KeywordsDebris Flow Rock Avalanche Fault Scarp Okavango Delta Lava Tube
Subaerial alluvial fans are “cone-shaped or fan-shaped depositional landforms created due to loss of flow competence anywhere that a river (channelized flow) is released from confinement and discharge or environment conditions promote avulsion” (North and Davidson 2012).
Alluvial fans are gently sloping semi-conical fan-shaped ramps that radiate from steep mountain area drainage outlets emerging into low-relief basins (Moore and Howard 2005) of reduced stream power. The cone-shaped deposit is fan shaped in plan view (Bull 1977). Alluvial fans have a concave longitudinal profile and a convex transverse (cross-fan) profile. Alluvial fans in places have stream channels incised into the fan material (Lecce 1990*). Alluvial fans are different from alluvial plains in that in fans the fluvial system is distributary, whereas in plains it is through-flowing (Stainstreet and McCarthy 1993).
Stream-flow (fluidal/riverine flow) dominated fans characterized by <20 % sediment concentration, low, <20° slope gradient and braided channels (e.g., Kosi Fan)
Debris-flow dominated fans produced by hyperconcentrated and debris flows, with sediment and debris concentration >20 %, steeper slopes (>20°), characterized by boulder lined levees, boulder fields (Bull 1977) (e.g., Death Valley fans)
Low sinuosity/meandering (“losi-mean”) river dominated fluvial fans: ultra low-gradient slope, tens of km across, highly vegetated, with permanent swamps and vegetated levees that stabilize the positions of channels and meander belts (e.g., Okavango Fan) (Stainstreet and McCarthy 1993)
Alluvial fan: Fan-shaped stream deposits that occur on mountain foot plains or broad valley floors (Fig. 1b). They are found at the base of mountain slopes, proximal to valley or canyon mouths. Their gradient is generally <10–15° near the apex, <1–5° near the toe, but may reach the angle of repose (~32°). On Earth they may be up to >100 km or longer. They are composed of gravel and sand, immature and mature. The coarsest debris is found in the proximal zone. They are deposited by water flow (braided streams) or debris flows (Blikra and Nemec 1998). Flood events result in overland flow: shallow and rapid water flow across interfluves resulting in thin-bedded, high-energy deposits.
Alluvial ramp: A low-gradient alluvial fan (Moore and Howard 2005).
Bajada (alluvial apron): Depositional piedmont formed from expansion and coalescing of adjacent alluvial fans in semiarid environment. These are distinguished from pediment that is a superficially similar, erosional landform (Lecce 1990*). Pediments are also usually mantled with a veneer of debris, which can be called alluvium.
Megafan (also fluvial fan): Unusually large alluvial fans (Leier et al. 2005) (e.g., Kosi River Fan, India) (Gohain and Parkash 1990). They are characterized by channels that have a through-flow of water, and discharge is continuous into a channel downstream (Nichols and Fisher 2007).
Terminal fan: Fan in arid climate on Earth where non-floodwater flow percolates into the ground (Bridge and Demicco 2012, p. 448).
Inland delta (obs.): This term was originally used for the Okavango Delta (Okavango Fan), Botswana, whose bird’s-foot pattern led to this naming during the colonial period. “Such a classification cannot be maintained because the fan does not debouch into it major water body” (Stainstreet and McCarthy 1993). Kosi Fan, India, was also termed inland delta by Gole and Chitale (1966) and was later reclassified as alluvial fan. Biologists, however, give this term a new meaning: Thieme et al. (2005, p. 47) redefined it as “an area where a river slows and may divide as it enters a flat area or a standing body of water, and a broad area of seasonal or permanent wetland is created.”
Intracrater fan: see Regional variations.
Alluvial fans are depositional landforms created where high-gradient, confined, sediment-laden streams empty subaerially onto low-gradient, unconfined surfaces (Hardgrove et al. 2009), a zone of reduced stream power (Harvey 2004). Flows decelerate and spread laterally due to change in gradient, triggering sediment deposition (Hugenholtz and Wan Bun Tseung 2007). Avulsion frequently occurs in initial channels and close to fan apex (but is also common further down) because of weakened levees. Channels incised at the fan head deposit sediments further down. Incision of channels depends on mountain uplift rates and sediment supply (Stainstreet and McCarthy 1993). Unconfined flows may develop into channeled flows or dissipate (infiltrate or evaporate). Alluvial fan deposition may be dominated by unconfined flows (e.g., in Death Valley) or by channeled flows (e.g., at Kosi River) (North and Davidson 2012).
Alluvial fans in many places form along high-angle faults (Burbank and Anderson 2011), but their formation does not necessarily require active tectonics and aridity (North and Davidson 2012). On Mars, they typically develop on the inner rim of impact craters (Kraal and Asphaug 2006).
