channel of a powerful river.  Within the sandstone are stringers and lenses of pebbles and cobbles of well-rounded hard and durable rock, that indicate transport from long distances.  The red color indicates deposition in an oxidizing environment, presumably in a desert area without much vegetation, similar to the Colorado River delta where the river enters the Gulf of California.

Some rounded cobbles have been correlated to a volcanic area in Sonora, Mexico, or to outcrops of well-cemented quartz sandstone in Nevada.  River transport of these cobbles to the Western Transverse Ranges would have been impossible under the current geography.  Three map sketches below shows how source areas and depositional areas became separated – first by the Miocene rotation, followed by displacement on the San Andreas Fault and its predecessors.

Before and during Sespe time, volcanic activity and granite emplacement generated by oceanic plate subduction had stopped in California, but volcanism moved eastward to the Rocky Mountains, presumably because the downward angle of the subducted oceanic plate changed from steep to shallow.  Then subduction ceased entirely as a portion of the spreading ridge in the Pacific Ocean collided with the subduction zone, and portions of the continent were transferred to the Pacific Plate which was moving northwest in relation to North America.  The Miocene rotation of the Western Transverse Ranges was a result of the changed tectonic plate interaction from convergent to translational.

2008 SEPM Book 106

For close-up inspection, the Sespe Formation crops out in Red Rock Canyon Park, accessed from Old Topanga Canyon Road via Red Rock Road.

Sespe sandstone at Red Rock Canyon Park

Stop 4:  Conejo Volcanics on Kanan Road

As the Miocene rotation commenced, around 18 million years ago, the continental crust fractured and volcanic lava of variable composition was extruded on the surface.  Volcanic rocks thicken toward the western end of the range and are named the Conejo Volcanics.  The photos below show 3 road cuts on Kanan Road of basalt (dark color, high in iron); rhyolite (light color, high in silica); and andesite, most abundant, with intermediate color and composition.

Also along Kanan Road can be seen Andesite Breccia – accumulations of angular boulders deposited by debris flows shed from steep-sided volcanic cones, high above sea level.

Beyond the east end of the range, the deep Los Angeles sedimentary basin was formed near the axis of the rotation.  The basin contains about 3000 feet of volcanic rock overlain by about 30,000 feet of marine sandstone and shale deposited during and after the rotation.  The photo below shows dark volcanic rock in Bronson Canyon, at the southwest corner of Griffith Park.

Volcanic rocks of this age are also exposed at road cuts along Mulholland Highway east of Las Virgenes Road, and at rock pools of Malibu Creek State Park, downstream from Lake Malibu.

Stop 5: San Onofre Breccia at Lechuza Point

During the rotation of the continental fragment away from the Orange County coast, the underlying, subducted metamorphic rock – the Catalina Schist – was unroofed, elevated and eroded from areas now occupied by the Pacific Ocean.  Currently the Catalina Schist is exposed on the Channel Islands and a small outcrop on Palos Verdes Peninsula.  It contains the metamorphic mineral glaucophane, that indicates formation at relatively low temperature and high pressure in a subduction zone. 

When a large area of Catalina Schist was unroofed and elevated during the rotation, coarse debris was eroded and deposited on the current Orange County coast to form the San Onofre Breccia, where “breccia” refers to a conglomerate with angular rock fragments. Smaller outcrops of the breccia are found on the Channel Island and at Lechuza Point on the Malibu coast, so are included in this tour.

San Onofre Breccia at Dana Point in Orange County

San Onofre Breccia at Lechuza Point on Malibu coast

Stop 6: Topanga Formation near Calabasas Peak

The Topanga Formation is composed to tan-colored sandstone and dark shale deposited in deltaic and shallow marine environments, plus deep marine basins located not far from the coast.  The shallow marine sediments have a variety of fossil shells of clams, oysters and snails.  The deep marine sediments have tabular sandstone layers deposited by underwater “avalanches” of near-shore sand that flowed down submarine canyons to the basin floor as dense slurries of sand and mud, perhaps triggered by earthquakes.

Stop 7: Monterey Formation on Mulholland highway

The Monterey Formation, aka Monterey Shale, is composed mostly of diatomite – trillions of microscopic diatom shells that once housed diatoms, a type of microalgae and major component of plankton.  The fossil shells are made of amorphous silica, rather than calcium carbonate, so did not dissolve when deposited in deep ocean water.

During Monterey time, about 14 to 18 million years ago ago., the continental margin of California was greatly submerged.  Previous sites of deltaic and shallow marine deposition became located far offshore in deep water, not reached by sand and silt derived from the continent.  Therefore the primary sediment was diatom shells that resisted dissolution in deep water.

Near Lompoc, diatomaceous earth is quarried for use as filters, mild abrasives, absorbents, cat litter, potting soil and other purposes.  In the Santa Monica Mountains, diatomite has been recrystallized so that it is hard and impermeable.  At the time of deposition, dead diatoms contained abundant lipids that with burial became converted to petroleum.  When newly excavated, Monterey rock is dark from oil stain, but in outcrops the rock is light colored because remnant oil has been oxidized.

In places along Mulholland Highway, the Monterey is tightly folded as a result of soft-sediment deformation, due to minor slumping of sedimentary layers on the continental slope before lithification.