Playa Lake System Research Articles

Revisión de la Formación La Veteada en su estratotipo (La Rioja, Argentina): edad, estratigrafía y significado paleoambiental

The La Veteada Formation, in the Sierra de Famatina (west of Argentina), is one of the few records of Early Triassic age in South America verified by isotopic age and palynological assemblages. This unit is composed of sandstones, mudstones, shales, limestones, and marls, with some levels of gypsum, chert, and tuff levels. In its stratotype, the La Veteada Formation rests on Late Permian red beds of the Talampaya Formation and is covered by Neogene breccias and conglomerates belonging to the Del Crestón Formation. The unit is divided into three associations of sedimentary facies. Facies association A (80 m thick) is a red bed succession composed of mudstone and fine-grained sandstone, together with some levels of chert and gypsum. Facies association B (58 m thick) is greenish-gray to yellowish-gray and comprises limestones, marls, shales, and fine- to coarse-grained sandstones. This facies includes stromatolites and several levels of shales and marls where rich palynological assemblages were recovered. Finally, facies association C (24 m thick) is made up of yellowish-gray marls, mudstones, and sandstones. In this section, stromatolites are missing, thin levels of gypsum appear, and evidence of subaerial exposition as mud cracks and raindrop imprints are frequent at the top of the facies. The lithology, sedimentary structures, and vertical stacking of facies suggest that the La Veteada Formation was deposited in a lacustrine environment, which evolved from a shallow ephemeral playa lake system (facies association A) to a perennial lake, in which carbonate production increased compared to clastic sedimentation (facies association B). Regarding the facies association C, the increase in clastic supply, the missing stromatolites, and the presence of gypsum levels suggest a progressive shallowing of the lake and the likely transition to a palustrine environment. The Early Triassic age of the La Veteada Formation is indicated from two different lines of research. Firstly, a zircon U-Pb age of 249.66±0.11 Ma obtained from a tuff level at the middle part of the facies association A. Secondly, the presence of palynological species identified in other Early Triassic units worldwide. The La Veteada Formation records the filling of the embryonic extensional basins formed at the earliest Triassic. Moreover, this unit shows the evolution of depositional environments after the late Permian massive extinction event.

Early diagenetic calcite replacement of evaporites in playa lakes of the Quiricó Formation (Lower Cretaceous, SE Brazil)

Evaporites are abundant in modern playa lakes, where efflorescence, inorganic and/or bioinduced precipitation coexist with siliciclastic deposition. Overall, the sensibility of salts to slight physicochemical changes results in partial to complete dissolution of evaporites in surface and shallow subsurface. However, in some cases, salt minerals are pseudomorphically replaced, providing a snapshot to depositional settings and diagenetic processes of evaporite-bearing successions. This is recorded in the Quiricó Formation (Lower Cretaceous), Sanfranciscana Basin, southeastern Brazil. This studied succession portrays a playa lake system subdivided in saline flat and saline lake deposits, whose diagenesis was marked by ubiquitous cementation and replacement reactions. Petrographic and geochemical techniques were undertaken to characterise primary and secondary textures, mineralogy and distribution of mudstone- and sandstone-hosted pseudomorphs. The pseudomorphs frequently underlie the stratigraphic contact of playa lake-aeolian beds. Pseudomorphs occur as coalescent nodules and as chaotically-distributed chevron crystals, composed of inequicrystalline calcite mosaics. Nodular pseudomorphs have cubic/sub-cubic “ghost” inclusions and displacive contacts with the matrix, whereas the contacts of chevron pseudomorphs and the matrix are undisturbed. Both types exhibit morphological replication of precursor minerals and non-luminescent euhedral relicts. Stable isotopic signatures are covariant and range between −1.6–0.6 ‰ δ13C and 1.4–2.5 ‰ δ18O for nodular pseudomorphs, and −2.2 to −1.5 ‰ δ13C and 0.3–1.3 ‰ δ18O for chevron pseudomorphs. Nodular crystals reflect subaerial and intrasedimentary crystallisation of Ca-sulphates in saline flats, whereas chevron-shaped pseudomorphs indicate bottom-growth halite in shallow lakes. Textural evidences from pseudomorphs and host lithologies constrain the diagenetic overprint to surface and shallow burial settings (syndepositional/eodiagenetic stages). Despite multiple generations of calcite, the constant isotopic signatures of pseudomorphs reflect water chemistry fractionation in a closed-lake basin. Widespread calcitisation of playa lake sediments was caused by calcium-rich groundwaters, influenced by limestones from the Precambrian basement. The playa lake deposits record the onset of aridification processes that culminated in the Thermal Cretaceous Maximum.

