Interdune Corridors Research Articles

Sand Supply Affects Wind Erosion Efficiency and Sand Transport on Sand-Cemented Body Mulch Bed

Sand-cemented bodies (SCBs) are naturally distributed in some interdune corridors in the central Taklimakan Desert, northwest China. In this study, field-collected SCB particles were used as the experimental material, and wind tunnel experiments were conducted with different sand supplies, wind velocities, and SCB coverages to evaluate SCB wind erosion efficiency and vertical mass flux. The results showed that wind erosion efficiency decreased as SCB coverage increased. When the SCB coverage was above 40%, sand deposition processes occurred only under saturated sand flow, while sand transport remained unaffected by increases in SCB coverage under unsaturated sand flow. Under saturated flow, the highest concentrations of transported sand were found at 0–6 cm above the surface, and the main sand bed process was deposition. The sand bed process changed from aeolian erosion to deposition with increasing SCB coverage and tended to remain stable until the SCB coverage exceeded 40%. By contrast, under unsaturated sand flow, the sand bed process was primarily aeolian erosion, and the highest concentrations of transported sand were found at 0–4 cm above the surface. At high SCB coverage levels (more than 40%), a general balance between aeolian erosion and deposition processes was reached. In summary, increasing SCB coverage had a significant impact on surface wind erosion processes. Thus, SCBs can be used as a novel sand retention material.

ZIBARS: Distribution, morphology and environmental controls

Zibar is an Arabic word for aeolian bedforms that are coarse-grained, of limited relief, have no slipfaces, and occur on sand sheets and within interdune corridors of many sand seas. They may also be called granule-armored dunes, undulations, transverse aeolian ridges, mega-ripples, giant ripples, and chevrons and whalebacks. Zibars, though very extensive, are by no means ubiquitous in the world’s aeolian environments. They occur in thirteen main locations in dry, warm deserts: Algodones, USA; Gran Desierto, Mexico; eastern Mauritania; Ubari, Libya; Libyan Desert; Erg of Fachi-Bilma/Tenéré; Selima, Sudan; Namib, Namibia; Lut, Iran; southern Rub’ al Khali, Arabia; Thar, India; Kumtagh, China; and Atacama, Peru. They can occur as transverse ridges, as parabolic shapes, and as oblique features. In many regions they tend to have a spacing of around 8 to 14 per km. They tend to be modest in height, varying between tens of centimeters to up to c 6–8 m. All researchers seem to agree that they are mound-like forms without slipfaces and that their slope angles are no more than 5-15o. Nearly all zibars occur in the interdunes between various types of linear dune. They are composed of ill-sorted sand, often with a large coarse component.

Hybrid dry-wet interdune deposition in Precambrian aeolian systems: Galho do Miguel Formation, SE Brazil

It is currently not known whether the construction and accumulation of Precambrian aeolian interdunes occurred in the same way as for their Phanerozoic counterparts. To investigate this, a Mesoproterozoic aeolian succession, the Galho do Miguel Formation (SE Brazil), was studied using facies and architectural-element analyses. The sedimentary succession comprises deposits of simple dunes, draas (megadunes), and dry and wet interdunes. Dry and wet aeolian sub-environments were coeval and their development was likely controlled by local topographic relief that governed a hybrid dry-wet aeolian system. The dry sub-environment was composed of interconnected interdune corridors between large and well-developed bedforms (simple dunes and draas). The water table did not influence the construction of climbing dunes and dry interdunes, but this was fundamental for long-term aeolian accumulation. Due to the proximity of the water table to the depositional surface, the dry interdune flats were eroded only up to the groundwater level, where the wetness inhibited sand removal during dune migration. This condition enabled the accumulation of thick packages of dry interdune deposits (up to 3 m thick). The wet sub-environment is represented by extensive interdune flats, non-climbing dunes, and a continuously near-surface water table. In this context, the interdune deposits underwent only minimal reworking associated with dune migration. This setting allowed the progressive rise of the interdune substrate and the amalgamation of interdune deposits of different generations, thereby producing thick wet-interdune stratal packages (up to 8 m thick). The accumulation of thick packages of interdune strata in both sub-environments was generated by: (i) high rates of vertical accumulation of the interdune substrate due to high rates of sediment input for aeolian construction in Precambrian systems, and (ii) progressive relative rise of the water table. These conditions enabled the long-term accumulation and successive preservation of Precambrian aeolian systems, in which the stabilising effects of vegetation did not operate. Therefore, the aeolian architecture of the Galho do Miguel Formation suggests that: (i) Precambrian aeolian systems probably produced thicker dry and wet interdune deposits than their Phanerozoic counterparts, and (ii) hybrid dry-wet aeolian systems likely provided the most favourable conditions for long-term accumulation and successive preservation of these types of deposit in the Proterozoic.

