Saline Playa Lake Research Articles

230Th dating of gypsum from lacustrine, brackish-marine and terrestrial environments

Gypsum is an important environmental archive in hyperarid settings, but its application in earth sciences has been hampered by the limited availability of suitable dating methods. Here we re-evaluate the potential of 230Th/U dating for sedimentary gypsum as well as anhydrite samples in different depositional environments. We provide a robust analytical protocol based on a simple dissolution using nitric acid followed by a two-column separation procedure using anion exchange resin. Isotope analyses were performed using MC-ICPMS and a mixed 229Th-233U-236U tracer. We applied the method to a suite of samples that likely reached secular equilibrium as well as to three suites of younger (<300 ka old) samples, two from the Atacama Desert in Chile and one from a drill core of a saline playa lake in southern Spain. We employed a selective sampling approach targeting the least visually altered and clearest parts of the samples largely devoid of detritus and analysed multiple subsamples.The results demonstrate the general applicability of the 230Th/U method to gypsum as well as anhydrite, and recrystallization of gypsum to anhydrite does not significantly affect the U-series isotope system. However, there is evidence for open-system behaviour, which needs to be carefully assessed. Our results suggest that α-recoil processes as well as uptake or loss of uranium are important processes biasing the results of gypsum dating. Furthermore, we see signs of multiple-phase contamination in our case studies involving detrital components and authigenic phases. Based on these results we propose geochemical criteria in to identify biased 230Th/U gypsum ages.The application of our protocol to a suite of selenite samples deposited by brine outflows on the lowest terrace of Cerro Soledad (Atacama Desert, Chile) provides a mean age of 212 ± 8 ka, indicating a relatively humid climate interval with local rainfall in this area that is coinciding with marine isotope stage 7C. Selenites deposited in the paleo-lake Soledad system (Atacama Desert, Chile) yield 230Th/U ages ranging from 250 to 320 ka, in agreement with previous age estimates for the drainage of this paleo-lake obtained by cosmogenic nuclide dating. Application to a suite of gypsum crystals recovered from a drill core in the Laguna de Fuente de Piedra (South Spain) shows good agreement with their stratigraphic context, however, multiple-phase contamination produces larger age uncertainties for these ages.

Widespread freshwater carbonate in the Olduvai Basin, a precursor to a major eruption in the East African Rift System

AbstractFreshwater limestones are uncommon in the sedimentary record of the East African Rift System. Recent research in the Olduvai Basin, Tanzania has revealed an extensive carbonate unit ca 1.8 Ma in age that varies spatially in thickness, petrography, trace element and rare earth geochemistry, C and O isotope values and freshwater microfossils. Five distinct lithologies are recognized. Four reflect deposition in carbonate‐rich environments that occurred at the same time in a catena‐like pattern over a heterogeneous landscape: spring, wetland, freshwater lake/pond and saline playa lake. The fifth is carbonate nodular soil that formed by paedogenesis on areas of topographically higher terrane. The δ18O of the carbonate, as negative as −6.5‰, indicates that the groundwater is meteoritic derived. The origin of the carbonate comprising this thick, extensive, isochronostratigraphic deposit has not been determined, but it is believed that carbonatite volcanism may have supplied elevated levels of carbonate to the basin which then entered the groundwater system. The carbonate was deposited just prior to a major trachyte‐phonolite eruption of Mount Olmoti, and the deposition of a basin wide tuff, Tuff IF. The region is seismically active today and seismic events were likely associated with Olmoti volcanism. The widespread carbonate unit may have preceded the Olmoti eruption and formed from a recurring seismically induced increase in aquifer porosity and permeability. The carbonate‐enriched groundwater episodically discharged onto the land surface, interacting locally with both fresh and saline ponded water.