Climatic control on the Holocene hydrology of a playa-lake system in the western Mediterranean

Evaporitic lakes such as playa-lakes are characteristic of many arid regions and are unique environments with respect to fauna and flora, while being very vulnerable to climate and environmental fluctuations and threatened by the current global change scenario. Water balance oscillations in these systems can trigger the precipitation or dissolution of different evaporitic minerals, negatively impacting local biodiversity and economic activities. Here, we study the sedimentary record of a small saline pond from a playa-lake complex in southwestern Iberia in order to reconstruct the paleohydrological evolution of this area and assess potential anthropogenic disturbances. The different proxies studied in the ∼11.9 ky old sedimentary record of the Laguna de la Ballestera suggest that the greatest lake extension and the highest water levels occurred during the Early Holocene, pointing to the wettest period of the record. Climate transitioned towards more arid conditions during the Middle Holocene, and even more dramatically during the Late Holocene. In this last stage the wetland surface and the water level largely diminished and gypsum precipitation gradually increased pointing towards a negative precipitation/evapotranspiration balance and lowest water levels. Summer desiccation likely occurred under this scenario, especially after ∼1.0–0.9 cal ky BP coeval with the Medieval Climate Anomaly, when gypsum content started to rise abruptly. However, this significant gypsum precipitation was only associated with a massive drop in the siliciclastic content and scarce carbonates (dolomite and calcite) during the last ∼400 years. This evidence suggests a shift from a (semi) permanent to a temporal/seasonal hydrological regime. The environmental evolution of this wetland responded to the general climatic evolution of the western Mediterranean during the Holocene, being mostly controlled by changes in insolation. Our data also show that the environmental response of the studied wetland to natural climate variations was only significantly disturbed by human activities since the 20th century, especially in the second half of the century, deduced by abrupt fluctuations in the siliciclastic, gypsum and organic content in the sediments, as well as by the enhanced sedimentary accumulation rates, probably as a response to changes in the hydroperiod of the lake and in the catchment land use.

Climatic  <i>Vs</i> Anthropogenic Control on the Holocene Hydrology of a Playa-Lake System in the Western Mediterranean

Climatic  <i>Vs</i> Anthropogenic Control on the Holocene Hydrology of a Playa-Lake System in the Western Mediterranean

Late Triassic ichnoassemblage from a playa-lake system of the Coastal Meseta, Morocco: Palaeoenvironmental and palaeoecological implications

Here we present the first comprehensive ichnotaxonomic analysis of a Late Triassic playa system from the Sidi Saïd Maachou Basin, Coastal Meseta (Western Meseta, Morocco). The Late Triassic deposits consist of sediments deposited in various nonmarine environments dominated by dry red-bed facies. These deposits yielded, so far, an ichnoassemblage consisting of the invertebrate ichnogenera Cochlichnus, Cruziana, Diplichnites, Rusophycus, Palaeophycus, and Taenidium associated with the tetrapod tracks Brachychirotherium and Rhynchosauroides. These invertebrate and vertebrate trace fossils are preserved in concave epirelief and convex hyporelief on upper and lower surfaces of laminated mudstones and fine-grained sandstones. The invertebrate ichnofossils were probably made subaqueously and indicate different behaviours of the tracemakers: walking (cursichnium, Acripes), resting (cubichnium, Rusophycus) and combined locomotion-feeding or plowing (pascichnium, Cruziana). Additionally, many other fossils are collected from the same formation, including plant impressions, rhizoliths and fish scales. The invertebrate ichnoassemblage described herein is referred to the Scoyenia ichnofacies, which indicates opportunistic behaviours in temporarily or periodically inundated nonmarine environments, such as playa systems, floodplains and lake margins. The invertebrate ichnoassemblage reported here is the first well documented from the Sidi Saïd Maachou Basin and the Late Triassic of Morocco in general. The strata have a high potential to yield other ichnoassemblages, therefore further exploration is needed.

Paleohydrogeologic reconstruction of Triassic carbonate paleolakes from stable isotopes: Encompassing two lacustrine models