Lateral stability of central Australian longitudinal dunes

ABSTRACTSome authors consider that linear dunes migrate laterally on a grand scale, much as rivers move across floodplains. Others have concluded that lateral movement is limited in time and space. During the phase of dune initiation, small ribbons of sand formed in the lee of obstacles merge and coalesce to form recognisable linear dunes. Repeat observations demonstrate that mature sand ridges change position laterally a few metres either way within close limits defined by sand spillages on the slip or avalanche slopes that change orientation in response to changing sand-moving wind directions. This essay considers evidence by which to differentiate these two points of view. Supporting evidence for limited migration take the form of dunes resting on plinths of older alluvial substrates, some capped by gibber and that stand higher than adjacent interdune corridors. Such exposures show that the sand ridges have not migrated laterally during the last several tens of thousands of years. Migration is inhibited by clay cores, calcareous (calcrete) horizons, and the development of vegetated flanks. For these reasons, it is proposed that lateral migration be defined as a change of dune plan position that is greater than the maximum extent of sand deposition that develops in response to changes in the direction of sand-moving winds.

A classification scheme for fluvial–aeolian system interaction in desert-margin settings

Abstract This study examines 130 case examples from 60 desert regions to propose a generalised framework to account for the diverse types of interaction known to exist between active aeolian and fluvial depositional systems at modern dune-field margins. Results demonstrate the significance of aeolian and fluvial system type, orientation of aeolian versus fluvial landforms, distribution of open versus closed interdune corridors, and fluvial flow processes in controlling the distance and type of penetration of fluvial systems into aeolian dune fields. Ten distinct types of fluvial–aeolian interaction are recognised: fluvial incursions aligned parallel to trend of linear chains of aeolian dune forms; fluvial incursions oriented perpendicular trend of aeolian dunes; bifurcation of fluvial flow between isolated aeolian dune forms; through-going fluvial channel networks that cross entire aeolian dune fields; flooding of dune fields due to regionally elevated water-table levels associated with fluvial floods; fluvial incursions emanating from a single point source into dune fields; incursions emanating from multiple sheet sources; cessation of the encroachment of entire aeolian dune fields by fluvial systems; termination of fluvial channel networks in aeolian dune fields; long-lived versus short-lived modes of fluvial incursion. Quantitative relationships describing spatial rates of change of desert-margin landforms are presented. The physical boundaries between geomorphic systems are dynamic: assemblages of surface landforms may change gradationally or abruptly over short spatial and temporal scales. Generalised models for the classification of types of interaction have application to the interpretation of ancient preserved successions, especially those known only from the subsurface.

Grain-size characteristics of linear dunes on the northern margin of Qarhan Salt Lake, northwestern China

In order to clarify the formation mechanism of linear dunes on the northern margin of Qarhan Salt Lake, northwestern China, we analyzed the grain-size and sorting parameters of the dune and interdune sands. The surface sands (0–30 mm) from the dune base to the crest of both flanks and interdune corridors were sampled along transects from upwind to downwind through the dune field. The results indicated that the grain-size distribution differed at different positions between and within the dunes. The frequency curve for dune sands mainly showed a bimodal distribution, while the interdune sediments showed a trimodal distribution. The grain size distribution of the linear dunes showed a finer crest pattern, i.e. the crests were composed of sands that were generally finer, better sorted than those of base sands. In addition, at the dune field scale, the dune crest sands were tending to become much finer but sorting became worse along the downwind transects. However, the grain-size parameters of sediments in the interdune corridors showed no clear pattern. The results demonstrated that the grain size and sorting parameters exhibited a systematic change not only at the individual scale but also at the dune field scale. Our results quantitatively estimate the limited role of cohesive sediments on the formation of linear dune under unidirectional wind regime. More attention should be paid to a long-term wind regime observation, internal sedimentary structures and their formation ages.

