Quaternary Dune Research Articles

Palaeoclimate interpretations of Late Pleistocene vegetated linear dune mobilization episodes: evidence from the northwestern Negev dunefield, Israel

The vegetated linear dune (VLD) field of the northwestern (NW) Negev Desert, situated at the downwind eastern end of the northern Sinai - NW Negev Erg, constitutes an ideal setting for dating and interpreting its Late Quaternary dune encroachment episodes. This study builds upon the results of Roskin et al. (Age, origin and climatic controls on vegetated linear dunes in the northwestern Negev Desert (Israel), Quaternary Science Reviews 30 (2011), 1649–1674) that presented the stratigraphy of 35 sections and 97 optically stimulated luminescence (OSL) ages from the NW Negev dunefield. Here we refine our analysis of the Negev Late Pleistocene dune mobilizations and stabilizations and interpret their palaeoclimatic controls in light of regional and global sediment records and proxies. While initial dune encroachment into, and stabilization in, the NW Negev took place during the Last Glacial Maximum (LGM) at ∼23−18 ka, spatial and statistical analyses of the OSL dataset suggest that since the LGM, Negev dune activity was concentrated in two significant mobilization-stabilization episodes: a main episode at ∼16–13.7 ka and a minor one at ∼12.4–11.6 ka when the dunes reached their maximum spatial extent and stabilized. These episodes include rapid dune encroachment and accretion events and coincide with the Heinrich 1 and Younger Dryas cold events, respectively. The Late Pleistocene sand-transporting winds were characterized by a westerly direction that resulted in west-east VLD elongation. Dune mobilizations may have occurred in response to wintertime East Mediterranean cyclonic systems that brought storms of rainfall and strong winds. The rapid dune mobilization events and their concurrence with the Heinrich 1 and Younger Dryas cold events suggest a more global control. Despite the rainfall, the elongating VLDs were probably sparsely vegetated because of the high wind power; their stabilization resulted from a decrease in storminess, with the onset of a more arid Holocene climate. Other global low-latitude dune mobilizations and stabilizations are concentrated at the end of the Late Pleistocene, leading us to suggest that these were also controlled mainly by global cold-events and subsequent changes in windiness. The recurring discontinuous aeolian sedimentation pattern found in OSL-dated VLDs provides new and important chronological and sedimentological insight into prominent dune mobilization and stabilization processes. The suggested link between global drops in cold-event windiness and low-latitude dune stabilization episodes emphasizes the prevalence of winds over aridity regarding major dune mobilizations for low-latitude dunes, even if vegetated.

Response of surface processes to climatic change in the dunefields and Loess Plateau of North China during the late Quaternary

ABSTRACTThis paper draws on recent optically stimulated luminescence (OSL) dating to evaluate the long‐held assumption that dust accumulation rates in the Loess Plateau and the extent of active aeolian sand in the dunefields to the north have varied together over time, because both are controlled by the strength of the Asian monsoons and also possibly because the dunefields are proximal loess sources. The results show there is little evidence that high rates of loess accumulation coincided with well‐dated episodes of extensive dune activity in the Mu Us, Otindag, and Horqin dunefields, at 11–8 ka and 1–0 ka. Explanations for the apparent lack of coupling include local variation in the trapping of dust and post‐depositional preservation of the loess and dune sediments, in response to varying local environmental conditions. In addition, a substantial portion of the loess may be transported directly from source areas where dust emission has somewhat different climatic and geomorphic controls than aeolian sand activity within the dunefields. The results of this study cast doubt on the use of loess accumulation rate as a palaeoclimatic proxy at millennial timescale. The dunefield and loess stratigraphic records are interpreted as primarily recording changes in effective moisture at a local scale, but the timing of late Quaternary dune activity, along with a variety of other evidence, indicates that moisture changes in many of the drylands of northern China may not be in phase with precipitation in core regions of the Asian monsoons. Copyright © 2011 John Wiley & Sons, Ltd.