Alluvial fans prograde by lobe building and lobe avulsion. Lobe switching leads to the map-view fan shape. Progradation results in a stratigraphy of coarsening-upward intervals truncated by abandonment, then buried by the next lobe-building cycle (Lecce 1990*). The rate at which alluvial fans grow is controlled by tectonic activity, climatic conditions, and rate of sediment supply (Zanchetta et al. 2004).
Primary processes transporting sediment from the drainage basin to the fan, resulting in fan construction or aggradation, manifested in the deposition of unique sediment lobes. Material of lobes is typically transported during intense runoff periods. Water may arrive at the drainage basin as rapid snowmelt-induced flash floods or intense cloudbursts (Moore and Howard 2005*). Processes involved are stream flow (channel and sheet floods) and mass wasting active in the drainage basin, for example, rockfalls, rock avalanches, landslides, debris flows, etc.
Secondary processes reworking previously deposited sediment (e.g., deflation, gullying, slope wash, piping, soil development, weathering, faulting, case hardening, bioturbation, etc.), resulting in fan erosion and degradation (Blair and McPherson 1994).
Alluvial fan aggradation on Earth can span several hundred thousand years (Williams et al. 2011*). On Mars, pristine craters lack fluvial modifications (Grant and Wilson 2012). The age of craters with fans seems to be confined to one period in Martian history (Modified Crater) (Mangold et al. 2012). Assuming hillslope erosion in the presence of precipitation, formation of large Martian alluvial fans, could take >108 years (in hyperarid conditions), 107–108 years (in arid conditions), or 106 years (in semiarid to temperate conditions), depending on climate scenario (Armitage et al. 2011).
Alluvial deposits: Alluvial fans can have a complex depositional history. Alluvial fans can develop multiple inset surfaces, with unique relief and height above the active channel, soil characteristics, rock coating (varnish), and degree of dissection (Ferrier and Pope 2012). The terms “alluvial deposits” and “alluvium” are poorly defined and loosely used in North America to give a name to unconsolidated debris such as found on alluvial fans and pediments. See also “Colluvium.” Once the sediment is buried, one may apply the appropriate rock name, i.e., conglomerate, diamictite, litharenite, etc.
Fan apex (fan head): the highest point of the fan where the feeder channel leaves its confinement (Lecce 1990*).
Incised channel: downslope extension of the feeder channel on the fan, commonly but not always present.
Fan intersection point: where the active incised channel emerges onto the fan. At this point the channel depth becomes zero and stream flow becomes unconfined below it (Hooke 1967).
Medial part of fan (mid-fan) with active and abandoned depositional lobes, where sediment aggradation occurs or have occurred.
Fan toe (the distal end of a fan): the intersection of an alluvial fan with its adjacent terrain (North and Davidson 2012).
Sandflat (Hardie et al. 1978) or distal-fan sandskirt (Blair and McPherson 1994) on stream-flow dominated fans: “flat unchannelled sandy apron at base of fan” where “floodwaters disperse as unchannelled, unconfined sheetfloods across a narrow flat (<1°slope) sand plain” that leads out to ephemeral lakes (Hardie et al. 1978). Type example: Copper Canyon fan, Death Valley. North and Davidson (2012) do not recommend the use of the term because of its ambiguity. This zone is also called floodout zone characterized by terminal floodouts (Tooth 1999) or terminal splays (Lang et al. 2004).
Additional units related to channels: distributary channels originating from the incised channel, headward eroding gullies at the distal part of fans that may eventually intersect the incised channel levees crevasse splays (Blair and McPherson 1994).
The fault scarp, characterized by talus cones.
Alluvial fan deposits can include talus, rock avalanche, and aeolian components. Deposits are coarse grained, poorly sorted due to short transport distance and mass wasting processes.
These intracrater fans originate from crater rim and deposit their apron into the crater floor (Kraal and Asphaug 2006). Fans extend 10–40 km downslope, descending >1 km with low gradient (2°). Many of them display long and narrow ridges radiating downslope interpreted as distributaries. Their steep gradient is suggestive of gravel-sized particles (Moore and Howard 2005).
Some of the alluvial fans on Mars apparently emplaced subaerially or into ephemeral or shallow body of water, while the steep-fronted ones may have emplaced into deeper standing water (e.g., in Eberswalde crater) (Grant and Wilson 2012).
In Margaritifer Terra, fans display series of ridges radiating from near their apex, interpreted to be inverted relief channels, resulted from differential erosion between channel and lobe materials that indicates aeolian erosion of exposed sand and possible silt-sized particles.
