Alluvial Fan System Analysis Biology Essay

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Depositional systems develop over time and shape the many fluvial, marginal-marine and submarine (deep sea) environments on the earth. Facies models have been developed to detail these depositional systems and show the types of features found in the different environments (some may be palaeo-environments). Terrestrial alluvial fans and deep-sea sand fans are just two of these facies models developed and are the subject of this analysis and discussion. Even though there are many differences between each individual environment, some of the depositional bodies can be compared with each other. Looks can be deceiving and this is evident when looking at alluvial and deep-sea fans at surface value. Alluvial fans and deep- sea fans have underlying similarities and differences in the morphology, depositional processes, processes affecting sedimentation, architectural elements and the general development of the fans themselves. Deep-sea fans in readings have been called submarine and abyssal fans, which can be used interchangeably. "A submarine fan is a body of sediment on the sea floor deposited by mass-flow processes that may be fan-shaped." whilst "alluvial fans are cones of detritus that form at a break in slope at the edge of an alluvial plain." (Nichols 2009) Mere definitions of the two types of fans cannot detail how much one contrasts from the other, hence, further analysis is presented later on.

Similarities

The obvious indicator of some kind of similarity between the alluvial and deep-sea fans comes directly from the names. The term "fan" has been coined for these facies models because they both have the same general shape. As seen in Fig. 1,2 below, they resemble a cone or fan shape.

Fig 1 showing schematic diagram of an alluvial fan.

Fig 2 showing schematic diagram of a deep-sea fan

Deposition takes place where there is an abrupt change in the gradient. The fans can form at the base of a canyon and in the case of deep-sea fans (the canyon is submerged in a submarine setting). The two fan systems are divided into three parts: upper fan (inner fan), mid-fan and lower fan (outer fan). There are common morphological or architectural elements between the two fan types: incised channel, distributary channels and depositional lobes. Several kinds of primary processes act on these fan systems and these mass wasting processes are common to the two fans: fall, avalanche, slides and mass flows, mainly debris flows. Both climate and tectonics play major roles in the sedimentation of each fan type and controls these systems. However, these controls affect each in different ways and will be discussed in further analysis.

Differences

Where they are deposited?

Alluvial fans accumulate at the base of mountain front or any upland area where an emerging mountain stream deposits a sediment body. They are well known in arid or semi-arid regions but can form in any climate where there is a change in slope abruptly. "In humid regions, they may merge downslope with alluvial or deltaic plains and beaches or tidal flats or may build into a lake or the ocean." (Boggs 2006) The case study in the Lesser Antilles showed that the volcanic island had alluvial fans formed by the lahars of the humid region. Deep-sea fans are deposited at the base of the continental slope-rise break (at ~1500M i.e lower bathyal) on the abyssal plain. The climates where these fans are deposited are not limited to any specific region like that of alluvial fans. Since these fans are submerged, the action of the turbidity currents and the supply of sediment tend to be the main controls on the formation of deep-sea fans.

Geometry and Deposits of the Fans

Fig 3.1 shows the generalized model of alluvial fan (from Spearing,1974) : fan surface view

Fig 3.2 shows the generalized model of alluvial fan (from Spearing,1974) : cross-fan profile

Fig 3.3 shows the generalized model of alluvial fan (from Spearing,1974) : radial profile

Alluvial fans cone-shaped to arcuate (Fig3.1) and as the channels shift laterally over time. The clastic wedges are fan-shaped in plan or surface view and convex-upward in the cross-fan profile as seen in Fig 3.2. The radial profile of the fan is concave-upward from fanhead to fantoe with the greatest slope at the apex and decreases down the fan (Fig3.3).

As the turbidity current flows down the slope, it loses competency and starts to dump sediment to form a turbidite when velocity decreases (Fig 4.1). The turbidites that make up deep-sea fans deposit in a cycle. This is called the Bouma cycle as seen in Fig 4.2.

Fig 4.1 illustrates the transport and deposition process when a turbidite of a deep-sea fan forms.

Fines upward and flow velocity decreases

Te Clay - current no longer flows

Td Silt -silt laminations deposited from suspension

Tc - cross-bedded unit of fine sands

Tb - parallel laminations of sheet of medium sands

Ta - massive sand with slight grading and no structures

Lower flow regime

Upper flow regime

Fig 4.2 The Bouma Cycle of a Typical Turbidite.

