Longitudinal Dunes Research Articles

Snow accumulation and surface topography in the katabatic zone of Eastern Wilkes Land, Antarctica

Snow accumulation and surface microrelief distributions, together with the surface katabatic wind pattern and elevation profiles, are presented for the eastern Wilkes Land katabatic wind zone. The broad-scale net accumulation distribution displays a strong negative correlation with elevation but on the mesoscale there are significant variations with respect to the elevation profile. The accumulation distribution was found to be dependent on slope aspect. Higher accumulation rates were observed on the north-east (windward) slope than those on the north-west (leeward) slope for the elevation range of 1870 m–2230 m. These higher accumulation rates are associated with the occurrence of longitudinal dunes deposited by precipitation, during synoptic events. The dependence of the accumulation distribution on aspect implies that synoptic and orographic processes are the major control on the depositional regime, and that maritime synoptic systems regularly penetrate eastern Wilkes Land to at least 2300 m elevation.

Flume experiments on the alignment of transverse, oblique, and longitudinal dunes in directionally varying flows

ABSTRACTFor more than a century geologists have wondered why some bedforms are orientated roughly transverse to flow, whereas others are parallel or oblique to flow. This problem of bedform alignment was studied experimentally using subaqueous dunes on a 3–6‐m‐diameter sand‐covered turntable on the floor of a 4‐m‐wide flume.In each experiment, two flow directions (relative to the bed) were produced by alternating the turntable between two orientations. The turntable was held in each orientation for a short time relative to the reconstitution time of the bedforms; the resulting bedforms were in equilibrium with the time‐averaged conditions of the bimodal flows. Dune alignment was studied for five divergence angles (the angle between the two flow directions): 45°, 67–5°, 90°, 112–5° and 135°. The flow depth during all experiments was approximately 30 cm; mean velocity was approximately 50 cm s‐1 and mean grain diameter was 0–6 mm. Each experiment continued for 30–75 min, during which time the flume flow was steady and the turntable position changed every 2 min. At the end of each experiment, water was slowly drained from the flume and dune alignment was measured. Transverse dunes (defined relative to the resultant transport direction) were created when the divergence angle was 45° and 67–5°, and longitudinal dunes were created when the divergence angle was 135°. At intermediate divergence angles, dunes with both orientations were produced, but transverse dunes were dominant at 90°, and longitudinal dunes were dominant at 112–5°.One experiment was conducted with a divergence angle of 135° and with unequal amounts of transport in the two flow directions. This was achieved by changing the orientation of the turntable at unequal time intervals, thereby causing the amount of transport to be unequal in the two directions. The dunes formed during this experiment were oblique to the resultant transport direction.These experimental dunes follow the same rule of alignment as wind ripples studied in previous turntable experiments. In both sets of experiments, the bedforms developed with the orientation having the maximum gross bedform‐normal transport (the orientation at which the sum of the bedform‐normal components of the two transport vectors reaches its maximum value). In other words, the bedforms develop with an orientation that is as transverse as possible to the two flows. In those cases where the two flows diverge by more than 90° and transport equal amounts of sand, bedforms that are as transverse as possible to the two separate flows will be parallel to the resultant of the two flow vectors. Although such bedforms have been defined by previous work as longitudinal bedforms, they are intrinsically the same kind of bedform as transverse bedforms.

Megabarchanoids of the Thar: Their Environment, Morphology and Relationship with Longitudinal Dunes

The megabarchanoid dunefields in the very arid western part of the Thar Desert, India, exist within a vast field of longitudinal sand dunes. Studies on their morphology and environment indicate a close interrelationship between the megabarchanoids and the longitudinals. While the present fields of megabarchanoids receive supply from the longitudinals upwind and develop at the cost of some longitudinals, there are indications that the fields also supply sand to the longitudinals downwind. Even the internal arrangement within the fields indicates a gradual importance of the longitudinal element within the transversals. Similar development in the simple barchanoid fields of the region and in the growth sequences of the longitudinal dunes provides the basis for suggesting a unified concept of the growth of these aeolian bedforms, related to the primary aerodynamic system over the region.