Geomorphological Map of the Tadrart Acacus Massif and the Erg Uan Kasa (Libyan Central Sahara)

Here we present a geomorphological map of the Tadrart Acacus Massif and Erg Uan Kasa (SW Libya, central Sahara). The geomorphological mapping of the area was carried out by means of satellite imagery analysis followed by detailed control of units in the field. The Tadrart Acacus is a sandstone massif delimited to the West by a high scarp and cut by a dendritic fossil drainage network of W-E-oriented wadis. The massif has been shaped since the Tertiary by etchplanation and solutional processes; the latter is demonstrated by the ruiniform landscape and the high number of caves and rock shelters that dot the vertical cliffs of the wadis. To the East, the Tadrart Acacus merges below a complex system of sand ridges that form the Erg Uan Kasa. The large interdune corridors of the sand sea are dotted by lake sediments, which formed during the Pleistocene interglacials and the African Humid Period of the Holocene, when piezometric lakes were sustained by intense monsoonal rainfall. Geomorphological investigation demonstrates that the extant landscape originated thanks to the overlap of surface processes triggered by divergent (humid/arid) environmental conditions.

Caracterização geo-ecológica dos sistemas de cordões dunares da Estremadura

GEO-ECOLOGYCAL STUDY OF SOME DUNE SYSTEMS IN THE PORTUGUESEESTREMADURA. The Portuguese Estremadura coastal area between Nazaré and Peniche displays great variety of landforms. Most are cliff systems cut in sedimentary rocks with incisions controlled by tectonic activity. In the river mouths sand bars and beaches are present. Beach systems make up about 40% of the referred coastline and have ridges of diverse form and size, and rates of vegetation cover. These dunes appear on the inland side of the beach or even backshore as a result of erosion. They may have piled up into sand backs that cut off the lagoon estuaries to the West in the historic period. The geomorphologic and sedimentary characteristics that appear permit the identification of double ridges separated by interdunal corridors or depressions in West Nazaré, S. Martinho do Porto and Peniche (Consolação) and in a simple and continuous form in Baleal. This difference could be related with local topography, wind/wave climatology and/or vegetation cover. With regard to flora and vegetation, psammphitic and halonitrophilous pioneer plants that colonise foreshore ecosystems and transitions to backshore are Salsolo kali-Cakiletum aegyptiacae in the studied area. Springtime terophitic is sparse and grows where organic remains have accumulated. Flat backshore areas which are overrun by sea during spring-tides and storms are colonised by open vegetation, predominantly Elymus boreli-atlanticus (Euphorbio-Agropyretum junceiformis). Alongside the clumps of marran grass, on the backshore and on the crests of white dunes, Loto-Ammophiletum grows abundantly together with Otanthus maritimus and Lotus creticus. Grey dunes, which are further away from the effects of the ocean (winds and salinity), are characterised by relative sand stability and a succession of dune peaks and interdune corridors with thermophilic associated Armerio welwitschii-Cruciannelletum. The openings display terophitic associated Violo-Silenetum litoreae with Atlantic features. The mature stage of the system consist in the more interior stabilised dune of dense shrubs, consisting mostly of Juniperus turbinata (Osyrio-Juniperetum turbinatae).

Use of ground‐penetrating radar to investigate feathery complex linear dunes in the Kumtagh Desert, North‐west China

The Kumtagh Desert, the last desert in China to be explored, contains a distribution of feathery dunes, a type of complex linear dune composited by linear dunes and lingoid zibars. A debate about the existence, underlying relief and formation process of these dunes has been ongoing since 1974. The authors use ground‐penetrating radar (GPR) images across dunes and interdune areas to better understand the shallow underlying relief and sedimentary structure of the dunes in the Kumtagh Desert. The results are compared with adjacent outcrops and exposures in an excavated trench. No evidence was found for underlying relief, such as yardangs, consistent with the trend and distribution of linear dune ridges that might have controlled the orientation of the dune. However, outcrop observation and remote sensing investigation indicate little gravel bar topography across or along the dunes. Lingoid zibars are common in interdune corridors. Sedimentary structures imaged by GPR in the zibar dunes indicate low‐angle strata dipping towards the SW, indicative of sand accumulation on the leeside of the lingoid zibar with dune migration towards the SW. Strata within the linear dunes are less well‐resolved but are interpreted to indicate a bimodal dip that has been recognized in previous studies of linear dunes.

