Geologic Wonders

by Thomas W. Dibblee, Jr. - Geologist
July 2001

Introduction

The terrain in Los Padres National Forest is the most mountainous part of Ventura, Santa Barbara, San Luis Obispo, Kern and Monterey Counties. The nearly 2,000,000 acre Los Padres National Forest is in two sections, the southern and northern, separated by a gap in the mountains in northern San Luis Obispo County. Most of this wild mountainous terrain is covered by chaparral, but includes conifer forests in the higher elevations. Lower elevations include grasslands and oak woodlands.

Mountain Ranges
The major part of the Forest is in the outer Southern Coast Ranges of California: Santa Lucia, La Panza, Sierra Madre - Pine Mountain, San Emigdio and San Rafael Ranges. Southeastward, these ranges converge into the east-west trending coastal transverse ranges such as the Santa Ynez - Topatopa and San Cayetano Ranges. Where these ranges converge, mostly in and near Ventura County, they form the highest ranges of the Los Padres.

Mountain Building
The mountain ranges now within the Forest were squeezed up over the last 6 million years, as what was once a part of the eastern Pacific Plate shoaled against the western margin of the Sierra Nevada. They were emerged and elevated by great compressive forces in the earth's crust that ruptured it along a series of faults (breaks) during several stages or episodes. The latest mountain building episode is still active, as is indicated by numerous earthquakes in this area. As the terrains are elevated to form the mountains, they are intensively eroded by rainfall and wind to form the many valleys and canyons.

Uplift and erosion of the mountain ranges within Los Padres has exposed the rocks of the earth's crust. They are composed of crustal basement rocks, overlain by sedimentary rocks that accumulated to enormous thickness under the eastern Pacific Ocean prior to 6 million years ago.

Crustal basement rocks:
The crustal basement rocks upon which the sedimentary rocks were deposited are of two major types. The most extensive type exposed is a complex of hard crystalline metamorphic and igneous rocks, much like those of the Sierra Nevada. The metamorphic rocks include gneiss, schist, quartzite and marble, which all crystallized at great depth under high temperature from intensely folded sedimentary rocks. These are the oldest rocks in the Forest, now exposed in the northwest Santa Lucia Mountains and mountains of northeast Ventura County, and southern Kern County. They are older than 130 million years and include a Precambrian exposure on Frazier Mountain of "augen gneiss" as old as 1300 million years, one of the oldest rocks of California.

During Jurassic through Cretaceous time, about 100 million years ago, molten magma intruded into these metamorphic rocks. As the magma cooled, it crystallized and engulfed the metamorphic rocks in a massive granitic complex. These granitic rocks were once deeply buried and are now exposed in the northwestern Santa Lucia Mountains, La Panza Mountains, and mountains of northeast Ventura County, including the San Emigdio Mountains.

Less extensive crustal basement rocks include what geologists call the Franciscan rocks, named in 1904 after exposures at and near San Francisco. They are exposed in the coastal mountain areas of the Forest, such as the southwestern Santa Lucia and San Rafael Mountains.

The Franciscan rocks are composed of detrital sedimentary and some volcanic rocks that accumulated to an unknown, but enormous thickness in a deeply subsided trough under the eastern Pacific Ocean in late Jurassic through Cretaceous time (120 - 90 million years ago). The whole series was intensely sheared and slightly metamorphosed when, more than 80 million years ago, it was subducted eastward, thereby thrust under the crystalline basement rocks. In many places, this series is injected by igneous rocks hydrothermally altered to serpentine, which results in shiny slick greenish outcrops.

Sedimentary Rocks
The crustal basement rocks of the Los Padres are overlain by mostly marine sedimentary rocks that range in age from latest - Jurassic - Cretaceous to Miocene (120-24 million years), as determined from marine fossils. Fossils in the older rocks of late Jurassic - Cretaceous and Eocene ages are very scarce, small, and found at only a few places. Fossils in sandstone layers of Miocene age are abundant in some places, and are mostly shells of large oysters, pectens, clams, gastropods (snails) and sand dollars.