Caracterización sedimentológica de un Sistema Fluvial Distributivo de clima árido: arroyo Papagayos, en el piedemonte oriental de las Sierras La Huerta-Imanas, San Juan, Argentina

Recent studies proposed that distributive fluvial systems (DFS) occupy a great proportion in modern continental sedimentary basins, challenging classical sedimentary models proposed for similar ancient basin. Because of this, arises the importance of studying modern DFS with the aim of obtaining facial models to apply them in ancient deposits. In this paper, we analyze and discuss the sedimentary environments included in the modern deposits of the Arroyo Papagayos, Western Pampean Ranges, San Juan province, interpreted as a Distributive Fluvial System (DFS). We perform the study of the Papagayos creek by describing different architectural elements, which characterize the sub-environments organized in four zones from the mountain front to the distal playa lake. A low sinuosity gravelly-sandy bed braided fluvial system, characterizes the proximal zone. A medial to high sinuosity sandy-gravelly anastomosed-anabranching fluvial system describes the medial zone, where floodplain aggradation is domain by crevasse splays accretion. At this point, 10 km far from the mountain front, the deposits starts to develop a cone shaped morphology. A higher sinuosity flashy ephemeral meandering sandy-muddy fluvial system with eolian interaction defines the distal zone, where the main channel of the system disappears by low gradient of the topography and infiltration depositing muddy terminal splays. Finally, eolian dune and wet interdune deposits characterizes the fourth zone, interpreted as a transitional area between the DFS and the saline playa lake. A particular feature observed along the DFS system is that mudflows are the main sedimentary process in the floodplain, probably as the result of the combination of the seasonal arid climate and the Arroyo Papagayos drainage basin morphometry and the low gradient of the topography. The sedimentary process and environments recognized along the Arroyo Papagayos will allow a better understanding of arid DFS depositional dynamics as well as, to identify them in the geological record. Lastly, high aggrading conditions for DFS development will response to climate interaction between upland catchment area (semiarid) and valley (arid), drainage basin morphometry, smooth piedmont gradient and high tectonic subsidence of the basin, located in the Andean broken foreland.

Palaeohydrological evolution and implications for palaeoclimate since the Late Glacial at Laguna de Fuente de Piedra, southern Spain

Here, we present a terrestrial multi-proxy record of Late Quaternary environmental changes in the southern Iberian Peninsula covering approximately 30 ka. This sedimentary record originates from a saline playa lake (Laguna de Fuente de Piedra) hosted within a complex geological setting dominated by Triassic claystones and evaporites, Jurassic carbonates and Miocene deposits leading to a complex hydrogeological setting. Dissolution of evaporites in the catchment and intense evaporation are responsible for saline waters fluctuating in the basin. Thus, salinity as palaeohydrological proxy, requires a decoupling of internal and external hydrogeochemical processes. The greatest accumulation of evaporites in the LFP late Pleistocene–Holocene record coincides with a more humid or, at least, less evaporative, period. Based on multi-proxy data we describe five lacustrine lithofacies (2–5), and fluvial deposits (1) from sediment cores. The proposed conceptual lake margin model contains three main lake water stages repeated within the sedimentary succession and building up the characteristic lithofacies. Lake water stages refer to a flooding stage (influx > outflux), high water stage (influx = outflux), and low water stage (influx < outflux). The lithostratigraphy reveals a palaeohydrological record suggesting climate changes and associated lake level fluctuations. Lake level oscillations of different amplitudes have been identified. Low amplitude changes have been revealed for the periods from 28 ka cal BP to 17.5 ka cal BP and from 8.2 ka cal BP to present, whereas in between (17.5 ka cal BP to 8.2 ka cal BP) the Late Pleistocene/Holocene transition shows high amplitude lake level changes. The latter coincides with an increased influence of saline subsurface waters, due to groundwater level rising (sulfate signature). In contrast, the Holocene, records the low amplitude oscillations and a drop of the groundwater levels, which creates a less saline or fresher footprint in the sediments (carbonate signature). Thus, the periods of low amplitude lake level oscillations, low inputs of clastics and low groundwater levels (drier) coincide with periods of minimal seasonal insolation difference. In contrast, the period of higher amplitude lake level oscillations, higher input of clastics and higher groundwater table (wetter) is correlative to periods of maximum difference between summer and winter insolation.

