Geologic Setting of the Transverse Ranges Province

Although referred to collectively as the Transverse Ranges, the province consists of several discrete mountain ranges and intervening valleys, including:

• The Santa Ynez and Topatopa Mountains
• Oak Ridge and the Santa Susanna Mountains
• Santa Clara and Simi Valleys
• Santa Monica Mountains
• San Gabriel Mountains
• San Bernardino Mountains
• Little San Bernardino Mountains
• Pinto Mountains
• Eagle and Cottonwood Mountains

Geologists group these discrete landforms within the Western, Central, and Eastern Transverse Ranges.

• Prebatholitic Rocks
• Mesozoic Batholithic Rocks
• Post-batholithic Geologic Structures
• Upper Cenozoic Sedimentary Rocks
• Quaternary Surficial Materials
• Post-batholithic Geologic Structures

The main mass of the San Bernardino Mountains occurs east of the San Andreas Fault zone, although a thin slice of rocks native to the mountains occurs on Yucaipa Ridge outboard (southwest) of the Mill Creek strand of the San Andreas and has been displaced about 8 km northwest of its original position (Matti and others, 1985, 1992a; Matti and Morton, 1993).

Rocks of the main San Bernardino Mountains are similar to those in the Mojave Desert Province. They span a broad range of compositions and geologic ages and have been deformed by a variety of geologic structures. This web discussion describes these geologic materials and structures in terms of their relation to Mesozoic intrusions of granitic rock that formed major batholithic belts in this part of southern California.


Prebatholithic Rocks

Prior to intrusion of the first granitic magmas, pre-batholithic (pre-intrusion) rocks in the San Bernardino Mountains consisted of two packages: (1) very old crystalline rocks that represent the ancient Proterozoic North American continental crust as it existed more than 1.7 billion years ago, and (2) very old sedimentary rocks that were deposited in marine environments that lapped over the ancient Proterozoic continental mass during the Paleozoic and latest Precambrian eras, starting about 900 million years ago. The discussion below covers Proterozoic crystalline rocks, Late Proterozoic and Paleozoic metasedimentary rocks, metquartzite and metacarbonate sequences of undetermined ages, and the structure and metamorphism of these rocks.

Proterozoic crystalline rocks. These rocks consist of granitoid and gneissose rocks that constitute the Baldwin Gneiss of Guillou (1953). Three major rock types occur:

• well foliated to compositionally layered granitic gneiss that includes augen gneiss having "eye-shaped" crystals of potassium feldspar and (or) clots of quartzofeldspathic minerals;
  
  

  View of an outcrop of a compositionally layered (foliated) gneiss similar to Proterozoic gneiss in the San Bernardino Mountains (image source: S. Duncan Herron, Duke University).

  
  

  Photograph of compositionally-layered gneiss outcrop southeast of San Gorgonio Mtn in the San Bernardino Mountains, southern California (Section 16 of T.1S., R.2E.); pencil is about 6 inches (15 cm) long. This outcrop shows metamorphic rocks that look considerably different today than when the geologic materials first were formed. The rocks began life either as sedimentary materials deposited in a marine environment, or as igneous materials of granitic composition. Whatever their original origin, the parent rock (protolith) was subjected to high temperatures and strong directed forces (stress) that reconfigured the original mineral components into the conspicuous dark- and light-colored (mafic and felsic) layers that characterize the outcrop. This mineral segmentation into discrete layers is a type of foliation that geologists refer to as gneissose layering, and the rock generally is called "gneiss". Note the contortion of the mafic and felsic layers into small folds. Photo by J.C. Matti, USGS, June, 1978.

  
  

  A polished slab of a foliated augen gneiss similar to Proterozoic gneiss in the San Bernardino Mountains (image source: University of Cape Town, Rock Art Gallery).

  
  
• bodies of texturally massive to foliated equigranular to porphyritic plutonic rock;
  
  

  View a polished slab of a texturally massive to slightly foliated equigranular granitic rock similar to Proterozoic rock in the San Bernardino Mountains (image source: ©Marble and Granite, Inc).

  
  

  A polished slab of a texturally massive (non-foliated) porphyritic granitic rock similar to Proterozoic rock in the San Bernardino Mountains (image source: © R.M. Reed, University of Texas, Rob's Granite Page).

  
  
biotite-rich well-foliated compositionally layered gneiss that probably is metasedimentary in origin.



