15.3: Rocks of the Peninsular Ranges

It’s All About the Rocks.

Plutonic igneous rock is the most common rock in the Peninsular Ranges province. Even a quick glance at the geologic map of the province (Figure \(\PageIndex{1}\)) shows that the majority of the outcrops are “granitic igneous rocks, mainly of Mesozoic age.” By comparison, outcrops of "Cenozoic volcanic rocks" are so small they do not appear on a map at this scale. However, they do exist. They occur within the roof pendants or as Cenozoic flows on the westernmost slopes of the Ranges. "Mesozoic metamorphic and sedimentary rocks" are also rare, occurring mostly on the northern or western parts of the mountains as roof pendants or in the central part of the province as shear zones. Sedimentary rock and sediment ("Quaternary surficial deposits", "Neogene and Paleogene sedimentary rocks", "Cretaceous sedimentary rocks" and "Quaternary and Neogene non-marine deposits") do occur in the intermontane valleys, but most sedimentary rock is found west of the mountains on either the coastal plain or in the northern valleys and plains where the Peninsular Ranges province abuts the Transverse Ranges province.

Although Paleozoic and Precambrian rock designations are present in the legend of the geologic map, there are no outcrops of these rocks indicated on the map of the Peninsular Ranges. The "Paleozoic sedimentary and metamorphic rocks" outcrops are too small to appear at this map scale, but are present as roof pendants. "Pre-Cambrian rocks" are found elsewhere in California, but not here. Also, two types of Mesozoic rock, famously associated with California, do not occur in the Peninsular Ranges province. Neither "Mesozoic serpentine and other ultramafic rocks", nor the Mesozoic "Franciscan Complex sedimentary and metamorphic rocks" occur here.

Figure \(\PageIndex{1}\): Geologic map of the Peninsular Ranges Province of California, a detailed description of the rock units is included within the text. “Geology of the Peninsular Ranges” by Emily Wright, a derivative from the "Geology of California" is licensed under CC BY-NC 4.0.

Metamorphic Rock of the Peninsula Ranges

The metamorphic rock in the Peninsular Ranges is mostly metasedimentary rock that was originally part of the passive margin that existed before subduction began and the batholith intruded. The schists and gneisses of today are related to either collisions as terranes were accreted, or as younger plutons metamorphosed rock in the older existing plutons or the sedimentary rock they were intruding.

Emplacement of the plutons during the Mesozoic metamorphosed the existing, mostly marine sedimentary rock of the passive margin. These rocks are the oldest rock in the Peninsular Ranges and are concentrated as roof pendants in the northeastern ranges or along the western slopes of the western ranges (Figure \(\PageIndex{1}\)). To the north and east these rocks are primarily a mix of gneiss, schist, quartzite, and marble. The marble has been extensively quarried in the greater Riverside, California area (see Tectonics and Geologic History of the Peninsular Ranges Province).  The Peninsular Ranges province is also the location of one of the major North American mineral type localities for calc-silicate minerals (Box 15.3.1).

Box \(\PageIndex{1}\): Crestmore Quarry, Riverside County, California

Crestmore Quarry in Riverside County, California is known globally for its rare minerals. It is the type locality, or place of original discovery, for minerals, such as jennite, merwinite, wrightmanite and others. Some of these minerals have not been found anywhere in the world other than Crestmore.

The rare minerals, and the not so rare minerals, at Crestmore occurred during metamorphism, but at Crestmore, there were multiple episodes of metamorphism and metasomatism also occurred. Metasomatism is when the minerals are metamorphosed in an extremely wet environment and the generous amount of fluid in the system allows ions to exchange with abandon and grow new minerals totally different from the original minerals. Most of the time, when rock is metamorphosed, metasomatism does not occur because there is usually not enough fluid. Also, most of the time, when magma is involved, it is a silicate magma intruding into a silicate rock. At Crestmore silicate magma intruded into carbonate rock.

When metasomatism does occur, the new minerals are usually not especially rare minerals, rather they are metallic ore minerals. The heat, pressure, and abundant fluid allows the metallic ions to form oxide and sulfide minerals. This is how many gold, copper, tin, lead, zinc, and other metallic ores occur. At Crestmore however the multiple episodes of metamorphism and the wide range in chemistry made the situation complex and created situations where different types of minerals, calc-silicate minerals, grew instead of metallic ore minerals.

The protoliths at Crestmore were marine sedimentary rock, of possible Mississippian age, that were metamorphosed by the intrusion of the Peninsular Ranges batholith and became roof pendants of schists, quartzite, and marble (Box Figure \(\PageIndex{1}\)).

Box Figure \(\PageIndex{1}\): Sky Blue Limestone (actually a marble), Crestmore Quarry, Riverside County, California. This sample is approximately 15 cm by 8 cm (6 inches by 3 inches). "Crestmore Sky Blue" by Debra Brooks is licensed under CC BY-NC 4.0.

