Alluvial Fan Environments

Introduction https://youtu.be/C7QtGpzgESQ (adsbygoogle = window.adsbygoogle || []).push({}); Alluvial fan represent an environment of sudden transition, where narrow, steep mountain or upland valleys disgorge sediment onto an alluvial plain. Rapid erosion supplies coarse, often bouldery detritus, which builds a debris cone out from the mountain base. (Figure 1, Block diagram showing alluvial fan deposition in a semiarid to arid environment.) Thus, both on the earth's surface and in the sedimentary record, fans are commonly banked against structurally complex mountain fronts. FIGURE 1 At times, they are juxtaposed with older, highly deformed sediments; at times, directly with basement. In gross compositional aspect, fans will usually show an upward progression that reveals the order in which material of the rising mountain block was attacked by erosion. Alluvial fans are often referred to as syntectonic ("syn" = during) deposits, being generated by tectonic regimes of rapid uplift, and should be expected to have developed in almost any setting where orogenic activity has occurred inland from coastal areas. Where aggressive mountain building has taken place along the margins of marine basins, fans commonly prograde from a strictly terrestrial setting into a more transitional-marine regime, known as a fan delta. In arid and semiarid climatic settings, alluvial fans often develop without significant depositional interruption until they coalesce laterally to form bajadas. These have extensive, sheet-like geometries with internal depositional complexity. Fanglomerate is the rock term employed for the coarsest, and in some ways, most diagnostic, proximal lithology of the fan environment. Much of the sedimentation that takes place on the fan itself is done by the work of ephemeral braided streams; accurate analysis, therefore, recognizes a normal, though partial, correspondence between the alluvial fan and braided alluvial environments. For the sake of relative convenience, we treat these facies separately. The student should, however, be aware that this introduces a certain degree of artificiality into normal facies analysis. (adsbygoogle = window.adsbygoogle || []).push({}); In the sedimentary record, preserved fan deposits are particularly evident in deposits of Late Paleozoic - Early Mesozoic and Late Mesozoic - Mid-Tertiary age. These represent times of significant tectonic activity throughout much of the world. In North America, the Pennsylvanian-Permian time span is especially characterized by fan and fan delta development in the Appalachian, Rocky Mountain, and New Mexico-Texas-Oklahoma regions. Generally speaking, the style of deposition on alluvial fans prevents them from acting as good reservoirs. To date, there are only a few, clear-cut examples of fields producing from terrestrial fan facies. (Fan deltas, meanwhile, are productive in many parts of north Texas and southern Oklahoma. Refer to Moore (1979) for an overview of Late Paleozoic paleogeography in this region.) They are often, however, extremely important to recognize and delineate in the subsurface due to their indication of both tectonic setting and source area composition. Given this, and the fact that fans commonly grade into alluvial plain environments - whose sediments have far greater potential to be good reservoirs — alluvial fans can serve as associated fades of crucial significance. (adsbygoogle = window.adsbygoogle || []).push({}); Depositional Processes and Lithologic Characteristics (adsbygoogle = window.adsbygoogle || []).push({}); Sedimentation on an alluvial fan takes place mainly by a combination of: braided stream channel flow debris and mud flow (gravity sliding) sheet flooding Most modern and ancient fans, if adequately mapped, can be divided into proximal and distal subfacies. For the first of these, proximity to the sediment source, very steep drainage slopes, and often irregular, but high discharge makes for deposition of detritus that is typically an unsorted mixture of boulders, gravel, sand, clay, and possibly organic (soil) material. (Figure 1, Plan view of cross section of a single canyon and the alluvial fan which developed at its mouth, Van Horn Sandstone, Precambrian (?) Texas. FIGURE 1 Shown are the downfan decrease in slope and grain size, as well as the higher concentration of sedimentary structures in the distal portions. The overall progression grades from massive conglomerate/gravel of the proximal fan, through alternating trough and foreset crossbedded sand of the midfan, to finer-grained, more thinly bedded crossbedded sands of the distal fan.) This type of sedimentation occurs right up against the mountain front itself. In addition to high energy stream deposition, landsliding, in the form of debris and mudflow, is also common. Down-fan, where braided channel flow begins, more gravelly beds alternate with crossbedded sands. With