Foliated biotite gneiss

The first two rock types mainly are biotite-bearing and granodioritic in composition, are plutonic and metaplutonic in origin, and locally are cut by pegmatitic and quartz-rich dikes and veins (Butters, 1981). The plutonic rocks have a minimum age of about 1.7 billion years (Silver, 1971; Barth and others, 1993); gneissose metasedimentary rocks they intrude are even older (Barth and others, 1993).

Late Proterozoic and Paleozoic metasedimentary rock: A thick sequence of metasedimentary rock crops out locally in the San Bernardino Mountains. In the eastern part of the range, basal units of these rocks rest depositionally on top of the Baldwin Gneiss, which formed a broad crystalline platform of ancient North American basement on which the sedimentary materials were deposited in marine environments. Elsewhere in the range, the depositional relation with the Baldwin gneiss can only be inferred because the metasedimentary rocks occur as thin screens to thick pendants surrounded by Mesozoic granitoid rocks.

Although originally deposited as silt, sand, and calcareous mud and shoal-water buildups in marine environments, the sedimentary materials ultimately were dragged deeper into the Earth's crust, warped, folded, faulted, and metamorphosed in late Paleozoic time to yield the metasedimentary rocks we see today . These consist generally of a lower metaquartzite sequence and an upper metacarbonate sequence, both deposited in shallow marine environments of the ancient North American continental shelf. The metaquartzite sequence ranges in age from late Proterozoic to early Cambrian; the carbonate sequence ranges in age from early Cambrian through Pennsylvanian.

• Metaquartzite Sequence

• thick basal units of light-colored metaquartzite and conglomeratic quartzite, an interval of dark-colored phyllite, metasiltstone, and metaquartzite, and a light-colored sequence of quartz-sand-bearing limestone and dolomite and ripple-laminated quartz-rich metaquartzite and conglomeratic quartzite (some parts of this sequence are lithologically similar to the Stirling Quartzite as used by Stewart, 1970, in the southern Great Basin);
• an interval of dark-colored metasiltstone and phyllite interlayered with light-colored laminated and cross-laminated metaquartzite (lithologically correlative with the Wood Canyon Formation as proposed by Stewart and Poole, 1975);
• an interval of white, texturally massive metaquartzite (lithologically correlative with the Zabriskie Quartzite as proposed by Stewart and Poole, 1975).

• Metacarbonate Sequence

• Lower Cambrian limestone, calc-silicate rock, phyllite, and schist of the Carrara Formation ;
• Lower and Middle Cambrian Bonanza King Formation and various mappable units of white, light-gray, and dark-gray, laminated to texturally massive dolomite, dolomitic limestone, and limestone;
• the Middle Cambrian Nopah Formation , separated into a thin basal member of hornfels, phyllite, calc-silicate rock, and quartz-sand-bearing limestone of the Dunderberg Shale Member and white to buff colored laminated to texturally massive dolomite of the upper member;
• the Devonian Sultan Limestone , including dark colored dolomite of the Ironsides Member, white to buff colored laminated and texturally massive dolomite of the Valentine Member, and generally white limestone of the Crystal Pass Member;
• the Mississippian Monte Cristo Limestone , including interlayered dark- and light-gray limestones of the Dawn and Anchor Members, white limestone of the Bullion Member, and heterogeneous limestone and dolomite of the Yellowpine Member;
• the Mississippian and Pennsylvanian Bird Spring Formation , including a basal member of quartzite, siltstone, and impure limestone; a lower member of white coarsely crystalline limestone; a middle member of medium- and dark-gray, quartz-sand and chert-bearing limestone, and an upper member of light- and medium-gray limestone.

• Structure and metamorphism

Metaquartzite and metacarbonate rocks in the San Bernardino Mountains are complexly deformed, and have been metamorphosed to conditions that locally reach fairly high (amphibolite) grade. The rocks have been folded under ductile conditions and refolded into two- or more generations of open to tight folds, and are cut by numerous low-angle faults that have both older-over-younger and younger-over-older geometries (Cameron, 1981; Sadler, 1981; Matti and others, 1993). The Doble Fault of Guillou (1953) and the Santa Fe Thrust of Woodford and Harris (1928) are examples of such structures.

An outcrop of small-scale folds in metamorphic rocks similar to folds that have deformed metasedimentary rocks in the San Bernardino Mountains (image source: Ron Perkins, Duke University, Introduction to Geology).