The original intrusion was described as a quartz diorite. The conventions for naming igneous rock were standardized in the 1960s and today it would be called a tonalite, which is the most common composition of the rocks in the batholith. Contact metamorphic aureoles formed from 5 to 60 cm (2 inches to 2 feet) wide.

Subsequently, a new magma with a different composition intruded into the quartz diorite (tonalite), a quartz monzonite, a rock with less plagioclase feldspar and quartz and more potassium feldspar. This second intrusion, of a rock with yet another different chemistry, started a separate phase of metamorphism. Then pegmatite dikes intruded. The composition of the minerals in the pegmatite dikes changes from north to south, most have quartz and feldspars, but the northern dikes also include minerals such as epidote, prehnite, and zircon, while the southern dikes are more likely to have andradite garnets instead. With three different episodes of intrusion, each one with a different silicate chemistry, into predominantly carbonate rock there was ample opportunity to create new and varied minerals.

As of 2022, 162 different, verified and validated mineral types have been found at Crestmore, and it’s the type locality of 9 minerals.

Similar rocks are found as roof pendants and inclusions in the northern San Jacinto and Santa Rosa Mountains. In the Santa Ana Mountains of the northwest, the metamorphic rocks are only slightly metamorphosed and the original protoliths are still discernible. This is the Bedford Canyon formation, it dips to the west, and its base is not exposed. It includes slatey siltstones, meta-qraywacke, metaconglomerate, and some marble. In places it has been intruded by the batholith. The rare fossils in the marble have been provisionally dated to the middle Jurassic. Farther south in San Diego county, the rocks of the Peñasquitos Formation are probably equivalent.

Within the batholith, the Julian Schist and its probable equivalents are the most common metamorphic rock. The protolith for the Julian Schist are metasedimentary rocks deposited in a deep-water environment and metavolcanic rock, which were then metamorphosed, deformed, and then intruded by batholithic rocks.

Plutonic Igneous Rock of the Peninsular Ranges

The most common rocks in the province are the plutonic igneous rocks that form the mountains of the Peninsular Ranges (Figure \(\PageIndex{2}\)). These mountains are a result of Mesozoic subduction on the west coast of California as described previously (see Tectonics and Geologic History of the Peninsular Ranges Province). The Peninsular Ranges batholith is complex. Most batholiths are more uniform, but the Peninsular Ranges batholiths is divided into eastern and western parts based upon the changes that occurred in subduction over time and the resulting differences in the chemistry and mineralogy of the rock, which also leads to differences in the geophysical properties of the rock. The plutonic igneous rock of the batholith as a whole encompasses a range of chemistry.

Figure \(\PageIndex{2}\): Simplified geologic map of the Peninsular Ranges and the Transverse Ranges with only plutonic igneous rock and major fault zones mapped. The plutonic rocks south of the Banning fault zone are in the Peninsular Ranges province. Rocks north of that fault are in the Transverse Ranges province and not part of the Peninsular Ranges batholith. "Generalized Geology" by A. K. Baird and A. T. Miesch is in the public domain.

The three major types of plutonic igneous rock: gabbro, diorite, and granite (see Igneous Rocks and see also The Sierra Nevada Batholith), are all present in the Peninsular Ranges batholith. However, plutonic rocks can have a large variety of compositions, and each composition has its own name. The most common composition of plutonic igneous rock throughout the Peninsular Ranges batholith is tonalite. Tonalites are a type of granitic rock where the feldspars are more likely to be plagioclase than potassium feldspar (Figure \(\PageIndex{3}\)). Tonalite is a relatively new designation and when the map in Figure \(\PageIndex{2}\) was originally created the name tonalite was not used. Much of what is mapped as either quartz diorite or granodiorite on the map would instead be mapped as tonalite today.

Figure \(\PageIndex{3}\): A sample of tonalite, composed predominantly of feldspars and quartz with some hornblende and possible biotite. "Rocks - Tonalite" by Khruner via Wikipedia is licensed under CC BY-SA 4.0.

The western part of the batholith formed as part of an island arc system offshore of North America during the Jurassic. During the Cretaceous, part of another island arc moving obliquely up from the south accreted onto the existing island arc system (see Assembling a Continent to review the concept of terrane accretion). The collision of the two produced the oldest deformation in the Cuyamaca-Laguna Mountain shear zone, one of the many transitions that divide the Peninsular Ranges batholith into western and eastern parts. The western part of the Peninsular Ranges batholith includes the remains of this accreted island arc.

In the western Peninsular Ranges, the plutonic rocks are generally older than 105 million years; have more compositional variation and include rocks that are mafic through felsic. They have a slightly different chemical signature with higher concentrations of magnesium and also isotopic values that indicate that the magma formed in the asthenosphere and migrated up through mostly oceanic lithosphere. The rocks of the western batholith have compositions from gabbro to diorite to tonalite but are mostly tonalite. Notice in Figure \(\PageIndex{2}\), that the outcrops of gabbro are in the western half of the batholith. These rocks also have different geophysical properties and are more dense, more magnetic, and have a higher seismic velocity than the rocks of the eastern Peninsular Ranges.

