HOME

TheInfoList




An alluvial fan is an accumulation of sediments shaped like a section of a shallow cone, with its apex at a point source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but are also found in more humid environments subject to intense rainfall and in areas of modern glaciation. They range in area from less than to almost .Leeder 2011, p.285 Alluvial fans typically form where flow emerges from a confined channel and is free to spread out and infiltrate the surface. This reduces the carrying capacity of the flow and results in deposition of sediments. The flow can take the form of infrequent debris flows or one or more ephemeral or perennial streams. Alluvial fans are common in the geologic record, such as in the Triassic basins of eastern North America and the
New Red Sandstone The New Red Sandstone, chiefly in British geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks ...
of south
Devon Devon (, archaically known as Devonshire) is a county A county is a geographical region of a country used for administrative or other purposesChambers Dictionary The ''Chambers Dictionary'' (''TCD'') was first published by William Ch ...

Devon
. Such fan deposits likely contain the largest accumulations of gravel in the geologic record.Leeder 2011, p.290 Some of the largest alluvial fans are found along the
Himalaya The Himalayas, or Himalaya (; Sanskrit: , "snow", "dwelling", "abode"), are a mountain range in Asia separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the ...

Himalaya
mountain front on the
Indo-Gangetic plain#REDIRECT Indo-Gangetic Plain Image:India-Pakistan Borderlands at Night.JPG, 250px, Clusters of yellow lights on the Indo-Gangetic Plain reveal numerous cities large and small in this astronaut photograph of northern India and northern Pakistan, ...

Indo-Gangetic plain
. A shift of the feeder channel (a ''nodal avulsion'') can lead to catastrophic flooding, as occurred on the
Kosi River The Kosi or Koshi ( hi, कोसी, , ne, कोशी, ) is a trans-boundary river A transboundary river is a river that crosses at least one political border, either a border within a nation or an international boundary. Bangladesh has the ...
fan in 2008.Leeder 2011, p.289


Size and geomorphology

Alluvial fans can exist on a wide spectrum of size scales, from only a few meters across at the base to as much as 150 kilometers across, with a slope of 1.5 to 25 degrees. The slope measured from the apex is generally concave, with the steepest slope near the apex (the ''proximal fan''Boggs 2006, p.247 or ''fanhead''Blatt ''et al.'' 1980, p.629) and becoming less steep further out (the ''medial fan'' or ''midfan'') and shallowing at the edges of the fan (the ''distal fan'' or ''outer fan''). ''Sieve deposits'', which are lobes of coarse gravel, may be present on the proximal fan. The sediments in an alluvial fan are usually coarse and poorly sorted, with the sediments becoming less coarse toward the distal fan. When there is enough space in the alluvial plain for all of the sediment deposits to fan out without contacting other valleys walls or rivers, an unconfined alluvial fan develops. Unconfined alluvial fans allow sediments to naturally fan out and the shape of the fan is not influenced by other topological features. When the alluvial plain is narrow or short parallel to depositional flow, the fan shape is ultimately affected. Wave or channel erosion of the edge of the fan sometimes produces a "toe-trimmed" fan.Leeder 2011, p. 282 When numerous rivers and streams exit a mountain front onto a plain, the fans can combine to form a continuous apron. In arid to semi-arid environments, this is referred to as a '' bajada'' and in humid climates the continuous fan apron is called a piedmont alluvial fan.American Geological Institute. ''Dictionary of Geological Terms''. New York: Dolphin Books, 1962.


