THE GEOLOGIC HISTORY
OF
ORANGE COUNTY
(modified from "Orange County Geology - Teachers Guide", by Carol
J. Stadum, Chapman College, for the Orange County Department of Education)
| The Triassic-Jurassic Bedford Canyon Formation (Jbc) |
The backbone of Orange County is the Santa Ana Mountain Range which extends
from the Puente Hills near Prado Dam southeast beyond the county limits.
The highest, northern-most peaks in the range are composed of Triassic-Jurassic
Period metasedimentary rocks of the Bedford Canyon Formation. This formation
contains the oldest exposed rocks in Orange County which were formed during
the earliest part of the great "Age of Reptiles", about 225 million years
ago. These rocks include argillite, quartzite, slate, and small exposures
of shale and limestone which contain poorly preserved mollusk fossils.
Underlying the Bedford Canyon Formation and exposed to the southeast
of Santiago Peak, is the granitic core of the mountains. The large, round,
white, granodiorite boulders are easily seen from the Lower San Juan Creek
Campground, over the summit of the Ortega Highway, and along the eastern
flank of the mountains above Lake Elsinore.
The Bedford Canyon Formation was subjected to long periods of erosion,
then it was submerged and younger sediments were deposited upon it. The
formation was exposed again and this old erosional surface may be seen
today where it is in contact with the Santiago Peak Volcanics of the Jurassic
Period.
The silver, lead, tin, and zinc mines in the Santa Ana Mountains are
located in the Bedford Canyon Formation, and limited quantities of ore
have been recovered since 1870. Mining in the Santa Ana Mountains has
had little success because the formation has been so greatly fractured
and faulted that the ore veins are offset and impossible to trace. A promising
vein will appear for a few hundred feet then disappear. The boom days
of the silver mines in the late 1800's and the search for tin by the Borden
Company (tin to make cans for their milk) at the turn of the century provide
excellent study for county historians. Limestone has been quarried in
some parts of the mountains, but the mines are presently inactive. Some
deposits of gypsum have also been mined.
| The Jurassic Santiago Peak Volcanics (Jsp) |
Lavas flowed across the Bedford Canyon Formation during the Jurassic Period
plucking pieces of the metasedimentary rocks from the erosional surface
as they passed. The lava cooled quickly and chunks of Bedford Canyon rocks
can be seen included in the lava at the contact between the two formations.
These angular chunks of argillite and quartzite in the lava look like raisins
in raisin bread.
The volcanic vent is gone and much of the volcanics have eroded from
above the Bedford Canyon Formation. The latter formation is about 20,000
feet thick whereas the overlying Santiago Peak Volcanics are about 2,300
feet thick. The volcanics exposed in Silverado Canyon consist of hornblende
andesite.
| The early Cretaceous Trabuco Formation (Kt) |
The county landscape during the Mesozoic Era appeared as vast stretches
of an old erosional surface which was created by a fluctuating marine and
continental environment. Then the land was partly covered by extruded volcanics.
Unconformably overlying the volcanics is the easy-to-identify Trabuco Formation.
This formation consists of rounded cobbles and boulders which accumulate
in a semi-arid environment. The red binding clays in the Trabuco Formation
conglomerate are indicative of continental deposits and the size and shape
of the cobbles suggest that the formation may have consisted of ancient
coalesced alluvial fans.
| The Middle Cretaceous Baker Canyon Conglomerate Member of the
Ladd Formation (klb) |
The Trabuco Formation is the earliest Cretaceous Period formation to be
found in Orange County. It is exciting to study the rocks of this period
because we see, as we enter this period, continental deposits and a semi-arid
land, which then begins to subside as shallow seas slowly cover the alluvial
fans with marine sandstone, beach cobbles, and sea shells. This Middle Cretaceous
marine formation is called the Baker Canyon Conglomerate Member of the Ladd
Formation. Shallow water clams and snails may be observed as fossils in
this fairly resistant sandstone.
| The Middle Cretaceous Holz Shale Member of the Ladd Formation
(Klh) |
The seas continued to occupy a subsiding basin. The sediment deposited in
the deep-seas was composed of fine, clay-sized particles. It drifted far
before sinking to the sea bottom to be mixed with a variety of invertebrate
fossils and bits of wood which have become carbonized (charcoal-like). The
deep-sea shale is the upper member of the Ladd Formation and is called the
Holz Shale. The most unusual fossil found in this shale is the ammonite,
an extinct octopus-like mollusk which had a coiled, wavy, chambered shell.