In the eastern Peninsular Ranges, the plutonic rocks are 105 to 80 million years old and formed after changes occurred in the subduction process. The angle of the plate became shallower, and the production of magma moved inland, further from the trench, and to the east of the Cuyamaca-Laguna Mountain shear zone. Instead of forming beneath oceanic lithosphere, the plutons were now forming under continental lithosphere. The magma was now rising through both more rock, and more felsic rock, and therefore the rocks of the eastern ranges are less varied in composition and are primarily tonalite with some granite. They contain less magnesium and also have different isotopic signatures because of interacting with a different lithospheric composition as the magma rises towards the surface. Because of their different chemistry and mineralogy, they are also less dense, less magnetic, and have a lower seismic velocity.

The rocks of the eastern Peninsular Ranges batholith are also extremely similar in timing, chemistry, and mineralogy to the rocks of the Sierra Nevada batholith. They both formed during the Cretaceous from 105 to 80 million years ago. The chemistry and mineralogy of these batholiths is similar – the eastern Peninsular Ranges rocks are mostly tonalite with some granite, while the rocks of the Sierra Nevada are mostly granodiorite (a rock intermediate in composition between granite and tonalite) with some tonalite and granite. Their isotopic signatures are similar as are the trace elements present in both.

Query \(\PageIndex{1}\)

Some of the individual plutons in the Peninsular Ranges are associated with complex dike systems related to successive intrusions. In the western batholith, this includes ring dike complexes related to emplacement of plutons. Also there are pegmatitic dikes that formed during the latest stages of the intrusion, many of which include gemstones and rare minerals (Box 15.3.2). Dikes form when magma cools in cracks in the rock; pegmatitic is the texture formed when unusually large crystals form (see Igneous Rocks).

Box \(\PageIndex{2}\): Pegmatites of the Pala District of San Diego County

Northeast San Diego county and southwest Riverside county are the location of 14 pegmatite districts. Pegmatite dikes are known for their large, well-formed crystals which are frequently prized by museums and private collectors. Occasionally, pegmatites may have uncommon chemical compositions and both rare minerals and gem quality minerals are found. The Pala District which is mostly on the Pala Indian Reservation in the San Luis Rey River Valley of northeast San Diego County is known for gemstones and spectacular crystals of lithium, bismuth, and phosphate minerals, but especially lithium minerals.

Formal mining of the Pala District began in the 1870s but mining was most active from 1900 to 1922. Even though it is commonly referred to as a mining district, it was never formally organized as one, such as what happens when mining precious metals. There are six mines in the district and the most commonly mined minerals are lepidolite (lithium mica), tourmaline (pink varieties include both lithium and manganese), and spodumene (a lithium bearing pyroxene mineral). Other minerals mined include amblygonite, beryl (aquamarine and morganite), feldspar, and quartz. Morganite, the pink variety of beryl, was originally discovered at Pala in 1902. In this same year, commercial qualities of kunzite, the gemstone variety of spodumene, was also discovered and Pala is still a major producer of kunzite. Tourmaline, however, is the gem mineral that made Pala famous. Variegated or color-zoned tourmaline in shades of pink and green are especially prized, in particular “watermelon tourmaline” where instead of the more common elongate crystals with color changes from pink to green (Box Figure \(\PageIndex{2}\)), the center of the crystal is pink and the outer zone of the crystal is green, which reminds people of watermelons.

Box Figure \(\PageIndex{2}\): Color-zoned, gem tourmaline from the Pala and Mesa Grande Districts, San Diego County, California. From the Don M. George, Jr. Mineral Collection, California Institute of Technology. "Color zoned crystals of gem tourmaline" by David P. Willoughby, in CDMG Special Report 7-A Gem- and Lithium-Bearing Pegmatites of the Pala District, San Diego County, California is in the public domain.

One of the reasons the mining was so intense during the early 1900s was Dowager Empress of China Cixi adored pink tourmaline, which increased its appeal and demand skyrocketed during her lifetime. Between 1902 and 1910, 120 tons of gem-quality pink tourmaline was shipped to China from mines in San Diego county. Demand later fell, but never entirely, and pink tourmaline is still prized today by many collectors.

The pegmatites are usually granitic, frequently with graphic granite as an outer or border zone and finer-grained granitic rock within. The gem-bearing pegmatites are granitic but occur in fractures in gabbro. They vary in size from centimeters to meters thick, and the gem bearing pegmatites almost always occur in the “pocket” pegmatites where the pegmatitic dikes bulge outward and the inner core has large quantities of mostly euhedral crystals ranging in size from fine- to very coarse-grained. There is little to no open space in the pockets or bulges, and what might have been “open” is generally filled with clay minerals. The Pala pegmatites are believed to have formed during the final stages of crystallization within the pluton by internal segregation of magmatic fluids, not from outside fluids moving into the pluton.