Reconciling interpretations of geochemical, sedimentologic, and stratigraphic observations is a common challenge with many datasets, especially those from paleolake systems. The Triassic Cerro de las Cabras and Cerro Puntudo formations, part of the sedimentary fill of the Cuyana rift Basin in central-west Argentina, represent carbonate-rich playa-lake systems with abundant microbialites, but different sedimentologic and stratigraphic attributes such as presence or absence of efflorescent crystalline crusts and aggrading versus prograding-aggrading stacking patterns. A detailed study of the stable-carbon and oxygen-isotope composition of the limestones deposited in these paleolakes, together with sedimentological and mineralogical evidence as petrography, microprobe, XRD and cathodoluminescence data, enabled interpretation of their complex hydrology.Lack of correlation between carbon and oxygen stable-isotope values for both Cerro de las Cabras (δ13C χ–6.03‰; σ 0.67‰/δ18O χ −12.64‰; σ 0.55‰) and Cerro Puntudo (δ13C χ −3.10‰; σ 1.15‰/δ18O χ −12.58‰; σ 1.8‰) paleolakes strongly indicates open hydrology (underground flow), in apparent contradiction with sedimentologic and stratigraphic evidence for intermittent to persistently closed hydrography (overland flow). Stable-isotopic values for the Cerro de las Cabras paleolake together with sedimentology data indicate a recharge (hydrologically open) lake, and for the Cerro Puntudo a through-flow (hydrologically open) playa-lake. To resolve the apparent contradictions, we propose an integration of playa-lake hydrology (groundwater) with lake-basin type sedimentology, stratigraphy, and hydrography. This allowed identifying and proposing a balanced-fill–through-flow lake subtype for the Cerro Puntudo system and an underfilled–recharge-lake subtype for the Cerro de las Cabras paleolake; both with hydrology open to groundwater flow. This work suggests a starting point for expanding the lake-basin-type model to explicitly incorporate different groundwater and overland flow pathways.

The Three Forks playa lake depositional model: Implications for characterization and development of an unconventional carbonate play

The Upper Devonian Three Forks Formation of the Williston Basin is an important reservoir interval in the Bakken petroleum system, with estimates suggesting greater than 3.7 billion bbl of recoverable oil. Detailed stratigraphic analyses are sparse, but current studies propose shallow marine, tidal flat, or coastal sabkha environments for its deposition. Based on stratigraphic, sedimentological, and petrographic analyses of 28 cores, we infer that the Three Forks Formation was deposited in a continental setting with little to no marine influence. The setting was subaerial to subaqueous environments comparable to those found in playa lake systems or continental sabkhas. Late Devonian paleography and tectonics provided the isolation and slow subsidence of the Williston Basin that led to the accumulation of the Three Forks Formation in a hot and arid climate. Occasional floods were responsible for the deposition of the terrigenous sediments, debrite-like deposits, reworking, bedforms, scouring, and erosional surfaces. Flood waters collected in ephemeral lakes and ponds evaporated, resulting in the deposition of subaqueous dolomite, anhydrite, and even halite. The ponds and lakes progressively evaporated and contracted until they eventually dried out. The drying is characterized by dewatering structures, subaerially exposed and transported sediments with ripples, scour surfaces, evaporite cemented surfaces, evaporite removal breccias, halite pseudomorphs, and a variety of cracks. Vertical successions record an upward trend of increasing proportions of subaqueous facies, indicating a transition from arid to relatively less arid climatic conditions during Three Forks deposition. This depositional system resulted in the accumulation of a complex mosaic of lithologies composed of different proportions and textures of silt-sized detrital dolomite and quartz, clay, and evaporites in patchy to bull’s-eye configurations. This depositional model provides a coherent understanding of the existing depositional lithofacies and their petrophysical and geomechanical properties and provides the background for the prediction of their lateral distribution from wire-line logs.

Late Holocene Environmental Reconstructions from Lake Solai, Kenya

Playa lake systems tend to be overlooked archives of paleoenvironmental change due to the likelihood of a short and intermittent record of deposition. Groundwater-fed wetlands associated with these climate-sensitive playas, however, preserve changes in hydrologic budget and are thus valuable archives for semiarid regions. This study examines the paleoecological record of a groundwater-fed wetland from Lake Solai, Kenya. Biological proxies are used to reconstruct paleoenvironmental change and climate impacts over the past millennium. Dry conditions persisted between CE 1115 and 1490, followed by wetter conditions during the Little Ice Age. Near surface sediments indicate increasing anthropogenic impact through pastoralism.

Hydrogeological behaviour of the Fuente-de-Piedra playa lake and tectonic origin of its basin (Malaga, southern Spain)

Changes in the quantity of groundwater input due to water extraction for irrigation and urban supply has modified the water balance in the Fuente de Piedra playa lake. We have analysed the hydrogeology of the playa-lake system and developed a water-level model by means of a simple long-term water balance and piezometric analysis. In addition, a tectonic model is proposed to explain the endorheic basin development that led to the formation of the playa. Upright folds developed since the late Miocene and density-driven subsidence favoured the setting-up of and endorheic system located between the Atlantic and the Mediterranean basins in the Quaternary. The underlying low permeability rocks beneath the playa form a very stable aquitard with highly saline groundwater that prevents groundwater recharge of the lake into the aquitard. The hydrological modelling allowed us to simulate the evolution of the wáter level under a scenario of unaltered conditions during a 13-year period, showing that the percentage of days with dry conditions varies from 24.8% of the time under altered conditions to 14.9% as far as an unaltered scenario is concerned.

