Erg Oriental Research Articles

DEM‐based sand dune analysis: Identification and morphological regionalization in the Grand Erg Oriental

DEM‐based sand dune analysis: Identification and morphological regionalization in the Grand Erg Oriental

Diversity and networking of uni-cyanobacterial cultures and associated heterotrophic bacteria from the benthic microbial mat of a desert hydrothermal spring.

Thermal springs harbour microorganisms, often dominated by cyanobacteria, which form biofilms and microbial mats. These phototrophic organisms release organic exudates into their immediate surroundings, attracting heterotrophic bacteria that contribute to the diversity and functioning of these ecosystems. In this study, the microbial mats from a hydrothermal pool in the Ksar Ghilane oasis in the Grand Erg Oriental of the DesertTunisia, were collected to obtain cyanobacterial cultures formed by single cyanobacterial species. High throughput analysis showed that while the microbial mat hosted diverse cyanobacteria, laboratory cultures selectively enriched cyanobacteria from the Leptolyngbya, Nodosilinea and Arthronema. Per each of these genera, multiple non-axenic uni-cyanobacterial cultures were established, totalling 41 cultures. Cyanobacteria taxa mediated the assembly of distinct heterotrophic bacterial communities, with members of the Proteobacteria and Actinobacteria phyla dominating. The bacterial communities of uni-cyanobacterial cultures were densely interconnected, with heterotrophic bacteria preferentially co-occurring with each other. Our study highlighted the complex structures of non-axenic uni-cyanobacterial cultures, where taxonomically distinct cyanobacteria consistently associate with specific groups of heterotrophic bacteria. The observed associations were likely driven by common selection pressures in the laboratory, such as cultivation conditions and specific hosts, and may not necessarily reflect the microbial dynamic occurring in the spring microbial mats.

Wind Tunnel Experiments and Field Observations of Aeolian Sand Encroachment around Vernacular Settlements in the Saharan and Arabian Deserts

One of the key repercussions of the desertification process in the Sahara and the Arabian desert is increased aeolian sand drift and sand deposition. Despite its isolated location and severe desert climate—particularly in terms of solar radiation, sand drift, and sand deposition—the region of El-Oued Souf in the Grand Erg Oriental in the Algerian Sahara has been a key cultural and trade center for many centuries. To resist the extreme weather conditions, the architecture of the vernacular settlements in the region of El-Oued Souf has unique design traits. Newly constructed buildings, on the other hand, frequently employ technologies based on models imported or imposed from climates that bear little resemblance to the desert. As a result, the inhabitants of these structures live more ‘in the desert’ than ‘with the desert’. This study investigates the impact and the effectiveness of vernacular desert architecture to resist aeolian sand encroachment problems in sandy desert areas. The sand accumulation patterns around vernacular buildings were identified and quantified using wind tunnel experiments. The size and shape of the buildings, as well as their geometric configuration within the settlement, were investigated in relation to the shape and dimension of sand accumulation formations. Field observations in north Algeria and the United Arab Emirates confirmed the accumulation patterns produced by the wind tunnel experiments. The study proposes possible design indicators for building forms to minimize the impact of sand deposition on such forms.

Finding slender-horned gazelles in the Sahara's Grand Erg Oriental, Tunisia

North-African slender-horned gazelles are facing extensive poaching pressure. The main remaining population patch occurs in Saharan habitat between Tunisia and Algeria. Access to the species' distribution range in both countries is limited by difficult terrain and strict permitting requirements. We explored a mix of aerial (1365 km) and ground transects (351 km) to monitor the Grand Erg Oriental which covers the species' range in Tunisia. Only ungulate spoors but no gazelles were observed during the aerial survey over the southern part of the study area. Ground survey in the northern part revealed more conclusive results with evidence and observation of one couple, one group of three and another of four individuals. Our results confirm the presence of a slender-horned gazelle population in Tunisia yet support the need for immediate intervention to halt its decline. Ex-situ conservation is essential to the species’ future survival and continuous monitoring of the wild range is recommended.