These attributes are consistent with formation by overland flow of fluids. It is proposed that their formation is related to the crater forming impact event that may have generated water as atmospheric precipitation or liberated it from ground sources and sustained fluid flows for surface runoff (Williams and Malin 2008; Goddard et al. 2012). Water may have been related to post-outflow channel activity redistribution of water from the lowlands to the highlands.
Holden crater fans are interpreted to have resulted from sudden precipitation (snow) and runoff, which terminated suddenly (Moore and Howard 2005). Snow was possibly concentrated into crater rim depressions by the wind as snowdrifts (Grant and Wilson 2012).
In Harris Crater several fans show signatures of fluvial emplacement, while one fan unit exhibits boulders consistent with a debris flow; this suggests a decline in available water and/or change in sediment supply (Williams et al. 2011).
Apollinaris Patera has a giant, 200 km-long deeply incised fan structure on its southern flank, thought to be composed of low-viscosity lavas or pyroclastic deposits (Kerber et al. 2011). Ghail and Hutchinson (2003) interpret it as volcanoclastic sediments emplaced by repeated pyroclastic flows, receiving water from a snowfall-fed caldera lake.
On Titan, several radar-bright dry valleys (Titan) terminate in fan-shaped radar-bright features that may coalesce and are interpreted as alluvial fans and bajada. Radar brightness differences in a fan system may result from different particles sizes that may indicate long-term changes in depositional processes (Jaumann et al. 2009; Burr et al. 2012; Radebaugh et al. 2013) (e.g., fans of Leilah Flucus, Elivagar Flumina).
Alluvial fans help “characterize the amount and style of runoff responsible for their formation and to constrain the climate in which they formed” (Grant and Wilson 2012; Mangold et al. 2012). Fan morphologies are indicative of climate conditions, precipitation rates, and transport characteristics (Radebaugh et al. 2013). Alluvial fans represent the distal end of an erosion/deposition system. Transition from a fluvial system to alluvial fans indicates break in slope and thus reveals local topography.
Alluvial fans that formed dominantly by fluvial, not debris-flow processes, provide evidence of water that may include standing bodies of water. The landing area of the Curiosity mission is located in the vicinity of a large alluvial fan (the Peace Vallis fan) inside Gale crater where water-carried sediments have been found (Palucis et al. 2013).
Typical alluvial fans are found in the Death Valley, California, while the largest on Earth is the Koshi River alluvial fan in the southern foreland of the Himalaya. Hugenholtz and Wan Bun Tseung (2007) proposed that interdigitate patterns of lobate deposits on Martian alluvial fans resemble terrestrial debris-flow dominated fans, and small-scale debris-flow fans developed in unconsolidated sand deposits.
History of Investigation
Alluvial fans were first discussed by A. Surell in 1841 (Lecce 1990*). Debris-flow was first recognized as alluvial fan building process by Blackwelder (1928) (Blair and McPherson 1994*). The investigation of alluvial fans reflects paradigms and paradigm shifts in geomorphology. In the pre-paradigm period landforms were described; terminology and classification criteria were defined. Explanations on their origin were speculative and qualitative. In this stage there is no common literature that can serve as a basis of discussion. The fist paradigm (evolutionary model) was based on the evolutionary concept of Davis, who emphasized stages. Eckis in 1928 proposed that alluvial fans are temporal features and indicate immature stage in the geomorphological evolution. This theory was replaced by the process paradigm emphasizing equilibrium processes and morphology in the 1960s (equilibrium model). These investigations were supported by quantitative methods. Initially authors proposed a time-independent steady state equilibrium; later studies suggested dynamic equilibrium between fans and their source areas. In this model, rates of erosion of the fan and deposition into the fan are in equilibrium, complicated by climatic and tectonic conditions. That is, aggradation occurs during humid periods, while dry climatic periods are characterized by trenching. In the 1970s observations showed that normal stream processes contribute only 10–15 % of the alluvial fan volume; its majority is deposited in catastrophic events (debris flows). Thus, early evolutionary and subsequent equilibrium concepts were integrated to form a third paradigm (Lecce 1990).
Origin of Term
The term “alluvial fan” was coined by F. Drew in 1873 (Lecce 1990*).
Lava fans are produced by volcanic processes. Lava fans may be produced by (1) lava tube breakouts, resulting from lava tube blockage, increases in lava flux, or changes in slope (Carr et al. 1977; Richardson et al. 2009) (Figs. 9 and 10), or (2) along thrust zones. Lava fans were observed on Olympus Mons and the Tharsis Rise volcanoes on Mars (Richardson et al. 2009; Bleacher et al. 2011) and on Venus associated with a festoon-type volcano (Moore et al. 1992). Lava fans are subaerial fans, unlike lava-fed deltas, which form subaqueously.
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