Sabkha Surface Research Articles

The preservation potential of environmentally diagnostic sedimentary structures from a coastal sabkha

The Abu Dhabi sabkha constitutes the landward portion of the shallow-sloping carbonate ramp situated on the northwestern coast of the United Arab Emirates (UAE). The sabkha in this region is composed of a complex assortment of carbonate, organic, and evaporitic facies that constitute a regressive sabkha sequence. The surface features of the sabkha can be grouped into distinct, laterally continuous facies associations that are directly correlatable with elevation above present-day sea level. Previous investigations have suggested the employment of the Abu Dhabi sabkha as a modern depositional analogue to ancient carbonate-evaporite formations; however, a quantitative analysis of the relationship between sabkha surface and subsurface characteristics has not previously been undertaken. This study describes in detail the sedimentary structures and textures seen at the sabkha surface and compares them to the Holocene-age facies retained in the shallow subsurface. Of the 36 sedimentary structures observed at the sabkha surface, only seven (19%) are still identifiable in the early burial realm. The majority of the surface structures were lost during early burial as a result of microbial mat decomposition, destructive evaporite precipitation, and compaction. The loss of diagnostic sedimentary features during early burial as a result of fabric-destructive processes has considerable implications for the recognition and interpretation of ancient sabkha sequences. Documenting these processes from a range of modern sedimentary environments will aid in the identification of those diagnostic features that may be reliably employed during paleoenvironmental reconstructions.

Effect of Geosynthetic on the Performance of Road Embankment over Sabkha Soils in Algeria: Case Study

This paper focuses on the construction technique of road embankment which crosses a section of about 11 km on sabkha soils of Chott El Hodna in Algeria. Due to the poor bearing capacity of sabkha surface and the rising water table over the surface, serious difficulties were encountered during the investigation of the subsurface soil and the construction of the first embankment layers. In this construction site, the use of basal geosynthetic layer to maintain the performance of embankment was found to have relevant reinforcing effect by increasing the soil bearing capacity to allow safe construction of the first layers and improving the compaction quality. Based on the in situ observations and the geotechnical investigation, numerical simulations were performed using the software PLAXIS. The results have shown the contribution conditions of geosynthetics to improve the bearing capacity of sabkha soils.

Formation conditions and diagenetic evolution of sand roses in clastic sabkhas along the Arabian Gulf Coastal Region, Eastern Saudi Arabia

This field, petrographic and geochemical study aims at constraining the formation and diagenetic evolution of sand roses (desert roses) in interdune sabkhas in Eastern Saudi Arabia. These “roses”, which are mainly cemented by gypsum, carbonate, and clay minerals, occur as disc-shaped and spherical flower-like crystals. Sands, within the sand roses, are moderately-sorted, medium-grained, and sub-arkosic. Gypsum typically exceeds 20% of the volume of the roses, and locally gypsum is partly transformed to anhydrite. In addition to gypsum and anhydrite, early diagenetic modifications include precipitation of grain coating clay, dissolution of unstable grains (e.g., feldspar grains), and weak mechanical compaction. Iron oxide cement was formed when the sand roses exposed to the surface. The XRD and petrographic data indicate an increase in amounts of gypsum cement from the water table upward towards the sabkha surface. The sand roses also are larger and lighter in colour away from the water table. This study is expected to provide a better understanding of the mode of sand roses formation in the interdunes areas, as well as the diagenetic alterations in both phreatic and vadose zones.

Development and evolution of subaerial halite crust morphologies in a coastal sabkha setting

The halite crusts of the coastal Abu Dhabi Sabkha were monitored and described in order to assess their temporal evolution and their potential for preservation and recognition in ancient sabkha sequences. The position of polygon ridges is controlled by subtle differences in the sabkha surface topography – typically in the form of low lying mud ridges and exposed halite. Where this topography remains undisturbed, then successive generations of halite polygons were observed to form with borders in the same location. As each generation of halite crust effectively ‘rebuilds’ the controlling mud ridge, then this process can be seen as self perpetuating until disturbed by a major erosional event. In marginal marine settings, such as the Abu Dhabi Sabkha, the preservation potential of the halite crust is negligible. While associated sedimentary structures have been employed to infer the presence of an earlier halite crust in other geographic settings, these all require a net influx of sediment to the depositional system. The absence of such a sediment supply in the coastal-fringing Abu Dhabi Sabkha is inferred to be a major constraint to the preservation and recognition of halite crusts and their associated features into the sedimentary record.

