Lunette Dunes Research Articles

GEOLOGICAL AND GEOMORPHOLOGICAL CONTROLS ON THE PHYSICAL LANDSCAPE OF THE FREE STATE

ABSTRACT This paper comprises two parts. The first provides a broad-brush review of the geology and geomorphology of the Free State Province. The Province's geology forms the basis for its mining activities, while soils in this important agricultural province are in large part a function of its geology and climate. The geomorphology of the Province impinges on transport routes, determines the suitability of specific parts of the Province for human settlement, and has a direct influence on tourism, where the aesthetics of the eastern Free State landscape contribute to the increasing importance of its tourism industry. The second part of the paper examines the potential of the Quaternary geomorphology of the western Free State to contribute to palaeoenvironmental reconstructions. In this area, which is peripheral to the Kalahari desert. Pans and associated aeolian-derived geomorphic features such as lunette dunes bear evidence of environmental change. Morgenzon Pan, one of the largest in the area, is used to assess the potential of such landforms for elucidating late Quaternary palaeoenvironments.

Origin and evolution of major salts in the Darling pans, Western Cape, South Africa

Sediment and water samples from 12 saline pans on the semi-arid west coast of South Africa were analysed to determine the origin of salts and geochemical evolution of water in the pans. Pans in the area can be subdivided into large, gypsiferous coastal pans with 79–150 g/kg total dissolved salt (TDS), small inland brackish to saline (2–64 g/kg TDS) pans and small inland brine (168-531 g/kg TDS) pans that have a layer of black sulphidic mud below a halite crust. The salinity of coastal pan waters varies with the seasonal influx of dilute runoff and dissolution of relict Pleistocene marine evaporite deposits. In contrast, inland pans are local topographic depressions, bordered on the north by downslope lunette dunes, where solutes are concentrated by evaporation of runoff, throughflow and groundwater seepage. The composition of runoff and seepage inflow waters is determined by modification of coastal rainfall by weathering, calcite precipitation and ion exchange reactions in the predominantly granitic catchment soils. Evaporation of pan waters leads to precipitation of calcite, Mg–calcite, dolomite, gypsum and halite in a distinct stratigraphic succession in pan sediments. Bicarbonate limits carbonate precipitation, Ca limits gypsum precipitation and Na limits halite precipitation. Dolomitisation of calcite is enhanced by the high Mg/Ca ratio of brine pan waters. Brine pan waters evolve seasonally from Na–Cl dominated brines in the wet winter months to Mg–Cl dominated brines in the dry summer months, when 5–20 cm thick halite crusts cover pan surfaces. Pan formation was probably initiated during a drier climate period in the early Holocene. More recent replacement of natural vegetation by cultivated land may have accelerated salt accumulation in the pans.

Eolian and lacustrine evidence of late Quaternary palaeoenvironmental changes in southwestern Australia

Lake Lefroy and Lake Cowan occur in the relic Tertiary palaeodrainage that used to drain to the east in central western Australia. Lacustrine conditions prevailed during the late Neogene and the early part of the Quaternary. Strong aridity sets in during mid-Quaternary, resulting in extensive gypsum precipitation and eolian deposition in and around the playa lakes. Lakes Lefroy and Cowan are now hypersaline and deflating, with a salt crust developed on the lake floor and lunette dunes constructed along the eastern margins. Optical dating (green light-stimulated luminescence (GLSL) dating) of the lunette dunes along the eastern and southeastern margins of the playas and electron spin resonance (ESR) dating of relic gypsum dunes on the lake floor show that intensified eolian activity occurred during the last glacial period, with a major lunette construction phase occurring during the Last Glacial Maximum (LGM). 14C dating of high-lake deposits, which are extensively developed along the margins of Lake Cowan, suggested a significant high-lake event during the early to mid-Holocene. The late Quaternary eolian and lacustrine evidence of palaeoenvironmental changes supports the model that during the Last Glacial Maximum, the subtropical high-pressure cell intensified and probably shifted southward, displacing the southern westerlies.