These structures are pre-batholithic (developed prior to intrusion of granitic materials) : although some faults have been reactivated during Quaternary uplift of the range, fold and fault structures in the pre-batholithic rocks generally do not affect adjacent Mesozoic plutonic rocks, including Triassic hornblende monzonite that is the oldest Mesozoic plutonic rock in the region.

The Proterozoic Baldwin Gneiss beneath the metasedimentary sequence locally is involved in low-angle faults that cut the folded quartzite and carbonate rocks, but the unit appears to have been too competent (rigid and brittle) to fold easily. As a result of these ductility contrasts, the quartzite section appears to have broken away from the underlying Baldwin Gneiss and has slid along the original depositional contact along a low-angle fault that can be mapped throughout the north-central San Bernardino Mountains (Cameron, 1981, 1982; Sadler, 1981; Powell and others, 1983).

The ductile nature of the deformation, the persistent nature of the metamorphic recrystallization, and correlation of metamorphic mineralization with dynamic structures all point to regional dynamothermal metamorphism as the process that converted the sedimentary materials to their present metasedimentary form (Cameron, 1981, 1982). Thermal contact metamorphism locally affects the pre-batholithic rocks adjacent to contacts with Mesozoic granitic rock (Richmond, 1960; Cameron, 1981). However, contact metamorphism related to these granitic intrusions appears to be minimal.

Mesozoic Batholithic Rocks

In latest Paleozoic and Mesozoic time, the ancient continental margin of North America now exposed in the San Bernardino Mountains was subjected to successive intrusions of granitic rock called plutons that coalesced regionally to form a vast regional plutonic complex or batholith . These plutonic rocks can be grouped into two packages: (1) older latest Paleozoic and Mesozoic plutonic and volcanic rocks, and (2) younger Mesozoic plutonic rocks.

Older Mesozoic plutonic and volcanic rocks. These include:

• granitoid rocks of Permian and Jurassic age: These plutonic rocks are quartz-poor to quartzose alkalic and potassic rocks that occur in two main areas: (1) adjacent to the Mill Creek strand of the San Andreas Fault in the south-central part of the range, where a megaporphyritic hornblende-biotite monzogranite has a U/Pb intrusive age of about 215 million years (Triassic) (Frizzell and others, 1986); (2) in the north-central and northwest part of the range, where alkalic hornblende monzonite has yielded an 40Ar/39Ar minimum age of about 214 million years (Cameron, 1981). The Triassic granitic rocks are lithologically and chemically distinct from younger Mesozoic granitoids, with the hornblende monzonite in the north part of the range representing a distinctive suite of alkalic rocks that developed along the continental margin of western North America during early Mesozoic time (Miller, 1977a,b, 1978; Smith, 1982a,b).

• Jurassic hypabyssal (shallow crustal) plutonic rocks and volcanic rocks: Jurassic plutonic rocks occur locally in the central San Bernardino Mountains, and these may be related to a hypabyssal dike complex that crops out in the same region. The plutonic rocks are dioritic, quartz dioritic, and tonalitic in composition, and yield 40Ar/39Ar minimum ages of 126.7±3.5 Ma and 148.1±3.1 Ma and model Ar-retention ages of about 156 to 158 Ma (Cameron, 1981, p. 334). The hypabyssal dike complex was mapped and described by Richmond (1960) and Smith (1982a,b), who concluded that it was emplaced at shallow crustal levels within a northwest-trending fault zone that developed in prebatholithic rocks. Cameron (1981, p. 177-179) observed textural and compositional similarities between andesitic components of the dike complex and tonalitic, dioritic, and quartz dioritic shallow-level Jurassic plutonic rocks that he and Richmond (1960) mapped north of Big Bear Lake; Cameron proposes that the dike complex and the shallow-level dioritic rocks may be coeval and comagmatic. If Cameron is correct, then the dioritic rocks and the hypabyssal dike complex in the north-central San Bernardino Mountains may be part of the Late Jurassic Independence dike swarm of the Owens Valley region that James (1989) has recognized elsewhere in southern California.

