Fan Toe Research Articles

Tectonic controls on the morphometry of alluvial fans in an arid region, northeast Iran

ABSTRACT The morphology of alluvial fans provides valuable information about changes in tectonic, climate, base level, and drainage basin characteristics. This study aims to evaluate the relationship between active tectonics and morphometric characteristics of alluvial fans in the less studied southern region of the Joghatay mountain, northeast Iran. The study area includes 13 alluvial fans and their corresponding catchment basins in a region with semi-arid climate. Morphometric characteristics of alluvial fans and their corresponding drainage basin including fan area, fan slope, sweep angle, fan toe length, fan radius, basin area, basin slope, fan’s width-to-length ratio, asymmetry factor, and basin shape were measured. Our results revealed a negative relationship between the slope of alluvial fans and their corresponding basin area. The weak correlation between the drainage basin area and the fans area may be attributed to the lithology and active tectonics in the study area. The small sweep angle values of the studied alluvial fans can be attributed to the activity of thrust faults and presence of an uplifting anticline that resulted in formation of elongated alluvial fans in our study area. Analysis of the relationship between the morphometric characteristics of the fans and their corresponding basins indicated a negative correlation, which can be attributed to the sediment transport efficiency of the basins.

Contributions of Sediment From the Slate Canyon Alluvial Fan to the Formation and Morphogenesis of the Keeler Dunes, CA

The Keeler Dunes Complex is an active dunefield located adjacent to Owens (dry) Lake, California. The source of sediment to the Keeler Dunes area is often assumed to be from the Owens Lake playa; however, the dunes lie at the toe of the Slate Canyon alluvial fan (the Fan). Here hydrologic and hydraulic modeling was conducted for the Fan to assess the contribution of fan sediment to the Keeler Dunes. Assessment of the potential for sediment deposition was conducted for two scenarios based on the relocation of State Highway 136 from the Owens Lake playa upgradient on the Fan and the subsequent construction of flow diversion berms. The berm construction (1954 and 1967) coincided with observations of the destabilization and migration of the Keeler Dunes. Runoff from Slate Canyon watershed was estimated using a Hydrological Simulation Program–Fortran (HSPF) model based on hourly precipitation records. The resulting hydrology output served as inputs to FLO-2D models of the Fan. With the model of hydraulic output, it was estimated that approximately one million tons of sediment were moved from the Fan hydrographic apex toward the Keeler Dunes area during the peak streamflow event of record. This represents a significant volume with respect to the total volume of the Keeler Dunes. Our modeling of the peak flow event indicates the construction of the highway diversion berms resulted in the partial redirection of fan flows and therefore sediment deposition in relation to the Keeler Dunes. This localized change in sediment availability and spatial distribution is a likely factor in the subsequent morphogenesis of the dunes.

Sedimentation of open-framework gravels in alluvial-fan settings: Quaternary Poplar Fan, northwest China

Open-framework gravels (OFGs) form common lithofacies of alluvial fans. However, inferences of their mechanisms of transport and deposition, and of controls on their distribution in alluvial-fan successions, remain difficult, limiting our ability to interpret the environmental significance of such deposits and to predict their distributions in ancient successions. From an applied standpoint, such predictions are important in the development of hydrocarbon reservoirs and remediation of aquifers hosted in alluvial-fan strata. To elucidate our understanding of formative conditions of different types of OFGs, a study has been undertaken on the Quaternary Poplar Fan, in the endorheic Heshituoluogai Basin, China.Based on observations from outcrops and trenches on the Poplar Fan, eight different types of OFGs are identified: these lithofacies types account for 7% of the total deposits of the fan. During high flow stages, OFGs are deposited in association with supercritical sheet-like flow (type 1), unconfined streamflow (type 2), and flow in groundwater-fed channels (type 3). During low flow stages, OFGs are associated with trough cross-stratification (type 4) and sigmoidal cross-stratification (type 5); these types occur over the entire fan. At the fan margins, OFGs are associated with gulleys (type 6), especially at the distal fan toe. Other OFG types are associated with bank collapse (type 7) or intercalated with aeolian deposits (type 8). A model is proposed to account for the distribution of different types of OFGs across the fan, wherein types 1, 2 and 4 seen in the medial fan are dominant. This provides a framework for the identification, prediction and correlation of OGFs in subsurface alluvial-fan successions.