Mining continues to this day, the sixth mine in the district was only patented in 1979. While lithium is a strategic mineral, most of the mines concentrate on mining gemstones and large collectable crystals of common minerals such as feldspar and quartz. Also, the mines frequently augment their income by allowing people to pay a fee and search through the tailings for minerals.

Volcanic Igneous Rock of the Peninsular Ranges

Volcanic igneous rock is comparatively rare in the Peninsular Ranges. Most of the volcanic rock from the Cretaceous island arcs eroded away long ago, along with the volcanoes. The rocks that remain most commonly outcrop in the western part of the Peninsular Ranges and their composition is generally basaltic or andesitic with a chemistry that corresponds to the gabbro and diorite plutons also found in this part of the ranges. Their existence is also inferred from volcanic clasts found in much of the sediment and sedimentary rock on the coastal plain. The two most common volcanic rock formations are the Santiago Peak Volcanics and the El Modena Volcanics.

Santiago Peak Volcanics outcrop in the western Peninsular Ranges discontinuously from Orange county through San Diego county and into Mexico ending at the Aqua Blanca fault zone in Baja California (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{4}\): Wave eroded boulders of the Santiago Peak Volcanics on a wave-cut platform at Cabrillo National Monument, San Diego County, California. "Santiago Peak Volcanics Boulders" by the National Park Service is in the public domain.

They extruded onto the metamorphic Bedford Canyon Formation and both the Bedford Canyon Formation and the Santiago Peak Volcanics were subsequently intruded by rocks of the Peninsular Ranges batholith, therefore in many locales the Santiago Peak Volcanics have been slightly metamorphosed. They include flows, volcaniclastic breccia, welded tuff, and dikes. Composition varies but is mostly andesitic. Radioactive age dating places them as Early Cretaceous (oldest date 128.3 ± 2.5 million years ago) and their age is used to date the beginning of island arc volcanism.

The El Modena Volcanics represent a much younger episode of volcanism. They are interbedded with marine, fossiliferous sedimentary rock, which establishes their age as Miocene. They also vary in composition from basalt to andesite and include palagonite tuffs and in some locations pillow structures indicating that they had at times erupted underwater (see The Josephine Ophiolite- A Little Slice of the Mantle). This timing also corresponds with after the plate boundary in southern California had transitioned from a subduction zone to a transform fault and during a period of extension. Outcrops are in the northwest part of the province in the foothills of the Santa Ana Mountains or in the coastal San Joaquin Hills to the immediate west.

Sedimentary Rock and Sediment of the Coastal Plain

West of the mountains, sedimentary rock and sediment reign. The coastal plain of the Peninsular Ranges is dominated by sedimentation, mostly from the mountains with marine terraces and beaches as the major landforms. Sedimentary rock formations are seldom continuous along the coast. Near the Orange-San Diego county line the mountains extend west almost to the sea (Figure \(\PageIndex{5}\)). It is therefore convenient to discuss the sedimentary rocks as the northern or Orange county formations and the southern or San Diego county formations. Even though the formations are not continuous, almost always the depositional environments are similar, or are environments that would reasonably coexist.

Figure \(\PageIndex{5}\): Satellite image of the western Peninsular Ranges and the coastal plain. Note that the southern Santa Ana Mountains swing to the west and almost extend to the coast providing a north-south barrier to the movement of sediment. Astronaut Photo ISS018-E-5058 by NASA via Wikimedia Commons is in the public domain.

Mesozoic Sedimentary Rock and Sedimentation

The history of Mesozoic sedimentation and sedimentary rock in the Peninsular Ranges province actually begins in the Jurassic metasedimentary rock, previously discussed under metamorphic rock. The sedimentary protoliths for much of this rock are easy to ascertain. The Bedford Canyon Formation in the north and the Peñasquitos Formation to the south both record a history of west-dipping conglomerates and sandstones with abundant volcaniclastic material. In places they are then overlain by volcanic flows followed by erosion.

Jurassic to Cretaceous deposition begins on this erosional surface with alluvial fan deposits along the mountain front that are thicker, more continuous, and start earlier to the north (Jurassic Trabuco Formation) and get thinner, less continuous, and start later to the south (Cretaceous Lusardi Formation). These deposits are then overlapped by marine sediment as sea level rises. As the Cretaceous continues, deposition of shallow marine sediments and turbidite deposits follow (north - Ladd and Williams Formations; south - Point Loma and Cabrillo Formations). Turbidites are underwater landslide deposits (Video 15.3.1) with recognizable patterns of graded bedding and they are usually indicative of earthquake activity (Figure \(\PageIndex{6}\)). The Late Cretaceous formations start as marine, but gradually become non-marine over time.