Formation

Alluvial fans usually form where a confined feeder channel exits a mountain frontBoggs 2006, pp.246-248Leeder 2011, pp.285-289 or a glacier margin. As the flow exits the feeder channel onto the fan surface, it is able to spread out into wide, shallow channels or to infiltrate the surface. This reduces the carrying power of the flow and results in deposition of sediments. Flow in the proximal fan, where the slope is steepest, is usually confined to a single channel (a ''fanhead trench''), which may be up to deep. This channel is subject to blockage by accumulated sediments or
debris flow Debris flows are geological phenomena in which water-laden masses of soil Surface-water- gley developed in glacial till, Northern Ireland.">Northern_Ireland.html" ;"title="glacial till, Northern Ireland">glacial till, Northern Ireland. S ...
s, which causes flow to periodically break out of its old channel (nodal avulsion) and shift to a part of the fan with a steeper gradient, where deposition resumes. As a result, normally only part of the fan is active at any particular time, and the bypassed areas may undergo soil formation or erosion. Alluvial fans can be debris-flow-dominated or stream-flow-dominated.Boggs 2006, p.247Leeder 2011, pp.287-289 Which kind of fan is formed is controlled by climate, tectonics, and the bedrock lithology in the area feeding the flow onto the fan.


Debris-flow-dominated alluvial fans

Debris flows are a type of landslide that takes the form of a continuous, rapidly moving mass of water and material that is composed mainly of coarse debris. Typically, 20 to 80 percent of the particles in a debris flow are greater than 2 mm in diameter.   Debris-flow-dominated alluvial fans occur in all climates but are more common where the source rock is
mudstone Mudstone, a type of mudrock Mudrocks are a class of fine-grained Granularity (also called graininess), the condition of existing in granular material, granules or Grain, grains, refers to the extent to which a material or system is composed ...

mudstone
or matrix-rich
saprolite Saprolite is a chemically weathered rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock Bedrock in geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-l ...
rather than coarser, more permeable
regolith Regolith () is a blanket of unconsolidated, loose, Homogeneity and heterogeneity, heterogeneous superficial deposits covering solid Rock (geology), rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moo ...

regolith
. The abundance of fine-grained sediments encourages the initial hillslope failure and subsequent cohesive flow of debris. Saturation of clay-rich colluvium by locally intense thunderstorms initiates slope failure. The resulting debris flow travels down the feeder channel and onto the surface of the fan. Debris-flow-dominated alluvial fans are found to consist of a network of mostly inactive distributary channels in the upper fan that gives way to mid- to lower-level lobes. The channels tend to be filled by subsequent cohesive debris flows. Usually only one lobe is active at a time, and inactive lobes may develop desert varnish or develop a soil profile from eolian dust deposition, on time scales of 1,000 to 10,000 years.Leeder 2011, pp.287-288 Because of their high viscosity, debris flows tend to be confined to the proximal and medial fan even in a debris-flow-dominated alluvial fan, and streamfloods dominate the distal fan.Blatt ''et al.'' 1980, p.631 However, some debris-flow-dominated fans in arid climates consist almost entirely of debris flows and lag gravels from eolian winnowing of debris flows, with no evidence of sheetflood or sieve deposits. Debris-flow-dominated fans tend to be steep and poorly vegetated.Boggs 2006, p.248


Stream-flow-dominated alluvial fans

Stream flow processes take place on all alluvial fans but are the main process for sediment transport on stream-flow-dominated alluvial fans.Boggs 2006, p.248 Stream-flow-dominated alluvial fans occur where there is perennial, seasonal, or ephemeral stream flow that feeds a system of distributary channels on the fan. In arid or semiarid climates, deposition is dominated by infrequent but intense rainfall that produces flash floods in the feeder channel. This results in ''sheetfloods'' on the alluvial fan, where sediment-laden water leaves its channel confines and spreads across the fan surface. These may include
hyperconcentrated flowA hyperconcentrated flow is a two-phase flowing mixture of water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is t ...
s containing 20% to 45% sediments. As the flood recedes, it often leaves a lag of gravel deposits that have the appearance of a network of braided streams. Where the flow is more continuous, as with spring snow melt, ''incised-channel flow'' in channels high takes place in a true network of braided streams. Such stream-flow-dominated alluvial fans tend to have a shallower slope but can become enormous, and include the Kosi and other fans along the Himalaya mountain front in the Indo-Gangetic plain.Leeder 2011, pp.288-289 Here, continued movement on the Main Boundary Thrust over the last ten million years has focused the drainage of of mountain frontage into just three enormous fans.Leeder 2011, p.285 An example of an active stream-flow-dominated alluvial fan is found in the semi-arid region between the Kunlun and Altun mountain ranges that form the southern border of the Taklamakan Desert in northwest China. This particular fan is in total length. One lobe of the fan has flowing streams that are continually depositing sediment so that the fan is still prograding into the alluvial plain. The feeder channels consist of straight channels as well as instances of braided channels because of the large volume of sediment sourced from the local uplands.  