| The Late Cretaceous Williams Formation (Pleasants Silty Sandstone
(Kwp) and Schulz Ranch Sandstone Members (Kws)) |
Land emergence began near the end of the Mesozoic Era, and the last of the
"Age of Reptiles" saw the county covered by a shallow, warm sea. The Williams
Formation of the Late Cretaceous is divided into two members, the Pleasants
Silty Sandstone and the Schulz Ranch Sandstone. This latter member contains
wavy bands of sandstone and beach (lag) cobbles indicating that it may have
been an ancient shoreline.
| The Tertiary (Paleocene) Silverado Formation (Tsi) |
The Silverado Formation which overlies the Williams Formation contains coal
seams and commercial clay deposits. It marks the beginning of the Cenozoic
Era, called the "Age of Mammals", 70 million years ago. The county was then
covered with swampy sea-marshes and lagoons. The coal deposits are in thin
seams and are low quality lignite. They were mined during the turn of the
century and the town of Carbondale near the mouth of Silverado Canyon was
established as a coal mining town. The town has since disappeared. Mollusk
fossils found in the Silverado formation indicate that it was deposited
during the Paleocene Epoch of the Tertiary Period. Pisoltitc (pea-shaped)
aluminum-rich clays are sporadically found at the base of this formation.
| The Tertiary (Eocene) Santiago Formation (Tsa) |
The Santiago Formation is Eocene in age and consists of yellow-marine sandstone,
siltstone, and cobble conglomerate. The formation has a gradational contact
with the underlying Silverado Formation. The yellow sandstone has weathered
into caves and irregular cliffs which provided a "hide-out" for banditos
during the last century. A good place to see the sandstone caves is near
the junction of Santiago Canyon with Fremont Canyon behind Irvine Park.
| The Tertiary (Late Eocene to Early Miocene) Vaqueros/Sespe Undifferentiated
(Tv/Ts) |
Locally it is difficult to designate rocks as being Oligocene in age or
to place them anywhere in the same time scale between the Late Eocene and
the Early Miocene. Elsewhere in California, Oligocene rocks have been differentiated
but generally in the Orange County area the Vaqueros-Sespe Formations are
interbedded and almost impossible to separate. Where a differentiation can
be made, the red-colored Sespe Formation is a continental deposit and the
buff-colored Vaqueros Formation is marine and contains marine invertebrate
fossils. The formations represent a period of transition from a nonmarine
to a marine depositional environment. The sediments may have accumulated
along the shore of a subsiding or fluctuating sea basin, or deposition occurred
in desert bolsas (bays) alternating with shallow sea incursions. These undifferentiated
sediments are spectacular and can be distinguished as red, maroon, buff,
and gray, highly-eroded sandstones. They are poorly consolidated and present
problems to land developers. "The Sinks" area near Bolero Lookout in the
Santa Ana Mountains is an example of the brightly-colored, highly-eroded
formations. More accessible sites are exposed along the west flank of Santiago
Creek near Silverado Canyon and in the Santa Ana Canyon near Gypsum Canyon.
| The Miocene Epoch "Smorgashbord" |
The Miocene Epoch represents a "smorgasbord" of rock types, minerals, fossils,
and structural activity. Miocene formations contribute a great thickness
of marine sediments. Faulting was widespread during the Late Miocene, and
these "fossil" faults are commonly observed in offset beds. Although researchers
are not completely in agreement about the sequence of geologic events in
the Miocene, the following interpretation is probably the most acceptable.
| The Early-Middle Miocene Topanga Formation (Tt) |
The Topanga Formation is present throughout the Los Angeles Basin (of which
Orange County is a part) and contains abundant marine fossils ranging from
sharks teeth to sea shells and microfossils. It was deposited during the
Early-Middle Miocene in a shallow, warm sea. Near Lake Forest, a reef of
pectens and other fossil mollusks is exposed in a dry wash. It was through
this widespread formation that volcanics pushed their way. The El Modeno
Volcanics, the first row of hills east of Orange, represent a series of
lava flows and ash flows that accumulated in a shallow sea. In the San Joaquin
Hills, Miocene volcanics intruded finger-like projections (diabase) through
the overlying marine sediments. Mercury deposits (cinnabarite) in the Red
Hill area of Tustin are apparently from this volcanic activity that occurred
throughout the Los Angeles Basin fifteen million years ago.
| The Early-Middle Miocene San Onofre Breccia (Tso) |
Concurrent with or immediately after the formation of the Topanga Sandstone,
great landslides fell down the slopes of a large landmass west of our present
coastline. Catalina Island is a remnant of that landmass and crystalline
Catalina-type rocks were eroded to form the San Onofre Breccia. The breccia
formation has been uplifted by faulting and forms the headland of Dana Point.
