Saline Playa Research Articles

<I>Tronamyia</I>, a New Genus of Shore Flies, and a Phylogenetic Reassessment of the Tribe Lipochaetini Becker (Diptera: Ephydridae)

The genus Tronamyia is described in the tribe Lipochaetini (subfamily Gymnomyzinae) with Asmeringa lindsleyi Sturtevant & Wheeler (new combination) as its type species. The type species had previously been listed as an unplaced species in the tribe Atissini (subfamily Hydrelliinae). In a phylogenetic analysis of the tribe Lipochaetini, Tronamyia is the sister group of Lipochaeta Coquillett, a genus that also occurs along maritime beaches or inland saline or alkaline playas of the New World.

Facies and evaporite genesis of the Kuşcular Formation (Lower Paleocene) saline playa complex, Eastern Turkey

The Kuşcular Formation (Lower Paleocene), located in Eastern Turkey, is an alluvial and playa foreland basin succession deposited in a semi-arid climate. The playa deposits of the Kuşcular Formation, composed of interbedded green sandstone–siltstone–mudrock, gypsiferous sandstone, red-green gypsiferous mudrock, and banded nodular secondary gypsum, are found among the distal alluvial fan deposits. Interbedded green sandstone–siltstone–mudrock with desiccation cracks are interpreted as dry mudflat–sandflat deposits. The banded nodular secondary gypsum and gypsiferous mudrocks, which contain desiccation cracks, are interpreted as a saline playa and saline mudflat. Distal alluvial fan deposits are represented by stratified conglomerate and stratified pebbly sandstone. The sulfur in the evaporites was very likely derived from the oxidation of sulfides in Upper Cretaceous magmatic rocks comprising the basement of the basin. Soft sediment deformation structures (slump folds), which indicate seismically active areas, were formed within the playa and dry mudflad–sandflat complex. The facies aspects of playa deposits would suggest that the climate in Turkey was semi-arid during the Early Paleocene.

Seasonal Variations of Saltation Activity on A High Plains Saline Playa: Yellow Lake, Texas

The Southern High Plains region of West Texas and eastern New Mexico is often described as a flat elevated tableland. Spaced across this vast and otherwise featureless plain are 21 large closed basins containing approximately 40 irregularly shaped saline playas. Yellow Lake, located on the Yellow House Ranch northwest of Lubbock, Texas, is among the largest of the High Plains saline playas. This paper represents the first report of a 4-yr. study of saltation activity at Yellow Lake. From December 30, 1998, to January 1, 2003, a fast-response piezoelectric saltation sensor was used to collect a continuous record of aeolian activity at a point on the playa surface. Since saltation activity is often associated with dust emissions, the saltation record also provides information regarding seasonal patterns of dust emissions from the Yellow House Basin. Results suggest that blowing events can occur at any time of the year when conditions are favorable; however, the necessary conditions are rarely satisfied. As a result, the saltation record is characterized by many hours of inactivity punctuated with brief periods of occasionally intense aeolian activity. From December 30, 1998, to January 1, 2003, saltation activity was detected for only 7% of the hours measured. Although there can be substantial deviations from one year to the next, it was found that saltation activity tends to peak during winter months when winds are moderately strong and precipitation is at a minimum. Hourly saltation activity values occasionally approached unity during intense winter blowing events, indicating nearly continuous sediment transport over a one-hour period. Saltation activity is at a minimum during summer months when winds are often weak and wet conditions prevail. Although winds are typically strongest during the spring season, the playa is relatively stable due to high threshold values produced by significant spring precipitation. This contrasts sharply with the surrounding cropland, which tends to be most active during the spring season.