Aeolian dune‐field development in a water table‐controlled system: Skeiđarársandur, Southern Iceland

AbstractAeolian dune fields characterized by partly vegetated bedforms undergoing active construction and with interdune depressions that lie at or close to the water table are widespread on Skeiđarársandur, Southern Iceland. The largest aeolian dune complex on the sandur covers an area of 80 km2 and is characterized by four distinct landform types: (i) spatially isolated aeolian dunes; (ii) extensive areas of damp and wet (flooded) interdune flat with small fluvial channels; (iii) small aeolian dune fields composed of assemblages of bedforms with simple morphologies and small, predominantly damp, interdune corridors; and (iv) larger aeolian dune fields composed of assemblages of complex bedforms floored by older aeolian dune deposits that are themselves raised above the level of the surrounding wet sandur plain. The morphology of each of these landform areas reflects a range of styles of interaction between aeolian dune, interdune and fluvial processes that operate coevally on the sandur surface. The geometry, scale, orientation and facies composition of sets of strata in the cores of the aeolian dunes, and their relationship to adjoining interdune strata, have been analysed to explain the temporal behaviour of the dunes in terms of their mode of initiation, construction, pattern of migration, style of accumulation and nature of preservation. Seasonal and longer‐term flooding‐induced changes in water table level have caused episodic expansion and contraction of the wet interdune ponds. Most of the dunes are currently undergoing active construction and migration and, although sediment availability is limited because of the high water table, substantial aeolian transport must occur, especially during winter months when the surface of the wet interdune ponds is frozen and sand can be blown across the sandur without being trapped by surface moisture. Bedforms within the larger dune fields have grown to a size whereby formerly damp interdune flats have been reduced to dry enclosed depressions and dry aeolian system accumulation via bedform climb is ongoing. Despite regional uplift of the proximal sandur surface in response to glacial retreat and unloading over the past century, sediment compaction‐induced subsidence of the distal sandur is progressively placing aeolian deposits below the water table and is enabling the accumulation of wet aeolian systems and increasing the likelihood of their long‐term preservation. Wet, dry and stabilizing aeolian system types all co‐exist on Skeiđarársandur and the dunes are variously undergoing coeval construction, accumulation, bypass, stabilization and destruction as a result of interactions between localized factors.

DMP III: Pleistocene and Holocene palaeonvironments and prehistoric occupation of Fazzan, Libyan Sahara

AbstractThe palaeoanthropological and geomorphological sub-projects of the Desert Migrations Project (DMP) focus on the Pleistocene and early Holocene environment and prehistory of Fazzan so as to assess the timing and extent of hominin and human movement across the Sahara through time. This paper reports on the findings of the 2008 field season, with a focus on the prehistoric evidence along the northern margin of the Ubari sand sea.The geomorphological record of the area preserves evidence of at least five past episodes of lake formation. The exact chronology of these, as well as the spatial extent of these lakes, remains the focus of further study.The archaeological record of hominin and human occupation of Fazzan prior to the establishment of the Garamantian civilisation is extraordinarily rich. Between 2007 and 2008, the DMP palaeoanthropological project surveyed thirty-five localities along the northern margin of the Ubari sand sea, recording a range of assemblages spanning all Palaeolithic industries. Most of the archaeological remains found consisted of stone-tools, while grinding stones were comparatively restricted geographically. Mode 1/Oldowan tools were found at two localities, contrasting with the widespread presence of Mode 2/Acheulean, Mode 3/Middle Stone Age and Mode 5/microlithic artefacts. This indicates that, although hominin presence in the area is probably earlier than previously thought, populations were comparatively sparse until the Middle Pleistocene. Twenty-one localities within the Ubari sand sea, as well as seven south of the Messak Settafet were also surveyed between 2007 and 2008. The detailed study of the lithics from these areas will be carried out next year, but preliminary results stress the different nature of the assemblages found within interdune corridors — very low frequency of cores, no Mode 1 and extremely rare Mode 2 lithics (found at a single locality).The 2009 field season will focus on obtaining further samples of palaeolake sediments for dating, on the evidence of Mode 1 assemblages south of the Messak, as well as on the refining of the archaeological indicators that may distinguish the different phases of hominin and human occupation of Fazzan during the Later Pleistocene and Holocene.