Linear dunes - forms and formation

Most geomorphologists and geologists do not differentiate the various types of linear dune and refer to them as one dune type mainly in the terms of linear, longitudinal or seif dunes. In recent morphodynamic and genetic classifications of dunes, all types of linear dunes are lumped under the heading of longitudinal dunes (Wasson and Hyde, 1983; Hunter et al., 1983). This review, however, substantiates three different simple linear dunes - lee dunes, seifs and vegetated- linear - each of which has a different shape and mechanism of formation and elongation.

Longitudinal dunes can move sideways

AbstractLongitudinal dunes occur in all major sand areas in the world. Their dominant mode of migration or extension is considered to be either prevailing‐ or resultant wind‐parallel, the dunes extending downwind via accretion of the terminal nose. In the Qaidam Pendi in Northwest China a series of active longitudinal dunes extend downwind at 5–10 m yr−1. Internal sedimentary structures examined in the dunes, however, display beds dipping in one direction rather than two opposed directions as is expected. Analysis of aerial photographs confirms that these dunes migrate laterally up to three metres per year, whilst maintaining a symmetrical longitudinal dune morphology.

Settling‐velocity distributions and sorting processes on a longitudinal dune: A case study

AbstractThe processes acting on a longitudinal dune are inferred from the response in the area size‐sorting characteristics of the sediment of the active layer. The sediment size distributions are determined by settling and are best described by a log‐hyperbolic model which provides the most information on size‐sorting processes. Size‐sorting characteristics are markedly different across the low round‐crested part of the dune from cross‐sections of the high more sharp‐crested part. This results in changes in textural parameters along the lee‐side consistent with changes in the lee‐side separation vortex. This is the first time that the effect of wind speed up on the lee‐side is observed to be reflected in the sediment; it can only be detected with the very sensitive log‐hyperbolic parameters.

Natuur–En Mensgeinduseerde Sandbeweging Aan Die Weskus

Abstract The sand covered West coast of South Africa (the Sandveld) between Eland's Bay and the Orange River is characterised by extensive small stock farming, diamond mining and fishing, situated in a semi-desert landscape comprising coastal, longitudinal and coppice dunes as well as sand sheets. The environmental impact of historic and recent human activities are briefly described, followed by a Besler granulometric-based analysis of the aeolian mobility of 353 sand samples to determine those particular locations which are sensitive to sand movement. These are mapped and correlated with the observed human activities noted during previous field work, and the conclusion is drawn that nature and man contribute in a combined fashion towards sand movement and desertification of certain localities. Several guidelines are formulated for the conservation of this ecologically sensitive area.

Grain-size variations on a longitudinal dune and a barchan dune

The grain-size characteristics of the sand upon two dunes—a 40 m high longitudinal dune in the central Namib Desert and a 6.0 m high barchan in the Jafurah sand sea of Saudi Arabia—vary with position on the dunes. On the longitudinal dune, median grain size decreases, sorting improves and the grain-size distributions are less skewed and more normalized toward the crest. Though sand at the windward toe is distinct, elsewhere on the dune the changes in grain-size characteristics are gradual. An abrupt change in grain size and sorting near the crest—as described by Bagnold (1941, pp. 226–229)—is not well represented on this dune. Coarse grains remain as a lag on concave slope units and small particles are winnowed from the sand on the steepest windward slopes near the crest. Avalanching down slipfaces at the crest acts only as a supplementary grading mechanism. On the barchan dune median grain size also decreases near the crest, but sorting becomes poorer, though the grain-size distributions are more symmetric and more normalized. The dune profile is a Gaussian curve with a broad convex zone at the apex upon which topset beds had accreted prior to sampling. Grain size increases and sorting improves down the dune's slipface. However, this grading mechanism does not influence sand on the whole dune because variations in wind regime bring about different modes of dune accretion. On both dunes, height and morphology appear to influence significantly the grain-size characteristics.