Sabkhas of the Bitter-Lakes, Egypt: Composition and origin

Sabkhas adjoin the Bitter-Lakes area of Egypt (part of the Suez Canal complex), and occupy the topographic low areas east of the lakes. Each sabkha is subdivided into three zones: intertidal, transitional, and supratidal. Each of these zones has its own sedimentological characteristics, with a specific soil profile, and with marked transitional areas between them. The broad transitional zones arise from a seasonal areal expansion of the saline pools that occupy portions of the sabkha surface, particularly in winter. Thus, areas of mixed ancestry evolve because of the repeatedly shifting boundaries.

Capital Reef Complex (Permian), Guadalupe Mountains, Southwestern United States: A Classic Sedimentologic Model in Flux: ABSTRACT

The Capitan reef complex of west Texas and New Mexico has been an important sedimentologic model since a reef origin was proposed for the Capitan Limestone 1929. The Capitan's magnificent exposures the Guadalupe Mountain area; its large scale; its variety of carbonate, sandstone, and evaporite facies; and its relationship to major petroleum resources of the Permian basin have made it a justly famous sedimentary geologic model for academic and industrial geologists alike. Since 1950, extensive research has yielded markedly contrasting sedimentologic interpretations of key features, such as the nature and origin of the Capitan massive (reef wall); the back-reef pisolite, sandstone, and evaporite facies; the depositional profile of the shelf and shelf edge; the impor ance and magnitude of sea level fluctuations; and the role of submarine, vadose, and phreatic diagenesis. Early views of a barrier reef depositional profile have been replaced by a shelftop marginal mound profile, which the mound's gentle crest coincided with the backreef pisolite and tepee facies. The Capitan massive, earlier considered an ecologic barrier reef, is now interpreted as an outstanding example of massive limestone formed at a submerged shelf edge where extensive submarine cementation lithified sponge wackestones and formed massive cement boundstones. Permian vadose diagenesis, earlier accorded much importance and inferred as being related to major sea level falls, appears negligible. Phreatic diagenesis by mixed meteoric and marine fluids was at least locally important the Capitan massive and foreslope strata. The famous Guadalupe pisolite, interpreted until the mid-19 0s as lagoonal, and then widely accepted as Permian caliche of vadose origin, is now reinterpreted as largely synsedimentary, formed by subaqueous precipitation from hypersaline waters of a peritidal shelf crest; associated vadose fabrics are minor, overprint isopachous pisolite fabrics and are unrelated to major intraformational erosion surfaces. Subaerial erosion surfaces within the reef complex are largely localized high on the shelf marginal mound. Unequivocal evidence of emergence of the Capitan massive or its underlying foreslope has not been recognized, suggesting that any sea level lowering during Capitan deposition did not exceed a few tens of meters. Although a sabkha origin of the back-reef evaporites and eolian transportation of sand across a sabkha surface has conceptual appeal, a lagoonal origin of the evaporites and subaqueous deposition of the back-reef sandstone sheets better fits the available field evidence. Vast amounts of siliciclastic sand had to bypass the Capitan massive and upper foreslope facies, but neither channels nor scoured erosion surfaces have been identified. The Capitan reef complex can serve not only as a world class sedimentologic model, but also as a much studied model not yet well understood. The model is one whose scientific investigators have frequently been afflicted by over-reliance on Holocene models (modern model mesmerism), by overthrust of established authorities or dogma (dogma reverence), by uncritical acceptance of new concepts (bandwagonitis), by overuse of superficial look-alike features for End_Page 1441------------------------------ genetic analogies, and by just not discerning what was available for viewing the field or laboratory. Capitan remains a model in flux awaiting more critical field and laboratory research. End_of_Article - Last_Page 1442------------