The water balance equations in saline playa lakes: comparison between experimental and recent data from Quero Playa Lake (central Spain)

The Quero Playa Lake is an ephemeral saline playa lake located in the La Mancha region of central Spain. In this study, a daily monitoring of the brine physical properties, water activity, brine depth and main climatic parameters was simultaneously carried out together with determining the precipitation sequence of minerals. Field data were compared with the results of simulating the water evaporation in an environmental chamber. In this simulation, a similar hydrochemical composition for the saline lake was used, and the main climatic parameters, temperature and humidity, were controlled. The water balance equation for saline lakes has usually been described using the Wood and Sanford equation [Econ. Geol., 85(1990) 1226–1235]. Our experimental results required us to revise the water balance equation for the brine depth variations (dh/dt), that may be expressed as follows:dhdt=p1+kABAL+SI−SO−ξ−H+D,where p (mm) is the precipitation; k is the drainage coefficient of the lake; AL is the lake surface; AB is the drainage basin surface; SI and SO are the contribution of influent and effluent seepage to the depth of brine in the lake. The term ξ is the evaporation/condensation, defined as ξ=kpW(aW−RH), where k is the mass transfer coefficient (Dalton's equation); pW is the water pressure in equilibrium with the air; aW is the water activity of the brine; RH is the relative humidity. The other terms: H and D, correct the brine depth loss or/and gain a consequence of hydrated saline mineral precipitation and early diagenetic hydration/dehydration reactions. As a consequence of the above, we suggest that the water balance equation for saline lakes can be an important consideration in the interpretation of their evolution. The precipitation of hydrated saline minerals and the early diagenetic dehydration/hydration reactions imply changes in the dh/dt curves. As a result, the interpretation of the sequence of primary saline minerals in older evaporitic deposits may be used as a paleoenvironmental marker.

Supply‐limited horizontal sand drift at an ephemerally crusted, unvegetated saline playa

A site at Owens Dry Lake was observed for more than 4 years. The site was a vegetation‐free saline playa where the surface formed “ephemeral crusts,” crusts that form after rainfall. Sometimes these crusts were destroyed and often a layer of particles on the crust would engage in vigorous aeolian activity. Three “phases” of active sand drifting are defined as almost no movement (extreme supply limitation), loose particles on crust with some degree of sand drift (moderate supply limitation), and unlimited source movement corresponding to a destroyed surface crust (unlimited supply). These “phases” occurred 45, 49, and 6% of the time, respectively. The accumulation of loose particles on the crust was mostly the result of in situ formation. Crusted sediments with loose particles on top can exhibit mass flux rates about the same as for noncrusted sediments. Crusted sediments limit or eliminate sand drift in two conditions: for rough crusts that effect a sufficiently high threshold friction velocity (above the wind friction velocity) and for limited amounts of loose particles on the crust where particle supply is less than would be transported in normal saltation for a thick sandy surface. These “supply‐limited” cases are similar to wind erosion of limited spilled material on a hard concrete surface. We quantified “supply limitation” by defining a “potential” or “supply unlimited” sand drift function Q = AG where A represents supply limitation that decreases as the particle source is depleted. Here Q is the mass of sand transported through a surface perpendicular to the ground and to the wind and having unit width during time period t, and G = ∫ u*(u2* − u2*t) dt for u* > u*t. G is integrated for the same time period t as for Q, u* is the friction velocity of the wind, and u*t is the threshold friction velocity of the wind. Hard crusts (usually formed in the summer) tended to show almost no change of threshold friction velocity with time and often gave total protection from wind erosion. Rough crusts provided sufficient protection expressed as high threshold friction velocities. For these high threshold friction velocities, aeolian activity was greatly reduced or practically prevented. The softest crusts, usually formed in the winter, provided much less protection and sometimes were destroyed by the wind. Following this destruction the “potential” or “supply unlimited” sand drift would be observed.

Redbeds from the Middle Muschelkalk (Middle Triassic) of the SW Germanic Basin: arid environments from Pangea's interior

The facies of a redbed succession from the lower Middle Muschelkalk of the SW Germanic Basin allows reconstruction of the depositional environment and inference with regard to the climate in a little studied part of Pangea's interior. The redbeds are dominated by fine-grained terrigenous deposits, with both intercalated thin sandstone and carbonate beds. The clastics were derived from the nearby hinterland, and their facies suggest episodic deposition on a playa mud flat or flood plain. The occurrence in the lower part of the succession of sand-patch breccias, ooids, and dolomicrite beds suggests an origin in saline pans and playa lakes. These diverse facies, and phreatic dolomites, reflect an arid climate during deposition.