Migration, modification and merging in aeolian systems and the significance of the depositional mechanisms in Permian and Triassic dune sands of Europe and North America

The main depositional mechanisms acting in aeolian systems can be assessed in two different and independent ways: first by the internal anatomical approach with analysis of the cross-stratification fabric, and second by the external morphological approach with study of shape and surface features of dunes. Anatomical investigations of various ancient aeolian systems in the Permian and Triassic of Europe and North America and an extensive literature review concerning both internal anatomy and external morphology of ancient and modern dunes reveal three main depositional mechanisms operating in aeolian systems: migration, modification and merging. Migration comprises downwind movement of isolated dunes or trains of aeolian bedforms under predominantly constant wind direction. The orientation of the interset surfaces partially depends on the angular relationship between the path of bedform climb and the depositional surface and also on subsidence. Simple systems are characterized by successive dunes of similar size, whereas compound systems originate by superimposition of smaller aeolian bedforms on larger dunes or draas. Compound migration gives rise to the hierarchy of diastems. Modification comprises overprinting of existing dunes by winds coming from directions other than those that govern migration of the original aeolian bedforms. The anatomical elements representing modification include both intrasets and extensive intersets, depending on duration and on the scale of the changes of the winds. Simple modification comprises occasional origin of either only cross-dipping erosional boundaries or both diastems and intraset laminae oriented in different directions. A special case of modification is the creation of intrasets dipping in the same direction as the foreset inclination, comprising either reactivation or relaxation surfaces, depending on whether the foreset slope is steepened or lowered in angle. Complex modification takes place under repeatedly alternating winds of changing effectiveness. Small-scale modification comprises minor alterations of existing dune types, and medium-scale modification is characterized by conversion of primary aeolian bedforms into secondary dune types. Small-scale and medium-scale modification take place during short-term to smaller-scale long-term changes of effective wind direction. Large-scale modification originates by large-scale long-term changes, accompanied by a certain subsidence, which result in burial of the older dunes by a younger train of aeolian bedforms within the same sand sea migrating obliquely to perpendicularly to the former wind direction. Very large-scale modification is induced by very large-scale long-term changes which result int deflationary erosion of older dune fields and subsequent superimposition of a new sand sea on top of the eroded remnants of the former erg. Merging comprises coalescing of bedforms with only negligible erosion during burial of the first bedform by subsequent dunes. Merging is induced mainly when migration of dunes is stopped by a morphological barrier or ceases as a consequence of weakening winds. Merging further occurs during coalescence of numerous isolated barchans in rows to parallel wavy barchanoid ridges as a consequence of increasing availability of sand. The main depositional mechanism in compound systems is migration of hierarchically superimposed bedforms with subordinate modification under predominantly unidirectional winds. In complex systems, migration, modification and merging take place in different amounts under unidirectional, bidirectional and multidirectional winds. Depending on direction and strength of both earlier and subsequent winds, complex systems are either charcterized by mainly migration with subordinate modification or by considerable modification with either conversion or dissection of older dunes. The effects of migration, modification and merging are discussed in the light of both internal anatomy of ancient cross-stratification fabrics and external morphology of recent dunes. A classification of dune types based on these mechanisms divides the aeolian bedforms into primary, secondary and tertiary types. Primary dunes originate by simple or compound migration with minor modification and comprise mainly transverse dunes. Secondary dunes originate by complex interaction of migration and modification or even predominantly by modification, with the best examples being longitudinal dunes. Tertiary dunes form by merging of transverse and longitudinal elements with varying amounts of migration and modification and are mainly represented by star dunes. The results of various anatomical case studies of ancient aeolianites indicate that simple migration was the dominant mechanism for construction of ergs in the northern hemisphere in Permian and Triassic times.

Why deposits of longitudinal dunes are rarely recognized in the geologic record

ABSTRACTDunes that are morphologically of linear type, many of which are probably of longitudinal type in a morphodynamic sense, are common in modern deserts, but their deposits are rarely identified in aeolian sandstones. One reason for non‐recognition of such dunes is that they can migrate laterally when they are not exactly parallel to the long‐term sand‐transport direction, thereby depositing cross‐strata that have unimodal cross‐bed dip directions and consequently resemble deposits of transverse dunes. Dune‐parallel components of sand transport can be recognized in ancient aeolian sands by examining compound cross‐bedding formed by small dunes that migrated across the lee slopes of large dunes and documenting that the small dunes migrated with a component in a preferred along‐crest direction over the large dunes.