Dome dunes: Distribution and morphology

Dome dunes are mounds of sand that generally lack external slipfaces. There are two main types. Mini-domes are typically just a few metres in height and some tens of metres across. Some of these may either be proto- or degraded barchans and the two forms occur in close proximity. Large areas of mini-domes occur in Mauritania, the Libyan Desert of southern Egypt, the Danakil Depression of east Africa, and the Qaidam Basin of China. Mega-domes are tens of metres high and hundreds of metres in width. In Africa, they are found in the Grand Erg Occidental, the Grand Erg Oriental, the Murzuk and Ubari sandseas of Libya, the Qattara Depression of Egypt, and the Red Sea Province of Sudan. In Asia they occur in the Nefud of Saudi Arabia, the Rub’ al Khali, and the Taklimakan and Wulanbuhe Deserts of China. They have a range of morphologies, from almost perfectly circular through to elongated, triangular and barchanoid types. Morphometric analysis using Google Earth Pro demonstrates that the mean densities of mega-domes (per 100 km2) are 32.8, their heights average 66.1 m, and their widths average 1459 m.

AEOLIAN SANDS IN NORTH AFRICA AND THE SOUTHWEST OF THE ARABIAN PENINSULA

Background. The majority of the world’s deserts were formed within the valleys and coasts of ancient alluvial rivers and lakes. The material carried across many hundreds and thousands of kilometres by the wind was mixing with sediments of various genetic types. In addition, due to the similar physical nature of the wind and water transport of sediments, the recognition of the aeolian environment can be problematic. Aim. Using innovative research approaches, to provide a qualitative structural description of sands; to identify and characterize factors contributing to the formation and preservation of a ferruginous film on grains; to establish the genesis of the transferred sediments, as well as the relationship with the parent rocks. Materials and methods. Samples for analysis were collected from aeolian deposits in three deserts: Rub al Khali (United Arab Emirates), Nubian desert (Egypt) and Grand Erg Oriental (Algeria). Particle size analysis was used to characterize samples in terms of three parameters (length, width, thickness). X-ray diffraction analysis was carried out using an ARL X’tra diffractometer (Switzerland). The collected samples were also studied using a binocular microscope in reflected light and a scanning electron microscope. Results. The mineral composition of the sands under study was examined. The graphs (histograms) of the size distribution of quartz grains (being the most common sand mineral in terms of size and frequency of occurrence) were plotted. The shape and nature of the grain surface were analysed. Conclusion. The frosting on aeolian sand grains can result not only from micro-cracking as the most common sign of wind transfer, but also from the presence of a calcite film on grains. Factors contributing to the formation and preservation of a ferruginous film (desert varnish) were identified. The conditions for the formation of aeolian sands, as well as the genesis of such sand sediments, were established.

Measuring Sand Dune Migration Rates with COSI-Corr and Landsat: Opportunities and Challenges

It has been over a decade since COSI-Corr, the Co-Registration of Optically Sensed Images and Correlation, was first used to produce a raster map of sand dune movement, however, no studies have yet applied it to the full Landsat archive. The orthorectified and geolocated Landsat Level-1 Precision Terrain (L1TP) products offer the opportunity to simplify the COSI-Corr pre-processing steps, allowing an automated workflow to be devised. In the Bodélé Depression, Chad, this automated workflow has calculated average dune speeds of 15.83 m/year and an increase in dune movement of 2.56 m/year ±12.58 m/year from 1987 to 2009. However, this increase does not stem from a systematic increase in dune mobility. The fastest 25% of dunes from 1987 to 1998 reduced their speed by 18.16%. The overall increase stems from the acceleration of features previously moving under 13.30 m/year. While successfully applied to the Bodélé Depression, the automated workflow produces highly variable outputs when applied to the Grand Erg Oriental, Algeria. Variations within path/row scene pairings are caused by the use of mobile features, such as dune crests, as ground control points (GCPs). This has the potential to warp Landsat scenes during the L1TP processing, potentially obfuscating dune migration. Two factors appear to be crucial in determining whether a Landsat scene is suitable for COSI-Corr analysis. Firstly, dune mobility must exceed the misregistration criteria. Secondly, GPCs should be located on static features such as bedrock outcrops.