Te is deposited between turbidity currents while Ta-d is deposited by the turbidity current. The thickness of Te depends on the amount of clay being introduced into the area and the time between the next turbidite. The turbidite progrades outward from shelf to the plain ( fines outward). At the proximal part, the whole cycle is shown while at the distal, only Td-e is shown. Te is found across the whole cycle as shown in Fig 4.3.

Fig 4.3 shows the radial profile of a typical turbidite of a deep-sea fan.

Sedimentation Process

"Alluvial fans form where there is a distinct break in topography between the high ground of the drainage basin and the flatter sedimentary basin floor " (Nichols 2009) A feeder canyon drains into the basin margin. The change in gradient allows water and sediment to spread out and the flow quickly loses competence (reduction in kinetic energy of the water) and deposits the sediment load. Over time, repetition of these depositional events will form a build up of sediment in the form of a segment of a cone radiating from the feeder canyon. Three main types of flows transport the sediments of the alluvial fan system: stream flow, debris flow and mudflow.

Stream flow (fluid flow) processes occur on all types of alluvial fans and make up the main transportation mechanism for stream-flow dominated fans. There are two types of stream flow processes; sheet flood and incised channel flow (Blair and McPherson, 1994b). Sheet flood is a broad expanse of unconfined, sediment- laden runoff water moving downslope, which is formed as a result of catastrophic discharge such as during very heavy rainfall. Incised-channel flow takes place through 1-4m high channels incised into the upper fan. The channels allow for downslope sediment transport of sediment-gravity flows and sheetfloods. After the deposition, subsequent surficial reworking can take place by discharge from rainfall or snowmelt, eolian (wind) activity and bioturbation by plants and animals.

"Debris- flow deposits are known to be poorly sorted and lacking in sedimentary structures" (Boggs 2006) except for the case where there are possible reverse graded bedding in the basal parts of the fan. The deposits consist of blocks of various sizes, which can be both impermeable and nonporous due to their high content of muddy matrix. Clast-rich and clast-poor debris flows can be differentiated from the other. Debris flows tend to "freeze up" and stop flowing after flowing after short distances of transport over lower slopes on the fans.

Mudflows are similar to debris flows. However, they consist mainly of sand-sized and finer sediments. Landslides are associated with debris flow and supply the sediment for this flow.

The sediment content of deep-sea fans is majorly dependent on the turbidite system. The architectural elements described are made up of different grain sizes of material depending on the characteristics and volume of the sediment supplied to the submarine fan.

Different combinations of deep-sea fans systems are possible but can be summarized into the following models: (1) gravel-dominated, (2) sandy, (3) mixed sand and mud, and (4) muddy, with the usual caveat that any intermediate form can exist. The examples shown in Figs 5.1-5.4 are for systems that have a single entry point supplying a fan-shaped body of sediment, but for each case there are also scenarios of multiple supply points, which form coalescing bodies of sediment that do not form an overall fan-shape (Reading & Richards 1994; Stow et al. 1996).

Deep-sea fan systems are commonly divided into the following areas:

upper fan (inner fan): dominated by channel and levee complexes

mid-fan : dominated by depositional lobes

lower fan (outer fan): dominates by sheets

Although these classifications are suitable for some examples (e.g. sandy and mixed systems) the divisions are not so suitable for gravelly and muddy systems.

Gravel-rich systems

Coarse sediment may be deposited at the edge of a basin in coarse-grained deltas supplied by braided river or alluvial fans. As seen in Fig. 5.1, the deeper parts of these deltas can merge into small submarine fans forming wedge-shaped bodies at the base of the slope. The gravel sediments are primarily deposited by debris flows with the sands being rapidly deposited by high-density turbidity currents. These fan bodies tend to pass abruptly into thin-bedded distal turbidites and hemipelagic mudstones.

Fig 5.1 diagram showing the facies model for a gravel-rich deep-sea fan.

Sand-rich systems

Fig 5.2 diagram showing the facies model for a sand-rich deep-sea fan.