Photograph of granitic-rock outcrops in the San Jacinto Mountains, southern California; this scene typifies a geomorphic landscape underlain by plutonic rocks of granitic composition. The bedrock has weathered into the rounded to angular shapes that form the landscape surface, including some large boulders lying on the surface that formerly were weathered bedrock outcrops that now have toppled over onto the landscape floor. The prominent outcrop on the middle skyline has a rounded left side and a sheer right side; the latter was created when the outcrop spalled off its once-rounded right side, which now lies on the ground at the right edge of the photograph (with a tree-trunk lying on it). The spalling process probably was facilitated by a fracture in the bedrock that allowed weathering agents to attack the fresh rock, and that allowed access by rain and snow. Expansion and contraction that accompany the freezing and thawing of trapped water are agents that can literally force a fracture to widen, and can lead ultimately to the collapse of bedrock outcroppings. Photo by J.C. Matti, USGS, June, 1978.

Photograph of granitic-rock outcrop in the San Jacinto Mountains, southern California; pencil is about 6 inches (15 cm) long. This rock is texturally massive to slightly foliated-that is, its mineral components (white and dark-colored specks) either are randomly distributed throughout the rock matrix, or they are slightly oriented (foliated) by fluid-flow or by directed forces. In the lower part of the rock below the pencil the dark-colored (mafic) minerals especially appear to be aligned into a sub-parallel to parallel orientation. Photo by J.C. Matti, USGS, June, 1978.

Photograph of granitic-rock outcrop (tonalite to quartz diorite) in the southeastern San Bernardino Mountains, southern California; pencil is about 6 inches (15 cm) long. This rock is well foliated-that is, its mineral components (white and dark-colored specks) have been aligned into a sub-parallel to parallel orientation, most likely by directed force (stress) that physically re-arranged the mineral crystals after they had crystallized and begun to cool. Photo by J.C. Matti, USGS, June, 1980.

A polished slab of a texturally massive, equigranular granitic rock similar to late Mesozoic granitic rock in the San Bernardino Mountains. This example probably is monzogranitic in composition (image source: © Marble and Granite, Inc).	This is a view of a polished slab of a slightly foliated equigranular granitic rock similar to Late Mesozoic rock in the San Bernardino Mountains. This example probably is monzogranitic in composition (image source: © Marble and Granite, Inc).	Click on this thumbnail to view a polished slab of a texturally massive, equigranular granitic rock similar to late Mesozoic granitic rock in the San Bernardino Mountains. This example probably is granodioritic in composition (image source: © Marble and Granite, Inc).

Plutonic and Metamorphic Complex

A poorly understood belt of gneissose crystalline rocks crops out in the western San Bernardino Mountains and along the south-central and southeast margins of the range. The dominant characteristic of these rocks is gneissose fabrics created by faint to conspicuous compositional layering of mafic-rich (dark-colored) and mafic-poor (light-colored) layers. Layering is developed at all scales, ranging from millimeter and centimeter lamination to layering on outcrop and hillside scales. Dark-colored layers are biotite-rich and typically are foliated; light-colored layers are quartzofeldspathic and texturally massive to foliated, depending on whether the fabric is equigranular or lenticular. Mafic-rich layers generally are granodioritic to tonalitic; mafic-poor quartzofeldspathic layers mainly are biotite-bearing granodiorite, but include monzogranite, tonalite, and less common quartz-poor rock that is quartz monzodioritic. Mappable bodies of plutonic rock occur throughout the gneiss terrane (Matti and others, 1992b). The crystalline complex locally is traversed by low-angle shear zones, and the rocks are highly fractured.

Granitic-rock outcrop (granodiorite to tonalite) in the San Bernardino Mountains; pencil is about 6 inches (15 cm) long. Photo by J.C. Matti, USGS.

This is a polished slab of gneissose and foliated rock similar to rock in the plutonic and metamorphic complex of the southern margin of the San Bernardino Mountains. It is not clear whether rocks of this type in the San Bernardino Mountains are deformed Mesozoic granitoids or older Proterozoic metasedimentary or metaigneous rocks. This example probably is granodioritic to monzogranitic in composition (image source: © Marble and Granite, Inc).