Proximal bedded deposits related to pyroclastic flows of May 18, 1980, Mount St. Helens, Washington

Thin-bedded, dacitic, pumiceous pyroclastic-flow deposits partly cover the steep northern flank of Mount St. Helens volcano, Washington. They are termed proximal bedded pyroclastic-flow (PBPF) deposits and were formed during the eruption of May 18, 1980. These unconsolidated deposits, as much as 20 m thick, are characterized by well-defined, chiefly plane-parallel bedding sets separated by erosion surfaces. Most beds dip generally parallel to the slope of the underlying volcano flank, which averages 15° to the north but locally is as much as 30°. Individual beds range in thickness from 2 mm to >1 m. Cross-bedding, in which bedding sets dip gently to the north or locally to the south, is abundant. Cross-bedding is generally associated with large longitudinal dunes, apparently antidunes. Some cross-bedding sets, however, constitute small longitudinal anti-dunes (chute-and-pool structures) containing stoss-side beds that migrated south and dip steeply south toward the crater source. We suggest that PBPF deposits, base-surge deposits, and ignimbrite-veneer deposits are products of a broad spectrum of high-flow–regime pyroclastic processes. PBPF deposits at Mount St. Helens formed during deposition of high-velocity, partly turbulent, dry, fluidized pyroclastic flows that poured profusely down the volcano flank on May 18. Some beds of the PBPF deposits probably were deposited from typical high-concentration laminar pyroclastic flows. Other beds, especially those that display evidence of high-angle stoss-side accumulation, were probably deposited from low-concentration turbulent flow under high velocity as pyroclastic surges. Most beds in the PBPF deposits, however, may have formed during transitional flow conditions. Pyroclastic-surge and transitional flow-surge conditions may have resulted when the flows exceeded the threshold velocity or threshold minimum particle concentration for pyroclastic flows while lofting over obstacles or depressions or after collapse from Plinian ash columns.

Deflection of sand movement on a sinuous longitudinal (seif) dune: use of fluorescent dye as tracer

Sand was marked by fluorescent dye in order to trace sand movement and deposition on a longitudinal (seif) sand dune in the Sinai desert. The wind regime was monitored simultaneously. Tracing the dyed sand was possible after light to moderate sand storms and was graphically represented on maps. The dune was subjected to a seasonally bidirectional wind regime, with the wind hitting the dune obliquely on either side. On the windward flank the sand was transported parallel to the wind direction. On the lee flank sand movement was deflected towards parallelism with the crest line. Sand movement was deflected if the dune had a sharp profile which favored separation of wind flow on the lee flank. The deflection depended on the angle of incidence between the wind and the crest line: when the angle of incidence was < 40°, sand on the lee flank was transported parallel to the crest line; when the angle of incidence was nearly perpendicular to the crest, movement along the lee flank abated and deposition occurred. Where the dune was low, flat and blunt, as in a zibar dune, there was no boundary-layer separation and no deflection of sand movement on the lee flank. The deflected movement along the lee flank resulted in elongation of the longitudinal (seif) dune.

Size—frequency distribution of longitudinal dune rippled flank sands compared to that of slipface sands of various dune types

ABSTRACTRippled flanks of longitudinal dunes and the slip faces of various dune types can be distinguished granulometrically by comparing the top to bottom trend of changes of sorting in the sand population of each individual slope. Flank sands, which are prone to sorting processes through migration of ripples alongslope, are always better sorted upslope whereas slipface sands, which are controlled by avalanche and grainfall of sands, become better sorted downslope. Considering the absolute values of the bottom samples, the sands of the slip faces are both better sorted and the size distribution more symmetrical than those of the rippled flanks.Applying this approach in investigating ancient sediments could improve palaeogeographical interpretations.