Evaluating Groundwater Flow Effects for Enhancement of Ground-Source Heat Pipes in the Case of the Toyohira River Alluvial Fan, Japan

The purpose of this study was to evaluate the potential enhancement of ground-source heat pipes by groundwater advection at two sites within an alluvial fan of Toyohira River, Sapporo. Two sites were selected: one in the fan toe, for negligible groundwater flow (Site 1), and the other in the apex for fast flows, the latter characterized by a specific discharge of 1.0 m/d from the losing river (Site 2). The evaporator section(s) of a single (double) heat pipe(s) was installed in a borehole at each site; the condenser section(s) on the ground was placed inside cooled brine at a set temperature, resulting in heat extraction under steady conditions. The single heat pipe experiments showed that the heat extraction rates ranged between 0.23 and 0.79 kW and were not clearly different at the two sites, considering some uncertainty. For double heat pipes, the heat extraction rates were unchanged at Site 1, but were about 146% higher at Site 2 compared to the single tests, due to groundwater advection. This study revealed that the number of ground-source heat pipes required could be reduced from three to two in areas near Site 2.

Landscape and depositional controls on palaeosols of a distributive fluvial system (Upper Cretaceous, Brazil)

The stratigraphic record of distributive fluvial systems is commonly characterised by frequent and complex interstratification of palaeosols among channel and overbank deposits. However, current models focus primarily on sedimentation and pay only limited attention to palaeopedogenesis, thereby failing to incorporate important palaeoenvironmental and stratigraphic information. This study proposes a pedosedimentary model for distributive fluvial systems that depicts and accounts for two palaeopedogenetic trends: one downdip, in relation to distality from the fan apex, and one along-strike, in relation to distance from active channel belts. Palaeosols are reported in detail from an Upper Cretaceous succession of the Bauru Basin, southeastern Brazil, through the application of macro-, micromorphological and geochemical studies, combined with facies and architectural-element analyses of sediments. In the downdip palaeopedogenetic trend, the proximal zone of the depositional system is characterised by a dominance of well-drained Inceptisols that develop on amalgamated channel fills; in the medial zone, Inceptisols occur interlayered with overbank deposits containing Entisols and poorly drained Vertisols. The distal zone preserves more mature and poorly drained Inceptisols developed on deposits of overbank and sporadic distal channel fills. These pedotypes show an increase in maturity and hydromorphism, moving away from the apex to the fan toe. This is likely linked to (i) the progressive approach of the topographic surface to the water table, and (ii) the average increase in distance to an active channel belt in distal locations. The along-strike palaeopedogenetic trend culminates in poorly developed palaeosols in floodplain regions that correspond to topographic depressions located between channel belts and which were subject to recurrent floods. Because palaeopedogenesis in the floodplain region is penecontemporaneous to sedimentation, pedotypes show an increase in maturity, bioinduced calcification and hydromorphism with distance from the active channels; they pass laterally from Entisols and Inceptisols near active channels, to Vertisols away from active channels. Conversely, following avulsion, abandoned channel belts remain as topographically elevated alluvial ridges located at some distance from the newly active channels and positioned above the water table and this leads to the development of better drained and better developed Inceptisols relative to pedotypes of the floodplain region. Overall, both palaeopedogenetic trends demonstrate the overriding controls of topography, sedimentation rate and parent material on pedogenesis, with only minor climatic influence. This work offers a novel pedosedimentary model for distributive fluvial systems and highlights the palaeoenvironmental significance of palaeosol trends, providing new constraints for the recognition of distributive fluvial systems in the rock record.

Geomorphological controls on soil fertility of semi-arid alluvial fans: A case study of the Joghatay Mountains, Northeast Iran