Regolith geochemical mapping as an adjunct to geological mapping and exploration; examples from three contiguous Proterozoic basins in Western Australia

Regolith and regolith-geochemical mapping of the Proterozoic Padbury, Bryah and Yerrida basins of central Western Australia can be used to define the extent of the underlying bedrock and indicate areas of potential mineralisation. The three basins cover approximately 20,000 km 2, more than 90% of which is covered by regolith, dominated by colluvium/sheetwash. About 30% of the area lacks well-defined drainage and, as the climate is arid, most sediment is transported during cyclonic flooding. Soils are skeletal, and colluvially modified, except in areas of aeolian sand. Most stream sediment and sheetwash samples retain some components of the bedrock from which they were derived, apart from sediments found in major (but ephemeral) river channels on plains and aeolian sands. Prolonged exposure over many millions of years has resulted in deep chemical weathering; the weathering products are currently subject to mechanical erosion, and transport by flood or wind. Alkali and alkaline earth elements are leached at the fresh rock–saprock interface, and are in part redeposited in valleys, calcrete and saline playas. Though base metals are variably leached, many elements of economic interest are retained in coarse-grained altered lithic fragments and iron-rich debris, whereas rare earth elements, Th, Zn and Zr are concentrated in the fine fractions. In several instances regolith composition can be used to delineate the extent of, or sub-divide bedrock units. Regolith over a black sulphidic shale in the Yerrida Basin shows that unit to have three subfacies. Regolith derived from mafic rocks has higher CaO, Ga, Sc, and V than that over nearby sedimentary rocks, whilst different types of mafic units can be separated using MgO, Cr, Cu, and Ni. Regolith in the Padbury Basin is more Fe-rich and characterised by higher concentrations of MnO, P 2O 5, As, Ba, F, Pb and Th, except in one area of lower MnO and F. Variations in the K 2O, Ba and Sr contents of regolith, relate to different granitic sources to the underlying sedimentary rocks. Statistical treatment of regolith-chemical data suggests areas of mis-identified bedrock and, using normalised additive indices, highlights mineralised areas and points to other prospective areas.

Aeolian dust in a saline playa environment, Nevada, U.S.A.

Saline playas in north-western Nevada, U.S.A., remnants of pluvial periods of the Pleistocene, represent a tremendous source of unconsolidated sediments available for aeolian transport. This study investigated the transport of aqueous-soluble solutes in dust from July 1994 through June 1996 along a transect from a barren salt-encrusted playa surface (elevation=1224 m), to a former pluvial lake beach (elevation=1228 m), to a dune-mantled upland (elevation=1248 m). The content of aqueous-soluble solutes in aeolian dust showed a significant (p≤0·05) interaction with dust trap location (playa, beach, dune) and time of collection. Dust collectors on the playa surface generally contained significantly more aqueous-soluble solutes and had greater total flux of solutes than either the beach or the dune locations. The solute content of aeolian dust was usually higher, in some cases several orders of magnitude, than that in the surface 5 cm of soil. Recent changes in playa hydrology may explain this result. Pulses of nitrate-rich dust, synchronous with spring emergence, and other nutrient additions via aeolian dust may have stimulated invasion of dune-mantled uplands by the weedSalsola paulsenii (barb-wire Russian thistle).

GEOCHEMICAL CHARACTERISTICS OF AUSTRALIA'S SOUTHERN MARGIN PETROLEUM SYSTEMS

The hydrocarbons discovered to date on the southern margin of Australia have been assigned to the Austral Petroleum Supersystem based on the age of their source rocks and common tectonic history. Modelling of the source facies distribution within this supersystem using tectonic, climatic and geographic history of the southern margin basins, suggests the presence of a variety of source rocks deposited in saline playa lakes, fluvial, lacustrine, deltaic and anoxic marine environments.Testing of the palaeogeographic model using geochemical characteristics of liquid hydrocarbons confirms the three-fold subdivision (Al, A2 and A3) of the Austral Petroleum Supersystem.Bass Basin oils are assigned to the Austral 3, Eastern View Petroleum System. The presence of oleanane in the biomarker assemblages of these oils, together with their negatively sloping, heavy, isotopic profiles, indicate derivation from Upper Cretaceous-Tertiary fluvio–deltaic source facies.In the eastern Otway Basin, oils of the Austral 2, Eumeralla Petroleum System are sourced by Lower Cretaceous (Aptian–Albian) coaly facies. Oil shows reservoired in the Wigunda Formation at Greenly-1 in the Duntroon Basin are possibly sourced from the Borda Formation and are assigned to the Austral 2, Borda Petroleum System.In the western Otway, Duntroon and Bight basins, a lack of definitive oil-source rock correlations precludes the identification of individual Austral 1 petroleum systems.