Continental sabkha pans and associated nebkhas in southern Kuwait, Arabian Gulf

Summary Small evaporitic pans within a sand-sheet/gravel-plain terrain characterize the coastal margins of southern Kuwait. Sedimentological studies indicate that the pans commenced as ephemeral lakes during a Pleistocene pluvial phase, and later evolved into sabkhas during the Holocene with the changing climatic and hydrological regimes. The lacustrine episode is represented by laminated calcite micrite in the lower parts of the section, grading upward into a diagenetic gypsum facies of the sabkhas, with diagnostic evaporite textures. Dunes are represented only by nebkhas, which occur within the sabkha pans. Their initial build-up is controlled by a halophyte which colonizes the sabkha surface, trapping any moving sand. The nebkhas pass through phases of growth, stabilization and decay that are controlled by fluctuations in the water table. A rise of the water table can waterlog the roots and kill the plants—in which case the trapped sand starts drifting. A continued fall in the water table can lead to deflation of the entire complex. The nebkhas have a poor preservation potential. In this area the aeolian system is highly starved of sand in relation to the available wind energy. Stable isotope data indicate that the groundwaters are derived from old formation waters, with only minor meteoric recharge.

A siliciclastic coastal Sabkha, capricorn coast, Queensland, Australia

A small (3 km 2), modern, siliciclastic sabkha occurs landward of Kemp Beach on the Capricorn Coast of central Queensland, Australia. Five sedimentary facies were recognized in shallow cores: (1) surface sand layer and associated halite crust; (2) a mottled clayey peat unit displaying interstitial gypsum (this layer has formed by vertical accretion of algal mats); (3) a unit of mangrove peat and clay with abundant jarosite corresponding to the locations of previous mangrove-lined tidal channels; (4) a carbonaceous sandy unit which represents the sediment that accumulated in a lake bordered by mangroves; and (5) a basal, well-sorted sand unit. Evaporative pumping during dry winter months is the dominant hydrologic process in this sabkha. Water lost from subsurface brines is replenished by subsurface flow of sea water via a pre-existing tidal channel now covered by the prograding sequence. The main aquifer below the sabkha surface is the carbonaceous sandy unit. The construction of a weir across the tidal channel maintains a relatively uniform water level below the sabkha surface during the wet summer period. However, during dry winter months, the piezometric gradient is such that the water flow is primarily unidirectional from the ocean inlet, through the mangrove-lined channel, to the subsurface aquifer below the sabkha. Analyses of subsurface waters revealed a gradient from almost normal marine salinity in the lake to a four-fold increase in subsurface brines. Correspondingly, there is a linear increase in major cation concentrations, with the exception of Ca 2+ which becomes depleted due to precipitation of gypsum. During dry winter months a halite crust forms at the surface. In contrast to most previously described sabkha environments, aragonite does not occur. Its absence may be explained by the large organic matter content of the sediments within the aquifer, inhibiting calcium-carbonate precipitation.

Holocene Dolomitization of Calcium Carbonate Sediments from the Coastal Sabkhas of Abu Dhabi, U.A.E.: A Stable Isotope Study

Secondary dolomitization of Holocene calcium carbonate sediments occurs beneath the coastal sabkhas of Abu Dhabi, U.A.E. A stable isotope study of the sabhka sediments and brines indicates that the calcium-rich dolomite precipitates in isotopic equilibrium with brines at temperatures between 34° and 49°C. These calculated temperatures correspond to measured temperatures in the sediments where the dolomite is forming. Aragonite and perhaps high Mg-calcite serve as intermediates in the formation of the dolomite, which proceeds via a dissolution-reprecipitation process. With time, the dolomite appears to "age"; there is increased ordering in the crystal lattice, an increase in the crystal size, and continued isotopic equilibration with brines at the lower temperatures found deeper under the sabkha surface. A process of diagenesis and isotopic reequilibration also occurs in the coexisting calcium-carbonate phases. The final product of the "aging" is a calcite and dolomite mixture in which the carbonate phases are in isotopic equilibrium with each other and their environment at approximately 35°C (36 and 34°C calculated for calcite and dolomite, respectively, and 32.4°C measured in late summer). The oxygen-18 fractionation between the two end-products () falls within the predicted range suggested experimentally for secondary replacement dolomite in isotopic equilibrium with coexisting calcite (Fritz and Smith 1970; Matthews and Katz 1977).