A deep-sea fan system can be characterized as being sand rich if at least 70% of the deposited sediment is sandy material (Fig. 5.2). The source of the sediments is usually sand-rich shelves, which have already sorted the material by waves, storms and tidal currents. The mud has been removed from the sediments, leaving the sand-rich deposit that is reworked via the turbidity currents.

Since the sand-rich turbidity currents have low transport efficiency and do not travel very far, the fan body is expected to be relatively small, and can be less than 50 km in radius (Reading & Richards 1994). Deposition is known to be chiefly by high-density turbidity currents and the fan is characterized by sandy channels and lobes.

Channels with some lobes dominate the inner fan area. The mid-fan area is mainly coalesced lobes, often channelized. Due to the low transport efficiency the transition to finer grained sheet deposits of the lower fan is abrupt. Inactive areas of the fan (abandoned lobes) become blanketed by mud. Strata formed by these systems comprise thick, moderately widespread packages of sandy high-density turbidites separated by mud layers. The mud layers represent the periods of lobe abandonment.

Mixed sand-mud systems

Fig 5.3 diagram showing the facies model for a gravel-rich deep-sea fan.

Where a river/delta system provides large quantities of both sandy and muddy material, a mixed sand-mud depositional system results; these systems are defined as consisting of between 30% and 70% sand (Fig. 5.3). These higher efficiency systems are 10-100km in diameter and consist of well-developed channel levee systems and depositional lobes.

Deposits in the channels in the inner and mid-fan areas include lags of coarse sandstone, sandy, high-density turbidite beds and channel abandonment facies that are muddy turbidites (Reading & Richards 1994). They form lenticular units flanked by levee deposits of thin, fine-grained turbidites and muds. The depositional lobes of the mid-fan are very variable in composition, including both high- and lower density turbidites, becoming muddier in the lower fan area. In a sedimentary succession, the lobe deposits form very broad lenses encased in thin sheets of the lower fan and muds of the basin plain.

Muddy systems

Fig 5.4 diagram showing the facies model for a gravel-rich deep-sea fan.

The largest deep-sea fan systems in modern oceans are mud-rich (Stow et al. 1996), and are fed by very large rivers. Examples of these include the Bengal Fan fed by the Ganges and Brahmaputra rivers and the large submarine fan beyond the mouth of the Mississippi. These submarine fan systems are over 1000km in radius and consist of less than 30% sand (Fig. 5.4).

Channels are characteristically known to be the dominant architectural element of these systems. (Reading & Richards 1994) The channels deposits are sandy while on the channel margins, well-developed levees are seen made up of some sand and more mud deposits.

Depositional lobes tend to be poorly developed and thin. "The outer fan area consists mainly of thin sandstone sheets interbedded with mudrocks of the basin plain. In a succession of mud-rich fan deposits the sandstone occurrences are limited to lenticular channel units and isolated, thin lobes and sheets in the lower fan." (Nichols 2009)

Morphologic features

In an alluvial fan system, the major morphologic features are the drainage basin, feeder channel, apex, incised channel, distributary channels, intersection point and active depositional lobes. The drainage basin is said to be steep and low order and the apex of the fan is classically found in the mountain front. Deep-sea fans also have similar features associated with the fan channels; however, the details of these differ from those of alluvial fans. .

The fan apex is the highest, most proximal part of the fan and is located next to the feeder canyon from which the fan form radiates. A fan-head canyon may be incised into the fan surface near the apex. The depositional slope will usually be steepest in the proximal area. The slope over most of the fan may be about a degree, but it is a relatively steep depositional surface and there is a distinct break in slope at the fan toe, the limit of the deposition of coarse detritus at the edge of the alluvial fan. The fan deposits are thickest at the apex and taper as a conical wedge towards the toe.

The canyons that incise into the shelf edge direct a mixture of sediment and water to the specific areas of the basin where turbidity currents begin to flow down the canyon forming the channels of the deep-sea fan. This is the point where we being to notice one of the main differences between alluvial and deep-sea fans. The principal components of the deep-sea fan are incised channels (including submarine canyons, slope channels), leveed channel systems (with amalgamated sand in channels and turbidites in the levees), frontal splays and distal lobes. The following table shows where the elements are placed in the different types of deep-sea fan systems.