Alluvial fans in the geologic record

Alluvial fans are common in the geologic record, but may have been particularly important before the evolution of land plants in the mid-Paleozoic.Boggs 2006, p.249 They are characteristic of fault-bounded basins and can be or more thick due to tectonic subsidence of the basin and uplift of the mountain front. Most are red from hematite produced by diagenetic alteration of iron-rich minerals in a shallow, oxidizing environment. Examples of paleofans include the Triassic basins of eastern North America and the New Red Sandstone of south Devon, the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the H ...
Hornelen Basin of Norway, and the Devonian-
Carboniferous The Carboniferous ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other ...
in the
Gaspé Peninsula The Gaspé Peninsula, also known as Gaspesia ( French: ''Gaspésie'') is a peninsula A peninsula ( la, paeninsula from ' "almost" and ' "island") is a landform surrounded by water on most of its border while being connected to a mainland from ...
of Canada. Such fan deposit likely contain the largest accumulations of gravel in the geologic record.


Depositional facies

Alluvial fans are characterized by coarse sedimentation, though with an overall proximal to distal fining. Gravels show well-developed imbrication with the pebbles dipping towards the apex. Fan deposits typically show well-developed reverse grading caused by outbuilding of the fan. However, a few fans show normal grading indicating inactivity or even fan retreat. Normal or reverse grading sequences can be hundreds to thousands of meters in thickness.Boggs 2006, p.249 Depositional facies that have been reported for alluvial fans include debris flows, sheet floods and upper regime stream floods, sieve deposits, and braided stream flows. Debris flow deposits are common in the proximal and medial fan. These consist of coarse-grained massive gravel and blocks which contain relatively large portions of fine-grained matrix. Debris flow deposits lack sedimentary structure, other than occasional reverse-graded bedding towards the base, and they are poorly sorted.Boggs 2006, pp.247-249 The proximal fan may also include gravel lobes that have been interpreted as sieve deposits, where runoff rapidly infiltrates and leaves behind only the coarse material. However, the gravel lobes have also been interpreted as debris flow deposits.Boggs 2006, pp.247-249
Conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** Con ...
originating as debris flows on alluvial fans is described as ''fanglomerate''. Stream flow deposits tend to be sheetlike, better sorted, and sometimes show well-developed sedimentary structures such as cross-bedding. These are more prevalent in the medial and distal fan.Boggs 2006, p.248 In the distal fan, where channels are very shallow and braided, stream flow deposits consist of sandy interbeds with planar and trough slanted stratification.Blatt ''et al.'' 1980, p.630 The medial fan of a streamflow-dominated alluvial fan shows nearly the same depositional facies as ordinary fluvial environments, so that identification of ancient alluvial fans must be based on radial paleomorphology in a piedmont setting. Where alluvial fans are overlain by clay or marl sediments, they can be a potential trap for hydrocarbons and a possible exploration target.


Controls on depositional system evolution

Alluvial fans are built in response to erosion induced by tectonic uplift, and upwards coarsening of beds reflects cycles of erosion in the highlands feeding sediments to the fan. However, climate and changes in base level may be as important. Alluvial fans in the Himalayas show older fans entrenched and overlain by younger fans, which in turn are cut by deep incised valleys showing two terrace levels. Dating via optical stimulated thermoluminescence (OSL) suggests a hiatus of 70 to 80 thousand years between the old and new fans, with evidence of tectonic tilting at 45 thousand years ago and an end to fan deposition 20 thousand years ago. Both the hiatus and the more recent end to fan deposition are thought to be connected to periods of enhanced southwest monsoon precipitation. Dating of beds in Death Valley suggest that peaks of fan deposition during the last 25 thousand years occurred during times of rapid climate change, both from wet to dry and from dry to wet.Leeder 2011, pp.291-293