Hydrologic Framework of A Sabkha Along Arabian Gulf

During the last 4,000 to 5,000 years, sedimentary offlap and a relative fall in sea level have resulted in the development of broad, gently seaward sloping (about 1:3,000) planar areas, or sabkhas, along much of the south shore of the Arabian (Persian) Gulf. The depth of the water table below the sabkha surface increases with distance from the sea until it reaches 1.0 to 1.5 m, after which it remains constant. When the water table depth is in the range of 1.0 to 1.5 m, the sabkha surface has reached a state of deflational equilibrium. The water table beneath the sabkha always slopes seaward indicating that the direction of ground-water flow is toward the sea. The subsurface flow system beneath the sabkha is part of a regional seaward flowing groundwater regime. In the seaward part of the sabkha, a downward flow component is superimposed on the general seaward ground-water flow pattern as a result of the infiltration and percolation of water from shallow bodies of seawater that are propelled inland over the sabkha surface by strong onshore winds. Farther inland where marine flooding does not occur, the vertical flow component is directed upward in response to evaporative losses at the sabkha surface. The net rate of evaporation from the central parts of the sabkha is on the order of 6 cm ( ± factor of 2) of ground water per year. A regional ground-water flow system which causes the water table level to rise as the sabkha progrades appears to be a requirement for the development and the preservation of broad sabkhas. Fluctuations in sea level during sabkha development influence the rate of progradation and the thickness of sedimentary facies. The refluxing of sabkha brines beneath the lagoons in the Abu Dhabi area indicates a mechanism for preserving anhydrite during marine transgressions, thus permitting the development of stacked sabkha successions which are common in the rock record.

Genetic Characterization of Recent and Ancient Sabkha Systems: ABSTRACT

Ever since modern sabkhas were introduced to the geologic community and perceived as a concept applicable to ancient facies analysis, the literature has abounded with reports of inferred sabkha facies, many of which were compared with the modern Trucial Coast model of sabkha sedimentation. However, this well-described model probably has been overworked and stretched too far to be directly applicable to many ancient shallow-water to supratidal evaporite sequences. This predicament has risen partly because there is relatively little detailed information available on other modern sabkhas, despite the fact that there are other sabkhas around the world, few if any exhibiting the same characteristics as those of the Trucial Coast. Because all major depositional systems, such as marine deltas, for example (Galloway, 1975), exhibit a continuous spectrum of morphologic types, it must be recognized too that not all sabkhas are the same. They show significant variations in terms of physical processes that govern their development, and of the sedimentary responses to those processes. Thus, sabkhas should be distinguished or characterized on a genetic basis to prevent oversimplification and overuse of any one model. On the basis of dominant physical processes, three end members are recognized: (1) marine-, (2) fluvial-lacustrine-, and (3) eolian-dominated systems. Collectively, they may be displayed in a triangle diagram, upon which the positions of Recent sabkha systems can be superimposed. In general, playas fall into the fluvial-lacustrine-dominated field, interdune sabkhas are eolian-dominated, and coastal sabkhas are normally dominated by marine processes. Marine processes are mainly storm-, spring-, and wind-tidal flooding, and discharge of marine-derived brines at the sabkha surface. Playas, as well as some coastal sabkhas, are dominated by fluvial-lacustrine processes. Alluvial fans, flanking the landward side of coastal sabkhas and surrounding playa basins, contribute most of the sediment and consist of aquifers through which continental ground water flows, dissolves minerals, and discharges at the sabkha surface as saline brines. Wind, the third major process, is a factor in any sabkha, but is especially prevalent in interdune deflation depressions in both continental and coastal settings. It removes sediment above the ground-water table and enhances evaporation, resulting in precipitation of evaporites in interdune sediment. Sediment composition is the other significant factor in characterizing sabkha types. Carbonate-sulfate minerals (gypsum-anhydrite), soluble salts (halite, sylvite, polyhalite, etc), and terrigenous clastics are the dominant sedimentary components of sabkhas, but most are either dominated or distinguished by one or a mixture of those sediment types. By treating each of the three major sediment types as an end member, sediment variation between sabkhas can be displayed. A genetic examination of sabkha systems clearly reveals that the Trucial Coast is only one end member to a spectrum of sabkha types, both in terms of physical processes and sediment composition. Thus, care must be taken when comparing modern and ancient sabkha systems to correctly characterize the processes responsible for deposition of the strata in question and the dominant sediment composition. End_of_Article - Last_Page 1685------------