Characterizing bacterial assemblages in sediments and aerosols at a dry lake bed in Australia using high-throughput sequencing

Dust storms are responsible for the transport of a large quantity of bacteria from arid regions. A severe drought in the first decade of the new millennium in Australia increased the incidence of dust transport further. The major aims of this study were to characterize the bacterial communities in aerosols and their associated source sediments using high-throughput sequencing (HTS) and to investigate the possibility of using HTS to link dust to its source, which has not been previously performed in this way. Four field campaigns were conducted at the recently evaporated saline playa Lake Gnarpurt in the Australian state of Victoria between 2008 and 2010 (3 in the austral summer, 1 in winter) to collect aerosol and sediment samples. Aerosol samples were collected on filters up to 150 m above the lake bed using a tethered helium-filled balloon. DNA was extracted from all samples using commercial kits, and the bacterial communities were examined using 454 HTS on the 16S rRNA gene. Over 200,000 sequences from 29 samples were analysed. In both sediment and aerosol samples, Salinimicrobium was the most abundant taxon; however, there was great variation and diversity across all samples. Analysis of similarities of the bacterial communities indicated that there was a significant overlap between the sediment samples and the aerosols collected above that location, showing that the bacteria in the air was derived from a subset of dust from a nearby source. The challenge remains to use bacterial profiling to link an aerosol sample to a distant source.

High-resolution, multiproxy palaeoenvironmental changes recorded from Two Mile Lake, southern Western Australia: implications for Ramsar-listed playa sites

Numerous saline playa lakes exist across the arid, semiarid and temperate regions of Australia. These playa lakes exhibit a diverse range of hydrological conditions to which the Australian aquatic invertebrate biota have become adapted and which the biota can utilise as refugia in times of hydrological deterioration. Saline playas also yield palaeoenvironmental records that can be used to infer lacustrine and catchment responses to environmental variability. We present a palaeoenvironmental record recovered from Two Mile Lake, a saline playa from southern Western Australia. Dating, based on quartz optical luminescence and 14C accelerator mass spectrometry of biogenic carbonates and organic fibres, suggests that most of the sediment was rapidly deposited at 4.36 ± 0.25 thousand years ago. Ostracods and non-marine foraminifera preserved in the sediment show periods of faunal colonisation of the lake with oscillations between hypersaline and oligosaline conditions. The geochemistry of ostracod valves and foraminifera tests suggests higher-frequency variability within the lake, and palynological changes indicate landscape changes, possibly in response to fire. The Two Mile Lake record highlights the utility of saline playas as archives of environmental change that can be used to guide wetland health management, particularly under the impacts of a changing climate.

Modeling the interaction between evaporation and chemical composition in a natural saline system

Summary Evaporation controls the salinity of many natural and anthropic brine systems, but the reverse is also true. By controlling water activity, salinity affects evaporation rates. We present a method to compute the evolution of water activity in high salinity systems so as to evaluate evaporation rates. We place special emphasis on the assessment of invariant points, where activity is controlled by the set of precipitated minerals. The algorithm is tested on a natural Mg–SO 4 rich brine evaporation experiment. In accordance with the experiments, the model predicts two intervals (invariant points) in which water activity, the concentration of all species and the amount of liquid water remain constant, because evaporating water comes from the dissolution of hydrated minerals. This suggests that mineral paragenesis might have a considerable influence on shallow brine lake evolution by fixing chemical composition for a significant portion of time. This conjecture was tested with a simplified model of a perennial saline playa lake. An analytical solution was developed to illustrate the evolution of the proposed system. According to the calculations, the system tends to a cyclical steady state for both lake level and chemical composition. The latter remains fixed at invariant points during long time intervals, where hydrated minerals act as the water source for evaporation. This situation is to be expected in epsomite lakes where two invariant points can be found in equilibrium with relative humidities of 57% and 50%.