Figure \(\PageIndex{6}\): Point Loma Formation in San Diego County, California. "Turbidite-3" by Zimbres via Wikimedia Commons is licensed under CC BY-SA 3.0.

Video \(\PageIndex{1}\): Turbidites

A more detailed description of how turbidites form and what they geologically represent.

This repeating depositional pattern of the sea comes in (it onlaps) and the sea appears to move out (either it offlaps or uplift occurs), only to repeat again, is the story of the coastal plain throughout the Mesozoic and the Cenozoic. This pattern continues into present time with formation names changing and timing tracked by the usually abundant fossils that occur in the marine deposits.

The Point Loma Formation of San Diego county and the Williams Formation of Orange county, are also among the few stratigraphic units in California where dinosaur fossils have been found (Box 15.3.3).

Box \(\PageIndex{3}\): Southern California’s Dinosaurs

Incomplete hadrosaur fossils have been found in both the Point Loma Formation and the Williams Formation. In the Point Loma Formation, these consist of incomplete back and tail vertebra, a femur, and a partial lower jaw. In the Williams Formation, it is foot bones, a couple of back vertebrae, and a toe. In both locations it is not possible to determine the exact species of hadrosaur from the fossils that have been found.

Box Figure \(\PageIndex{3.1}\): Conception of how Aletopelta coombsi might have looked. "Life Restoration" by Nobu Tamura via Wikimedia Commons is licensed under CC BY-SA 4.0.

The best dinosaur find in California is that of an incomplete skeleton of an ankylosaur, Aletopelta coombsi or “wandering shield” in the Point Loma Formation which is now on display in the San Diego Natural History Museum (Box Figure \(\PageIndex{3.1}\)). While it may be incomplete, it is the most complete dinosaur fossil ever found in California (Box Figure \(\PageIndex{3.2}\)). It was found by the museum’s field paleontologist, Brad Riney, in 1987 in Carlsbad, California, during a project to extend a road. The complete hindquarters of the animal were recovered, including its back legs still attached to its pelvic bones, along with armor plates, spikes, and eight teeth. Unfortunately, even though much of the skeleton was found, neither its skull nor tail bones were found.

The Point Loma Formation is marine. Therefore, the fossils are believed to have been washed out to sea and buried and preserved in marine sediment. Some of the ankylosaur’s bones had been encrusted with oyster fossils and a shark tooth was also found mixed in with the remains.

Even though it is the most complete dinosaur fossil ever found in California, it’s not California’s State Dinosaur. That honor goes to Augustynolophus morrisi, whose skull was found in 1936 in the Panoche Hills in central California, and two other skulls were subsequently found in the same hills in 1939 and 1941.

Box Figure \(\PageIndex{3.2}\): The ankylosaur, Aletopelta coombsi, on display in the San Diego Natural History Museum. "Aletopelta" by ケラトプスユウタ (2019) via Wikimedia Commons is licensed under CC BY-SA 4.0.

To summarize the geological story of the Mesozoic, it starts with an arid climate and alluvial fan deposits along the mountain front that are thicker and more continuous in Orange county and thinner and less continuous in San Diego county. These deposits are then overlapped by marine sediment as sea level rose, and then deposition of turbidite and shallow marine sediments occurred, followed by either uplift or a fall in sea level, or both, because an unconformity separates Mesozoic from Cenozoic sedimentary rock. This also means that any evidence of the Cretaceous-Paleogene boundary no longer exists in the coastal Peninsular Ranges province, so there is no evidence here of how the dinosaurs became extinct.

Cenozoic Sedimentary Rock and Sedimentation

The Cenozoic begins in both the northern and southern parts of the coastal plain with an unconformity. In the southern part of the coastal plain, there is no record of early Paleogene sedimentary rock, any deposition from this time has been eroded away. In the northern part of the coastal plain the Silverado Formation is deposited on the unconformity and repeats the story of ongoing changes in sea level. The lower three members of the formation are non-marine, while the uppermost member is marine indicating rising sea level. This rise in sea level continues into the mid-Paleogene with rocks of this age abundant on both the northern and southern parts of the coastal plain.

On the southern coastal plain, the mid-Paleogene sedimentary rock is the La Jolla Group. The members of the La Jolla Group record a history of non-marine to coastal to marine to non-marine environments. This pattern continues on the northern coastal plain with the Santiago Formation unconformably deposited on the Silverado Formation and also recording a history of a marine environment transitioning into a non-marine environment. The Santiago Formation is conformably overlaid by the non-marine Sespe and then the marine Vaqueros Formations; the La Jolla Group to the south is unconformably overlaid by the Poway Group.