Facies Anatomy of Modern Continental Sabkha, Bristol Dry Lake, California: ABSTRACT

Bristol Dry Lake is a 155-sq km fluvial-lacustrine dominated, continental sabkha, or playa, in the Mojave Desert of southeastern California, and is filled with at least 300 m of interbedded terrigenous clastics, gypsum, anhydrite, and halite. The evaporite facies roughly form a bull's-eye pattern with abundnat gypsum and anhydrite surrounding a basin center accumulation of halite. Transects through Bristol Dry Lake, from the alluvial fan and sand flat to the center of the playa, reveal (1) crudely bedded, coarse-grained clastics prograding over and interfingering with either (2) wadi (alluvial/eolian) sand and silt, or (3) mud-flat facies of nodular to enterolithic gypsum or anhydrite and blades of gypsum in red-brown silt and clay, followed by (4) saline mud-flat facies f red-brown silt and clay crowded with giant (15 cm diameter), displacive, hopper-shaped crystals of halite, and (5) salt-pan beds of chaotic mud-halite up to 4 m thick in the center of the playa. Deposition of terrigenous clastics was by fluvial-sheetflood processes around the toes of alluvial fans, fluvial flow through very shallow rills and suspension settling in the mud-flat environments. Much of the sediment is reworked by eolian processes. Evaporites are precipitated at or just below the sabkha surface from discharging brines. Lithofacies of this modern continental sabkha are nearly identical to those comprising the Middle-Upper Permian evaporites of the Texas and Oklahoma panhandles, and they are excellent process analogs for ancient facies analysis. End_of_Article - Last_Page 934------------

Coastal Sabkha and Salt Pan Deposition of the Lower Clear Fork Formation (Permian), Texas

ABSTRACT Middle Permian (Leonard Series) evaporites, carbonates, and red beds of the lower Clear Fork Formation in the Texas Panhandle display many features common to modern coastal and continental sabkha facies. Similarities among lithofacies patterns and sequences, sedimentary structures, and anhydrite-halite textures and fabrics of both the lower Clear Fork Formation and modern sabkha facies permit construction of a model of halite deposition in coastal-sabkha salt pan environments which may be generally applicable to other ancient sabkha facies in intracratonic basins and around stable or rifted continental margins. Inferred coastal sabkha and marine inner-shelf environments and lithofacies recognized in the lower Clear Fork Formation are largely based upon analogous modern depositional se tings and include the following: 1) red siltstone to mudstone deposited in a wadi or desert alluvial-eolian plain and inner-sabkha mud flats, both passing basinward into 2) chaotic mudstone-halite of an inner-sabkha saline mud flat, which flanked 3) inner-sabkha salt pans bearing banded to massive halite and laminated anhydrite, and 4) nodular, nodular-mosaic, and massive anhydrite and dolomite deposited in marginal sabkha and inner shelf environments. Salt pans may have formed in the inner sabkha as a result of displacive gypsum-anhydrite growth and jacking up of the marginal sabkha surface, which impeded seaward drainage of floodwater off the sabkha. Modern process-sediment analogs include alluvial fans, saline mud flats, and salt pans of the coastal sabkhas in Baja, Mexico, the Trucial Coast, and Kutch, India. Evaporites and terrigenous clastics in Bristol Dry Lake, California, exhibit many features analogous with the Permian, suggesting that this setting, though strictly continental, serves as a process analog. Bromide values in lower Clear Fork halite average 40 ppm, which is less than an unaltered marine halite but still within the range of marine halite that has undergone diagenesis. Upward decreasing values in the lower Clear Fork upper-cycle may indicate a decrease in salinity during deposition or that the younger halite beds suffered more diagenesis than the underlying halite.