Transport of aerosols over the French Riviera – link between ground-based lidar and spaceborne observations

Abstract. For the first time, a 355 nm backscatter N2-Raman lidar has been deployed on the western part of the French Riviera to investigate the vertical aerosol structure in the troposphere. This lidar system, based at the AERONET site of Toulon–La Garde, performed continuous measurements from 24 June to 17 July 2014, within the framework of the multidisciplinary program Mediterranean Integrated Studies at the Regional and Local Scales (MISTRALS). By coupling these observations with those of the spaceborne instruments Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP), Spinning Enhanced Visible and InfraRed Imager (SEVIRI), and Moderate Resolution Imaging Spectroradiometers (MODIS), the spatial extents of the aerosol structures are investigated. The origins of the aerosol plumes are determined using back trajectories computed by the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT). This synergy allowed us to highlight plumes of particulate pollutants moving in the low and medium free troposphere (up to ∼5 km above the mean sea level) towards the French Riviera. This pollution originates from the Spanish coast, more particularly from Costa Blanca (including Murcia) and Costa Brava–Costa Daurada (including Barcelona). It is mainly due to traffic, but also to petrochemical activities in these two regions. Desert aerosol plumes were also sampled by the lidar. The sources of desert aerosols have been identified as the Grand Erg Occidental and Grand Erg Oriental. During desert dust events, we highlight significant differences in the optical characteristics in terms of the backscatter-to-extinction ratio (BER, inverse of the lidar ratio) between the planetary boundary layer, with 0.024 sr−1 (∼42 sr), and the free troposphere, with 0.031 sr−1 (∼32 sr). These differences are greatly reduced in the case of pollution aerosol plume transport in the free troposphere (i.e., 0.021 and 0.025 sr−1). Transported pollution aerosols appear to have similar BER to what is emitted locally. Moreover, using the correlation matrix between lidar aerosol extinction profiles as a function of altitude, we find that during transport events in the low free troposphere, aerosols may be transferred into the planetary boundary layer. We also note that the relative humidity, which is generally higher in the planetary boundary layer (>80 %), is found to have no significant effect on the BER.

An inclusive view of Saharan dust advections to Italy and the Central Mediterranean

We address observations of physical and chemical properties of Saharan dust advections (SDA) as observed in the Central Mediterranean basin, within the framework of the LIFE+10, DIAPASON project (www.diapason-life.eu). DIAPASON aimed at the definition of best practices and tools to detect and evaluate the contribution of Saharan dust to ground particulate matter (PM) loads. Polarization-sensitive, automated lidar-ceilometers (PLC) are one of the tools prototyped and used in the Rome area to reach this goal. The results presented in this study focus on: 1) the effectiveness of various observational tools at detecting and characterizing atmospheric dust plumes, and 2) processes and properties of Saharan dust advections reaching the central Mediterranean region. In this respect, the combination of numerical model forecasts and time-resolved (at least hourly) PLC or chemical observations was found to constitute an efficient way to predict and confirm the presence of Saharan dust. In the period 2011–2014, Saharan dust advections were observed to reach over Rome on about 32% of the days. In some 70% of these days the dust reached the ground in dry conditions, while 30% of advection days involved wet deposition. Dry (wet) deposition was found to maximize (minimize) in summer. The northern Sahara between Algeria and Tunisia (Grand Erg Oriental), was confirmed as the most frequent region of origin of the dust mobilized towards central Italy. Secondary source regions include northern Morocco and Libya. On a statistical basis, Saharan advections to Rome were preceded by increasing atmospheric pressure and stability. These conditions were found to favor the accumulation of aerosols related to local emission sources before the SDA reached the ground. Meteorology (precipitation and turbulence in primis) resulted to be an important modulator of PM concentrations during SDAs. Magnitude and timing of these factors should be well considered to correctly evaluate the dust share in PM loads or the related health effects. Saharan advections observed during DIAPASON affected particle concentrations down to diameters of about 0.6–1 μm, with number concentrations peaking at the 2.5 μm diameter range. These advections were associated with a significant increase in Si-rich particles containing a non-negligible fraction of water. Rainfall was observed to preferentially remove dust particles larger than 2 μm, causing a significant depletion in the Ca-rich fraction with respect to the Si-rich one. The increase in PLC depolarization ratios above 5%, as well as the hourly PIXE records of the Si/Ca ratio increasing above 1 were found to represent good markers for the actual presence of Saharan dust particulate matter, when Saharan advection conditions are occurring.