Channels

Lobes

Sheets

Gravel-rich systems

Poorly channelised

Sand-rich systems

Braided channels

Channelized lobes

Mixed sand and mud systems

Channel and levee complexes

Depositional lobes

Muddy systems

Channel and levee complexes

Depositional lobes

Sheets

Fig 6 detailing the different architectural elements on deep-sea fans as determined by the dominant grain size deposited on the fan.

The action of turbidity currents on deep-sea fans underlies the formation of the fan. The channel deposits are typically coarse sand and gravel. These deposits form thick, structureless or crudely graded beds characterised by Tab of the Bouma sequence. "The lateral extent of these turbidite beds is restricted by the width of the channel, which, when it is filled, forms a lenticular body made up of stacked coarse-grained turbidites." (Nichols 2009)

Most of an individual turbidity flow is confined to the channel but the upper, more dilute part can spill out laterally (similar to fluvial environments). The overbank flow contains fine sand, silt and mud and this spreads out into a fine-grained turbidity current away from the channel that makes a submarine channel levee. The levee turbidites are made up of the upper parts of Bouma sequences (Tc-e and Tde) and thin away from the channel margin and form a low-angle, wedge-shape. When the channel is aggrading (i.e. filling up with sediment and building up at the same time), the levee successions may build up to form thick bed units.

The formation of deep-sea fan's depositional lobes is also a form of turbidite deposit. The turbidity current spreads out at the distal ends of the channel to produce a turbidite deposit lobe as part of the fan surface. An individual lobe is constructed by a succession of turbidity currents that tend to prograde on the lobe through time. Usually if the deposition on the fan is ordered, this would be indicative of a simple progradational geometry, with each turbidity current having about the same magnitude and each depositing progressively further out from the channel mouth. However, realistically the currents vary in magnitude making the patterns rather complex. The more proximal parts of the flow tend to become channelized as the lobe and the lobe progrades up to when the channel avulses to another part of the fan. "Avulsion occurs because an individual lobe will start to build up above the surrounding fan surface and eventually flows start to follow the slightly steeper gradient on to a lower area of the fan." (Nichols 2009)

The important point to note is that the succession formed due to depositional lobe progradation is a coarsening-up succession capped by a channelized unit. Notice that the individual turbidites are normally grading but as the lobe progrades currents will carry coarser sediment further out. It could be inferred that coarser sediment are contained in successive deposits lending to a general coarsening-up pattern. Commonly this overall coarsening-up and thickening-up is not seen because of the complex, often random pattern of deposition on depositional lobes (Anderton 1995). The fan lobe, therefore, does not show any consistent vertical bed patterns. Depositional lobe deposits often contain the most complete Bouma sequences (Ta-e and Tb-e).

Turbidite sheets are highly common in the deep-sea fan system."Turbidite sheets are deposits of turbidity currents that are not restricted to deposition on a lobe but have spread out over a larger area of the fan." (Nichols 2009) These turbidite sheets are made up of fairly thin, fine-grained turbiditescharacterised by Bouma divisions Tc-e and Tde with lacking trends in grain size and bed thickness. Interbedding with hemipelagic mudstones is common.

Controls on Fan Processes

In alluvial fans, according to Blair and McPherson (1994), there are numerous processes that affect fan formation. Drainage basin bedrock lithology corresponds to the ability of the fan to resist weathering, grain size controls, fracture patterns and also process styles, such as colluvial vs. alluvial processes. The shape and size of the drainage basin controls the fan morphometry and drainage basin slop. Effects of neighbouring environments, such as, aeolian, lacustrine, fluvial, volcanic and marine interactions, control depositional environments. Also, the size of the basin controls scale of geomorphic process: large basins mean more storage points and small basins mean geomorphically active. Size is controlled by the sediment production rate and the rate of the development (that is, the vertical space where the sediment can collect; Heward 1978, Nilsen, 1982). Following the previous, the rate of sediment production is a function of the drainage basin area, rock types and climate. Fan volume is result of sediment production rate and the rate of development of accommodation space. These can be summarized as follows: Fan Area, Af is function of drainage area (Ad), source area lithology, climate and tectonic activity (Bull, 1964, 1968; Denny, 1965, 1967; Hooke, 1968).