Alluvial fans are ideal locations to examine the effect of geomorphic processes and landforms on the soil characteristics. The rate of soil development and fertility differs between active surfaces, exposed to aggradation, and old or relict surfaces, subject to degradation. The aim of this research is to evaluate the effect of landforms and processes on soil fertility based on the studies on three alluvial fans of different ages (relict, old and young). A total of 36 soil samples were collected from these three alluvial fan surfaces. The samples were gathered from different locations of fans (apex and toe) and landforms (gully beds, interfluves, swales, and bars) and for each sample, nine variables including soil texture, acidity (pH), electrical conductivity (EC), total neutralizing value (TNV), available phosphorus (AP), available potassium (AK), total nitrogen (TN), total organic carbon (TOC), and hydraulic conductivity (K) were measured. The results show that the values of TNV, AP, AK, TN and TOC increase as clay% increases. In the relict and old fans, the values of TNV, TOC, TN, AP, AK, clay%, and silt% are higher in the interfluves than in the gully beds. Also, in the relict and old fans, the rates of clay%, silt%, TOC, TN, and AP are higher in the apexes than in the toes. In the young fan, the values of most variables are higher in the toe compared to the apex. The wide and relatively flat interfluves of the apex of old and relict fans have soils that are more fertile compared to the extensively entrenched surfaces of relict and old fan toes with relatively steep sloped interfluves. The less fertile soils in the apex of young fan can be attributed to its coarse-textured soils and also to repeated flooding and depositional processes. Overall, this study reveals that geomorphological landforms and processes such as aggradation and degradation play an important role in the spatial variation of soil fertility.

Transience of the North American High Plains landscape and its impact on surface water.

Ecosystem diversity and human activity in dry climates depend not just on the magnitude of rainfall, but also on the landscape's ability to retain water. This is illustrated dramatically in the High Plains of North America, where despite the semi-arid modern and past climate, the hydrologic conditions are diverse. Large rivers sourced in the Rocky Mountains cut through elevated plains that exhibit limited river drainage but widespread surface water in the form of ephemeral (seasonal) playa lakes1, as well as extensive groundwater hosted in the High Plains aquifer of the Ogallala formations2. Here we present a model, with supporting evidence, which shows that the High Plains landscape is currently in a transient state, in which the landscape has bifurcated into an older region with an inefficient river network and a younger, more efficient, river channel network that is progressively cannibalizing the older region. The older landscape represents the remnants of the Ogallala sediments that once covered the entirety of the High Plains, forming depositional fans that buried the pre-existing river network and effectively 'repaved' the High Plains3-6. Today we are witnessing the establishment of a new river network that is dissecting the landscape, capturing channels and eroding these sediment fans. Through quantitative analysis of the geometry of the river network, we show how network reorganization has resulted in a distinctive pattern of erosion, whereby the largest rivers have incised the older surface, removed fan heads near the Rocky Mountains and eroded the fan toes, but left portions of the central fan surface and the Ogallala sediments largely intact. These preserved fan surfaces have poor surface water drainage, and thus retain ephemeral water for wetlands and groundwater recharge. Our findings suggest that the surface hydrology and associated ecosystems are transient features on million-year timescales, and reflect the mode of landscape evolution.

Growth and shape of a laboratory alluvial fan.

Using laboratory experiments, we investigate the influence of water and sediment discharges on the morphology of an alluvial fan. In our flume, a single-thread laminar river deposits corundum sand into a conical fan. We record the fan progradation with top-view images and measure its shape using the deformation of a Moiré pattern. The fan remains virtually self-affine as it grows, with a nearly constant slope. We find that, when the sediment discharge is small, the longitudinal slope of the fan remains close to that of a river at the threshold for sediment transport. Consequently the slope depends on the water discharge only. A higher sediment discharge causes the fan's slope to depart from the threshold value. Due to the downstream decrease of the sediment load, this slope gets shallower towards the fan's toe. This mechanism generates a concave fan profile. This suggests that we could infer the sediment flux that feeds a fan based on its proximal slope.

The Source, Flow Rates, and Hydrochemical Evolution of Groundwater in an Alluvial Fan of Qilian Mountain, Northwest China

Major ions and selected environmental tracers (D, 18O, 3H and chlorofluorocarbons (CFCs)) were employed to determine the source, flow rates, and hydrochemical evolution of groundwater in an alluvial fan along the front of the Qilian Mountains, which serves as an important groundwater reservoir in northwest China. Temporal and spatial variations in ion concentrations were limited near the upper portion of the alluvial fan. However, groundwater quality deteriorated along the flow path as concentrations of Mg2+, Na+, SO42−, and Cl− increased toward the fan toe and into the center of the valley. The relative abundance of the major cations and anions also changed down-fan from Ca2+ to Na+ and Mg2+, and from HCO3− to SO42− and Cl−, respectively. Isotopic data suggests that precipitation within the Qilian Mountains, which recharged the alluvial deposits near the mountain front, was the primary source of groundwater. Apparent groundwater ages determined by CFCs varied from 23 to 40 years, and in general, increased in age from south to north (down-fan). It is concluded that groundwater in the study area was recharged by the Qilian Mountains, flowed through the alluvial fan from south to north, and was subsequently discharged several decades later from springs located along the toe of the fan, after which it re-entered the Shule River.