DEM-based morphometry of large-scale sand dune patterns in the Grand Erg Oriental (Northern Sahara Desert, Africa)

Formerly, sand dune patterns were investigated mostly by aerial and satellite images, but more recently, geomorphometric analysis based on digital elevation models (DEMs) has become an important approach. In this paper, sand dune patterns of the Grand Erg Oriental (Sahara) are studied using the De Ferranti (2014) DEM, which is a blending of SRTM (Shuttle Radar Topography Mission), ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) and other elevation datasets. In the Grand Erg Oriental, there are four large-scale dune pattern types with gradual transitions between them and with several subtypes, namely P1, consisting of large, branching linear dunes; P2, a complex pattern including small-size and widely spaced star and dome dunes; P3, a network type created mostly from crescentic dunes; and finally, P4, consisting of large and closely spaced star dunes. The largest dunes with 90–100-m mean height can be found in the southern parts of the Grand Erg Oriental, where P1 and P4 patterns dominate, and these areas are also characterised by the most intensive sand accumulation with 25–30-m equivalent sand thickness. In the present study, we use regression analysis to investigate the functional relationships between sand dune characteristics. Further on, we have elaborated a DEM-based method to delineate dunes and calculate sand volumes and dune orientations. Comparing wind rose data and sand dune axis rose diagrams, it is concluded that in some parts of the Grand Erg Oriental, the present dune types and patterns are in agreement with the actual wind regime, but in other cases, the present dune patterns are at least partially the results of former wind regimes.

Incrustation ability of geothermal waters of the Continental Intercalaire aquifer in the Grand Erg Oriental basin (Algeria and South Tunisia)

The chemical composition of groundwater from wells tapping the Continental Intercalaire (CI) aquifer in the Grand Erg Oriental basin and the detailed analysis of saturation indices enable identification of the mineralization origin. CI groundwater is characterized by a Ca–SO4/Cl abundant water type and a Na–Cl second type. Chemical pattern is controlled by evaporites dissolution (halite, anhydrite and gypsum). Recently, the decrease in these CI geothermal water wells performance is explained by an incrustation in the well screen. To study the scaling process, we used the degree of calcite saturation, hardness and Langelier Saturation Index (LSI). It has been demonstrated that scaling potential is the most important in the basin axis, around El Oued region (southern Algeria) and in the south-east of Tunisia and the Chott Djerid borders (El Hamma of Gabes, Tozeur and Kebili) corresponding to the discharge zones. However, the scaling potential decreases towards Algerian reliefs (Tademait and Tinrhert) representing aquifer recharge zones.

Rapid recovery of photosynthesis and water relations following soil drying and re-watering is related to the adaptation of desert shrub Ephedra alata subsp. alenda (Ephedraceae) to arid environments

Ephedra alata subsp. alenda is the most important pioneer plant of the moving and semi-stable sand dunes in the deserts and steppes of south Tunisia and occurs naturally in the Grand Erg Oriental, one of the most extreme habitats for plant growth on the planet. A new analysis of physiological performance of this medically important and internationally threatened xerophytic shrub was conducted to assess possible mechanisms of drought tolerance and how these relate to its ecological success. Five-month old plants, grown under controlled climatic conditions, were subjected to a well-watered control treatment or progressive drought by withholding water for 14d with subsequent recovery for 7d. Soil water depletion significantly reduced stem relative water content (RWC) water potential (Ψw) and osmotic potential (Ψπ). Ephedra displayed more negative Ψw and Ψπ values of ca. −3.5 and −4.1MPa, respectively, at the end of the drought treatment, and were associated with turgor loss. Low stem Ψw reduced stomatal conductance (gs), photosynthetic CO2 assimilation rates (ACO2), transpiration (E) and internal CO2 concentration (Ci). However, instantaneous (WUE; ACO2 E−1) and intrinsic (WUEi; ACO2 gs−1) water use efficiency (WUE) increased gradually as water deficit was intensified. Stomatal closure therefore only exerted limited control against dehydration and could not compensate for decreases in soil water status, typical of anisohydric behavior. Drought-stressed stems accumulated high levels of proline up to 480% of control values, highlighting a pivotal role in osmotic adjustment during intense water deficit. In contrast, the osmotic adaptation to soluble sugars was limited. Drought-stressed plants increased ACO2, E, gs and Ci and decreased WUE and WUEi during the first 48h after re-watering, such that they reached similar values to those of control plants by the end of the experiment. Stem proline levels of drought-stressed plants returned to near control values with re-watering. Overall, rapid recovery of photosynthesis following drought-breaking moisture appears to be a critical mechanism allowing E. alata to withstand and survive dry environments.