Stratigraphic evolution of an aeolian erg margin system: the Permian Cedar Mesa Sandstone, SE Utah, USA

AbstractThe Permian Cedar Mesa Sandstone of south‐east Utah is a predominantly aeolian succession that exhibits a complex spatial variation in sedimentary architecture which, in terms of palaeogeographic setting, reflects a transition from a dry erg centre, through a water table‐controlled aeolian‐dominated erg margin, to an outer erg margin subject to periodic fluvial incursion. The erg margin succession represents a wet aeolian system, accumulation of which was controlled by progressive water table rise coupled with ongoing dune migration and associated changes in the supply and availability of sediment for aeolian transport. Variation in the level of the water table relative to the depositional surface determined the nature of interdune sedimentary processes, and a range of dry, damp and wet (flooded) interdune elements is recognized. Variations in the geometry of these units reflect the original morphology and the migratory behaviour of spatially isolated dry interdune hollows in the erg centre, locally interconnected damp and/or wet interdune ponds in the aeolian‐dominated erg margin and fully interconnected, fluvially flooded interdune corridors in the outer erg margin. Relationships between aeolian dune and interdune units indicate that dry, damp and wet interdune sedimentation occurred synchronously with aeolian bedform migration. Temporal variation in the rates of water‐table rise and bedform migration determined the angle of climb of the erg margin succession, such that accumulation rates increased during periods of rapidly rising water table, whereas sediment bypassing (zero angle of climb) occurred in the aftermath of flood events in response to periods of elevated but temporarily static water table. During these periods in the outer erg margin, the expansion of fluvially flooded interdunes in front of non‐climbing but migrating dunes resulted in the amalgamation of laterally adjacent interdunes and the generation of regionally extensive bypass (flood) supersurfaces. A spectrum of genetic depositional models is envisaged that accounts for the complex spatial and temporal evolution of the Cedar Mesa erg margin succession.

Geo‐ecological soil features and the vegetation pattern in an arid dune area in the Northern Negev, Israel

AbstractSoils are suggested to be an important factor influencing the vegetation pattern. In order to prove this hypothesis in an arid and sandy Negev ecosystem of longitudinal dunes and interdune corridors near Nizzana, Israel, the distribution of soil units and plant communities were compared on a very small‐distance level. The Nizzana site has a size of nearly 100 ha. 250 soil pits with 3–5 horizons were mapped and sampled within the soil geographical monitoring. Soil water capacity and cation exchange capacity were estimated from the field data. In addition 176 plots of 100 m2 were established around the soil pits with emphasis on plants, coveting the environmental gradient across the dune ridges. Plant mapping plots were arranged into groups with the same dominating perennial species and a key for the vegetation mapping was developed. In the investigated desert site topography, soil surface crust, soil texture and salt content partly can explain the distribution of plant communities. A soil classification proposal is based on these parameters and is a useful tool to indicate correlations between soil units and the vegetation pattern. Nevertheless, dependent on topography and surface and/or subsurface water flow as well as the occurrence of “fertile islands” the moisture and nutrient regimes influence the vegetation pattern with respect to plant growth to a greater extent than the soils themselves.

Mixed aeolian sandsheet and fluvial deposits in the Tumblagooda Sandstone, Western Australia