Petrography and geochemistry of late Quaternary dune fields of western Argentina: Provenance of aeolian materials in southern South America

Abstract Landscapes of western Argentina are dominated by aeolian sand of diverse composition, reflecting multiple sediment sources. This study focuses on determining the petrography and geochemical composition of sand from three western Argentina dune fields, Medanos Grandes, Medanos Negros and San Luis, to better constrain the provenance of aeolian sand and its relation to Pampean loess. Medanos Grandes sands are litharenites to feldspathic litharenites, with metamorphic and volcanic rock fragments and lesser amounts of quartz and feldspar. Trace elements (U, Th, Sc, V) indicate the dominance of felsic source. A mixed provenance, with contributions from Sierras Pampeanas metamorphic–igneous complex, pre-Quaternary volcanic rocks and direct input from Andean explosive volcanism, is assumed. Medanos Negros sands are lithic feldsarenites, with abundant feldspars and quartz and lesser amounts of rock fragments. Trace elements indicate a mafic source for these aeolian sands, geochemically and petrographically distinct likely due to the input of ultramafic-mafic lithologies of the Sierra Pampeanas. The San Luis sand has substantial petrographic variability with lithic feldsarenites, feldspathic litharenites and lithic arenite. Trace element composition indicates a felsic source. A diagnostic attribute is the dominance of fresh pumice and volcanic glass shards. Contributions from Andean volcanic sources and local metamorphic and igneous rocks are ascertained. Pampean loess and western Argentina dune field sand show broad petrographic and geochemical similarities indicating aeolian sand, silt particles and, eventually, far travelled dust may have a relatively common source. Another viable source for loess is associated with aeolian abrasion in the many dune fields in western Argentina.

The Acridian plagues, a new Holocene and Pleistocene palaeoclimatic indicator

Five palaeosols, intercalated within the Quaternary dune beds of Fuerteventura and Lanzarote (Canary Islands), off the Moroccan coast, mark wetter climatic episodes. In all of them, billions of calcified insect ootheca testify to past occurrences of Acridian plagues, such as those reaching the western Sahara following heavy rainfall events over the Sahel. The most massive infestation is in the Holocene, and should coincide with the climax of Saharo–Sahelian humidity at the peak of the present interglacial.

Quartz grain surface features in environmental determination of aeolian Quaternary deposits in northeastern Tunisia

Abstract Quartz grain surfaces from the Quaternary sand deposits of northeastern Tunisia were examined by scanning electron microscopy. Superficial textures on the quartz grain surface are commonly well preserved and are characteristic of specific formation conditions, allowing the reconstruction of the transport and sedimentation chronology and environment of individual grains. In the northeastern coastal part of Tunisia (Cap Bon peninsula), the quartz grain exoscopy analysis involves a complex sedimentary history during the Quaternary. The Quaternary dune samples provided a mixture of sub-angular and rounded quartz morphotypes with well preserved mechanical impacts on the grain surfaces. The investigated quartz grains revealed reasonably distinct textural combinations and are characterized firstly, by mechanical features, and secondly, by chemical processes. In all the materials examined, differing environmental evolutions, marine, aeolian and pedologic were detected.

Post-cropping change and dynamics in soil and vegetation properties after forest clearing: Example of the semi-arid Mikea Region (southwestern Madagascar)

This study concerns post-cultural dynamics in the semi-arid South-West of Madagascar (Analabo area, near the Mikea Forest). A synchronic comparison was performed on a set of abandoned fields plots aged from 2 to 30 years and on forest and savanna reference ecosystems, located on cambic arenosols developed from same Quaternary dune sands. The studied parameters concerned mainly a few physical and chemical soil properties (density, permeability, compaction, texture, C, N, K, P content). Important changes occur in the post-cultural succession: an increase of the soil density and compaction, and decrease of the soil upper layer permeability. Consequently, slash and burn cultivation leads to a packing and an induration of the soil surface. Results about edaphic indicators have shown that the physical parameters used better discriminate various stages of evolution than the chemical parameters do. The multivariate analysis of soil indicators shows that vegetative succession over 30 years in a forest ecosystem cleared, burnt, cultivated and left, does not lead to features corresponding to a closed-canopy forest but rather to those of a tree savanna with open, mixed woody-herbaceous vegetation. The primary dense dry deciduous regeneration of the primary forest is very low or nil. In the semi-arid SW Madagascar (Analabo region), post-cultural dynamics conditions consists in a savanna-formation process, controlled by: (1) the intensity and duration of the disturbance (during the cultivation phase); and (2) the more drastic environmental conditions (including both climate and soil).