These rocks traditionally have been interpreted as a Precambrian gneiss complex intruded by Mesozoic plutons (Dibblee, 1975, 1982b; Rogers, 1967, who compiled the work of Dibblee, 1964b,c, 1967a,b, and unpublished). However, the origin and age of the gneissose terrane are problematical. Without question, gneissose fabrics of the crystalline complex reflect metamorphic deformation under dynamothermal conditions. However, it is not clear if all of these gneisses formed during Precambrian metamorphism of Precambrian protoliths. For example, the gneissose rocks locally are intermingled with bodies of calcite marble, calc-silicate rock, metaquartzite, and garnetiferous biotite-sillimanite schist (Miller, 1979) that may well be Paleozoic in age, in which case the enclosing gneissose rocks also may be high-grade Paleozoic metasedimentary rocks or even deformed and metamorphosed older Mesozoic granitic rocks. To reflect these relations, Matti and others (1992b) interpret this terrane as a metamorphic and plutonic complex that largely was created in Mesozoic time, even though it contains tracts of Precambrian and Paleozoic metamorphic rocks. By this interpretation, some of the gneissose rocks are deformed and metamorphosed granitic bodies of Mesozoic age.

Post-batholithic Geologic Structures

Post-batholithic geologic structures in and around the San Bernardino Mountains fall into three categories:

Late Miocene uplift structures: These are associated with uplift of the ancestral San Bernardino Mountains in late Miocene time (about 12 to about 5 million years ago) (Meisling and Weldon, 1982, 1989). These include the Squaw Peak thrust fault in the Cajon Valley region and east-trending north-dipping reverse faults and left-lateral faults in the western and central part of the mountains, including the Santa Ana Fault. The Santa Ana Fault is an east-striking reverse fault located in the interior of the range. Displacement on this fault has placed crystalline rocks against the Santa Ana Sandstone. The fault is obscured by landslides and colluvial debris along much of its length. Near the fault, the bedding of the Santa Ana Sandstone steepens and locally is overturned (Sadler, 1993).

San Andreas Fault zone: As discussed above, several strands of the San Andreas Fault zone traverse the southeastern San Bernardino Mountains and flank the southwestern base of the range (Matti and others, 1992a; Matti and Morton, 1993). Older strands of the zone include the Wilson Creek, Mission Creek, and Mill Creek faults; the modern trace of the fault in this region is represented by the San Bernardino strand. The older strands have generated considerable right-lateral displacements that over the last few million years have juxtaposed far-traveled crystalline basement rocks against the main mass of the San Bernardino Mountains. The modern San Bernardino strand is capable of generating large earthquakes, although the strand apparently did not rupture during the 1857 earthquake that occurred along the Mojave Desert segment to the northwest. Locally, as in the San Gorgonio Pass region and in the Yucaipa area, complexities in the San Andreas Fault have created associated reverse and thrust-fault zones and normal dip-slip fault zones (Matti and others, 1992a,b).

Quaternary uplift structures: Structures associated with Quaternary uplift of the range include the north-frontal fault zone (Meisling, 1984; Miller, 1987; Sadler, 1982a) and faults along the south part of the range that facilitated uplift (the San Gorgonio Pass Fault zone of Matti and others, 1992a). Meisling and Weldon (1989) indicate that uplift was accomplished in early Quaternary time by north-directed upward movements of the San Bernardino Mountains block along south-dipping low-angle structures that underlie the range. This uplift created the impressive topographic relief along the north face of the San Bernardino Mountains. Although largely complete by middle Quaternary time (Meisling and Weldon, 1989; Spotila and others, 1998), tectonism presumably associated with uplift of the range has continued into the late quaternary, giving rise to strike-slip and thrust-fault scarps that locally break late Quaternary alluvial deposits adjacent to the northern range front (Miller, 1987; Miller and others, 1998; Powell and Matti, 2000).

Upper Cenozoic Sedimentary Rocks

Isolated patches of upper Cenozoic sedimentary occur throughout the San Bernardino Mountains. The major outcrop belts occur in four main areas:

• The Santa Ana Sandstone and related deposits

• Deposits of the nonmarine Old Woman Sandstone

• Deposits of nonmarine Mill Creek Formation

The Mill Creek Formation (of Gibson, 1964, 1971 as used by Matti and others, 1992b) crops out extensively in the vicinity of Yucaipa Ridge and lower Mill Creek between the Mission Creek and Wilson Creek strands of the San Andreas fault. The sedimentary formation consists of mudstone, sandstone, and conglomerate deposited in alluvial-fan, riverine, and lacustrine (lake) environments during the Miocene Epoch . Woodburne (1975) reviewed permissive evidence supporting a late Miocene age for the unit, but its age is not well established. Layered strata of the formation are beautifully exposed in roadcuts along State Route 38 in Mill Creek Canyon and in the canyon cliffs above the creek.