Abstract Field investigation of the Late Silurian Tumblagooda Sandstone of Kalbarri, Western Australia demonstrates that the lower part of the formation comprises a unit of fluvial trough crossbedded sandstone (450 m) overlain by previously unrecognized aeolian sandsheet deposits (350 m) which include small (to 2 m) aeolian dunes. Interbedding of aeolian and fluvial facies is present and fluvial facies decrease in importance down the fluvial transport path which was to the west and northwest. Aeolian transport was to the southeast and resulted in formation of medium- to coarse-grained low-angle sandsheets and small parallel-crested dunes. Fluvial transport in aeolian-dominated areas was deflected to the southwest along interdune corridors. Interdune deflation areas were frequently flooded, probably by rising ground water. Traces fossils are abundant, including beds bioturbated by Heimdallia and numerous arthropod trackways which cross aeolian strata. The dominance of sandsheet relative to dune morphologies is ascribed to the following factors: (1) mediumcoarse sand grain size, (2) high water table, and (3) short aeolian transport distance between fluvial channels not permitting extensive dune formation. A lack of vegetation is essential for development of fluvial and aeolian sandsheet facies. Fluvial sandstones were cemented by quartz overgrowths with only minor compaction. Aeolian dune sandstones have only minor quartz cement but show greater chemical compaction effects. Porosity retention is better in the aeolian sandstones. The Taylor Group of Antarctica which is thought to be of early Devonian age shows strong similarities to the Tumblagooda Sandstone in terms of trace fossil assemblages, and sedimentology.

Runoff generation in a sandy area—the nizzana sands, Western Negev, Israel

AbstractThis paper presents the results of sprinkling experiments conducted over the lower part of vegetated and crusted linear dunes as well as over flat playa surfaces that appear in the interdune corridors. Data obtained show that these two surface units respond quickly to rainstorms. Runoff generation can be expected for any storm exceeding 2‐3 mm and runoff coefficients are high. When the topsoil algal crust, 1 to 2 mm thick, is removed from the surface of the dune, infiltration increases drastically and eliminates any possibility of runoff generation under present‐day rainfall conditions. This data may be of great help in the understanding of the geomorphology and sedimentary sequence of the corridors separting linear dunes.

Sedimentology and paleohydrology of Late Quaternary lake deposits in the northern Namib Sand Sea, Namibia

The Namib Sand Sea is the largest active desert dunefield in southern Africa, and is comprised mainly of large north-south linear dunes. In the interdune areas of the northern Sand Sea eleven small areas of calcareous lacustrine sediment have been studied. These beds are typically less than a metre thick and are dominantly comprised of calcareous sandstones to mudstones and sandy limestones. The carbonates are mainly magnesian calcites (1–14% MgCO 3) with some protodolomite and aragonite. Calcified reed casts and fresh to brackish water gastropods, diatoms, and ostracods are present in some beds. δ 18O values indicate a hot and dry climate. A number of enriched δ 13C values may reflect high salinity, low organic populations, or carbonate recrystallization. These carbonate-rich lacustrine deposits are indicative of increased periods of moisture availability in this normally hyperarid region during the Late Quaternary. The origin of the water responsible for depositing these sediments may be: (1) ponding at the end point of the Tsondab River, which at one time extended farther west into the Sand Sea; (2) flooding into interdune corridors when water levels rose in rivers such as the Kuiseb; (3) groundwater seepage into depressions either through dunes that border rivers or from the underlying Tsondab Sandstone; and (4) increased rainfall. We do not believe that there is evidence to support a major increase in precipitation over the region. However, even a small increase in precipitation in the headwaters of valleys that drain toward the Sand Sea might: (1) generate enough additional runoff to extend the terminal point of rivers such as the Tsondab farther into the dunes; (2) cause lateral flooding from major valleys into interdune corridors; and (3) recharge aquifers. The sedimentary records at Narabeb, Ancient Tracks, and West Pan, which lie along the old course of the Tsondab River, favor a ponded river origin for them, whereas groundwater seepage is favored at other sites. The chronology of deposition, based on radiocarbon dates, suggests that ponding and recharge occurred earlier in the lower, western part of the area, and later in the east. This is in harmony with the view that the end point of the Tsondab River progressively retreated eastward between about 30 and 14 ka BP, as dunes blocked its route.