Dynamic processes acting on a longitudinal (seif) sand dune

ABSTRACT Field measurements were made on a longitudinal dune in the Sinai Desert in order to understand its morphology and dynamics. The field measurements contradicted the wind structure indicated by the helicoidal flow theory. Rather, it was found that winds coming from two basically different directions at different times and striking the dune obliquely were responsible for sand transport and erosion or deposition along the lee flank.The essence of this mechanism is the deflection of the wind airflow on the lee flank of the dune to a direction parallel to the crest line. The occurrence of erosion or deposition depends upon the angle of incidence between the wind and the crest line. When this angle is &lt; 40° the velocity of the deflected wind is higher than on the crest line or the windward flank and longitudinal sand transport occurs. When the angle is less acute (&gt; 40°) the velocity of the deflected wind drops and deposition takes place on the lee flank.The angle of incidence in each wind storm is changed intermittently between 30° and 100° along the dune because the dune meanders and because of the sinuous outline of the crest line. In this manner sand transport and erosion or deposition occurs along the lee flank depending on the angle of incidence between the wind and the crest line. As a result of the deflection of the wind the dune elongates at an average rate of more than 1 m per month. Peaks and saddles along the crest line advance at an average rate of 0.7 m per month.The lack of uniformity in the effects of the wind on both sides of the dune creates a lack of uniformity in the rate of erosion and deposition. This can explain the formation of peaks along the crest line of the dune.

Great Sand Sea and Selima Sand Sheet, Eastern Sahara: Geochronology of Desertification

The relation of playa sediments and associated archeological sites with longitudinal dunes allows estimation of ages for the two uppermost strata of the Great Sand Sea. Active dune formation corresponds with interpluvial periods of hyperaridity; dune stability corresponds with semiarid pluvial periods. Archeological sites associated with truncated paleosols in the Selima Sand Sheet suggest a similar climatic relation and indicate that the isohyets of central Sudan shifted at least 400 kilometers northward during the peak of pluvials.

Internal Structure and Surface Geometry of Longitudinal (Seif) Dunes

ABSTRACT A mutual relationship exists between the morphology of longitudinal (seif) dunes and their dynamics and internal structure. The basic mechanism of longitudinal dune movement results from the deflection of oblique wind flow into a longitudinal flow on the lee flank. The magnitude of the longitudinal flow depends on the angle of the wind incidence with the crest line. Deposition occurs when this angle of incidence increases through meandering of the longitudinal dune crest line. After heavy rain exposed the internal structure of the dune, two kinds of depositional unit were found: the main unit forms in the meandering area on both sides of the dune; the other, secondary unit is a narrow belt on both sides and parallel to the crest line. The two units have different strikes. Because the laminae deposited on the dune are of the three primary stratification types--climbing translatent strata, sand flow cross-strata, and grainfall laminae--the range of dips is wide (10° to 30°). These dips accord with topographic profiles across the dune. A model of the internal structure of a longitudinal dune has been developed from the field data. The model shows the erosional and depositional areas along this linear dune.

Stabilized dune ridges in northern Saskatchewan

Stabilized dune ridges occurring in northern Saskatchewan have previously been identified as variedly as "ice-crack moraines" and longitudinal dunes. Investigations of their morphological, structural, and sedimentary attributes reveal that they are, indeed, of eolian origin, but they form a particular group within the parabolic dune association, namely, the "Cree Lake type dune ridges." The ridges occur in association with other types of parabolic dunes and other eolian features, such as loess and wind-abraded glacial blocks and bedrock outcrops. The dunes and the associated eolian features were all formed by southeasterly paleowinds of uniform direction. The dune ridges developed from primary parabolic dunes of simple and composite types through the process of dune elongation. At the same time, exposed rock surfaces were abraded by the wind and loess was deposited downwind from the developing dune fields. The southeasterly direction of the paleowinds, which is almost directly opposite to the direction of the present-day winds affecting dunes in the Lake Athabasca area, was due to adiabatic air masses coming off the ice sheet from the east and affected eolian activity in quite a large region in northern Saskatchewan and Alberta. The somewhat cool and sufficiently dry adiabatic winds checked the vegetation on the dunes and in the areas around them. The development of the dune ridges came to an end when a sudden climatic change evoked the rapid stabilization of the dunes by vegetation but not before most of the ridges became partly deformed by southwesterly crosswinds resulting from the same climatic change. The period of eolian activity is estimated from the age of the local ice frontal positions to have been between 10 000 and 8800 years BP. Only one other region is known from North America, namely, the St. Lawrence Lowland in the east, where analogous eolian environment prevailed in the zone peripheral to the continental ice sheet and produced comparable eolian features.