The limnology of Lake Torrens, an episodic salt lake of central Australia, with particular reference to unique events in 1989

Lake Torrens, a large (∼ 6000 km2), episodic, saline playa lake in arid central Australia, filled for the first recorded time in March 1989. This unique event followed unprecedently heavy rain over its catchment. Water remained in the lake until early 1990. Salinities remained low ( 200 g l-1 before the water evaporated completely. Na and Cl probably dominated the ions. Twenty-nine taxa of aquatic animals were recorded. Crustaceans dominated and of these Parartemia minuta (anostracan), Daphniopsis queenslandensis and Moina baylyi (cladocerans), and several species of ostracods were the most important. Other significant taxa included Tanytarsus barbitarsis (chironomid), Brachionus plicatilis (rotifer) and Craterocephalus eyresii (fish). The fauna was dominated by widespread forms that are either part of a distinct assemblage found widely in central Australian salt lakes (especially P. minuta, D. queenslandensis, M. baylyi and Trigonocypris subglobusus [ostracod]), or cosmopolitan forms (B. plicatilis), or forms found widely in Australian salt lakes (e.g. Diacypris compacta, Tanytarsus barbitarsis). However, possibly three new species also occurred: a new species of a new genus of mytilocypridinid, Branchinella nov.sp. and Heterocypris nov. sp. Their occurrence is anomolous. It is suggested that further work will show that they too are widely distributed. Both genera of the latter two taxa are typical of temporary waters and specimens may have been washed in from nearby water-bodies. Sixty-four species of bird were recorded when the lake contained water, many breeding. The breeding of the banded stilt (Cladorhynchus leucocephalus) was notable; breeding of this species had been unknown in South Australia since 1930. A large population built up until breeding was stopped by predation from the silver gull (Larus novaehollandiae). When dry, the lake has a depauperate but well-defined and regionally restricted ‘terrestrial’ fauna (scorpions, spiders, beetles, ants).

Relation of vertical flux of particles smaller than 10 μm to total aeolian horizontal mass flux at Owens Lake

The vertical flux of particles smaller than 10 μm for a saline playa surface, the particle size composition of which was classified as loam‐textured, was estimated for a highly wind‐erodible site on the playa of Owens (dry) Lake in California. The ratio of this vertical flux to the horizontal flux of total airborne material through a surface perpendicular to the soil and to the wind, Fa/qtot, is 2.75×10−4 m−1. This is consistent with that ratio for sand‐textured soils and suggests that the binding energy and size of saltating particles for the tested surface material at Owens Lake is of the same order as that for sandier soils. The horizontal mass flux of saltating grains, q, in the reported wind erosion event is 51.3% of the total horizontal mass flux qtot. Therefore the ratio of Fa/q is 5.4×10−4 m−1.

Sedimentological characteristics and facies of the evaporite‐bearing Kirmir Formation (Neogene), Beypazari Basin, central Anatolia, Turkey

ABSTRACTA thick sedimentary sequence comprising fluvial, lacustrine and volcano‐sedimentary rocks is present in the Neogene Beypazari Basin, central Anatolia. These units display considerable lateral facies variation and interfinger with alkaline volcanic rocks along the north‐eastern margin of the basin. The uppermost Miocene Kirmir Formation contains numerous evaporite horizons. The evaporite sequence is up to 250 m thick and may be divided into four lithofacies. In ascending stratigraphical order these are: (1) gypsiferous claystone facies, (2) thenardite‐glauberite facies, (3) laminar gypsum facies and (4) crystalline gypsum facies. These facies interfinger with one another laterally along a section from the margins to central parts of the basin. The lithological and sedimentological features of the Kirmir Formation indicate fluvial, saline playa mudflat, hypersaline ephemeral playa lake and very shallow subaqueous playa lake depositional environments, which probably were influenced by alternating semi‐arid and evaporative conditions.