Steep slopes underlain by the Mill Creek Formation are prone to failure, particularly where mudrock is interlayered with sandstone and conglomerate, and the formation is marked by numerous small and large landslides. Youthful and older landslides are particularly prominent west and east of State Route 38 in Mill Creek Canyon and along the southwest base of Yucaipa Ridge east of the mouth of Mill Creek.

• Deposits of the nonmarine Formation of Warm Springs Canyon

The Formation of Warm Springs Canyon is a name applied informally by Matti and others (1992b) to a poorly exposed sequence of nonmarine sandstone and conglomerate that crops out along the south margin of the San Bernardino Mountains from the Mill Creek Canyon area west to the Waterman Canyon area and beyond. The unit occupies the same structural position as unnamed sedimentary rocks mapped by Morton and Miller (1975, figs. 1c-1g) along the southwest margin of the San Bernardino Mountains. Gibson (1964, 1971) mapped this interval within his Mill Creek Formation (also see West, 1987), but Dibblee (1982b) referred to the rocks as Potato Sandstone and Hillenbrand (1990) as Potato Formation.

Sedimentary rocks of the Warm Springs Canyon formation crop out in a narrow belt between the Mill Creek and Wilson Creek strands of the San Andreas Fault. Its internal stratigraphy is broken and disrupted by faults confined to the formation's outcrop belt. No age has been determined for the rocks, but lithologic comparison with other nonmarine sedimentary rock sequences in the region suggests that it probably is late Miocene in age. The formation consists of heterogeneous sedimentary rocks dominated by sandstone, conglomerate, and conglomeratic sandstone interlayered with mudrock in some stratigraphic intervals.

The following text from Matti and others (1992b, p. 1-2) discusses uncertainties concerning how the Mill Creek and Warm Springs Canyon formations relate to each other spatially and geologically:

"Stratigraphic relations between the Mill Creek Formation and the Warm Springs Canyon formation east of the Wilson Creek fault are problematical, and are interpreted in different ways by different workers. Most investigators suggest a stratigraphic and paleogeographic link between the two units. For example, Dibblee (1982b, fig. 7) grouped the two units together within the Mill Creek Formation, and implied that they were deposited together elsewhere in southern California before being displaced jointly into the Mill Creek region by his north branch of the San Andreas fault (the Mill Creek strand of our usage). Demirer (1985), Sadler and Demirer (1986), and West (1987) elaborated this concept by proposing that the two sedimentary sequences are halves of a two-sided depositional basin that developed within a rift zone of the ancient San Andreas fault (also see Sadler and others, 1993). Despite the conclusions of these earlier studies, we maintain that sedimentary rocks we assign to the Mill Creek and Warm Springs formations are two distinct and different sequences separated by the Wilson Creek fault . We acknowledge lithologic similarities between them, but we emphasize three points that in our judgment warrant separation of the two units: (1) the Warm Springs Canyon formation is a sequence that overall is compositionally and texturally more immature than the Mill Creek Formation; (2) the geometry and pattern of sedimentary facies in the Mill Creek Formation is far more orderly than in the Warm Springs Canyon formation; and (3) we have not observed the two sequences to interfinger with each other as indicated by Demirer (1985) and West (1987).

Quaternary Surficial Materials

Alluvial deposits. Quaternary sand-and-gravel deposits are extensive in parts of the San Bernardino Mountains, including the following major outcrop belts:

• alluvial deposits on the south margin of the mountains that accumulated within the intra-mountain drainages of City Creek, Plunge Creek, and Santa Ana River. In the Barton Flats area of the upper Santa Ana River, alluvial-fan deposits are interlayered with riverine deposits and glacial-outwash deposits, all of which have been deformed by landslide processes that have created a complex geomorphic setting of scarps and hummocky ground (Sadler and Morton, 1989)

• the Big Bear Lake area, where lowlands now occupied by Big Bear and Baldwin Lakes have been the sites of Quaternary alluvial accumulation (note, however, that broad expanses of sedimentary material formerly interpreted as Quaternary in the Big Bear Lake region [for example, units interpreted by Richmond, 1960, as Quaternary talus deposits and by Sadler, 1981, as relict fanglomerate] are interpreted by Matti and others (1993) as mainly Tertiary in age)

• multiple alluvial-fan units that extend north onto Lucerne Valley from the north front of the San Bernardino Mountains (Miller and others, 1998; Powell and Matti, 2000).