Late Quaternary lunette dune sedimentation in the southwestern Kalahari desert, South Africa: luminescence based chronologies of aeolian activity

Through the application of luminescence dating techniques, great advances have recently been made in the understanding of the late Quaternary histories of many dryland regions, including southern Africa. Here, this work has largely applied optical dating to sediments from the extensive linear dune systems which dominate the Kalahari region of the central southern African interior. Relatively little attention has been given to localised aeolian accumulations such as lunette dunes, despite their wide distribution over many parts of the region, in particular the arid southwest Kalahari. To redress this, we consider the palaeoenvironmental significance of pan margin lunettes, and investigate the timing of lunette dune development in the southwest Kalahari through the application of optical dating to sediments from lunette dunes associated with four closed basins (pans). Ages show that localised aeolian activity occurred frequently throughout the past 18 ka, indicating that the factors controlling lunette sedimentation are markedly different from those determining regional linear dune development. It is proposed that the mechanisms controlling sediment supply to lunette dunes are controlled at both the regional and local (pan-specific) scales by factors such as groundwater levels. Where pans support more than one discrete lunette, the time since any individual lunette crest received significant accumulations of sediment increases with distance away from the pan margin. Outer and inner lunette sequences may be chronologically disparate features. Inner lunette formation may arise in response to modifications to the local atmospheric boundary conditions by the presence of large, downwind outer lunette features, or by shifts in sediment supply.

Holocene aeolian activity in the southwestern Kalahari Desert, southern Africa: significance and relationships to late-Pleistocene dune-building events

Little is known about the timing of dune-building episodes in the southwestern Kalahari Desert. Previously, arid phases when dune activity occurred have been inferred from gaps in chronologies derived from information that attests to former humid periods. To redress this, optically stimulated luminescence (OSL) dating was applied to sediment samples taken from a range of aeolian landforms in the SW Kalahari. Four periods of dune building are identified in the late Pleistocene and Holocene. Holocene aeolian activity at 6 ka and 1-2 ka led to the construction of localised dune forms, in contrast to late Pleistocene times when the extensive field of linear dunes that dominates the region was emplaced. Late-Holocene aridity was however sufficient to allow rapid emplacement of the lunette dune at Witpan. The record of aeolian activity is evaluated in the context of humid chronologies and an ocean-atmosphere model of Holocene climate change, with good agreement obtained. It is concluded that the application of optical dating to a range of aeolian landforms has the potential to establish the style as well as timing of dune-building. It is also demonstrated that the climate during dune-building periods must have differed significantly from the prevailing semi-arid climate with drought periods of today, which may have been the predominant climate regime through much of the Holocene.

Lunette dunes in the northeast Cape, South Africa, as geomorphic indicators of palaeoenvironmental change

Three lunette dunes occur on the southern shore of a pan on the farm Buffelsfontein above the Great Escarpment in the northeastern part of the Eastern Cape Province, South Africa. The two inner lunettes are formed of silt and clay, and the third of fine sand. The location of these dunes is anomalous as lunette dunes are rare in the wetter parts of the southern African subcontinent. The two inner lunette dunes lie along the downwind margin of a present day pan, while the third lunette is associated with the margin of a large palaeopan. The morphometry of the Buffelsfontein pan/lunette dune series was investigated using aerial photography and field surveys. Sedimentological and geochemical techniques were applied to investigate the lunette and pan floor sediments and to assist in the elucidation of processes responsible for the development of the lunettes. Sedimentological differences in the pan fringing lunettes suggest two very different sediment sources related to changing geomorphic conditions, possibly since the last glacial maximum. The pan/lunette complex offers a unique opportunity to quantify these changes as local rainfall records are available for the past 100 years.

The nature, distribution and formation of pans in arid zones

Pans, closed depressions, are a widespread feature of many of the world's drylands. By using literature survey, air photographs, topographic maps, orbital photographs and imagery, combined with field work it is possible to describe the major areas where these features occur. Their distribution is controlled to a great extent by the availability of susceptible surfaces. They also develop in certain particular environmental settings: palaeolacustrine basins, palaeodrainages, interdunes, and on coastal plains. Many of the pans have a distinctive morphology while on their lee sides they may have lunette dunes. Many processes combine to create and maintain pans, and these can be considered in a general model which has certain key elements.The first of these is that the area should not be one where fluvial processes are fully integrated. It should also not be one where aeolian accumulation is such as to infill any irregularities in the land's surface. If these two predisposing conditions are fulfilled then under dryland conditions, if susceptible surfaces are present, there are various circumstances that may lead to hollow development and enlargement. Although in some cases such processes as solution, suffosion and animal activities may play a role, we believe that the predominant reason why pans have the characteristics that they do (including their shapes, lunettes, alignments, etc.) is that they result from the operation of the twin processes of salt weathering and aeolian deflation.