Origins of Relief Along Contacts Between Eolian Sandstones and Overlying Marine Strata

Origins of large-scale relief along eolian-marine unit contacts, which form significant stratigraphic traps for hydrocarbons, can be recognized as inherited, reworked, and/or erosional. The Permian Rotliegende-Weissliegende Sandstone and Yellow Sands of Europe may best exemplify inherited relief in that dunes are preserved largely intact. Reworked relief, which shows significant destruction of original dune topography but with remnants of the bedforms preserved, is shown by relict Holocene dunes of coastal Australia, the Jurassic Entrada Sandstone of the San Juan basin, and the Pennsylvanian-Permian Minnelusa Formation of Wyoming. Erosional relief results from post-eolian processes and is exemplified by the the Jurassic Entrada Sandstone of northeastern Utah. Whether inherited to reworked relief is preserved depends on factors that relate to the budget of the dunes and the erg and to the nature of the transgressive environment. Sand budget refers to the maintenance, gain, or loss of dune prior to and during initial transgression. Sand can be conserved and the dune form stabilized by vegetation, early cementation, and a wind regime conducive to sand trapping by the dune. Dune building can continue during initial transgression by maintenance of sand-drift paths, best accomplished by offshore winds. In contrast, dunes are degraded by deflation, a cutoff of the supply, and a variety of marine processes. Factors related to the nature of the transgressive environment include (1) orientation of the dunes to destructive marine pro esses, (2) types of processes, (3) energy level, (4) type and rate of sedimentation, and (5) rate of marine encroachment. Preservation of inherited to reworked relief requires that the dunes become stabilized or remain active into the early phases of the transgression. Effects of destructive marine processes must be minimized by resistant, early-cemented dunes, by an orientation of the dunes such that marine energy is dissipated along a minimum area of the dunes, by a low-energy, non-wave-dominated marine environment, by a rapid transgression that reduces the length of time for reworking, or by high rates of marine sedimentation burying dune forms. Erosional relief requires selective downcutting through erg deposits and can (1) reflect original erg topography (e.g., by preferential channeling through interdune corridors), (2) be totally independent of erg topography and deposits, or (3) be controlled by erg facies with preferential downcutting through less resistant deposits. The last case could result in relief that mimics original topography where interdune deposits are less resistant, or show an inverse of original topography where dune sands are less resistant.

Climbing Zibars of the Algodones

Zibars are coarse-grained, low-relief, slipfaceless, eolian bedforms that occur on sand sheets and within interdune corridors of many eolian sand seas. Zibar surface features consist exclusively of wind ripples and local shadow and shrub-coppice dunes associated with vegetation. A study of zibars located on the southwestern margin of the Algodones dune field, California, shows that zibars, and their associated interzibar areas, are migrating features oriented transverse to oblique to the resultant sand-transport direction. Zibars leave deposits formed by deposition on the zibar lee face. The zibar shape modifies the near-surface airflow to produce a subtle grain-size change across the zibar/interzibar surface. Flow expansion (not separation) leeward of the zibar crest causes the lee-face deposits (those preserved upon bedform migration) to have the finest grain size on the zibar/interzibar surface. Sediment coarser than that deposited on the zibar lee face is transported more slowly and less frequently, and thus becomes concentrated on the zibar stoss slope and, ultimately, becomes interzibar deposits. Migration of zibars and interzibar areas can produce two characteristic depositional sequences depending upon the angle of climb. For low angles of climb, a sequence of stacked amalgamated interzibar deposits is preserved; zibar deposits are absent. Amalgamated interzibar deposits consist of low-angle (usually less than 5°) wind-ripple laminae of medium to coarse sand. Given higher angles of climb, deposits of both the entire interzibar and zibar lower lee face are preserved. Low-angle interzibar deposits of medium to coarse sand are overlain tangentially by finer grained (fine to medium sand) wind-ripple laminae (dipping less than 15°) of the zibar lee face. Lee-face wind-ripple laminae are sharply truncated and overlain by the succeeding set of interzibar wind-ripple deposits. This zibar/interzibar sequence is distinctive and allows for the detailed interpretation of some ancient sand-sheet deposits as left by migrating zibars. A possible example of ancient zibar deposits is present in the Entrada Sandstone (Jurassic) of southwestern Colorado.

A handaxe of Libyan Desert glass

The handaxe of Libyan Desert glass (PL. XIIIb & FIG. I) was found on 23 June 1979, in the Sand Sea of S.W. Egypt, at latitude 25°o8' N, longitude 25° 35·5' E, near the southern boundary of the known distribution area of the glass. Lying just north of the Gilf Kebir, this part of the Sand Sea is characterized by an extensive field of linear dunes, trending almost exactly N-S, which are up to Ioom high, tens of km long, and separated by interdune corridors or ‘streets’ 2–5 km wide.