Agricultural suitability of dune system and Limpopo Basin soils near Xai-Xai, Mozambique

Quaternary dune systems, as well as the Limpopo Basin near the rural city of Xai-Xai in southern Mozambique were studied concerning their soil fertility and sustainability for crop cultivation. Due to population growth, most sites, which are not overbuilt, are used for crop production. The Arenosols and Regosols of the dune area have slope of 20° and sandy textures. A poor water-holding capacity and a low capability of bonding nutrients are the consequence. A lack of food prevents a disciplined crop rotation including longer fallow periods. Despite persistent cultivation for decades, the soils have a high base saturation, but already a slight decline in the base saturation has considerable effects on crop quality. Shifting cultivation along the coastal dunes leads to a massive clearing of woodlands which destabilises the sensitive dune systems and exposes them to erosion. The Fluvisols of the Limpopo River valley on the other hand cannot be used without restrictions either. High salt contents influence plant growth and the clayey soils constrain root penetration, water availability, drainage and cultivation. Nevertheless, the soils are rich in nutrients and organic matter, and there is no water stress, even during the dry season. The valley soils appear to be a reasonable platform for agriculture.

Late Quaternary dune development along the western margin of South Africa and its relationship to paleoclimatic changes inferred from the marine record

Late Quaternary dune development along the western margin of South Africa and its relationship to paleoclimatic changes inferred from the marine record

Geomorphology and chronology of Late Quaternary dune fields of western Argentina

Stabilized dune fields are common in western Argentina and potentially reflect drought variability in the Late Quaternary. This study focussed on deciphering geomorphic and stratigraphic record of three vegetated dune fields from east to west, San Luis, Médanos Negros and Médanos Grandes, with a mean annual precipitation from ∼ 700 to 91 mm. Optically stimulated luminescence single-aliquot regeneration protocols yield ages on eolian quartz grains, constraining depositional events. The San Luis dune field, the wettest area, exhibits evidence for repeated activation during the last glaciation, ca. 33–20 ka, generally coincident with regional loess deposition These dunes were locally reformed in the past 100 years which is apparently coincident with intensified agrarian use and drying in the late 19th and early 20th century. The two driest and western-most dune fields, Médanos Negros and Médanos Grandes, show reactivation at ca. 2.5 ka 0.9 and 0.5 ka; and ca. 4.3–4, 2.1 and 0.6–0.4 ka, respectively. Eolian depositional events at ca. 2.5–2.1 ka and 0.6–0.4 ka may be coeval for Médanos Grandes and Negros dune fields. Sustained drought conditions conducive for eolian activity is associated with warmer sea surface temperatures in the South Atlantic, a weakened South American Convergent Zone and a dispersed and southeasterly flow of tropical moisture from the Low Level Chaco Jet. This nascent analysis indicates drought variability beyond historic observations with potentially at least four dune reactivation events in the past ca. 4.5 ka.

Late Quaternary dune accumulation along the western margin of South Africa: distinguishing forcing mechanisms through the analysis of migratory dune forms

The west coast of South Africa is situated at a critical location between temperate and subtropical oceanic and atmospheric circulation systems, and palaeoenvironmental proxies from this region have the potential to elucidate issues concerning variations within these systems over glacial–interglacical cycles. While semi-arid climates have hindered the preservations of organic proxies, a variety of aeolian bedforms have been analysed in an effort to improve our understanding of environmental change in the region. Optically stimulated luminescence measurements of 51 samples from 15 reticulate dune sites along the west coast have enabled the identification of phases of aeolian activity, as well as periods of relative geomorphic stability. Combined with data derived from previous studies of the region's sediment accumulating deposits and other palaeoenvironmental proxies, periods of increased windiness are identified at 16–24, 30–33, 43–49 and 63–73 ka From approximately 17–12 ka, decreasing transport capacity resulted in the stabilisation of the west coast's dune fields. During the Holocene Altithermal (∼ 4–8 ka), despite reduced wind strength, increased aridity resulting from higher temperatures and a reduced influence of moisture bearing westerly systems appears to have trigged widespread remobilisation of the region's dune fields. The combination of ages from a variety of dune forms with different development mechanisms, and comparisons with a range of proxy data sources, have allowed for an enhanced interpretation of the region's aeolian archives, moving beyond simple correlations between dune activity and “aridity.”