An estimate is that the speed of the landslides must have at times exceeded
100 miles an hour to create such a large boulder breccia. The rocks and
minerals include asbestos, serpentine, actinolite, fuchsite, epidote, chlorite,
glaucophane, pyrite, magnetite, and quartzite.
| The Middle Miocene Monterey Shale (Tm) |
In the Middle Miocene Epoch a great sea inundation, associated with the
development of deep basins, resulted in massive accumulations of diatomite,
an almost pure deposit of one-celled, glassy, plant shells which are commercially
mined for use as swimming pool filters. The white (diatomaceous) cliffs
of Newport Back Bay are formed by the Monterey Shale Formation and contain
fish scales and fish bones as well as microfossils.
| The Late Miocene Puente Formation (La Vida (Tplv), Soquel (Tps),
Yorba (Tpy), Sycamore Canyon (Tpsc) Members) |
The area again changed as the deep basins filled and the sea began to retreat.
The Puente Formation was deposited in the Late Miocene and is composed of
four members. From oldest to youngest the members are as follows: The La
Vida Member consists of laminated diatomaceous siltstone with thin interbedded
sandstone and coincides in depositional time with the top of the Monterey
Formation; the Soquel member is sandstone with some interbedded siltstone
and local conglomerate beds; the Yorba Member has thin-bedded siltstone
and local beds of sandstone and conglomerate; and the Sycamore Canyon Member
consists of inter-bedded conglomerate, sandstone, and siltstone. This may
be confusing to the layman; for the purposes of this Guide the entire formation
is composed of Late Miocene marine sediments containing some fossils and
is found in the northern and northeastern part of Orange County. Sandstone
provides a good reservoir rock for oil which has been trapped by faulting
and folding. Sandstone of the Puente Formation composes most of the Puente
Hills. The Burruel Hills are also cloaked by the Puente Formation which
has a maximum thickness of 5,400 feet.
| The Late Miocene to Early Pliocene Capistrano Formation (Tcs,
Tct, Tcs) |
The marine Capistrano Formation is found in the southern part of the county.
It ranges from Late Miocene to Early Pliocene in age and consists of poorly
consolidated, fossiliferous, sandy-siltstone and mudstone. Sediment failures
have caused extensive landsliding in San Juan Capistrano and San Clemente
areas. Turbidite deposits may be seen in the sea cliffs between Dana Point
and San Onofre. Turbidites are deposits of marine sediment formed by undersea
mud slides or turbidity currents that dump sediment into graded layers:
the larger-sized gravel falls first, followed upward by progressively finer
sediment grains. The base of the turbidite is a sharp accumulation of the
coarse gravels. The top of a turbidite deposit is often difficult to define
because the fine-grained, last to settle sediments, blend with the natural
"grain-size" of marine sediments.
| The Pliocene Niguel Formation (Tn) |
The Niguel Formation is a shallow, marine deposit that overlies the Capistrano
Formation in the Mission Viejo area. It is Pliocene in age and has a maximum
thickness of 350 feet. This gray, Silty sandstone contains abundant fossil
sea shells and is unconsolidated.
| The Pliocene Fernando Formation (Repetto Sandstone (Tfl), Pico
Sandstone (tfu) Members) |
The 5,000-feet thick Fernando Formation is composed of two members which
have been considered by many geologists to be separate formations. The lower
unit is the Repetto Sandstone and the upper unit is the Pico Sandstone.