In arid climates

Alluvial fans are often found in
desert A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to the processes of ...

desert
areas often subjected to periodic
flash flood A flash flood is a rapid flooding of low-lying areas: Washland, washes, rivers, dry lakes and Depression (geology), depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ...
s from nearby
thunderstorms A thunderstorm, (also known as an electrical storm or a lightning storm) is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. (There are relativitly weak thunderstorms, ...

thunderstorms
in local
hill A hill is a landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the scie ...

hill
s. The typical
watercourse A watercourse is the channel Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Cou ...

watercourse
in an
arid A region is arid when it is characterized by a severe lack of available water, to the extent of hindering or preventing the growth and development Development or developing may refer to: Arts *Development hell, when a project is stuck in d ...

arid
climate Climate is the long-term pattern of weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a la ...

climate
has a large, funnel-shaped basin at the top, leading to a narrow defile, which opens out into an alluvial fan at the bottom. Multiple
braided stream A braided river, or braided channel, consists of a network of river channels separated by small, often temporary, island upright=1.15, Great_Britain.html"_;"title="Ireland_(left)_and_Great_Britain">Ireland_(left)_and_Great_Britain_(right),_a ...
s are usually present and active during water flows. Phreatophytes (plants with long tap
root In vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia artēria'' 'windpipe' + φυτά ''phutá'' 'plants'), form a large grou ...

root
s capable of reaching a deep
water table The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated. Th ...

water table
) characteristically form fan-toe phreatophyte strips. The phreatophytes may form sinuous lines radiating from the fan toe. These trace buried channels of coarse sediments from the fan that have interfingered with impermeable playa sediments.


In humid climates

Alluvial fans also develop in wetter climates. In
Nepal Nepal (; ne, नेपाल ), officially the Federal Democratic Republic of Nepal ( ne, सङ्घीय लोकतान्त्रिक गणतन्त्र नेपाल ), is a landlocked country in South Asia. It is ma ...
the
Koshi River The Kosi or Koshi ( hi, कोसी, , ne, कोशी, ) is a trans-boundary river A transboundary river is a river that crosses at least one political border, either a border within a nation or an international boundary. Bangladesh has th ...

Koshi River
has built a megafan covering some below its exit from
Himalayan foothills The Himalayas, or Himalaya (; Sanskrit Sanskrit (; attributively , ; , , ) is a of that belongs to the branch of the . It arose in South Asia after its predecessor languages had there from the northwest in the late . Sanskrit is t ...
onto the nearly level plains where the river traverses into
India India, officially the Republic of India (Hindi Hindi (Devanagari: , हिंदी, ISO 15919, ISO: ), or more precisely Modern Standard Hindi (Devanagari: , ISO 15919, ISO: ), is an Indo-Aryan language spoken chiefly in Hindi Belt, ...
before joining the
Ganges The Ganges ( ) (in India: Ganga ( ); in Bangladesh: Padma River, Padma ( )). "The Ganges Basin, known in India as the Ganga and in Bangladesh as the Padma, is an international river to which India, Bangladesh, Nepal and China are the riparian ...

Ganges
. Along the upper Koshi tributaries, tectonic forces elevate the
Himalayas The Himalayas, or Himalaya (; Sanskrit Sanskrit (; attributively , ; nominalization, nominally , , ) is a classical language of South Asia that belongs to the Indo-Aryan languages, Indo-Aryan branch of the Indo-European languages. It ar ...

Himalayas
several millimeters annually. Uplift is approximately in equilibrium with erosion, so the river annually carries some 100 million cubic meters (3.5 billion cu ft) of sediment as it exits the mountains. Deposition of this magnitude over millions of years is more than sufficient to account for the megafan. In
North America North America is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continen ...