Climatic cycles inferred from the aminostratigraphy and aminochronology of Quaternary dunes and palaeosols from the eastern islands of the Canary Archipelago

AbstractAminochronological and aminostratigraphical methods have been used to study the Quaternary aeolian deposits from the islands located east of the Canary Archipelago (Fuerteventura and Lanzarote islands and La Graciosa, Montaña Clara and Alegranza islets). The extent of racemisation/epimerisation of four amino acids (isoleucine, aspartic acid, phenylalanine and glutamic acid) was measured in land snail shells of the genus Theba. The age calculation algorithms of these amino acids have been determined to permit the numerical dating of these deposits. Eight Aminozones, each defining dune/palaeosol‐formation episodes, have been distinguished and dated at 48.6 ± 6.4, 42.5 ± 6.0, 37.8 ± 4.6, 29.4 ± 4.8, 22.4 ± 4.5, 14.9 ± 3.6, 11.0 ± 4.0 and 5.4 ± 1.1 ka BP, the first five of them defining cycles of 5–7 ka.The alternation of palaeosols and aeolian deposits, which are related to abrupt transitions from humid to arid conditions, are the reflection of globally induced changes in North Africa palaeoenvironmental conditions linked to the effect of African palaeomonsoons on the trade winds and the Saharan Air Layer. Probably these aeolian cycles, with a recurrence period of 5–7 ka, are the expression of multiples of the ∼2.4 ka solar‐cycle. Copyright © 2005 John Wiley & Sons, Ltd.

Unsaturated zones as archives of past climates: toward a new proxy for continental regions

Natural chemical and isotopic tracers contained in unsaturated-zone moisture profiles are being developed as potential new archives for reconstructing recharge history, as well as palaeoclimatic or palaeobotanical conditions over time scales ranging from 20–120,000 years. Results worldwide to date are reviewed, and examples from northern Africa and the western USA are discussed in detail. Encouraging results are obtained from relatively homogeneous deposits such as Quaternary dune sands, where Cl profiles are compared both with the instrumental record, such as rainfall and river-gauging records, and 3H profiles. Model studies have helped to define the persistence time of unsaturated-zone signals, where evidence of a 20-year event such as the Sahel drought may persist for 1,000 years.

Origin of late Quaternary dune fields on the Southern High Plains of Texas and New Mexico

Mostly stabilized late Holocene eolian sands on the Southern High Plains of the United States were studied to determine their origins and to assess whether present dune stability depends more strongly on sediment supply, sediment availability, or transport limitations. Geomorphic, sedimentological, and geochemical trends indicate that late Holocene dunes formed under westerly paleowinds, broadly similar to those of today. Mineralogical and geochemical data indicate that the most likely source for the sands is not the Pecos River valley, but the Pleistocene Blackwater Draw Formation, an older, extensive eolian deposit in the region. These observations suggest that new sand is supplied whenever vegetation cover is diminished to the extent that the Blackwater Draw Formation can be eroded, in agreement with modern observations of wind erosion in the region. We conclude, therefore, that Southern High Plains dunes are stabilized primarily due to a vegetation cover. The dunes are thus sediment-availability limited. This conclusion is consistent with the observation that, in the warmest, driest part of the region (where vegetation cover is minimal), dunes are currently active over a large area. Geochemical data indicate that Southern High Plains dunes are the most mineralogically mature (quartz rich) sands yet studied in the Great Plains, which suggests a long history of eolian activity, either in the dune fields or during deposition of the Blackwater Draw Formation.

Preliminary investigation of Indigenous campsites in late Quaternary dunes, Port Augusta, South Australia