Rifted margin source rock deposition: a carbon isotope and biomarker study of a West African lower cretaceous “Lacustrine” section

Lower Cretaceous passive margin sections, penetrated in wells peripheral to the deep basinal development of pre-salt sediments of the Liawenda-Ponta Vermelha Troughs in the Lower Congo Coastal Basin, revealed sustained source potential. Kerogen type, kinetic ( E a/ A) and carbon isotopic data plus rock extract biomarker signatures confirmed large, systematic organofacies variations within these rift phase sediments. An inverse correlation between 25,28,30-trisnorhopane and gammacerane abundances suggested three major depositional regimes reflecting salinity and water depth control. 13C-depleted basal sediments, showing strong gammacerane/4-methyl sterane signatures, were segregated by a major isotopic excursion. This event represented the transition between saline playa lake to deeper water stratified conditions supporting a high level of anoxia. For these intermediate sediments, sterane and tricyclic diterpane abundances, plus sterane/hopane ratios, had marine connotations and could be interpreted as the result of intermittent marine incursions. Fore-shadowing irreversible oceanic ingression, a resurgence of gammacerane abundance in the uppermost sediments typified littoral-shallow marine depositional conditions. Recognition of a possible non-ubiquity in lacustrine depositional conditions during the opening of a rifted margin provides further insight into the complexity and attendant controls on hydrocarbon generation in such habitats.

New Branchinella (Anostraca) From Texas, U.s.a., and the Problem of Antennalike Processes

With this report, four species of the anostracan genus Branchinella are known from North America. Two occur in the state of Texas and one each in the states of Georgia and Florida. The two east coast species probably need to be brought under protection of endangered species laws, since they have been reported only from their type localities. For the first time outside the genus Dendrocephalus, we found antennalike processes between the first and second antennae on the heads of males in a new Branchinella from Texas and the two east coast species with which it seems closely related. Linder (1941) reported what he called fingerlike processes proceeding frontally from the bases of the eyes of males of an Australian species of Branchinella and wondered if they were comparable with the antennalike processes found in species of the New World genus Dendrocephalus. Our finding of antennalike processes in three New World species of Branchinella suggests that homology is a strong possibility. While species of Branchinella are the numerically dominant anostracans in Australia (Geddes, 1983), they are among the least common fairy shrimps in North America. Including a new species described herein, all four North American Branchinella have distributions confined to the United States. Of these, B. lithaca (Creaser, 1940) and B. alachua Dexter, 1953, have been reported only from their type localities in Georgia and Florida, respectively. The status of these merits investigation, since they are very likely in need of protection under threatened and endangered species legislation. A third previously described species, B. sublettei Sissom, 1976, has an apparently safe distribution in western Texas. Home (1974) found it in six saline playas in three counties on the Llano Estracado south of Lubbock.

Sedimentology of the saline lakes of the Cariboo Plateau, Interior British Columbia, Canada

There are several hundred saline lakes in Interior British Columbia, including muddy siliciclastic playas, saline playas, perennial lakes (including meromictic sulphate lakes), and ephemeral lakes, some with permanent salts. The lake waters have highly variable compositions, with Na-CO 3-Cl, Na-CO 3-(SO 4)-Cl, Mg-Na-SO 4 and Na-Mg-SO 4, the dominant types of brine. On the Cariboo Plateau, where they are most abundant, the saline lakes are small, shallow, and occupy depressions within glacial and glacio-fluvial deposits. Most are groundwater-fed. The region is characterized by extremely cold winters and short hot summers. Dense coniferous forest mantles much of the plateau and surrounds most of the lakes. Most basins comprise three main subenvironments—hillslope, mudflat (saline and dry) and lake (ephemeral or perennial). Fluvial sediments are of little significance. Mudflats are primarily a zone of extensive interstitial carbonate precipitation from shallow groundwaters, including abundant magnesite and hydromagnesite. The amount of carbonate formed varies with groundwater composition. Some mudflats are carbonate-dominated; others are predominantly siliciclastic with only highly soluble interstitial salts forming. Sedimentary structures are disrupted by carbonate precipitation and displacive salt crystallization. Springs and ephemeral seepages are locally present. Microbial mats form extensively along many littoral zones and around springs; laminates are preserved in some cores. Efflorescent salt crusts cover saline mudflats around most lakes and playas. Subaqueous salts (including natron, epsomite, bloedite, mirabilite) are precipitated during late summer, autumn and winter in several hypersaline lakes, some by evaporative concentration, others by brine cooling and freeze-out. Several hypersaline, ephemeral lakes have an unusual “spotted” morphology, with hundreds of individual brine pools within carbonate-siliciclastic muds. Most recent sedimentation in the Cariboo lakes has been autochthonous, with a predominance of carbonates, evaporites and organic-rich muds. This reflects the dense vegetation in the catchment which has stabilized soils and limited clastic influx, the predominance of groundwater recharge, the lack of basin marginal relief, and the common occurrence of inflow waters with ( Ca + Mg) ( HCO 3 + CO 3 ) near unity which provide abundant carbonate sediment.