Origin of coarsely crystalline gypsum domes in a saline playa environment at the White Sands National Monument, New Mexico

Dome‐like structures up to 3 m in height, composed predominantly of coarse crystals of selenite gypsum, occur on the surface of Alkali Flat, a saline playa lake in White Sands National Monument, New Mexico. The structures were investigated using field observations, aerial images, and geochemical methods. The domes are inferred to be remnants of lake sediments that have been preserved from Holocene eolian erosion due to higher cementation resulting from crystallization of large gypsum (selenite) crystals in the playa's clay‐rich sediments. Low δ2H values of the selenite, from −90 to −69 ‰, suggest precipitation from saturated solutions associated with groundwater seepage in the playa related to the upward flows of groundwater, primarily fed by meteoric water rather than the evaporation of the playa water. Values of δ34S for selenite range from 12.1 to 13.8 ‰ (median 13.0 ‰) and are notably higher than the dissolved sulfate in the modern groundwater system that has δ34S values from 10.2 to 11.8 ‰ (median 11.4 ‰). The two main regional sources of sulfate are Middle Permian evaporites with median δ34S values of 12.0 ‰ and Lower Permian evaporites with median δ34S values of 13.0 ‰. The δ34S values measured in domes suggest their formation during a period of time when groundwater seepage might have been dominated by deeply circulating water that interacted with Lower Permian evaporites buried in the basin center. The apparent alignments of domes suggest that past groundwater seepage was controlled by fractures beneath Alkali Flat which are associated with a regional fault system in the area.

Sulfur isotope signatures in gypsiferous sediments of the Estancia and Tularosa Basins as indicators of sulfate sources, hydrological processes, and microbial activity

In order to reconstruct paleo-environmental conditions for the saline playa lakes of the Rio Grande Rift, we investigated sediment sulfate sources using sulfur isotope compositions of dissolved SO 4 2 - ions in modern surface water, groundwater, and SO 4 2 - precipitated in the form of gypsum sediments deposited during the Pleistocene and Holocene in the Tularosa and Estancia Basins. The major sulfate sources are Lower and Middle Permian marine evaporites (δ 34S of 10.9–14.4‰), but the diverse physiography of the Tularosa Basin led to a complex drainage system which contributed sulfates from various sources depending on the climate at the time of sedimentation. As inferred from sulfur isotope mass balance constraints, weathering of sulfides of magmatic/hydrothermal and sedimentary origin associated with climate oscillations during Last Glacial Maximum contributed about 35–50% of the sulfates and led to deposition of gypsum with δ 34S values of −1.2‰ to 2.2‰ which are substantially lower than Permian evaporates. In the Estancia Basin, microbial sulfate reduction appears to overprint sulfur isotopic signatures that might elucidate past groundwater flows. A Rayleigh distillation model indicates that about 3–18% of sulfates from an inorganic groundwater pool (δ 34S of 12.6–13.8‰) have been metabolized by bacteria and preserved as partially to fully reduced sulfur-bearing minerals species (elemental sulfur, monosulfides, disulfides) with distinctly negative δ 34S values (−42.3‰ to −20.3‰) compared to co-existing gypsum (−3.8‰ to 22.4‰). For the Tularosa Basin microbial sulfate reduction had negligible effect on δ 34S value of the gypsiferous sediments most likely because of higher annual temperatures (15–33 °C) and lower organic carbon content (median 0.09%) in those sediments leading to more efficient oxidation of H 2S and/or smaller rates of sulfate reduction compared to the saline playas of the Estancia Basin (5–28 °C; median 0.46% of organic carbon). The White Sands region of the Tularosa Basin is frequently posited as a hydrothermal analogue for Mars. High temperatures of groundwater (33.3 °C) and high δ 18O(H 2O) values (1.1‰) in White Sands, however, are controlled predominantly by seasonal evaporation rather than the modern influx of hydrothermal fluids. Nevertheless, it is possible that some of the geochemical processes in White Sands, such as sulfide weathering during climate oscillations and upwelling of highly mineralized waters, might be considered as valid terrestrial analogues for the sulfate cycle in places such as Meridiani Planum on Mars.