Dissolved uranium, radium and radon evolution in the Continental Intercalaire aquifer, Algeria and Tunisia

Natural, dissolved 238U-series radionuclides (U, 226Ra, 222Rn) and activity ratios (A.R.s: 234U/238U; 228Ra/226Ra) in Continental Intercalaire (CI) groundwaters and limited samples from the overlying Complexe Terminal (CT) aquifers of Algeria and Tunisia are discussed alongside core measurements for U/Th (and K) in the contexts of radiological water quality, geochemical controls in the aquifer, and water residence times. A redox barrier is characterised downgradient in the Algerian CI for which a trend of increasing 234U/238U A.R.s with decreasing U-contents due to recoil-dominated 234U solution under reducing conditions allows residence time modelling ∼500 ka for the highest enhanced A.R. = 3.17. Geochemical modelling therefore identifies waters towards the centre of the Grand Erg Oriental basin as palaeowaters in line with reported 14C and 36Cl ages. A similar 234U/238U trend is evidenced in a few of the Tunisian CI waters. The paleoage status of these waters is affirmed by both noble gas recharge temperatures and simple modelling of dissolved, radiogenic 4He-contents both for sampled Algerian and Tunisian CI and CT waters. For the regions studied these waters therefore should be regarded as “fossil” waters and treated effectively as a non-renewable resource.

Crystal chemistry of the polysome ferrohogbomite-2N2S, a long-known but newly defined mineral species

Ferrohogbomite-2 N 2 S is a polysomatic hogbomite-group mineral composed of two nolanite modules ( N ) and two spinel modules ( S ) where the spinel modules have ideally hercynite composition. Ferrohogbomite has been described in the literature for a long time but was not distinguished from magnesiohogbomite and simply named hogbomite. The revised nomenclature of hogbomite minerals required definition of ferrohogbomite polysomes as new mineral species. The sample from Ain Taiba, northwestern edge of Grand Erg Oriental in the Algerian Sahara, has the simplified composition IV (Fe 2+ 3 ZnMgAl) Σ = 6 VI (Al 14 Fe 3+ Ti 4+ ) Σ = 16 O 30 (OH) 2 . The crystal structure was refined from single-crystal X-ray data. The mineral crystallizes in the acentric space group P 6 3 mc with a = 5.712(1), c = 18.317(7) A, and shows twinning by merohedry with the inversion centre as twinning operation. This ferrohogbomite-2 N 2 S is associated with ilmenite, hematite, minor magnetite, and pseudorutile. Some of the ferrohogbomite-2 N 2 S grains are rimmed by hercynite, Fe 0.57 Zn 0.26 Mg 0.18 Al 2 O 4 , which separates them from hematite. The original assemblage probably consisted of ilmenite and Al-bearing magnetite or another Ti-bearing spinel phase. During oxidation ilmenite was replaced by ferrohogbomite on its rims and magnetite was oxidized to hematite. Finally, residual ilmenite was partly transformed along cracks and grain boundaries to pseudorutile. A HRTEM study showed homogeneous ferrohogbomite-2 N 2 S with minor planar faults increasing towards the grain boundaries. The observed orientation relation between ferrohogbomite-2 N 2 S and hematite suggests that ferrohogbomite-2 N 2 S formed topotactically from a primary spinel or magnetite phase that had been subsequently oxidized to hematite.

Regional gravity analysis of the crustal structure of Tunisia

Gravity data were integrated with seismic refraction/reflection data, well data and geological investigations to determine a general crustal structure of Tunisia. The gravity data analysis included the construction of a complete Bouguer gravity anomaly map, residual gravity anomaly maps, horizontal gravity gradient maps and a 2.5-D gravity model. Residual gravity anomaly maps illustrate crustal anomalies associated with various structural domains within Tunisia including the Sahel Block, Saharian Flexure, Erg Oriental Basin, Algerian Anticlinorium, Gafsa Trough, Tunisian Trough, Kasserine Platform and the Tell Mountains. Gravity anomalies associated with these features are interpreted to be caused either by thickening or thinning of Palæozoic and younger sediments or by crustal thinning. Analysis of the residual gravity anomaly and horizontal gravity gradient maps also determined a number of anomalies that may be associated with previously unknown structures. A north-south trending gravity model in general indicated similar subsurface bodies as a coincident seismic model. However, thinner Mesozoic sediments within the Tunisian Trough, thinner Palæozoic sediments in the Gafsa Trough, and a greater offset on the Saharian Flexure were required by the gravity data. Additionally, basement uplifts under the Kasserine Platform and Gafsa Trough, not imaged by seismic data, were required by the gravity data. The gravity model revealed two previously unknown basins north and south of the Algerian Anticlinorium (5 km), while the Erg Oriental Basin is composed of at least two sub-basins, each with a depth of 5 km.

