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Southern California Geology

 

Surficial Geologic Materials in Southern California

 

Alluvial Deposits

Definition

Alluvium is:

  • ".a more or less stratified deposit of gravel, sand, silt, clay, or other debris, moved by streams from higher to lower ground." Encyclopedia of Geomorphology (Fairbridge, 1968, p. 10).
  • "A general term for clay, silt, sand, gravel, or similar unconsolidated detrital material, deposited during comparatively recent geologic time by a stream or other body of running water, as a sorted or semi-sorted sediment in the bed of the stream or on its flood plain or delta, [or] as a cone or fan at the base of a mountain slope.." Glossary of Geology (Bates and Jackson, 1987, p. 18). SCAMP classifies alluvial deposits into five major categories:
Wash deposits
Alluvial-valley deposits
Alluvial-fan deposits
Slope-wash deposits
Pediment-veneer deposits
Click here to view the SCAMP working classification of Quaternary alluvial deposits

Wash deposits (map units of Qw, Qyw, Qow, Qvow)

Definitions:

Geomorphologic usage .--"A term applied in the western U.S. (esp. in the arid and semi-arid regions of the SW) to the broad, gravelly, normally dry bed of an intermittent stream, often situated at the bottom of a canyon; it is occasionally filled by a torrent of water.." Glossary of Geology (Bates and Jackson, 1987, p. 730).

Sedimentologic usage .--"Loose or eroded surface material (such as gravel, sand, silt) collected, transported, and deposited by running water.." Glossary of Geology (Bates and Jackson, 1987, p. 730).

SCAMP usage .--SCAMP combines these two definitions of wash -one definition pointing to the geomorphic setting of the materials and the other definition pointing to the geologic character of the materials. Thus, modern wash deposits are unconsolidated sand-and-gravel deposits that occur in active channels of streams and rivers. The deposits typically have fresh flood scours and channel-and-bar morphology, and occur in the following settings:

  • as active deposits in steep-walled channels and arroyos incised into older alluvial units
  • as non-incised networks of active channels distributed across valley floors and alluvial fans
  • as thin active veneers that occur at the bottom of mountain canyons.

To the extent that all alluvial deposits are formed by running water (as defined above), then all alluvial-deposit types mapped by SCAMP are "wash deposits". Even the "alluvial-valley" and "alluvial-fan" units described below are "wash" deposits-given that they formed largely from the accumulation of channel-deposited sediment. However, it is not helpful to lump all alluvial deposits as "wash deposits", so SCAMP geologic maps differentiate among various deposit-types that owe their origin ultimately to stream deposition in channel washes.

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Alluvial-valley and alluvial-fan deposits

SCAMP's distinction between alluvial-valley map units and alluvial-fan map units is based on the two major kinds of recently deposited alluvium that we see around us on the southern California landscape today:

  • gently to moderately sloping cone-shaped fans of gravel and sand that have built out away from the mouths of mountain and foothill canyons;
  • relatively flat terraces of sand-and-gravel that underlie narrow to broad valley lowlands.

Not only does this differentiation apply to the modern landscape, but similar relations can be demonstrated for older inactive surficial deposits whose depositional and geomorphic history can be reconstructed from geologic clues contained in the deposits.

Classification of a surficial unit into either alluvial-valley or alluvial-fan categories is based on interpretation of its geomorphic setting , its genesis , and its physical properties (such as layering characteristics, grain size, sorting, depositional mechanisms). Distinguishing alluvial-fan and alluvial-valley deposits based on differences in their physical properties can be difficult (as between alluvial-valley deposits and deposits formed in the downstream toes of alluvial fans). General differences in physical properties between the two categories include:

  • alluvial-fan deposits typically are coarser grained and more poorly sorted than alluvial-valley deposits, and commonly contain debris-flow accumulations not found in the riverine settings of alluvial-valley flood plains;
  • alluvial-valley deposits typically have more layers of clay, silt, sand, and organic-rich sediment (peat) than do alluvial-fan deposits;
  • where modern surficial deposits (or their older counterparts) can be traced upstream toward known or inferred highland reaches, alluvial-fan deposits tend to coarsen in grain size and lose fine-grained interbeds more rapidly than do their alluvial-valley counterparts.

Alluvial-valley deposits (map units Qa, Qya, Qoa, Qvoa)

Definitions:

Geomorphologic usage. --SCAMP uses the term alluvial valley in a way similar to the Glossary of Geology's definition of alluvial plain:

  • "A level or gently sloping tract or a slightly undulating land surface produced by extensive deposition of alluvium, usually adjacent to a river that periodically overflows its banks.." Glossary of Geology (Bates and Jackson, 1987, p. 18).