Deposits of ephemeral and perennial lakes in the tertiary Daban Basin (Northern Somalia)

The Daban Basin is filled with 2700 m of Middle Eocene to Oligocene clastic deposits. From the bottom of the sequence upward six sedimentary environments are distinguished: restricted lagoon, delta, lagoon, alluvial plain, ephemeral and perennial lakes. The latter show a lateral transition into thick fan-delta sequence (Kalajab fan-delta). A general vertical trend from an ephemeral, saline lake to perennial fresh-water lake has been recognized. The lower portion of the lacustrine sediments, which is 1100 m thick, consists of red and green siltstones, interbedded with cross-laminated sandstones representing dry mudflat (playa) deposits. An interval of saline playa sedimentation, interbedded in the playa mudflat sequence, includes red siltstones, green siltstones and laminated gypsum arranged in cycles; these were deposited in dry mudflats and in a chemically stratified lake. The upper portion of the lacustrine deposits, which is 900 m thick, is composed of green siltstones, cross-lamimated sandstones, chalky laminated limestones and some tuff layers. Fresh-water fishes, ostracods, gastropods and tree trunk remains are locally present. These sediments were laid down in a fresh-water perennial lake. The Daban Basin was a rapidly subsiding depression during the period that rifting was taking place in the adjacent Gulf of Aden. Lacustrine deposition was controlled by the paleogeographic and paleoclimatic changes induced by this event.

Sedimentology of a saline playa in the northern Great Plains, Canada

ABSTRACTCeylon Lake, a small salt playa located in southern Saskatchewan, is typical of many shallow ephemeral lacustrine basins found in the northern Great Plains of western Canada. The present‐day brine, dominated by magnesium, sodium, and sulphate ions, shows wide variation in composition and concentration on both a temporal and a spatial basis. The modern sediments overall exhibit relatively simple facies relationships. An outer ring of coarse grained shoreline and colluvial clastics surrounds mixed fine grained clastics and salts and, in the centre of the basin, salt pan evaporites composed mainly of mirabilite, thenardite, and bloedite.Coring of the late Pleistocene and Holocene sedimentary fill shows that the lake has evolved from a relatively dilute, deep water, clastic dominated basin through a shallower, brackish water, carbonate‐clastic phase, and finally into the present salt dominated playa. The thick sequence of evaporites preserved in the basin suggests evolution of the brine from a Na‐rich solution to a mixed Mg‐Na system. The most important post‐depositional processes affecting Ceylon Lake sediments are mud diapirism and salt karsting.

Hydrochemical evolution of regional groundwaters to playa brines in central Australia