Although alluvial fan deposits are not the most significant in a sedimentary basin in terms of volume, they are important because fan deposition is sensitive to tectonic and climatic controls. Alluvial fans develop at the margins of sedimentary basins and these can be sites of tectonic activity, with faults along the basin margin creating uplift of the catchment area and subsidence in the basin. It is therefore probable to see evidence of tectonic activity within an alluvial fan succession, such as an influx of coarse detritus onto the fan resulting from renewed tectonic uplift (Heward 1978; Nichols 1987). According to Ferril et al (1996), fans that form in tectonically active areas are affected by continental uplift and erosion of high land and tend to be supplied with the coarse sediment. Hence, they occur along rising fault scarps. Also, in a tectonically inactive setting, the rate of vertical accommodation space development is low.

A change in climate can also result in changes in the processes of deposition on a fan (Harvey et al. 2005): for example, with an increase in rainfall more water is available and this may result in a predominance of sheetflood and stream-channel processes, with less debris-flow events occurring. The character of the conglomerates deposited on the fan will reflect this climatic change, with more clast-supported and fewer matrix-supported conglomerate beds. A further factor controlling fan deposition is the nature of the bedrock in the catchment area: lithologies that weather to form a lot of mud will tend to generate muddy debris flows, whereas more resistant rocks will break down to sand and gravel, which is transported and deposited by sheetflood and stream-channel processes (Blair 2000a, Nichols & Thompson 2005). Ferrill et al (1996) believed alluvial fans in semiarid to arid climates tend to have length/width ratios of ~1-5 and tend to also have channels that are entrenched at the upstream end and that deposit downstream of the intersection point (that is, the intersection of fan surface and channel; Hooke, 19967; Heward, 1978).

The controls on deep-sea fan development have some differences when compared to those of alluvial fans. According to Bouma (2004), the turbidite systems have four key controls (that is, tectonics, climate, sedimentary characteristics and processes and sea-level fluctuations) which interact with each other. Fans form when there is sufficient sediment being supplied to the shelf margin. Since the sediment plays a major role in the formation, it follows that the type of sediment affects the development of the fan.

Their formation tends when the sea level is at a lowstand since the fall in sea level activates the fan due to an increase in sediment transport from the river and shelf toward the deeper parts of the basin. Also, this is corroborated by the fact that sediment is trapped in the nearshore zone (stopping the transport to the deep) during a relative rise in sea level (e.g., Stow, 1985; Mutti and Normark, 1987; Posamentier and Vail, 1988). However, the formation of turbidites is favoured during a highstand (e.g., Amazon Fan, Flood et al., 1991; Mississippi Fan, Kolla and Perlmutter, 1993; Navy Fan, Piper and Normark, 1983; Bengal Fan, Kuehl et al., 1989; Weber et al., 1997).

Two major end members of turbidite systems are coarse-grained/sand-rich and fine-grained/mud-rich. Coarse-grained fans typically belong to active margin settings. These fans progradegradually into a basin and show a decrease in thickness and grain size in the downflow direction. The sediment source is near the coastline, and the turbidite basins formed are generally small to medium. The fine-grained fans appear on passive and active margins and prograde rapidly into a basin. They deposit most of the input sand in the distal fan as oblong sheet sands. Tectonically confined basins normally have their sediment source nearby, and therefore will be filled with coarse-grained fans. Most of the open (unconfined) basins are medium to large in size, have their sediment source far from the coast, and therefore lose the coarser fractions during continental transport. Diapirically controlled basins are small- to medium-sized confined basins that have a fine-grained turbidite fill, but may not reveal the bypassing of the majority of the sand to the outer fan because of the abundance of sediment transport to the basin.

Hydrocarbon accumulation

According to Nilsen, alluvial fan deposits are not typically reservoir rocks for hydrocarbons because the deposits lack the connectivity laterally to the source rock, are not very deeply buried, do not have proper seals, have low permeability and porosities after diagenesis and tend to not have facies that are good source rocks. In the case of deep-sea fans, the restricted basins have the poor circulation and oxygen-depleted bottom water to favour rich accumulations of organic debris in hemipelagic muds. Howell and Normark in their findings found that these potential hydrocarbon source beds are interbedded with the coarser grained mass flow deposits, which are also most abundant in the more distal parts of the fans. The thick-bedded sands and gravels of channels and depositional lobes provide ideal conditions for the migration and storage of oil (Nelson and Wright, 1979).

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