GEOBIA-based identification of alluvial fans and bajadas through geomorphometry, image analysis and fuzzy ontology

ABSTRACTAlluvial fan and bajada mapping is important for practical and economic importance to society. In this study, advanced geomorphometric, image processing and knowledge representation methods were investigated, enhanced and developed, towards the simultaneous identification of alluvial fans and bajadas. The developed method was based on the landform-pattern element approach. Data used included the 10 m spatial resolution National Elevation Dataset, provided by USGS and a pan-sharpened 15 m Landsat OLi imagery. The study area was located in the Death Valley, Nevada, USA. Digital Terrain Model (DTM) processing included noise-removal filtering and depression treatment. A two-phase GEographic Object-Based Image Analysis approach involving multi-scale segmentation and fuzzy ontology-based reasoning was designed. On the first phase, topographic forms were identified, by examining their morphometry and spatial relationships. On the second phase, the alluvial fans and bajadas were identified based on drainage pattern texture and topographic features such as landform shape. Due to a systematic omission on the fan toe outline, each fan and bajada spectral signature was taken into account to determine the omitted areas through region growing on the multispectral imagery. Accuracy assessment indicated satisfactory results, since a 90.3% completeness and 88.8% quality were achieved for both alluvial fans and bajadas.

Surface morphological types and spatial distribution of fan-shaped landforms in the periglacial high-Arctic environment of central Spitsbergen, Svalbard

ABSTRACTA 1:19,500 map of the Petunibukta region documents the spatial distribution of almost 300 colluvial and alluvial fans (together with their catchments) in the periglacial high-Arctic environment of Svalbard. Fan-shaped landforms were mapped using an orthophoto and digital elevation model generated from 2009 aerial photographs and a 2013 high-resolution satellite image using Geographic Information Systems. Four additional maps at a scale of 1:40,000 provide details about the morphometric characteristics of the studied fans: long- and cross-profiles, slope and aspect. Additionally, parameters such as fan and catchment area, relief, length, width, elevation (the lowest point at the fan toe, apex, and the highest point of the catchment boundary), fan mean plan and profile curvatures, fan relation to neighbouring fans, are also presented. Fans were classified according to the dominant processes shaping their surface: colluvial fans (n = 229), alluvial fans dominated by debris flows (n = 49), and alluvial fans dominated by fluvial flows (n = 19).

The formation and evolution of a coastal alluvial fan in eastern Taiwan caused by rainfall-induced landslides

The formation and evolution of a coastal fan in eastern Taiwan associated with a sequence of rainfall-induced landslides during the 2009–2013 period are explored in this study. The evolution of these landslides is mainly attributed to the head-cutting process initiated by Typhoon Parma in October 2009. During the attack of Typhoon Megi in October, 2010, a subaerial coastal fan with a surface slope of 8.9° was formed after the mobilization of the rainfall-induced landslides. The geomorphic features both in the steep gully and on the coastal fan were categorized as the sequence of granular debris flows and sheet floods. Severe fan toe erosion occurred thereafter due to the wind-wave forcing. Even if the variations of both the cumulative rainfalls and the drainage areas are one or two orders of magnitude among devastating fan-forming landslides worldwide, the mean annual precipitation and the basin ruggedness index (Melton ratio) are effective indicators to normalize the rainfall threshold and to characterize the fan surface slope, respectively. Severe catastrophic landslides generally occur when the normalized cumulative rainfalls with respect to mean annual precipitation are greater than 0.1. The fan slope generally increases with the increasing Melton ratio for the catchment. Uchiogi’s empirical model is applicable for predicting the rainfall-induced area ratio of newly generated landslides. In this case study, the relationship of the fan area to the total landslide area follows a linear regressive curve when the ratio of landslide area with respect to the drainage area exceeds 0.0056.