Geochemistry and mineralogy of a recent sabkha along the coast of Sinai, Gulf of Suez

Most efforts in the study of sea-marginal sabkhas have concentrated on the Persian Gulf, but little is known about the sediments and mineralogy of sabkhas marginal to other seas. The purpose of this paper was to present some geochemical and mineralogical observations in a recent sabkha on the coast of Sinai along the Gulf of Suez. The sabkha is composed of coarse clastic sediments with marine-derived groundwater at depth of about 1 m. The general morphology, climate and water salinity of the Gulf of Suez resemble those of the Persian Gulf, despite the fact that the content of authigenic evaporites in this sabkha is more sparse. The evaporite minerals accumulated only in the upper 30–40 cm of the sabkha, below that and down to the groundwater table, there is no accumulation of evaporites. Laterally, the salinity of the groundwater in the sabkha and the concentration of evaporites in the sediments above it increase constantly with distance from the shore. In contrast to the Persian Gulf where anhydrite is a major evaporite mineral, in Belayim gypsum is the only calcium sulphate mineral in the recent sabkha. Anhydrite is found only in an old elevated sabkha where it recrystallized from gypsum. The gypsum occurs as interstitial crystal concentrations or lithified horizons almost exclusively at the depth of 20–40 cm below the sabkha surface. Above that, in the uppermost horizons, there is in situ accumulation of interstitial halite crystals. The total concentrations of gypsum and halite are almost equal in this sabkha. The sea water recharge in El Belayim is almost exclusively by seepage through the sabkha sediments and not by flooding. The groundwater under this sabkha is only slightly more saline than the Gulf water, thus, not heavy enough for extensive downward refluxing. The major hydrodynamic process must be upward migration of the brines from the groundwater, precipitating on the way gypsum and later halite with some magnesite. Since the sediments of the sabkha are too coarse to support extensive capillary movement, the brines must, therefore, migrate upwards due to ‘evaporative pumping’.

Aragonitic and High-Mg Calcite Caliche from the Persian Gulf--A Modern Analog for the Permian of Texas and New Mexico

Laminated, lacy, and pisolitic carbonate crusts have been found on and within some outcrops of Pleistocene limestones which rise above the modern sabkha surface in Abu Dhabi. These sediments are similar in many ways to typical caliche but have mineralogies and cement textures. Internally, the pisolites are uniformly aragonitic and are cemented together by fibrous aragonite and micritic and fibrous high-Mg calcite in isopachous, meniscus, and microstalactitic textures. It is suggested that caliche crusts with unstable carbonate mineralogies, together with marine or brine associated isotopic ratios and trace element concentrations, yet showing dominantly textures, can develop in marine influenced areas where and phreatic pore waters have unusually elevated salinities and high concentrations of Sr and Mg. Thus, in strongly evaporitic, low relief settings such as the Persian Gulf sabkhas, supratidal areas are characterized, not by fresh water, but by brines which allow the precipitation of cements of marine chemical character with textures. Similar examples are known from Bonaire, another evaporite locality. Thus, very clear distinctions should be drawn between the terms and fresh water; ancient textures do not immediately imply fresh-water pore fluids. One can have hypersaline vadose as well as meteoric vadose conditions. The Abu Dhabi pisolitic crusts with their unusual mineralogies (including some dolomite) appear to be quite analogous to the pisolitic facies of the Permian reef complex of Texas and New Mexico and may provide a model for their formation and diagenesis. The initial presence of aragonite and high-Mg calcite in the pisolitic and microstalactic crusts of the Permian back-reef sediments would be more compatible with the observed penecontemporaneous dolomitization than would a stable low Mg calcite caliche.