Geomorphology, stratigraphy and thermoluminescence dating of the lunette dune at Lake Victoria, western New South Wales

Abstract Lake Victoria, 13 km long and 10 km wide, is the largest lunette lake attached to the Murray River in southeastern Australia. The lake occupies the western part of a larger basin at least twice the size of the present lake. The larger basin has been incised nearly 30 m into the regional sand plain surface and into the early-middle Pleistocene Blanchetown Clay. Within the larger basin (east of the lunette of the present Lake Victoria), various landforms have developed, including alluvial terraces, salt pans, small dry basins and multiple small lunettes. Among these, an arcshaped sand ridge, possibly an old lunette, was dated by TL as 82 ka. The present lunette of Lake Victoria is up to 40 m high and consists of 3 units: Nulla Nulla Sand (30 m thick), Talgarry Sand (up to 10 m thick) and the reworked Dunedin Park Sand. The Nulla Nulla Sand consists dominantly of pure medium to fine-grained sand sequences with a few red palaeosols, indicating long-standing high lake levels with a few lunette stable intervals. Deposition of the Nulla Nulla Sand ended around 20 ka, starting at least 40 ka ago and probably much earlier. The Talgarry Sand, overlying a prominent red palaeosol on top of the Nulla Nulla Sand, consists of pure sand horizons interbedded with at least 3 gypseous clayey silt layers. The gypseous sediments indicate a major change of hydrologic conditions in the lake basin, fluctuation of lake levels and periodically high salinity. The Talgarry Sand was deposited from 17 to 10 ka as dated by TL and previous 14C dates. After 10 ka, lunette accretion stopped, possibly the result of incision of the Murray River channel, reducing the water and sediment input to the basin. However, erosion and deflation of the lunette formed an apron of sediment on the lower lake-facing slope and sand transport to the lee-side long slope, leaving “islands” of old sediment along the lunette crest zone.

Present day lunette sediment cycling at Witpan in the arid southwestern Kalahari Desert

Witpan is a pan depression in the southwestern Kalahari Desert. The pan fringing lunette dune has a number of notable characteristics: a highly active crest; the lower lunette pediment is cemented; and the lower lunette supports a highly developed and active gully system. Geomorphological and sedimentological considerations are used to evaluate the interaction of wind and water in shaping contemporary lunette processes. It is considered that like other pans in the area, groundwater activity influences pan floor sediment availability and transport, that the present highly sea-sonal climatic regime permits the interaction of aeolian transport of sediment to the dune and its return to the pan floor by water during storm events, creating a cycling process favoured by sedimentary properties that promote surface runoff.

Pans

Closed desert depressions, called pans, have an extensive distribution in the world's dry lands, particularly on certain types of susceptible surface, which include shales, fine-grained sandstones and unconsolidated materials. They also occur in certain characteristic geomorphological situations, including interdunal areas, old river channels, and deflated lake beds. Pans often, but not invariably, have lunette dunes on their lee sides. Aridity is an important predisposing factor for pan development, and permits deflation, salt weathering, and disruption of drainage systems, all of which contribute to pan formation. Large numbers of hypotheses for pan development have been proposed, including animal activities and solution, but all these can be incorporated in a general model of pan development.

Late Quaternary paleoenvironments in the southwestern Kalahari

Landforms of the southwestern Kalahari in southern Africa include linear dune systems, pans and associated lunette dunes and the valleys of ephemeral rivers. Present day climates in the region are transitional from arid to semi-arid, making the area sensitive to climatic change. The linear dunes are part of an extensive system of fixed dunes developed in the surface Kalahari sands. The dunes were in existence prior to 32,000 yr B.P., but have been reactivated on a number of occasions since then in conditions of increased wind velocities, more NW winds and reduced rainfall, especially in northern and eastern parts of the region. Periods of dune activity probably occurred at 19,000-17,000, 10,000-6000 and 4000-3000 yr B.P. The development of pans in the region has been strongly influenced by changes in groundwater conditions. Deflation from pans and formation of lunettes occurred in periods when groundwater levels were higher and fluctuated seasonally more than they do today. Such periods probably occurred 25,000–20,000 yr B.P., around 10,000 yr B.P. and just prior to 1000 yr B.P. Still higher groundwater discharges, promoted by increased regional rainfall, resulted in the formation of spring tufas and seasonal or permanent lakes in the pans, which were also fed by surface runoff, during the intervals between 33,000–28,000; 23,000–21,000; 17,000–15,000 and 12,000–11,000 yr B.P. The headwaters of the major ephemeral streams lie outside the region, but within the same climatic zone. Increased flow in the Molopo River occurred prior to 19,000, and between 15,000 and 13,000 yr B.P.

Lunette dunes in southern Africa

Lunettes are mounds of aeolian sediment that occur on the lee sides of pans (closed depressions) in Bostwana and South Africa. They are of frequent, though not universal occurrence, especially in south west Botswana, the northern Cape Province, and the north-western Transvaal. Those in the Kalahari are predominantly sandy, but in South Africa there are some examples which are either heavily calcreted or which contain substantial amounts of clay and silt.