greater distance from the mountain front, the proportion of sand increases, and the shallow cut and fill of braided channel deposits predominates. Mudflows may occasionally spread finer-grained clay material over parts of the fan. Preserved alluvial fan complexes, therefore, usually show the overall lateral and vertical progression shown in Figure 1. Vertical sections are usually dominated by a large number of stacked sequences that each show an upward change from conglomerate to coarse and medium-grained sand. Each sequence is truncated abruptly by the next overlying sequence, indicating the lateral shifting of the braided channels over the fan. Most identification of fan facies in the rock record, however, has concentrated on the distinctive fanglomeratic lithologies. (Figure 2, Photograph of fanglomerate, Colorado. This particular section was part of a fan system that spread from the basement block uplifts of the Ancestral Rocky Mountains in Pennsylvanian time.) (adsbygoogle = window.adsbygoogle || []).push({}); FIGURE 2 A number of specific sedimentary structures will be found in fan sediments. These will be concentrated in channels, where current action is most dominant. Cross-bedding, pebble imbrication and long axis orientation, current lineations (from smaller grain orientation, for example), dune and ripple morphology are those most commonly found. In addition, the long dimension of channels can be used to derive paleocurrent patterns. These will show the overall morphology of the fan, since most stream flow is directly down the steep slopes, i.e., "fanned" out. Detailed paleocurrent information is not often systematically collected for fan deposits. The downfan (basinward) direction is usually fully evident from large-scale stratigraphic parameters, or from seismic and structural data. Paleontology As might be surmised, macrofossils are relatively rare in alluvial fan complexes. Some vertebrate bones and plant fragments may exist, the result of sudden burial due to flooding. Normally, however, the depositional caprice of the fan environment prevents any significant floral or faunal colonization. Microfossils, such as spores and pollen, have a far better chance of being incorporated. At the same time, percolating groundwater within the fan would be acidic and these often-valuable remains have a strong chance of being strongly oxidized, thus partially dissolved and often altered beyond recognition. (adsbygoogle = window.adsbygoogle || []).push({}); Geometry (adsbygoogle = window.adsbygoogle || []).push({}); The size and detailed morphology of fans will vary according to climate, source rock, basin architecture, and the degree and style of tectonic activity. Individual fans are normally wedge-shaped in plain view and concave-upward in cross section. (Figure 1 Plan view of cross section of a single canyon and the alluvial fan which developed at its mouth, Van Horn Sandstone, Precambrian (?) Texas. FIGURE 1 Shown are the downfan decrease in slope and grain size, as well as the higher concentration of sedimentary structures in the distal portions. The overall progression grades from massive conglomerate/gravel of the proximal fan, through alternating trough and foreset crossbedded sand of the midfan, to finer-grained, more thinly bedded crossbedded sands of the distal fan.) They may also reveal a lens-like morphology in the subsurface. Fans can have radii that range from hundreds of feet to tens of miles. Bajadas can be over 100 miles long, though considerably narrow in width. Sections have been measured up to several thousand feet thick, with separate conglomerate-sand sequences showing lateral continuity of up to about 1500 ft. Clay layers are much more localized and thin. Commonly, a fan or fan complex will be thickest at its proximal end, having, for example, filled in a rapidly subsiding local graben. (adsbygoogle = window.adsbygoogle || []).push({}); Associated Facies Since many fans grade laterally into alluvial plains (Figure 1, FIGURE 1 (adsbygoogle = window.adsbygoogle || []).push({}); Distribution of different rock types in an actual alluvial fan complex of eastern Pennsylvania and Figure 2, Cross sections of alluvial fans showing common associated facies. FIGURE 2 Note that interfingering with braided stream alluvium may be subtle, barely detectable), their more distal channel deposits and sheet-flood material are not easily distinguished from those of braided streams. In general, alluvial deposits are characterized by sediments of finer-grain size, better sorting and rounding, and more abundant sedimentary structures. In arid environments, fans may be associated with the local salt deposits of ephemeral playa lakes. As discussed, the proximal end of most fans is directly adjacent to a mountain front, and this conspicuous association is usually considered to be diagnostic. (adsbygoogle = window.adsbygoogle || []).push({}); Diagnostic Evidence (adsbygoogle = window.adsbygoogle || []).push({}); Seismic Vertical seismic sections through alluvial fan complexes typically show discontinuous internal reflectors. (Figure 1, Seismic section through a probable alluvial fan that developed over structurally deformed basement. FIGURE 1 Note the relatively poor internal seismic character to the deposit.) This should be expected, given the great lateral and vertical variation in lithology. Furthermore, the concave-upward ideal profile of an individual fan is more often subdued in the subsurface, due to postdepositional compaction and tectonic tilting. Bajadas are more extensive and sheet-like and will display only a vague fan-type profile over their width. Where fan development continued for considerable periods of time, the original mountain front may be worn down and completely buried in its own debris; in this case, fan sediments are likely to grade upward into braided stream alluvium, which is also characterized by poor internal reflections. Seismic profiles can be very useful in delineating old mountain fronts and often the extent of the immediately adjacent sediments they produced. Specific lithologic types within the fan itself, however, are not normally discernible. Wireline Logs (adsbygoogle = window.adsbygoogle || []).push({}); Most geologists maintain that alluvial fan facies generate no important diagnostic response on wireline logs. The diverse processes acting to create and prograde the fan result in considerable vertical and lateral change, which means that log profiles will be correspondingly varied and therefore difficult to correlate. On the other hand, it should not be overlooked that this type of unpredictable log response showing substantial changes between even closely spaced wells, might itself be considered a characteristic of fan deposits. Dipmeter logs (Figure 2, Idealized gamma ray and dipmeter logs for an alluvial fan sequence, showing both fanglomerate and channel development. FIGURE 2 Note the three major patterns: lowest green dips, which represent shale breaks and correspond to spikes on the gamma ray, curve; random "bag o'nails" dips in fanglomerate; and dip clusters that show an upward-increasing blue pattern in channel sands.) will most often show a "bag o' nails" pattern for fanglomerate. A green ("shale", i.e., structural slope) pattern may be discernible, but depositional slopes are frequently high (5° to 25° ), and this could confuse correlations. Due to the braided stream flow on a surface of radiating geometry, dip switches are characteristic. Frequency diagram plots, ideally, should show approximately a 180° azimuth sweep with, perhaps, a subdued bisecting maximum that represents the influence of the feeding stream. Figure 2 shows an idealized dipmeter profile through a fan. Several shale layers are indicated simultaneously by spikes of high radioactivity on the gamma ray log and low dips (green motif) on the dipmeter log. These shales separate three channels, whose tadpole patterns show a clustering at a higher magnitude of dip. This is due to cross-bedding. These channels overlie the random "bag o'nails" pattern of fanglomerate, and thus show the outward progradation of the fan. Cores The general coarseness, poor-sorting, clast-angularity, and immaturity of alluvial fan sediments are the conspicuous features which dominate most core samples of the upperfan. Finer-grained, cross-stratified or flat-bedded channel sandstones can also be prevalent, particularly from midfan sediments. It is therefore, the association of these - as well as that of thin shales (mudflows), sand-silt beds (sheet-floods), and gravel layers - which is most prescriptive in terms of what is normally seen in core samples. Such diversity might be found in cores taken from a single well, or from a number of nearby wells. (adsbygoogle = window.adsbygoogle || []).push({}); Assessment (adsbygoogle = window.adsbygoogle || []).push({}); 1. Macrofossils are relatively rare in alluvial fan complexes. A .TrueB .False 2. The geometry of an alluvial fan is best described as: A .wedge-shaped, concave upwards and lensoid.B .box-shaped and concave downwards.C .wedge-shaped and concave downwards. 3. In alluvial fan deposits sedimentary structures are most common in: A .proximal fanB .mid-fanC .distal fan 4. Facies associated with alluvial fan environments can be either alluvial plain, eolian, or playa. A .TrueB .False 5. Alluvial fans have a diagnostic log response in the subsurface. A .TrueB .False 6. Alluvial fan deposits rarely serve as good petroleum reservoirs because the sediments: A .are poorly sorted.B .result from debris flow and ephemeral stream runoff.C .are deposited next to mountain fronts.D .A, B and CE .A and C Recommended for You (adsbygoogle = window.adsbygoogle || []).push({}); Classification of Nonmarine Sandstones The Principles of Geosteering | Geosteering Best Practices Basin Histories and Properties