A series of late Quaternary dunes are located in the vicinity of Port Augusta in the mid-north of South Australia. Observations of deflating archaeological material were first recorded by Norman B. Tindale during the 1939 Harvard- Adelaide University Anthropological Expedition (Tindale 1939:827).        A mile & 3/4 beyond the Port Augusta Bridge on the side of the road to Iron Knob, we hunted over a site where we had on a previous occasion found several old Kangaroo Island type implements. Found an old earthy layer from which series of large crude quartzite flakes were eroding also a few large and much altered shells. Much of the implements was already dropped onto a hard pan and the rest was on the surface of the weathering earth layer, so that absolute results not obtainable but it seemed likely that most if not all the   material weathering out belonged to a single period.  Records of similar material at nearby Dempsey's Lake were made by Cooper (1953) and Lampert (1976). Dempsey's Lake was the focus of palaeontological investigations during the 1950's from which time a certain amount of Diprotodon skeletal material was recovered. Stone tools described as consistent with 'Kartan' industries have also been recovered and are generally characteristic of the core tool and scraper tradition (Lampert 1976). Lampert (1976) also noted the absence of small tools 'such as pirris and tulas'. The fossil bone and stone tools were exposed on an red sand dune running in a straight line from north west to south (Lampert 1976: 12).  In 1997 a series of similar deflations exposing stone tools and small amounts of animal bone were identified in eroded red sand dunes trending generally north west to south east and bordering on the runway at Port Augusta Aerodrome (Walshe 1997; Walshe and Evans 2001). These sites are located approximately 3 km from the Port Augusta Bridge on the northwestern side of the Stuart Highway and may well be the same sites referred to by Tindale in 1939 and mentioned above. This paper reports on a salvaged assemblage from these dunes prior to their destruction, addresses the likely age range of the sites, and identifies the need for further research on Indigenous sites associated with late Quaternary dunes.

Lizards, Snakes, and Amphisbaenians from the Quaternary Sand Dunes of the Middle Rio Sao Francisco, Bahia, Brazil

Lizards, Snakes, and Amphisbaenians from the Quaternary Sand Dunes of the Middle Rio Sao Francisco, Bahia, Brazil

Origin of the late Quaternary dune fields of northeastern Colorado

Stabilized eolian deposits, mostly parabolic dunes and sand sheets, cover much of the landscape of northeastern Colorado and adjacent parts of southwestern Nebraska in four geographically distinct dune fields. Stratigraphic and soil-geomorphic relations and accelerator radiocarbon dating indicate that at least three episodes of eolian sand movement occurred between 27 ka and 11 ka, possibly between 11 ka and 4 ka, and within the past 1.5 ka. Thus, eolian sand deposition took place under both glacial and interglacial climatic conditions. In the youngest episodes of eolian sand movement, Holocene parabolic dunes partially buried Pleistocene sand sheet deposits. Late Holocene sands in the Fort Morgan and Wray dune fields, to the south of the South Platte River, have trace element ratios that are indistinguishable from modern South Platte River sands, but different from Ogallala Formation bedrock, which has previously been cited as the main source of dune sand on the Great Plains. Sands in the Greeley dune field, to the north of the South Platte River, have trace element concentrations that indicate a probable Laramie Formation source. Measurements of parabolic dunes indicate paleowinds from the northwest in all dune fields, in good agreement with resultant drift directions calculated for nearby weather stations. Thus, paleowinds were probably not significantly different from present-day winds, and are consistent with a South Platte River source for the Fort Morgan and Wray dune fields, and a Laramie Formation source for the Greeley dune field. Sand accumulated downwind of the South Platte River to form the Fort Morgan duen field. In addition, sand was also transported farther downwind over the upland formed by the calcrete caprock of the Ogallala Formation, and deposited in the lee of the upland on the southeast side. Because of high wind energy, the upland itself served as a zone of sand transport, but little or no sand accumulation took place on this surface. These studies, which demonstrate the importance of fluvial-source sediments for dune fields in Colorado, may be applicable to other dune fields in North America. Because modern drift potentials in northeastern Colorado are among the highest in the world, the present stability of dunes in the region may be in part a function of the dunes being supply-limited rather than solely transport-limited. Extensive (∼ 7700 km 2) late Holocene dunes document that eolian sand in northeastern Colorado is very sensitive to small changes in climate or fluvial source conditions.

Quaternary dune carbonates from the Mediterranean Coast of Egypt: Petrography and diagenesis