Sedimentologic usage .--Deposits of relatively well stratified alluvium (gravel, sand, silt, and clay) that accumulate on the floors of axial valleys. These have a variety of physical properties and bed forms typical of deposition from fluvial (river) stream flows on gently sloping beds (e.g., Miall, 1996).

SCAMP usage .--We use the term "alluvial-valley" instead of the term "alluvial-plain" because of the misleading image invoked by the latter-i.e., broad flood plains akin to those of the US High Plains or Midwest. Locally in southern California such plains do occur (for example, lowlands of the Santa Ana River in the vicinity of San Bernardino or the Los Angeles River in the LA Basin). However, more common in the lowlands of southern California are narrow valleys having relatively straight longitudinal axes traversed by ephemeral streams. Tributary canyons commonly terminate in alluvial fans that debouch onto the valley floors of the through-going axial valleys. We believe the term "alluvial-valley deposits" best describes the alluvial materials deposited in these geomorphic settings. The alluvium can accumulate from meandering streams, braided streams, or relatively straight, through-going channels.

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Alluvial-fan deposits (map units of Qf, Qyf, Qof, Qvof)

Definition:

Geomorphologic usage .--An alluvial fan is "A low, outspread, relatively flat to gently sloping mass of loose rock material, shaped like an open fan or a segment of a cone, deposited by a stream (esp. in a semiarid region) at the place where it issues from a narrow mountain valley upon a plain or broad valley, or where a tributary stream is near or at its junction with the main stream.;it is steepest near the mouth of the valley where its apex points upstream, and it slopes gently and convexly outward with gradually decreasing gradient.." Glossary of Geology (Bates and Jackson, 1987, p. 17). (Note: this definition does not encompass the occurrence on some alluvial fans of sediment-gravity flow processes. Many "alluvial fans" are built up in part through the action of semi-viscous debris flows and mud flows).

Sedimentologic usage .--The deposits of well stratified to crudely stratified sediment (gravel, sand, silt) that accumulate on the surfaces of alluvial fans. These have a variety of physical properties and bed forms typical of deposition from fluvial (river) stream flows and from sediment-gravity flows (debris flows and mud flows) (see Bull, 1964a,b,c; 1972; 1977; Nilsen, 1982).

space-shuttle image of an alluvial-fan complex
This is a space-shuttle image of an alluvial-fan complex and associated playa in the Death Valley region (source: NASA photograph STS058-083-028, October 1993, obtained from the NASA Earth from Space archive of space-shuttle images).
 
a space-shuttle image of a single giant alluvial fan in the Gansu Province, China
This is a space-shuttle image of a single giant alluvial fan in the Gansu Province, China (source: NASA photograph STS048-610-034, September 1991, obtained from the NASA Earth from Space archive of space-shuttle images).
 

SCAMP usage .-SCAMP combines the geomorphic and sedimentologic definitions of alluvial fans in our classification and mapping of surficial deposits interpreted to have accumulated in alluvial-fan environments.

One problematic issue raised by mapping alluvial-fan deposits involves how to treat the alluvial-fan-cone deposits and equivalent deposits farther upstream in the canyon narrows: should these deposits be grouped in a single geologic-map unit or split into two separate units?

Without doubt, the alluvial-fan shape and its sedimentary fill are a distinct geomorphic and geologic entity situated at the mouth of a canyon or tributary valley. However, a narrow extension of the fan cone commonly extends for some distance up-canyon, much like a handle extends away from a frying pan. When geomorphologists and geologists describe alluvial-fan processes and sedimentologic structures, they usually refer to processes and structures that led to the construction of the fan cone itself. The narrow up-canyon "handle" usually is differentiated from the alluvial-fan "pan", and is classified instead as wash deposits or canyon alluvium.

This practice can pose a problem for the geologic-map maker in cases where the "pan" and its "handle" are continuous with each other and have similar geologic features (bedding thickness, sedimentary structures, particle size, bedforms). This typically occurs at the mouths of canyons and tributaries, where the fan-cone clearly extends without physical interruption into the lower canyon narrows: to differentiate the fan-cone and its intra-canyon extension as separate map units would require arbitrary placement of a boundary (geologic contact) between them-a placement that could not be justified on geologic grounds and could not be duplicated with any precision.

For this reason, alluvial-fan map units on most SCAMP geologic maps commonly contain not only the fan-cone at canyon mouths but also the narrow intra-canyon extension that is traceable upstream for a feasible distance.