Effects of weathering and lithology on the quality of aggregates in the alluvial fans of Northeast Rivand, Sabzevar, Iran

Alluvial fans as depositional landforms can be considered as potential sources of aggregates. As the age of alluvial fans increases, their constituent sediments are exposed to longer periods of weathering and increased mineral alteration, resulting in a decrease in aggregate quality. In this study, physical properties and point load tests were used to assess the aggregate quality on three alluvial fan surfaces (relict, old and young) in the northeastern part of Rivand village in west of Sabzevar, Northeast Iran. Differentiating young from old and relict fans was carried out based on geomorphic criteria such as weathering features, fan surface morphology and drainage pattern. The young alluvial fan is characterized by sub-rounded and unvarnished clasts, distributary drainage patterns and a relatively flat surface, whereas old and relict fans are characterized by incised and rough surfaces, tributary drainage pattern and highly weathered and varnished clasts due to their long-term exposure to weathering. Due to a range of rock types occurring across each fan surface, lithological studies were performed to eliminate the effect of lithology on aggregate quality. A total of 18 rock types comprising comparable lithologies were sampled from each of the three alluvial fans. Results show that, in almost all 18 rock types, the point load test values increases from relict to young fans whereas porosity and percentage of water absorption decrease, implying that aggregate quality decreases with time as a function of duration of exposure to weathering. Also, the strength of aggregates in all three fans decreases from the fan apex to the fan toe. Data show that micaceous, intrusive igneous rocks, tuffs with high porosity and fine-grained extrusive igneous rocks with some porosity are more sensitive to physical weathering, and therefore have lower strength, particularly on the relict and old fans. Overall, variations in aggregate strength on these fans can be attributed to the relative ages of fans, with relict and old fans containing lower quality aggregates due to the longer-term exposure to weathering.

Short-term evolution of an active basin–fan system, Aspromonte, south Italy

Alluvial fans are widespread throughout Calabria (southern Italy), but only a minority of them show evidence of current activity. By means of interpretation of aerial photographs, orthophotos and satellite images, comparison of topographic maps, reconstruction of major rainfall events, and geomorphologic and sedimentologic field observations, we reconstructed the recent history of the geomorphic and sedimentation events and the consequent morphological changes of an active basin–fan system merging with the bed of an ephemeral stream locally named fiumara. The case study is located along the Ionian side of the Aspromonte massif, a sector of Calabria where mass movement, intensive erosion, and channel mass transport are the predominant morphodynamic processes.Evidence is available of large volumes of debris produced by mass movements and erosion processes affecting the upper sector of the feeder basin. The aggressive climate with intense winter storms is the principal cause of erosion and mass movements. Largest rainfall events occurred in clusters of 4–6 events between the 1930s and the early 1970s, at roughly 20-year time intervals. The development of the fan appeared as incipient in 1955 air photos. Activity continues today, with events of sediment output from the feeder basin. Large rainfall events are reflected, with a delay of ca. eight years, by aggradation/dissection and progradation/retrogradation cycles in the feeder channel and the fan, respectively. Mutual interference between fan development and human activity exists, as well as does the interaction between the fan progradation and the floods of the receiving fiumara stream.Based on the aforementioned survey tools, two almost 10-year cycles were recognised. Feeder channel aggradation, owing to the debris input of the feeder subbasin and to the presence of check dams built along the feeder channel, continues until the channel slope reaches a threshold value. This can also be caused by the trimming of the fan toe by the fiumara. The stored alluvium is therefore released, resulting in progradation at the fan site until the feeder channel slope decreases. Subsequently, a renewed cycle starts again in response to sediment input from the source area.The morphological and sedimentological evolution of the active basin–fan system indicates how short-term trends and cyclic modifications of such systems can be important, especially where tributary basins join larger stream beds of a fiumara-type torrent.

Mechanisms and timescales of fluvial activity at Mojave and other young Martian craters

AbstractMojave Crater, and five other relatively young Late Hesperian to Amazonian‐age Martian craters exhibit channelized alluvial fans that are sourced from bedrock‐eroded catchments. These catchments emerge from the crests of sloping surfaces, suggesting a formation mechanism that involved precipitation. The evidence for fluvial activity at all six craters is restricted to their interiors and the immediate surrounding regions. Detailed mapping at Mojave reveals the highest density of channels, catchments and fans interior to the crater. Similar landforms are identified outside of the crater, but not beyond ~200 km from the rim. Irregular pits on the floor of Mojave, interpreted as degassing structures from hot impact melt, directly superpose several fan surfaces, and partly destroy the fan toes. This suggests that sediment was mobilized immediately after crater formation, while the crater was still hot. Based on the patterns and timing of channel‐fan development at all six craters we favor several hypotheses for the precipitation mechanism: (1) snowfall and melt on young, hot impact craters, (2) impact plume precipitation, and (3) degassing of volatiles from impact melt terrain. Scenario (1) suggests a different global or regional climate relative to modern conditions, requiring equatorial and midlatitude snowfall accumulation. Scenarios (2) and (3) do not necessarily require unique climate conditions, as water may have been mobilized from the target or the impactor.