The Poway Group, formerly referred to as the Poway Conglomerate, includes some of the most interesting rocks of southern California. The cobbles in the conglomerates are large, well-rounded, and include a distinctive rhyolitic composition. The depositional environments are non-marine to marine and include stream channels to submarine fan deposits, the question becomes, where was their source rock? No suitable source rock for these cobbles has been found anywhere in southern California. The second question raised by these cobbles is how did they get so widely distributed? While they are mostly found in San Diego county, they are also found on islands in the California borderlands to the north and west, far from San Diego county.

To find the source required looking at the geography of the Paleogene and at that time, the plate boundary was a subduction zone, the Gulf of California had not opened, and the Peninsular Ranges were part of the North American plate (Video 15.3.2). Matching the geochemistry and mineralogy of the cobbles locates the source as a mountain range in Sonora, Mexico.

Video \(\PageIndex{2}\): History of the Poway Group Cobbles

The formation and subsequent dispersal of the cobbles in the Poway Group is illustrated in the following animation. This video has no sound. Access a written description.

The mid-Paleogene was a time of warm climates, and high sea level, the late Paleogene was a time of a cooling climate and drop in sea level as ice ages began in the polar regions. Sea level fell and erosion increased in the Peninsular Ranges. Late Paleogene rocks are missing in the northern coastal plain and while present on the southern coastal plain their deposits are thicker to the south and get progressively thinner and eventually disappear to the north. These rocks are part of the non-marine Otay Formation and known for the high levels of bentonite, a clay formed by the alteration of volcanic ash and are also a source of vertebrate fossils. The large variety and number of fossils found paint a picture of rolling hills, grasslands, herds of grazing animals: oreodonts, small rhinoceros and camels, with small burrowing mammals, such as mice, shrews, hedgehogs, gophers, and rabbits, along with fox-like dogs, short-faced dogs, and saber-toothed carnivores. Many of the fossils are of creatures that still exist today, but some, like oreodonts, have no living analogs.

The fall in sea level and the increasing erosion also mean that there is a regional unconformity between the rocks of the late Paleogene and the Neogene. The Neogene begins a time of massive tectonic change as the plate boundary transitions from a subduction zone to a transform boundary (Figure 15.2.4). In the northern coastal plain, much of the sediment came from sources to the north in addition to the Peninsular Ranges to the east. Here, the Neogene sequence begins with the deposition of the Topanga Formation, which interfingers with the El Modena Volcanics, indicating that extension and volcanism occurred together. The extension created a series of tilted fault blocks in the Borderlands to the west forming a now submerged topography of ridges and basins (Figure 15.2.5). As the ridges grew, they also became a sediment source for the rocks on the coastal plain, an example is the San Onofre Breccia.

The San Onofre Breccia outcrops discontinuously along the coast from Laguna Beach to Oceanside, with the best exposures near Dana Point. It has blueschist clasts that are large and angular, therefore their source must be close (see Sedimentary Rocks) and is inferred to be a now-submerged ridge to the west. Santa Catalina Island and the Palos Verde Peninsula in the Los Angeles basin are believed to be exposed remains of this ridge because the Catalina Schist outcrops in these locations today.

In the northern coastal plain both the Monterey and the Puente Formations are then deposited during the mid to late Neogene. The Puente Formation is of economic importance because it is a common source rock for the oil fields of northern Orange county. Further south is the marine Capistrano Formation. It is mostly marine sandstones and siltstones, but in the cliffs of Dana Point it includes a submarine fan deposit, which is well exposed and includes conglomeritic lenses (Figure \(\PageIndex{7}\)).

Figure \(\PageIndex{7}\): Geology students examining a large boulder eroded from a conglomeritic lens of the Capistrano Formation, Dana Point, Orange County, California. "Conglomeritic Boulder at Dana Point" by Debra Brooks is licensed under CC BY-NC 4.0.

Continuing south, the San Diego Formation unconformably overlies the San Onofre Breccia. It ranges in age from late Neogene to Pliocene and begins as a shallow marine sandstone, but gradually the environment changes to non-marine and the size of the clasts increases.

The lower marine section has abundant microfossils that have been used to analyze water temperature. At the base of the formation, the fossils indicate water temperatures were warmer, about 20° C (68° F) and they then decreased to approximately 15° C (59° F) at the top of the marine sequence. This correlates with the cooling temperatures and falling sea levels that occurred going into the Pliocene and the beginnings of the Ice Ages. This pattern of transitioning from marine to non-marine is repeated in the Niguel Formation of southern Orange county and the Fernando Formation of northern Orange county.

The Pliocene and Pleistocene also record the formation of marine terraces with increasing terrace formation into the Pleistocene. An estimated rise and fall of sea level of approximately 190 m (625 feet) has been documented in the greater San Diego region. During the Pleistocene, the earthquake activity along with the changes in sea level created most of the marine terraces seen today. The youngest sedimentation is from the failure of marine terraces along the coast, leaving deposits of landslide debris, and the Quaternary alluvial deposits being washed down and out from the slopes of the Peninsular Ranges mountains.

