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(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.

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