You may observe folded Repetto Sandstone along the east side of the Newport
Freeway-cut through the toe of Burruel hills near the town of Olive. These
marine sediments were also laid down during the Pliocene Epoch.
| The Late Pliocene Unnamed Formation |
A Late Pliocene unnamed sandstone formation deposited in the Newport Beach
area contains a wealth of vertebrate and invertebrate fossils but is on
private property and inaccessible. This formation has produced fossils of
marine mammals, sea birds and a variety of sea shells.
| Uplift and River Down-Cutting in the Pliocene |
The seas continued to withdraw as the land was uplifted. The Santa Ana River,
which drains the San Bernadino Mountains, flowed across the land's surface
in its present position before the Santa Ana Mountains were elevated. As
the mountains rose during the Pliocene, the persistent river continued cutting
in its channel, eroding , steep narrows known as the Santa Ana Canyon. A
stream or river which continues in its channel instead of being rerouted
by an uplifting landmass is called an antecedent stream. The river deposited
great quantities of alluvial material upon the land that is now Orange County,
providing the basis for an excellent soil for our agricultural industries.
The heavily populated, areas of the county are built upon this ancient flood
plain. The San Gabriel River is also, an antecedent river and continued
to cut across the Puente Hills as they were uplifting.
| The Pliocene La Habra (Qlh) and Coyote Hills (Qch) Formations |
The La Habra Formation is found in the Coyote Hills and along the southern
flank of the Puente Hills. It is a non-marine deposit and appears to have
been an old flood plain. Diatomaceous shale chunks or clasts occur near
the basal contact with the underlying Coyote Hills Formation. These clasts
were probably washed from the Puente Hills area onto the growing flood plain
in the Late Pleistocene Epoch. The Coyote Hills Formation is limited only
to the Coyote Hills and is a lagoonal deposit.
| The Cutting of Marine Terraces (Qt, Qtm) in the Plio-Pleistocene |
During the end of the Pliocene and into the Pleistocene Epoch, the seas
receded slowly. Each successive shoreline is represented today by a marine
terrace. Three marine terraces are seen north of Corona del Mar. These flat
terraces may be crossed by driving toward the beach on MacArthur Boulevard.
| The Pleistocene San Pedro Sandstone (Qsp), Timms Point Silt, Lomita
Marl, Palos Verdes Sand |
Although the Pleistocene Epoch is called the "Ice Age", no glacial ice covered
the county or southland area. A heavily vegetated, marshy area extended
inland beyond the shoreline, and a great variety of vertebrate "Ice Age",
animals lived in southern California. The tar seeps (La Brea Tar Pits in
Los Angeles) entombed many of those species. Peat bogs in the coastal area
near Huntington Beach were covered with sand and have become peat beds.
San Pedro Sandstone, which contains marine fossils; Timms Point Silt; and
Lomita Marl, a calcareous clay found near Palos Verdes, were all deposited
in the Pleistocene and underlie beach-terrace sands. The Palos Verdes Sand
(terrace) with abundant Pleistocene sea shells can be seen from Coast Highway
#l in the Newport Mesa cliffs.
| Faults and Earthquake Activity |
Marine sediment in the Los Angeles Basin is greater than 10,000 feet thick.
Near the Coyote Hills in the northwest part of Orange County the sediment
thins to 50 to 200 feet thick. When an earthquake occurs, the movement of
the tremors through the sediment tends to amplify the motion; the effect
is similar to shaking a bowl of jelly.
There are many faults and fault zones throughout southern California.
Certainly there has been no activity along most faults for millions of
years. Other fault systems have been active since the Pleistocene Epoch
but not in the last two hundred years, or within the recorded history
of Orange County. Scientists are presently developing new methods for
measuring the potential threat of earthquake activity along Pleistocene
faults. Two large fault zones in Orange County which have not been active
in recorded history are the Norwalk fault zone which trends northwest-southeast
near Anaheim and the Cristianitos fault zone which trends north-south
across the Ortega Highway east of San Juan Capistrano.
The three active and large fault zones that have been active in this
century are the Whittier fault zone, the Elsinore fault zone, and the
Newport-Inglewood fault zone. The Whittier fault zone extends along the
southwestern base of the Puente Hills and has trapped migrating oil. The
zone may be observed by driving into the Puente Hills from Brea on Brea
Boulevard. One mile north of Brea the road turns east along the foot of
the hills (the fault zone), then turns north again. The road was built
along a stream coarse which was offset by the Whittier fault. The Whittier
fault joins the Chino fault near Prado Dam, and they merge into the Elsinore
fault zone which trends along the eastern base of the Santa Ana Mountains.
The Newport-Inglewood fault zone extends northwest from offshore Newport
Beach to Inglewood (distance of forty miles) and, like the Whittier fault,
has warped and faulted marine sediments into ideal oil traps. The 1933
Long Beach earthquake resulted from movement along this fault.