The sedimentary series of latest Jurassic, Cretaceous, Paleocene and Eocene ages (100 to 38 million years old) are exposed extensively in most of the mountain ranges of the Forest. This sedimentary series is composed of dark gray crumbly micaceous shale, light brown, hard sandstone layers, and in some areas, brown cobble conglomerate. These sedimentary strata accumulated under deep marine conditions as the sea floor of this part of the Pacific Ocean subsided continuously for some 62 million years. During this time span, as much as 20,000 feet of these detrital sediments accumulated. They were derived from erosion of a metamorphic and granitic terrain, such as the Sierra Nevada.

The enormous thickness of this sedimentary accumulation indicates that the crustal basement rocks were very deeply buried beneath it. By Oligocene time (38 to 24 million years ago), this thick marine sedimentary accumulation and the basement rocks, in areas now known as the Coast Ranges, became severely disrupted by crustal movements. Uplift in the area partly exposed the marine accumulation from the sea, and also lifted the basement rocks to the northeast and southwest from their deep burial.

This emergence created new, mountainous land. Detritus eroded from the mountains was deposited in the valleys as the red conglomerate and sandstone of Oligocene age, which is now exposed in upper Cuyama Gorge, eastern Cuyama-Lockwood Valley area, and as red beds (Sespe Formation) on both sides of what is now the Santa Ynez - Topatopa range.

However, this disruption and emergence was short-lived and followed by a rise in sea-level. By Miocene time, as downward erosion of the mountains continued, the new land with its mountains and valleys was resubmerged under the Pacific Ocean. Deposition of the Miocene marine sediments then occurred, in many places unconformable over the uneven bedrock of the former land surface.

The lowest Miocene sediments are sandstone, with shallow marine fossils in some places and clay shale locally. These sandstones are generally thin, less than 600 feet thick in most places, but become as thick as 3,000 feet at Hurricane Deck in the San Rafael Mountains.

The lower Miocene sandstone and clay shale are overlain by an unusual, distinctive formation of white platy brittle siliceous shale of middle and late Miocene age, called the Monterey Shale. This shale contains abundant fish scales and microfossils such as Foraminifera. It is exposed extensively in the San Rafael and southern Santa Lucia Mountains. In a few places it contains a basaltic lava flow at the base, as well as rhyolitic tuff. The Monterey Shale was deposited under deep marine conditions and is as thick as 5,000 feet; it extends along almost the entire coast of California.

In some places, such as in upper Sespe Canyon, Huasna Canyon area, and west Cuyama Valley, the Monterey Shale is overlain by soft, white, shallow marine sandstone of late Miocene age, with large thick oyster shells and other fossils.

In the eastern Cuyama Valley and Lockwood Valley of the Forest, sediments of Oligocene and Miocene age are composed of detrital red conglomerate, sandstone and clays deposited by streams in a valley or coastal plain environment. These sediments include coarse conglomerate of granite boulders, which were deposited as alluvial fans in Miocene time. In eastern Lockwood Valley, red beds and lake beds of Oligocene age include an erupted volcanic flow of basalt. These terrestrial sediments are unconformable on marine Eocene sediments and crystalline basement rocks.

The Miocene terrestrial sediments of these areas are overlain by more terrestrial sediments and gravelly sediments of Pliocene age (6 - 2 million years old). All together, these terrestrial sediments of Oligocene, Miocene and Pliocene age are estimated to be as thick as 7,000 feet.

Thick sedimentary accumulations 100 to 38 million years old (described previously under Rock Types) were originally deposited horizontally on flat basement rock platforms, and then subsequently on top of each other. As the land that is now coastal California subsided under the eastern Pacific Ocean, the sediments accumulated to great thickness. This subsidence was in large part counteracted by episodes of uplift and emergence from shear and compressive tectonic forces. The earliest episode occurred in Paleocene / early Eocene time, which emerged much of what are now the coastal mountains. It was followed by an episode in Oligocene time (described in Rock Types), which affected about the same area. Subsidence of this area in Miocene time was followed by the latest and most widespread episode of emergence and uplift beginning about 6 million years ago. The uplift has occurred in several stages during Pliocene, and Pleistocene times and through to the present. Thus the present Coast Ranges and western Transverse Ranges evolved and are still evolving.

Faults

Previously described crustal movements occurred mostly along major faults that formed west of what is now the Great Central Valley. The largest and most active is the San Andreas fault, one of the world's largest vertical faults, that cuts southeast up Santiago Canyon, Mill Potrero, Cuddy, Frazier, and Gorman Valleys, and beyond, as a straight rift zone through the San Emigdio Mountains and vicinity. Movement along this great fault is right lateral: horizontal motion, along which the terrain on the southwest side, shifts northwest with respect to the northeast side. This shear movement has prevailed at least since Miocene time, possible since Oligocene, Eocene, or even Cretaceous time.