Glacial deposits. Sedimentary deposits interpreted as morainal accumulations from late Pleistocene glaciers occur on the north side of San Gorgonio Mountain and to the west on San Bernardino Ridge (Sharp and others, 1959). Riverine glacial-outwash detritus is common on the upper parts of Barton Flats where it forms a sedimentary veneer overlying the Santa Ana Sandstone.

Landslide deposits. Landslides are common in the San Bernardino Mountains. Large landslide complexes occur locally along the steep south margin of the range (for examples see Miller, 1979, and Matti and others, 1992b). Extensive landsliding has occurred in the Barton Flats area (Sadler and Morton, 1989) where older surficial gravel units that overlie the Santa Ana Sandstone have failed and slid northward toward the low-lying Santa Ana River, creating a complex pattern of crown scarps and depressions on the Barton Flats landscape. The steep terrane of Sugarloaf Mountain has shed numerous large landslide masses (McJunkin, 1978; Powell and others, 1983; Sadler and Morton, 1989), as has the precipitous west-facing slope of the Granite Peaks massif (Matti and others, 1982b). Spectacular landslides have originated on the north side of the San Bernardino Mountains. These include the well-known Blackhawk landslide, first recognized in the 1920's (Woodford and Harriss, 1928). This landslide traveled northward from the mountains a considerable distance onto the desert floor by riding a layer of compressed air (Shreve, 1968). Older landslides similar to the Blackhawk slide have been shed off of the north face of the San Bernardino Mountains throughout late Quaternary time (Sadler, 1981, 1982a; Matti and others, 1993).

Aerial view looking east at the toe of the ancient Blackhawk landslide in Lucerne Valley.

Aerial view looking west at the toe of the ancient Blackhawk landslide in Lucerne Valley.

Post-batholithic Geologic Structures

Post-batholithic geologic structures in and around the San Bernardino Mountains fall into three categories:

Late Miocene uplift structures. These are associated with uplift of the ancestral San Bernardino Mountains in late Miocene time (about 12 to about 5 million years ago) (Meisling and Weldon, 1982, 1989). These include the Squaw Peak thrust fault in the Cajon Valley region and east-trending north-dipping reverse faults and left-lateral faults in the western and central part of the mountains, including the Santa Ana Fault. The Santa Ana Fault is an east-striking reverse fault located in the interior of the range. Displacement on this fault has placed crystalline rocks against the Santa Ana Sandstone. The fault is obscured by landslides and colluvial debris along much of its length. Near the fault, the bedding of the Santa Ana Sandstone steepens and locally is overturned (Sadler, 1993).

San Andreas Fault zone. As discussed above, several strands of the San Andreas Fault zone traverse the southeastern San Bernardino Mountains and flank the southwestern base of the range (Matti and others, 1992a; Matti and Morton, 1993). Older strands of the zone include the Wilson Creek, Mission Creek, and Mill Creek faults; the modern trace of the fault in this region is represented by the San Bernardino strand. The older strands have generated considerable right-lateral displacements that over the last few million years have juxtaposed far-traveled crystalline basement rocks against the main mass of the San Bernardino Mountains. The modern San Bernardino strand is capable of generating large earthquakes, although the strand apparently did not rupture during the 1857 earthquake that occurred along the Mojave Desert segment to the northwest. Locally, as in the San Gorgonio Pass region and in the Yucaipa area, complexities in the San Andreas Fault have created associated reverse and thrust-fault zones and normal dip-slip fault zones (Matti and others, 1992a,b).

Quaternary uplift structures. Structures associated with Quaternary uplift of the range include the north-frontal fault zone (Meisling, 1984; Miller, 1987; Sadler, 1982a) and faults along the south part of the range that facilitated uplift (the San Gorgonio Pass Fault zone of Matti and others, 1992a). Meisling and Weldon (1989) indicate that uplift was accomplished in early Quaternary time by north-directed upward movements of the San Bernardino Mountains block along south-dipping low-angle structures that underlie the range. This uplift created the impressive topographic relief along the north face of the San Bernardino Mountains. Although largely complete by middle Quaternary time (Meisling and Weldon, 1989; Spotila and others, 1998), tectonism presumably associated with uplift of the range has continued into the late quaternary, giving rise to strike-slip and thrust-fault scarps that locally break late Quaternary alluvial deposits adjacent to the northern range front (Miller, 1987; Miller and others, 1998; Powell and Matti, 2000).

See Geologic Framework of the San Bernardino National Forest - Selected References

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