Due to channel-cutting processes or other geomorphic and geologic agents, the alluvial-fan complex (fan-cone and its intra-canyon extension) commonly becomes isolated from local sediment terraces and strath-veneers higher up-canyon. During any given alluvial cycle, the latter once were part of a continuous sediment package that extended from canyon-head to canyon-mouth; however, upstream components now are isolated, relict bodies of sediment scattered along the canyon bottom. To include these isolated sediment bodies as part of the alluvial-fan complex would be to broaden the alluvial-fan concept farther than most geomorphologists and sedimentologists would accept. Therefore, SCAMP maps refer to these upstream deposits variably as units of wash (map units of Qw, Qyw, Qow, Qvow) or units of alluvial-valley alluvium (units of Qa, Qya, Qoa, Qvoa), at the discretion of the map author.

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Slope-wash Deposits (map units of Qsw, Qysw, Qosw, Qvosw)

Definition:

(a) Soil and rock material that is or has been transported down a slope by mass wasting assisted by running water not confined to channels. Cf: colluvium. (b) the process by which slope wash material is moved; spec. sheet erosion.." Glossary of Geology (Bates and Jackson, 1987, p. 621).

Some workers do not see the need for "slope-wash deposits" as a category of surficial deposit separate from "colluvial deposits" --the two are viewed as synonymous, with "colluvial deposits" taking precedence. However, the definition of "colluvium" embraces a considerable range of geologic processes and geologic products-so much so, that "colluvium" (broad sense) becomes a catch-all phrase for hillslope materials that are formed by any one of several mechanisms in any one of several different environments.

By contrast, some workers in semiarid to arid environments recognize hillslope geologic products that originate from a fairly specific type of geologic process-the flow of rainfall-induced sheeted runoff on low-angle slopes between channels (rills). Abrahams and others (1994, p. 177-180) refer this process as "interrill overland flow" , which they (p. 177) describe as:

  • ".a sheet of water with threads of deeper, faster flow diverging and converging around surface protuberances, rocks, and vegetation.

Abrahams and others (1994, p. 179) go on to say that:

  • "The tendency for threads of overland flow to increase in depth and velocity downslope coupled with the convergence (and divergence) of these threads around obstructions may lead to the formation of small channels or rills. Such features are very common on desert hillslopes, particularly where the underlying material is easily eroded."

Some SCAMP scientists working in Mojave Desert areas (e.g., the Twentynine Palms Marine Corps base; Matti and Morton, 1994, 1995) have encountered broad, low-angle hillslopes mantled with thin veneers of fine gravel, sand, and silt derived from underlying substrates. The upper surfaces of these veneers either are smooth or are traversed with switching and anastomosing shallow rills. These geologic materials almost surely were formed through the inter-rill overland-flow mechanism described by Abraham and others (1994).

SCAMP usage .--To distinguish generalized colluvial material from the more specialized alluvial material produced by the sheet-flow mechanism described by Abraham and others (1994), some SCAMP scientists distinguish "slope-wash deposits" from the more generalized "colluvial deposits" . No SCAMP consensus on this practice has been reached, however, and the map reader must determine from the map-unit description and from the digital database how the term "colluvium" is used by the map originator.

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Pediment-veneer Deposits (map units of Qpv, Qypv, Qopv, Qvopv)

Definition:

Pediment: "A broad gently sloping rock-floored erosion surface or plain of low relief, typically developed by subaerial agents (including running water) in an arid or semiarid region at the base of an abrupt and receding mountain front or plateau escarpment, and underlain by bedrock (occasionally by older alluvial deposits) that may be bare but are more often partly mantled with a thin discontinuous veneer of alluvium derived from the upland masses and in transit across the surface.." Glossary of Geology (Bates and Jackson, 1987, p. 487-488).

Pediment: ".a gently sloping erosional surface developed on bedrock or older unconsolidated deposits. This erosion surface may be subaerially exposed or covered by a discontinuous to continuous veneer of alluvial deposits. Its downslope limit may be defined.as that point where the deposit thickness exceeds a small fraction of the pediment length (e.g., 0.5 to 1.0%) or some arbitrarily defined maximum thickness, whichever is less." (Dohrenwend, 1994, p. 323).

thumbnail to view a photograph of a desert landscape Click on this thumbnail to view a photograph of a desert landscape forming the floor of the Qaidam Basin, China. Similar landscapes are common in the Mojave Desert of southern California, where island-like hills or mountains (inselbergs) project above a gently-sloping desert floor whose bedrock surface (pediment) may be only a few meters beneath pediment-veneers of sand (source: University of British Columbia Earth and Ocean Sciences (EOS) Image Gallery , photograph by Fletcher and Baylis)

Pediments and their associated veneers are widespread in the Mojave Desert region of southern California. Veneer-forming depositional processes include slope-wash, stream-flow, and eolian (wind) processes that yield various kinds of sand, silt, and gravel deposits. These can be lumped into a pediment-veneer unit as represented by the SCAMP surficial-materials classification, or they can be broken out into separate wash, slope-wash, and alluvial-fan subunits within the pediment-covering package.


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