A study on the dynamics of sand dunes in Tunisia: Mobile barkhans move in the direction of the Sahara

The speed and direction of displacement of individual eyebrow‐shaped dunes (barkhans) was monitored at two experimental stations (Menzel El Habib and Noueil) in southern Tunisia. The displacement of the barkhans was followed from 1 June 1992 until 31 May 1993. After 1 year the barkhan at Noueil had moved 51 m in the southwest direction and the barkhan at Menzel El Habib had moved 48.5 m further westward (west by southwest). The dominant active winds with windspeeds higher than 3ms−1 sms1 were coming from east, southeast, and north. Hence the barkhans migrated in the direction of the Sahara and in particular to the Erg Oriental. The desertification process along the border of the Sahara was not due to sand movement coming from the Sahara, but resulted from a disequilibrium of the ecological system.

Microwave permittivity of dry sand

The complex, relative dielectric permittivity /spl epsiv/ of dry desert sand, collected at the Grand Erg Oriental in Ksar Ghilane, Tunisia, in October 1994, was measured with microwave resonators at frequencies between 0.243 and 6 GHz. The results are presented. Whereas the real part /spl epsiv/' is nearly independent of frequency, a monotonous decrease of the imaginary part /spl epsiv/" with increasing frequency is observed. The data can be fitted to a Debye relaxation spectrum with a relaxation frequency of about 0.27 GHz, leading to a nearly frequency-independent penetration depth of about 1 m over the range from 1-10 GHz. The spectrum can be explained by Maxwell-Wagner losses of semiconducting spheres embedded in a nonconducting sand medium.

Application of space imagery in Algerian Sahara

Grand Erg Oriental of the Algerian Sahara is one of the most difficult terrain having high altitude dunes ranging in height from 100 m to 300 m above ground level and 250 m to 530m above M.S.L. Interpretation of space Imagery for morphotectonic features like structures and lineaments over such a rough terrain had been an enigma. For the first time in 1978 an attempt was made in connection with exploration for hydrocarbons. A new approach on the hypothesis that the configuration of dunes is directly related to the subsurface topography which in turn being the product of geology and tectonics has been developed and the results obtained had been encouraging. The study resulted in delineating of more than 200 structures and 700 lineaments. Many of these have since been confirmed by geophysical survey and subsequent drilling. It is observed that the deep seated structures are more prone to interpretation on Landsat frames, in areas covered by high dunes as compared to lineaments. Contact of dune types, identified, may reflect variation in subsurface geology.

TALEMZANE: ALGERIAN IMPACT CRATER DETECTED ON SIR‐A ORBITAL IMAGING RADAR

In November, 1981, NASA's first Shuttle Imaging Radar mission (SIR‐A) began producing maplike photographic strips of Earth scenes from orbital altitude. A Saharan radar image acquired over Algeria clearly delineates two sedimentary basins, Erg Occidental and Erg Oriental, separated by an elongated zone of exposed bedrock, the M'Zab Chebka. At the NE margin of the Chebka, rimrocks, slopes, and ejecta deposits of Talemzane meteorite impact crater appear as a distinct two km wide radar‐bright ring. This unique circle of strong radar backscatter distinguishes the solitary impact structure from numerous dayas (similarly appearing karstic depressions) which characterize the region. The crater is prominant on radar, but is obscure on optically obtained satellite and aircraft images, as are partly buried fluvial drainage systems and fault‐block traces developed in bedrocks of the Chebka. Radar detection of an annular drainage system indicates possible presence of a ring graben at the crater. Brightest radar signals on the image are cultural features at recently developed gas fields near Hassi er R'Mel.

Distribution stratigraphique des acritarches dans le paléozoïque du sahara algérien

The Palaeozoic sequence of the Algerian Sahara is well characterized bytypical acritarch assemblages. Twenty-three palynological zones have been defined between the Lower Tremadocian and the Strunian with Hymenozonotriletes lepidophytus. They were based upon ranges of acritarchs, chitinozoans and spores. A range chart is presented, showing the distribution of 160 typical acritarch species alongside a composite litho-stratigraphical section. The data used originate from three main areas: Illizi—Tinrhert basin, Grand Erg Oriental and Grand Erg Occidental.