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Southern California Geology

Geologic Setting of the Transverse Ranges Province

San Bernardino Mountains

Prebatholithic 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;

    outcrop
    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).

    A gneiss outcrop
    A compositionally-layered gneiss outcrop southeast of San Gorgonio Mtn in the San Bernardino Mountains, southern California; pencil is about 6 in (15 cm) long. Photo by J.C. Matti, USGS.

    Polished slab
    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 of rocks
  • bodies of texturally massive to foliated equigranular to porphyritic plutonic rock;
    Another polished slab
    Click on this thumbnail to 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
    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

foliated biotite gneiss (unknown source).

 

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
Metaquartzites in the San Bernardino Mountains first were studied by Vaughan (1922) who grouped them within his Saragossa Quartzite. Working in the north-central part of the range, Guillou (1953) applied the name Chicopee Formation to the upper part of Vaughan's Saragossa Quartzite in order to distinguish distinctive quartzite and non-quartzite rock types that occur in that part of the sequence. Richmond (1960) adopted Guillou's nomenclature in revised form (Chicopee Canyon Formation), but for purposes of his regional mapping Dibblee (1964) continued to apply the name Saragossa Quartzite to all the quartzitic rocks.
Stewart and Poole (1975) first pointed out affinities between the San Bernardino Mountains quartzite sequence and similar rocks in the Great Basin and Mojave Desert Provinces, and later workers applied some of the southern Great Basin rock names in the San Bernardino Mountains (Tyler 1975, 1979; Cameron, 1981, 1982). However, lithologic and thickness differences between parts of the quartzitic sections in the San Bernardino Mountains and those in the southern Great Basin led Cameron (1981, 1982) to break out several new quartzitic formations within Vaughan's (1922) old Saragossa Quartzite, and Cameron (1982) grouped these formations within his Big Bear Group.
Although workers don't all agree about the nomenclature applied to the quartzitic sequences, all recent workers have split out multiple rock units in order to portray the considerable lithologic variation present within Vaughan's (1922) Saragossa Quartzite. In ascending order, this variation includes:
  • 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).
This sequence and nomenclature is usable mainly in the central San Bernardino Mountains where the quartzitic sections are relatively well preserved (Sadler, 1981); elsewhere in the range, the metaquartzite sections are highly intruded by Mesozoic plutonic rocks, are less complete, and are structurally more complex.
  • Metacarbonate Sequence
Metamorphosed limestone and dolomite in the San Bernardino Mountains first were studied by Vaughan (1922) who grouped them within his Furnace Limestone. Various informal units of the formation have been mapped by Guillou (1953), Richmond (1960, his Furnace Formation), Dibblee (1964), Hollenbaugh (1968), and Sadler (1981).
Stewart and Poole (1975) concluded that parts of the Furnace Limestone are lithologically similar to Paleozoic carbonate rocks of the Mojave Desert and Basin and Range Provinces, and Tyler (1975, 1979) first applied nomenclature other than Furnace Limestone to some of these rocks. He proposed that the lower part of Vaughan's Furnace Limestone is lithologically correlative with the Carrara and Bonanza King Formations of the southern Great Basin, a precedent followed by Cameron (1981, 1982). Cameron (1981) also indicated that the Furnace Limestone above the Bonanza King Formation included rocks like those in Devonian and Carboniferous units in the southern Great Basin, and Brown (1984a,b, 1987, 1991) mapped these units and determined their stratigraphy and formational contacts (see geologic maps by Matti and others, 1993, and Miller and others, 1998).
From oldest to youngest, the metacarbonate sequence in the north-central San Bernardino Mountains includes:
  • 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

An outcrop of small-scale folds in metamorphic rocks 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 webpages)

 

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.

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.

 


Continue to Mesozoic Batholithic Rocks

Selected References
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