Tectonic activity was also increasing during this time as the Gulf of California continued to open. The resulting transpression caused deformation of the sedimentary rock units of the northern coastal plain forming broad, shallow anticlines and synclines. This broad and shallow folding will help set up some of the traps, or structures within rock that can contain fluids, which make up the oil fields in the greater Los Angeles basin of the Transverse Ranges and the northernmost Peninsular Ranges provinces.

Query \(\PageIndex{2}\)

References

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3. Germinario, M. (1993). The Early Mesozoic Julian Schist, Julian, California. In R. G. Gastil & R. H. Miller (Eds.), GSA Special Paper 279: The Prebatholithic Stratigraphy of Peninsular California. (pp. 107-118). Geological Society of America. DOI: https://doi.org/10.1130/SPE279-p107
4. Harden, D. R. (2004). California Geology (2nd ed.). Pearson Prentice Hall.
5. Herzig, C. T., & Kimbrough, D. L. (2014). Santiago Peak volcanics: Early Cretaceous arc volcanism of the western Peninsular Ranges batholith, southern California In D. M. Morton & F. K. Miller (Eds.), GSA Memoir 211: Peninsular Ranges Batholith, Baja California and Southern California. (pp. 61-143). Geological Society of America. DOI: https://doi.org/10.1130/2014.1211(09)
6. Hildebrand, R. S. (2013). GSA Special Paper 495: Mesozoic Assembly of the North America Cordillera. Geological Society of America. DOI: https://doi.org/10.1130/SPE495
7. Jahns, R. H. (1954a). Chapter I: Geology of the Peninsular Range Province, Southern California and Baja California. In R. H. Jahns (Ed.), CDMG Bulletin 170: Geology of Southern California. (pp. 29-52). California Division of Mines and Geology.
8. Jahns, R. H. (1954b). Chapter VII: Pegmatites of Southern California. In R. H. Jahns (Ed.), CDMG Bulletin 170: Geology of Southern California. (pp. 37-50). California Division of Mines and Geology.
9. Le Bas, M. J., & Streckeisen, A. L. (1991). The IUGS systematics of igneous rocks. Journal of the Geological Society, London. 148 825-833. https://edisciplinas.usp.br/pluginfile.php/7940993/mod_resource/content/1/rockclassification.pdf
10. MacKevett, E. M. (1951) Special Report 5: Geology of the Jurupa Mountains, San Bernardino and Riverside Counties, California. California Division of Mines and Geology.
11. Nilsen, T. H., & Abbott, P. L. (1981). Paleography and Sedimentology of Upper Cretaceous Turbidites, San Diego, California. American Association of Petroleum Geologist Bulletin, 65 (7) 1256-1284.
12. Nordstrom, C. E. (1970). Lusardi Formation: A Post-Batholithic Cretaceous Conglomerate North of San Diego, California. Geological Society of America Bulletin, 81(2) 601-606. DOI https://doi.org/10.1130/0016-7606(1970)81[601:LFAPCC]2.0.CO;2
13. Norris, R. M., & Webb, R. W. (1990). Geology of California (2nd ed). John Wiley and Sons.
14. Perkins, D. (2022). Petrology: An Introduction to Igneous and Metamorphic Rocks and Processes. WordPress. https://opengeology.org/petrology/
15. Prothero, D. R. (2017). California’s Amazing Geology. CRC Press.
16. Rivero, C., & Shaw, J. W. (2011) Active folding and blind thrust faulting induced by basin inversion processes, inner California borderlands. In J. Shaw & J. Suppe, (Eds.) AAPG Memoir 94: Thrust fault-related folding. (pp. 187-214). American Association of Petroleum Geologists. DOI:10.1306/13251338M943432
17. Schoellhamer, J. E., Vedder, J. G., Yerkes, R. F., & Kinney, D. M. (1981). USGS Professional Paper 420-D: Geology of the Northern Santa Ana Mountains, California. U.S. Geological Survey.
18. Shaw, S. E., Todd, V. R., Kimbrough, D. L., & Pearson, N. J. (2014). A west-to-east geologic transect across the Peninsular Ranges batholith, San Diego County, California: Zircon ¹⁷⁶Hf/¹⁷⁷Hf evidence for the mixing of crustal- and mantle-derived magmas, and comparisons with the Sierra Nevada batholith. In D. M. Morton & F. K. Miller (Eds.), GSA Memoir 211: Peninsular Ranges Batholith, Baja California and Southern California. (pp. 61-143). Geological Society of America. DOI: https://doi.org/10.1130/2014.1211(03)
19. Silver, L. T., & Chappell, B. W. (1988). The Peninsular Ranges Batholith: an insight into the evolution of the Cordilleran batholiths of southwestern North America. Earth and Environmental Transactions of the Royal Society of Edinburgh, 79(2-3), 105-121. DOI: https://doi.org/10.1017/S0263593300014152
20. Wetmore, P. H., Herzig, C., Alsleben, H., Sutherland, M., Schmidt, K. L., Schultz, P. W., & Paterson, S. R. (2003). Mesozoic tectonic evolution of the Peninsular Ranges of southern and Baja California. In S. E. Johnson, S. R. Paterson, J. M. Fletcher, G. H. Girty, D. L. Kimbrough, & A. Martín-Barajas (Eds.), GSA Special Paper 374: Tectonic evolution of northwestern México and the Southwestern USA. (pp 93-116). Geological Society of America. DOI:https://doi.org/10.1130/0-8137-2374-4.93
21. Wetmore, P. H., Schmidt, K. L., Paterson, S. R., & Herzig, C. (2002). Tectonic implications for the along-strike variation of the Peninsular Ranges batholith, southern and Baja California. Geology, 30(3) 247-250.
22. Williams Formation. (n.d.). DBpedia. Retrieved Juky 22, 2024, from https://dbpedia.org/page/Williams_Formation
23. Yerkes, R. F. (1957). USGS Professional Paper 274-L: Volcanic Rocks of the El Modeno Area, Orange County, California. U.S. Geological Survey.