Modeling aluminum–silicon chemistries and application to Australian acidic playa lakes as analogues for Mars

Recent Mars missions have stimulated considerable thinking about the surficial geochemical evolution of Mars. Among the major relevant findings are the presence in Meridiani Planum sediments of the mineral jarosite (a ferric sulfate salt) and related minerals that require formation from an acid–salt brine and oxidizing environment. Similar mineralogies have been observed in acidic saline lake sediments in Western Australia (WA), and these lakes have been proposed as analogues for acidic sedimentary environments on Mars. The prior version of the equilibrium chemical thermodynamic FREZCHEM model lacked Al and Si chemistries that are needed to appropriately model acidic aqueous geochemistries on Earth and Mars. The objectives of this work were to (1) add Al and Si chemistries to the FREZCHEM model, (2) extend these chemistries to low temperatures (<0 °C), if possible, and (3) use the reformulated model to investigate parallels in the mineral precipitation behavior of acidic Australian lakes and hypothetical Martian brines. FREZCHEM is an equilibrium chemical thermodynamic model parameterized for concentrated electrolyte solutions using the Pitzer approach for the temperature range from <−70 to 25 °C and the pressure range from 1 to 1000 bars. Aluminum chloride and sulfate mineral parameterizations were based on experimental data. Aluminum hydroxide and silicon mineral parameterizations were based on Gibbs free energy and enthalpy data. New aluminum and silicon parameterizations added 12 new aluminum/silicon minerals to this Na–K–Mg–Ca–Fe(II)–Fe(III)–Al–H–Cl–Br–SO 4–NO 3–OH–HCO 3–CO 3–CO 2–O 2–CH 4–Si–H 2O system that now contain 95 solid phases. There were similarities, differences, and uncertainties between Australian acidic, saline playa lakes and waters that likely led to the Burns formation salt accumulations on Mars. Both systems are similar in that they are dominated by (1) acidic, saline ground waters and sediments, (2) Ca and/or Mg sulfates, and (3) iron precipitates such as jarosite and hematite. Differences include: (1) the dominance of NaCl in many WA lakes, versus the dominance of Fe–Mg–Ca–SO 4 in Meridiani Planum, (2) excessively low K + concentrations in Meridiani Planum due to jarosite precipitation, (3) higher acid production in the presence of high iron concentrations in Meridiani Planum, and probably lower rates of acid neutralization and hence, higher acidities on Mars owing to colder temperatures, and (4) lateral salt patterns in WA lakes. The WA playa lakes display significant lateral variations in mineralogy and water chemistry over short distances, reflecting the interaction of acid ground waters with neutral to alkaline lake waters derived from ponded surface runoff. Meridiani Planum observations indicate that such lateral variations are much less pronounced, pointing to the dominant influence of ground water chemistry, vertical ground water movements, and aeolian processes on the Martian surface mineralogy.

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.

GEOCHEMICAL CHARACTERISTICS OF AUSTRALIA&amp;apos;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).

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.

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.

Triassic rift sedimentation and palaeogeography of central East Greenland

The majority of the investigated Triassic (upper(?) Scythian – Rhaetian) rocks apparently formed in continental environments comprising alluvial fans, braided rivers, aeolian deserts, flood plains, saline playa lakes and freshwater shallow lakes. In the Middle Triassic a brief but widespread marine transgression affected large parts of the basin and resulted in the deposition of barrier limestones and lagoonal mudstones. Details of sedimentary structures, bed types, facies sequences and vertical and lateral variations are discussed for each of these environments. Special emphasis is given to the remarkable variety of lacustrine facies associations. The Triassic sediments probably accumulated in a N-S trending rift valley. This fault controlled depositional basin apparently formed in connection with overall rifting of the 'Laurasian' megacontinent. Tectonic movements along N-S trending fault lines appear to have been an important control on the thickness and nature of the Triassic facies, and the basal alluvial fan sediments formed simultaneously with or slightly after tectonic uplift of the borderlands. The climate throughout the Middle-Late Triassic period was warm and subtropical with alternating dry and wet seasons. The basin was under the influence of dominant north-east trade winds (winter) and less common south-east trade winds (summer). The vertical succession of climate-sensitive rocks further suggests a gradual shift towards increased humidity during the Middle-Late Triassic times. This climate trend is explained by a northwards continental drift of the area.