Statistical analyses and modeling approaches to hydrodynamic characteristics in alluvial aquifer

AbstractThis study is aimed at quantifying the difference in aquifer's response to recharge between some different locations in a fan aquifer and a delta aquifer for a preliminary study of revealing mechanisms of water transport in alluvial aquifer. The aquifer's response to recharge is statistically quantified with the two viewpoints: (1) timing and volume of recharge and (2) time length of aquifer's holding water. For the first point, a statistical model that links precipitation and groundwater level is introduced, and its parameters are identified using correlation analysis. Our results show that the recharge rate at the toe is higher than that at the apex and at the delta. For the second point, the concept of ‘memory effect’ of aquifer is adopted and quantified using the autocorrelation and spectral analyses. Our results show that the memory effect is longer at the toe of fan than at the apex, and thus, a temporary increase of water level has about five times as long‐term influence on subsequent water levels at the toe of the fan as at the apex. This study demonstrates that the statistical analyses and modeling of hydrological data are useful for characterizing aquifer's hydrodynamics. Copyright © 2012 John Wiley & Sons, Ltd.

Impact of the magnitude and frequency of debris‐flow events on the evolution of an alpine alluvial fan during the last two centuries: responses to natural and anthropogenic controls

AbstractThe dynamics and the surface evolution of a post‐LGM debris‐flow‐dominated alluvial fan (Tartano alluvial fan), which lies on the floor of an alpine valley (Valtellina, Northern Italy), have been investigated by means of an integrated study comprising geomorphological field work, a sedimentological study, photointerpretation, quantitative geomorphology, analysis of ancient to modern cartography and consultation of historical documents and records. The fan catchment meteoclimatic, geological and geomorphological characteristics result in fast rates of geomorphic reorganization of the fan surface (2 km2). The dynamics of the fan are determined by the alternation of low‐return period catastrophic alluvial events dominated by non‐cohesive debris flows triggered by extreme rainstorms which caused aggradation and steepening of the fan and avulsion of its main channel, with periods of low to moderate streamflow discharge punctuated by low‐ to intermediate‐magnitude flood events, causing slower but steady topographic reworking. The most ancient parts of the fan surface date back at least to the first half of the 19th century, but most of the fan surface has been restructured after 1911, mainly during the debris‐flow‐dominated events of 1911 and 1987. Phases of rapid fan toe incision and fan degradation have been recognized; since the 1930s or 1940s, the Tartano fan has been subjected to a state of deep entrenchment and narrowing of the main trunk channel and distributary area. Post‐Little Ice Age climate change and present‐day surface uplift rates have been considered as possible explanations for the observed geomorphic evolution, but tectonic or climatic controls cannot account for the order of magnitude of the erosional pace. Anthropogenic controls plausibly override the natural ones: in particular, the building of a dam in the late 1920s, about 2 km upstream of the fan, seems to have triggered fan dissection, having altered the sediment discharge through sediment retention. Copyright © 2011 John Wiley & Sons, Ltd.

Late Holocene development of a Norwegian alpine alluvial fan affected by proximal glacier variations, episodic distal undercutting, and colluvial activity