References for Box \(\PageIndex{1}\): Crestmore Quarry, Riverside County, California

1. Burnham, C. W. (1951). Chapter VII: Contact metamorphism at Crestmore, California. In R. H. Jahns (Ed.), CDMG Bulletin 170: Geology of Southern California. (pp. 61-70). California Division of Mines and Geology.
2. Devito, F., Parcel, R. T. & Jefferson, G. T. (1971). Field Trip No. 5: Contact metamorphic minerals at Crestmore Quarry, Riverside, California. In W. A. Elders (Ed.), Geological Excursions in Southern California. UCR Campus Museum.
3. Eakle, A. S. (1927). Famous Mineral Localities: Crestmore Quarry, Riverside County, California. American Mineralogist, 12 319-321. http://minsocam.org/MSA/collectors_corner/arc/crestmoreca1.htm
4. Glazner, A. F., Bartley, J. M. & Coleman, D. S. (2019). A more informative way to name plutonic rocks. GSA Today, 29(2) 4-10. DOI: https://rock.geosociety.org/net/gsatoday/science/G384A/article.htm
5. Mindat.org (n.d.). Crestmore quarries, Crestmore, Jurupa Valley, Riverside County, California, USA. [Data set]. https://www.mindat.org/loc-3449.html
6. Woodford, A. O., Crippen, R. A. & Garner, K. B. (1941) Section across Commercial Quarry, Crestmore, California. American Mineralogist, 26 351-381. http://www.minsocam.org/msa/collectors_corner/arc/crestmoreca2.htm

References for Box \(\PageIndex{2}\): Pegmatites of the Pala District of San Diego County

1. Herberling, L. (n.d.). History of Mining in Southern California. San Diego Natural History Museum. https://www.sdnhm.org/exhibitions/all-that-glitters/history/history-of-mining-in-southern-california/
2. Jahns, R. H., & Wright, L. A. (1951). CDMG Special Report 7-A: Gem- and Lithium-Bearing Pegmatites of the Pala District. California Division of Mines and Geology.
3. Mindat.org (n.d.). Pala Mining District, San Diego County, California, USA. [Data set]. https://www.mindat.org/loc-3560.html
4. Morton, D. M., Sheppard, J. B., Miller, F. K., & Lee, C-T. A., (2018). Petrogenesis of the cogenetic Stewart pegmatite-aplite, Pala, California: Regional implications. Lithosphere 11(1) 91-128. https://doi.org/10.1130/L1026.1
5. Pala, California. (2024, July 22). In Wikipedia. https://en.Wikipedia.org/wiki/Pala,_California

References for Box \(\PageIndex{3}\): Southern California’s Dinosaurs

1. Abbott, P. L. (1999). The Rise and Fall of San Diego. Sunbelt Publications.
2. Ford, T. L., & Kirkland, J. I. (2001). Carlsbad Ankylosaur (Ornithischia, Ankylosauria): An Ankylosaur and Not a Nodosaurid. In K. Carpenter (Ed.), The Armored Dinosaurs (pp. 239-260). Indiana University Press.
3. Hayes, G. (2017, September 24). Say Hello to California’s New State Dinosaur: Augustynolophus morrisi. Geotripper. https://geotripper.blogspot.com/2017/09/say-hello-to-californias-new-state.html
4. Katz, B. (2017, September 27). Meet California’s new State Dinosaur. Smithsonian Magazine. https://www.smithsonianmag.com/smart-news/meet-augustynolophus-morrisi-californias-new-state-dinosaur-180965038/
5. San Diego Natural History Museum (n.d.) Ankylosaur. https://www.sdnhm.org/exhibitions/fossil-mysteries/fossil-field-guide-a-z/ankylosaur/