The San Andreas Fault is thought to be the present boundary between the North American continental plate and the Pacific oceanic plate, along which the Pacific plate is shifting north with respect to the North American plate. This lateral shear movement on the San Andreas fault is designated as dextral shear movement. This motion is combined with compressive movement as each block on opposite sides of the fault is being pushed against the other. This compressive movement caused the uplift of the San Emigdio Mountains.

There are other vertical faults within the Coast Ranges on the Forest, which run parallel to the San Andreas, but Southwest of it; they accommodate similar, but lesser movement. Such faults include the Sur-Nacimiento fault in the northwest Santa Lucia Mountains and the related Rinconada fault in the Garcia and Sierra Madre Mountains. Dextral shear movements prevailed on these faults mostly in Pliocene time, but the faults have been inactive since. Although these faults are hidden by dense chaparral, they can be detected and mapped from the different rock formations they juxtapose and by the fold structure of those rocks.

Associated with these faults as well as the San Andreas fault are compressive thrust faults that bound the mountain ranges uplifted on those faults. The La Panza, South Cuyama, Ozena, San Guillermo, Frazier Mountain, Pine Mountain, Little Pine and San Cuyetano faults are all thrust faults related the aforementioned vertical faults.

In the southern part of the Forest are two major near vertical faults that trend nearly east-west: 1) the Big Pine fault, which intersects the San Andreas fault near Frazier Park and extends west to Big Pine Mountain, and 2) the Santa Ynez fault along the north side of the Santa Ynez - Topatopa transverse range. Movement along these two faults was mostly left lateral, where the block on the north side shifted west with respect to the block on the south side.

The Big Pine and eastern Santa Ynez faults are in the higher elevation terrain where the northwest-trending Coast Ranges and east-trending Transverse Ranges converge. This may be the effect of conflict between dextral shear movement on the northwest-trending San Andreas and its parallel, related faults, and sinistral shear movements on the east-trending Big Pine and Santa Ynez faults. This conflict of shear movements may have converted itself into compressive movements, which greatly elevated the mountainous terrain through compressive folding and slip along the associated thrust faults.

Fold Structures

Compressive folding along and between the major faults described above has been so intense, especially since Miocene time, that the once deeply buried crustal basement complexes have been squeezed up to the surface. The overlying, once flat lying, sedimentary rocks have been complexly tilted at high angles and folded, to generate the mountain ranges. This great fold structure is especially evident in the mountainous terrain where the Coast and Transverse Ranges converge, and in the San Rafael and Santa Lucia Ranges. These mountain ranges are major fold belts, with fold axes that trend generally west-northwest.

Minerals: Mineral deposits of economic value have been mined since the 1800's in Los Padres National Forest. Gold was mined in the Mt. Pinos, Ojai and Monterey Ranger Districts. Mercury was mined in the Santa Barbara and Santa Lucia Districts prior to the 1970's. In the rugged terrain of the northwestern Santa Lucia Mountains near Big Sur, high grade white limestone marble crops out, but limited mining of this material has occurred.

Sedimentary deposits of gypsum can be found within the upper Miocene lacustrine red clays of Apache Canyon in eastern Cuyama Valley. Another small gypsum layer and a formation of phosphatic pellets in or above marine upper Miocene sandstone in upper Sespe Canyon was prospected before 1975 but never mined. Uranium has been reported in the Santa Lucia Ranger District, but not mined.

Prior to 1930, a small amount of calcium borate (colemanite) was mined from Oligocene lake bed sediments below a basalt flow in northeastern Lockwood Valley. A moderate amount of slab rock in Miocene Monterey Shale has been quarried from Colson Canyon, San Rafael Mountains, northeast of Santa Maria.

Petroleum: Most of the forest sits on sedimentary rocks that in a few places contain petroleum. Minor amounts of oil and gas are still pumped from three oil fields that extend into Los Padres National Forest. One well is in the Sierra Madre Mountains at the south edge of the south Cuyama oil field, which is found within the underlying lower Miocene sand. The Sespe oil fields in the Ojai Ranger District have also been pumped extensively.