A large-scale groundwater system in central Australia discharges to a chain of playas. Recharge in calcrete and fractured rock aquifers gives rise to relatively low-salinity HCO 3 Cl SO 4 groundwaters, which evolve through regional saline groundwaters, to highly saline playa brines. The hydrochemical evolution of the groundwaters follows the anionic sequence HCO 3 Cl SO 4 → ClbHCO 3SO 4 → ClSO 4HCO 3 → ClSO 4 → Cl. With increasing salinity, there is a relative increase in Na, K, Mg, Cl and SO 4; however, there is a relative decrease in HCO 3, Ca, and SiO 2 owing to the precipitation of carbonate, sulphate and silicate minerals, and the resultant brines are depleted in these ions. Significant chemical variation in the composition of playa brines is a result of complex processes of solution, evaporative concentration, precipitation and mineralogical change, including dolomitisation. Thermodynamic calculations based on the Pitzer equations have enabled a general model to be developed for these evolutionary processes in saline groundwaters up to the stage of halite saturation. At an early stage the regional groundwaters are saturated with respect to the carbonate minerals, dolomite first, then calcite. With increasing salinity, sulphate minerals begin to precipitate: saturation with respect to gypsum is attained at a chlorinity of 19‰, and saturation with respect to anhydrite is attained at 122‰. The playa brines attain saturation with respect to halite at a chlorinity of 144‰. Solute budgets based on a chloride concentration factor show that final playa brines are 178 times more concentrated than recharge groundwaters, and confirm the virtually complete loss of HCO 3, Ca and SiO 2 through precipitation. There are subtle differences in the hydrochemistry of different central Australian playa brines and also vis-à-vis playa brines described from other parts of the world. Most Australian playas have brines of the ClNa type with SO 4 and Mg also important. The generally accepted Hardie-Eugster model for brine evolution and mineral precipitation sequences has therefore been modified and extended. Three pathways are defined, following calcite precipitation, on the basis of the ratio of molar Ca to alkalinity; these pathways lead to saline waters with different compositions. Subsequent evolution of the brines depends on the ratios between molar SO 4, Mg, Ca and alkalinity.

Modern dolomite deposition in continental, saline lakes, western Victoria, Australia

Microcrystalline dolomite forms a major constituent of Holocene sediments of numerous continental, saline playa lakes in southeastern Australia. The lake waters are highly supersaturated with respect to dolomite as well as other Mg carbonates, but undersaturated or near saturation with respect to calcite and aragonite. The dolomite shows no replacement textures and most likely formed by direct precipitation. Conditions in these lakes that appear conducive to the precipitation of dolomite are (1) high salinity, (2) high Mg/Ca ratios, and (3) high alkalinity. The moderate sulfate levels of the brines do not seem to be inhibiting carbonate precipitation.

Influences of quaternary climatic changes on processes of soil development on desert loess deposits of the Cima volcanic field, California

Summary Soils formed in loess are evidence of both relict and buried landscapes developed on Pliocene-to-latest Pleistocene basalt flows of the Cima volcanic field in the eastern Mojave Desert, California. The characteristics of these soils change systematically and as functions of the age and surface morphology of the lava flow. Four distinct phases of soil development are recognized: phase 1 - weakly developed soils on flows less than 0.18 M.y. old; phase 2 - strongly developed soils with thick argillic horizons on 0.18 – 0.7 M.y. old flows; phase 3 - strongly developed soils with truncated argillic horizons massively impregnated by carbonate on 0.7 to 1.1 M.y. old flows; and phase 4 - degraded soils with petrocalcic rubble on Pliocene flows. A critical aspect of the development of stage 1 soils is the evolution of a vesicular A horizon which profoundly affects the infiltration characteristics of the loess parent materials. Laboratory studies show that secondary gypsum and possibly other salt accumulation probably occurred during the period of phase 1 soil development. Slight reddening of the interiors of peds from vesicular-A horizons of phase 1 soils and presence of weakly developed B horizons indicates a slight degree of in situ chemical alteration. However, clay and Fe oxide contents of these soils show that these constituents, as well as carbonates and soluble salts, are incorporated as eolian dust. In contrast to phase 1 soils, chemical and mineralogical analysis of argillic horizons of phase 2 soils indicate proportionally greater degrees of in-situ chemical alteration. These data, the abundant clay films, and the strong reddening in the thick argillic horizons suggest that phase 2 and phase 3 soils formed during long periods of time and periodically were subjected to leaching regimes more intense than those that now exist. Flow-age data and soil-stratigraphic evidence also indicate that several major loess-deposition events occurred during the past ∼ 1.0 M.y. Loess events are attributed to past changes in climate, such as the Pleistocene-to-Holocene climatic change, that periodically caused regional desiccation of pluvial lakes, reduction of vegetational density, and exposure of loose, unconsolidated fine materials. During times of warmer interglacial climates, precipitation infiltrates to shallower depths than during glacial periods. Extensive, saline playas which developed in the Mojave Desert during the Holocene are a likely source of much of the carbonates and soluble salts that are accumulating at shallow depths both in phase 1 soils and in the formerly noncalcareous, nongypsiferous argillic horizons of phase 2 and 3 soils.