Actions éoliennes récentes aux abords du delta de l'Orénoque (Upper quaternary wind deposits near the Orinoco delta (Venezuela))

Abstract. - On the western side of the Orinoco delta, some patches of quartz sands, which have been leeched from their ironoxydes, cover irregularly the Mesa Formation, of Upper Pliocene age. Most patches don't have any typical eolian land- form, but some of them are associated with longitudinal dunes. These sands are typically eolian well sorted sands (fig. 2 and table). They are fossilised on the edge of the Orinoco delta, by holocene and contemporary alluvium. They have been deposited before the end of the Flandrian transgression, during a low sea level stage. These observations suport and complete a previous paleoclimatic reconstruction of the Northern part of South America. Further W, in the Apure and Colombian Llanos, an extensive dune field has been shaped during the last andean cold period (Wurm, Wisconsin). At the end of it, the climate being wet again, the rivers have flooded the dune field and excavated their valleys through it, reaching again the Orinoco. In Eastern and Central Amazonia, the relatively dry conditions changed to the humid present ones during the begining of the Flandrian transgression. The present rain forest took the place of a former xeric vegetation. Climatic changes have been very broad in Northern South America, as broad as in Western Africa, but their pattern is quite different, as a result of a different structural frame, clearly visible in the distribution of sea and land. In Africa, the distribution of present and past climates is clearly zonal. In America, on the contrary, the zonal influence is strongly obliterated by orography and N-S atmospheric circulation.

Eolian features in the western desert of Egypt and some applications to Mars

Relations of landform types to wind regimes, bedrock composition, sediment supply, and topography are shown by field studies and satellite photographs of the Western Desert of Egypt. This desert, which lies at the core of the largest hyperarid region on earth, provides analogs of Martian wind‐formed features. These include sand dunes, alternating light and dark streaks, knob ‘shadows,’ and yardangs. Surface particles have been segregated by wind into deposits (dunes, sand sheets, and light streaks) that can be differentiated by their grain size distributions, surface shapes, and colors. Throughgoing sand of mostly fine to medium grain size is migrating southward in longitudinal dune belts and barchan chains whose long axes lie parallel to the prevailing northerly winds, but topographic variations such as scarps and depressions strongly influence the zones of deposition and dune morphology. Sand from the longitudinal dunes on the plains is commonly redistributed into barchans in the depressions. These barchans are generally simple crescents that are morphologically similar to many of the dunes seen on Viking orbiter pictures of the north polar sand sea on mars. Light streaks are depositional features consisting of dune belts and elongate sheets of coarse to medium sand and granules. Intervening dark streaks are erosional features consisting of strips of desert‐varnished bedrock and lag gravel surfaces exposed between the sand deposits. The shape of both light and dark streaks is controlled by wind flow around topographic highs. Dark zones (shadows) in the lee of mountains, hills, and knobs are erosional products from the topographic highs; they change shape only in response to movement of the adjacent lighter‐colored sand deposits. Streamlined yardangs carved in crystalline limestone constitute one of the largest yardang fields on earth. Yardangs occur also in sandstone of the Nubian Series and in lacustrine sediments. The variables that affect the patterns of wind erosion and deposition in the Western Desert are topographic effects on wind velocities and directions, resistance of the bedrock, sand supply, and climatic change with time; vegetation is essentially absent and is not a controlling factor.

MARS: The North Polar Sand Sea and related wind patterns

Viking Orbiter 2 images of the north polar region reveal an enormous sand sea (erg) covering an area of &gt;5×105 km2 around the perennial ice cap. All dunes are either transverse or barchan. The various dune morphologies and modification of primary dune types reflect a wind regime having more than one wind direction. In the summer, two major wind directions prevail: (1) off‐pole winds that become easterly due to coriolis forces and (2) on‐pole winds that become westerly. During the winter and/or spring, only the on‐pole winds exist. Strong winds (&gt;75 m/s) are required for sand accumulation to form the thick transverse dunes. The strongest winds in the north polar region are thought to exist during summer over the transverse dune field between 110°W and 220°W; this area is a relatively warm belt (temperature &gt;230 K) between two ice zones (temperature &lt;220 K). A frequent cyclogenetic process in this area may cause sand storms. The dunes seem to be currently active. The lack of well‐developed longitudinal dunes implies that the dune field is young. The relationship of the present dune field to the perennial ice indicates that the dunes began to form after the formation of the present ice cap.