The late-Holocene, especially post-Little Ice Age, evolution of Nystølen alluvial fan, Langedalen, Jostedalsbreen region, southern Norway, is investigated using a combination of evidence from surface morphology, recent aerial photography, lichenometric dating, hydrological reconstruction, stratigraphy, and sedimentology. The fan is of predominantly fluvial origin, but it was effectively influenced by variations in the extent of the Nystølsbreen glacier, which advanced onto the fan apex at its Little Ice Age maximum about A.D. 1750, by colluvial activity (primarily snow avalanche) and by the Langedøla River undercutting the fan toe. The fluvial signature is evident in the pattern of distributary channels, lobate boulder berms on the proximal fan surface, three characteristic sedimentary facies associations, and a general down-fan fining identified within stratigraphical sections. The glacier influenced fan development through inputs of sediment (till and glaciofluvial material) and by modification of the flood regime. Snow avalanches added volumetrically small quantities of characteristically coarse and angular colluvial sedimentary material, but debris flows were unimportant. Episodic undercutting of the fan toe initiated a sequence of events consequent on the fall in the local base level, which rendered the distal fan susceptible to headward erosion of box-sided gullies during flood events and their subsequent infilling by the coarse-grained sediment. The evidence is used to develop a new dynamic evolutionary model that reflects these spatial and temporal complexities of glacier-fed, alpine alluvial fan development and incorporates submodels of process interaction, sedimentary architecture, and gully erosion.

Palaeoenvironmental evolution of the Barbate–Trafalgar coast (Cadiz) during the last ∼ 140 ka: Climate, sea-level interactions and tectonics

Coastal response to tectonic activity and eustatic-climate interactions during the Late Pleistocene and Holocene has been analyzed along the Barbate–Trafalgar shoreline. The study area consists of an uplifted platform (La Breña, ∼ 140 m) bounded by two major NW–SE faults that have created two subsiding areas: Meca in the west and Barbate in the east. The areas of subsidence have favoured the accumulation of a thick morphosedimentary sequence consisting of (in ascending stratigraphic order) beach, alluvial, and aeolian deposits, which repeatedly underwent soil-forming processes. This study outlines the palaeogeographical evolution of the area over the last ∼ 140 ka, as deduced from geomorphological mapping associated with a range of laboratory analyses (mineralogical, geochemical, magnetic susceptibility, and soil micromorphology analyses), optically stimulated luminescence (OSL dating), and existing U–Th dating. Special attention has been paid to the alluvial unit, made up of vertically stacked sandy sheet-flood deposits with interbedded red paleosols. OSL ages suggest that sediment supply to the alluvial/coastal environments took place mainly at the end of the two most recent glacial periods (Oxygen Isotopic Stage [OIS] 6 and OIS 3/OIS 2) and during the last interglacial period (end of OIS 5). This means that although alluvial sedimentation took place at times of relatively high sea level, these were not times of highstand because very high sea levels (like the present) allow marine erosion of the distal part of the fans (fan toes), cutting cliffed coasts. The repeated occurrence of paleosols in the alluvial sequence seems to indicate a recurrence of environmental changes that modified the feedback relationships between the catchment and the coastal areas. These changes are recorded in repeated oscillations of soil parameters, and are revealed from the results of geochemical and environmental magnetism analyses. We associate repeated pedogenesis and alluvial sedimentation with the glacial/interglacial global climatic changes due to an oscillating humidity/aridity ratio rather than with cyclicity in thermal regime.

GIS analysis of depositional slope change at alluvial-fan toes in Japan and the American Southwest

Slope change at alluvial-fan toes provides a clue to understanding differences between alluvial fans and other depositional landforms. This paper analyses topographic data for 430 alluvial fans and their adjacent lowlands in Japan and the American Southwest (ASW), typical humid and arid regions with abundant alluvial fans. Arcs showing fan toes were identified and representative areas above and below each arc were then delineated based on a GIS buffer analysis. The mean slope of the area above ( S A) and below the fan toe ( S B), and the ratio of S A to S B ( RS) were computed. Overall fan slope ( S O), total fan length ( L) and angle of the fan apex ( AF) were also measured. Relationships among these morphometric parameters reflect differences in climate and sedimentary processes. Fans in the ASW tend to have straight profiles with similar values for S A and S O, indicating that fan gradients from deposition resulted from low-frequency/high-magnitude storms have been preserved well under an arid climate. Higher RS values result from good preservation of both steep fan surfaces and gentle adjacent lowlands, which is also ascribable to infrequent sediment redistribution. In Japan, S A tends to be smaller than S O, and RS is often close to unity reflecting frequent sediment redistribution on fans and across fan toes under a humid climate. RS in both the regions tends to converge into unity with increasing L. Longer alluvial fans are more strongly dominated by fluvial processes than by mass-flows, especially at their toes. Longer fans also tend to have small AF, leading to fluvial processes similar to those on floodplains. Sediment re-mobilisation at the toes of such fans is more frequent, resulting in smaller differences between S A and S B.