North America
, streams flowing into Central Valley have deposited smaller but still extensive alluvial fans, such as that of the Kings River flowing out of the
Sierra Nevada The Sierra Nevada () is a mountain range A mountain range is a series of mountains ranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, and align ...
which creates a low divide, turning the south end of the
San Joaquin Valley The San Joaquin Valley ( ) is the area of the Central Valley of the U.S. state In the , a state is a , of which there are currently 50. Bound together in a , each state holds al jurisdiction over a separate and defined geographic ter ...
into an
endorheic basin An endorheic basin (; also spelled endoreic basin or endorreic basin) is a drainage basin A drainage basin is any area of land where precipitation In meteorology Meteorology is a branch of the atmospheric sciences Atmospheri ...
without a connection to the
ocean The ocean (also the sea The sea, connected as the world ocean or simply the ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
.


Flood hazards

The biggest natural hazard on alluvial fans are floods, hyperconcentrated flows, and debris flows, typically resulting from heavy and prolonged rainfall. Floods commonly take the form of short (several hours) but energetic
flash flood A flash flood is a rapid flooding of low-lying areas: Washland, washes, rivers, dry lakes and Depression (geology), depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ...
s that occur with little or no warning. These are characterized by high velocities and capacity for sediment transport. Debris flows resemble freshly poured concrete, consisting mostly of coarse debris. Hyperconcentrated flows are intermediate between floods and debris flows, with a water content between 40 and 80 weight percent. Floods may transition to hyperconcentrated flows as they entrain sediments, while debris flows may become hyperconcentrated flows if they are diluted by water. Because flooding on alluvial fans carries large quantities of sediment, channels can rapidly become blocked, creating great uncertainty about flow paths that magnifies the dangers. In August 2008 high
monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology ...

monsoon
flows breached the embankment of the
Koshi River The Kosi or Koshi ( hi, कोसी, , ne, कोशी, ) is a trans-boundary river A transboundary river is a river that crosses at least one political border, either a border within a nation or an international boundary. Bangladesh has th ...

Koshi River
, diverting most of the river into an unprotected ancient channel and across surrounding lands with high
population density Population density (in agriculture Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary Image:Family watching television 1958.jpg, Exercise tr ...

population density
that had been stable for over 200 years. Over a million people were rendered homeless, about a thousand lost their lives and thousands of hectares of crops were destroyed. The Koshi is known as the ''Sorrow of Bihar'' for contributing disproportionately to India's death tolls in flooding, which exceed those of all countries except
Bangladesh Bangladesh (, bn, বাংলাদেশ, ), officially the People's Republic of Bangladesh, is a country in South Asia South Asia is the southern region of Asia, which is defined in both geography, geographical and culture, ethno-c ...
.


In the Solar System


Mars

Alluvial fans are also found on
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
descending from some crater rims over their flatter floors. Three alluvial fans have been found in Saheki Crater. These fans confirmed past fluvial flow on the planet and further supported the theory that liquid water was once present in some form on the Martian surface. In addition, observations of fans in
Gale crater Gale is a crater Crater may refer to: Landforms *Impact crater, a depression caused by two celestial bodies impacting each other, such as a meteorite hitting a planet *Explosion crater, a hole formed in the ground produced by an explosion near ...

Gale crater
made by satellites from orbit have now been confirmed by the discovery of
fluvial In geography Geography (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ap ...
sediments by the
Curiosity rover ''Curiosity'' is a car A car (or automobile) is a wheeled motor vehicle Electric bicycles parked in Yangzhou's main street, Wenchang Lu. They are a very common way of transport in this city, in some areas almost outnumbering regular bic ...
.


Titan

Alluvial fans have been observed by the Cassini-Huygens mission on
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictional ...
using the Cassini orbiter's
synthetic aperture radar Synthetic-aperture radar (SAR) is a form of radar Radar (radio detection and ranging) is a detection system that uses radio waves to determine the distance (''ranging''), angle, or velocity of objects. It can be used to detect aircraft, ...

synthetic aperture radar
(SAR) instrument. These fans are more common in the drier mid-latitudes at the end of methane/ethane rivers where it is thought that frequent wetting and drying occur due to precipitation, much like arid fans on Earth. Radar imaging suggests that fan material is most likely composed of round grains of water ice or solid
organic compounds In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
about two centimetres in diameter.


See also

* * * * *


References and notes


External links

* {{DEFAULTSORT:Alluvial Fan Sedimentology Fluvial landforms Geography terminology