The Quaternary carbonates of the Mediterranean coast of Egypt between Alexandria and Salum appear as parallel limestone ridges rising up to 100 m above sea level. These ridges are dominated by dunal carbonates which differ not only in their primary composition but also by distinct grades of meteoric water diagenesis. Oolitic facies dominates the younger aeolianites of the first and second ridges. Bioclastic facies with abundant coralline algae, benthonic foraminifers, molluscs, echinoderms and intraclasts represents the major rock type in the older aeolianites. Features of meteoric water diagenesis include precipitation of increasing amounts of avoid-filling low Mg-calcite spar, dissolution of aragonite and stabilization of aragonite and high Mg-calcite to low Mg-calcite. Aeolianites below paleosol horizons contain abundant calcrete cements, micritized fossils and detrital grains which are commonly corroded and replaced by calcite. Three stages of progressive meteoric diagenesis are recognised in the Egyptian Quaternary aeolianites. In stage 1 minor precipitation of low Mg-calcite occurs at the grain boundaries. Stage 2 is marked by partial dissolution of aragonite, partial loss of high Mg-calcite and precipitation of low Mg-calcite in some pore spaces. In stage 3, most of the remaining pores are occluded by cementation. At the end of stage 3, Mg is removed from high Mg-calcite but some aragonite still persists. The early vadose cements are represented by miniscus, bridge and pendant cements. The phreatic cements were precipitated as bladed spar in the isopachous rims and equant spar in the intergranular and mouldic porosity. The late vadose cements are represented by micritic cements that were related to calcrete formation. Elemental behaviour during meteoric water diagenesis indicates that it leads to a gradual decrease in bulk Sr along with Na in progressively altered aeolianites. Mn distribution is controlled by the carbonate mineralogy (aragonite versus calcite) as well as the proximity of the aeolianites to the overlying paleosol horizons.

Quaternary Vegetation Change and Dune Formation on Bermuda: A Discussion

The Bermuda oceanic islands are comprised mainly of lithified dunes (i.e. calcareous eolianites) which formed during Quaternary sea-level regressions. This short communication considers the vegetation history of Bermuda in relation to these dune-building episodes. At 32?N, Bermuda is the northernmost limit of most frost-intolerant stibtropical vascular plant species in the northern hemisphere. The global cooling that caused the Quaternary glacial periods and also the lowering of sea level, may have led to the local extinction of Bermuda's subtropical interglacial floras. Located >900 km from the nearest continental source area, the filling of the vacated niches on Bermuda by long-distance dispersal may have involved low frequency events requiring more than 6000 years. The result would have been a very sparse vegetative cover during the dune-building episodes which would explain why the Bermuda eolianites reach elevations greater than 60 m above the present-day sea level. This hypothesis is supported by: (1) the absence of reliable fossil evidence for vegetation interfering with, or interbedded within, the eolian deposits; and (2) preliminary palynological analyses of.the clay-rich fossil soils intercalated between the eolianite formations which have revealed pollen that does not appear to correspond to the existing native flora of Bermuda.

Quaternary tectonic influences on coastal morphology, south-central California

The coastal morphology of south-central California is controlled largely by the rates and spatial distribution of uplifting and subsiding crustal blocks, which are bordered by Late Quaternary reverse faults that clearly disrupt marine terraces. We describe the style, rates, and locations of Late Quaternary deformation within the San Luis Range and the bordering Los Osos and Santa Maria valleys, and the influence of this deformation on evolution of the coast. The WNW-ESE trending San Luis Range, which is rising at a rate of 0.1 to 0.2 m/kyr, is bordered locally by a rugged coastline with narrow beaches and emergent marine terraces. The development and degree of preservation of the marine wave-cut platforms reflect the erosional resistance of bedrock lithologies. The marine terraces extend well inland south of Pismo Beach showing that a marine embayment existed during the Late Quaternary throughout large areas of the Santa Maria Valley. Coastal margins of the Los Osos and Santa Maria valleys, which are static or slowly subsiding, are characterized by the absence of marine terraces, and the presence of broad beaches, sand spits, and Late Quaternary dune fields. The coastal margins of these valleys are advancing seaward at average rates in excess of 5 m/kyr during the past 120 ka.

Late Quaternary dune incursions in the southern levant: Archaeology, chronology and palaeoenvironments

In this paper the geomorphic history of aeolian sand of western Negev and northern Sinai is summarized on the bases of archaeological data and rediocarbon dating. Aeolian sands and sand dunes are associated with Epipalaeolithic sites (18,000–10,000 BP) and they lie on loess reworked by fluvial transportation that includes Upper Palaeolithic sites (30,000–20,000 BP). The deposition of the aeolian sand, therefore, slightly predates the last glacial maximum of the Late Pleistocene (Isotopic Stage 2). The source of the sand has been identified as the Nile Delta, which was exposed to a wider extent than present due to sea level drop. The dune incursion, from west to east, was gradual and time-transgressive but relatively rapid and was induced by hyperarid climatic conditions.