Permian paleogeography of west-central Pangea: Reconstruction using sabkha-type gypsum-bearing deposits of Parnaíba Basin, Northern Brazil

Extreme aridity during Late Permian – Early Triassic period was the main factor for resetting the entire paleoclimate of the planet. Permian evaporite basins and lacustrine red beds were widely distributed along the supercontinent of Pangea. Sulphate deposits in Western Pangea, particularly in Northern Brazil, accumulated in an extensive playa lake system. Outcrop-based facies and stratigraphic analysis of up to 20m thick evaporite-siliciclastic deposits reveal the predominance of laminated reddish mudstone with subordinate limestone, marl and lenses of gypsum. The succession was deposited in shallow lacustrine and inland sabkha environments associated with saline pans and mudflats. Gypsum deposits comprise six lithofacies: 1) bottom-growth gypsum, 2) nodular/micronodular gypsum, 3) mosaic gypsum, 4) fibrous/prismatic gypsum, 5) alabastrine gypsum, and 6) rosettes of gypsum. Gypsum types 1 and 2 are interpreted as primary deposition in saline pans. Bottom-growth gypsum forms grass-like crusts while nodular/micronodular gypsum indicates displacive precipitation of the crust in shallow water and the groundwater capillary zone. Types 3 and 4 are early diagenetic precipitates. Abundant inclusions of tiny lath-like anhydrite crystals suggest a primary origin of anhydrite. Alabastrine gypsum, fibrous gypsum (satinspar) and rosettes of gypsum probably derived from near-surface hydration of anhydrite. The gypsum-bearing deposits in the Parnaíba Basin contribute towards understanding paleogeographic changes in Western Pangea. A progressive uplift of East Pangea, culminated in the forced regression and retreat of epicontinental seas to the West. Restricted seas or large lakes were formed before the definitive onset of desert conditions in Pangea, leading to the development of extensive ergs.

Palaeoenvironmental characterization of a crocodilian nesting site from the Late Cretaceous of Brazil and the evolution of crocodyliform nesting strategies

Despite the vast crocodyliform fossil record, little is known about the reproductive biology and nesting strategies of the extinct members of the group. Here we report a large accumulation of crocodilian fossil eggs from the type-locality of the baurusuchid Pissarrachampsa sera. Sedimentary facies and architectural elements of the site support a palaeoenvironmental model with a shallow lacustrine, playa-lake system interacting to ephemeral braided fluvial channels, with aeolian influence and development of sandy soils. The presence of pedogenic calcretes in the palaeosols indicates arid to semi-arid conditions. The crocodilian affinity of the eggs is supported by the thin eggshell which bears wedge-shaped shell units with tabular microstructures. Furthermore, taphonomic data support an autochthonous assemblage of eggs and skeletal remains, hinting at a monotypical stratigraphic horizon and suggesting P. sera as the egg-laying taxon. The repeated pattern of four (eventually five) eggs per clutch at the site demonstrates that P. sera laid fewer eggs compared to modern crocodilians, indicating that k-selected reproductive strategy pattern is pervasive in the fossil record of Notosuchia. In the crocodyliform phylogenetic framework, the k-strategy and the “egg clutch sizes” optimization of Notosuchia is opposite to the strategy with larger clutches consistently occurring in modern Crocodylia and Neosuchia, the sister clade to Notosuchia. Yet, the lack of data on more early-branching taxa renders unclear which pattern is plesiomorphic for Crocodyliformes as a whole.

Russia: TVEL/Siberian Chemical Combine – TiO2

Although it is commonly assumed that the trace-fossil record of eolian dunes and associated environments is invariably poor, a systematic review of the available information indicates that this is not necessarily the case. A model involving five main phases of colonization of desert environments through the Phanerozoic is proposed in this paper. The first phase (Cambrian–Silurian) involved animal incursions into coastal dune fields directly from the sea, although it is unlikely that these animals would have remained for long periods of time in coastal deserts. The second phase (Devonian) reflects the activities of dune pioneers that left their fluvial habitat to enter temporary or permanently into inland deserts. The third phase (Carboniferous–Permian) involved the colonization of deserts by tetrapods. The fourth phase (Triassic–Cretaceous) involved a major exploitation of the infaunal ecospace as reflected by the appearance of more varied behavioral patterns in sub-superficial structures. The fifth phase (Paleogene–Recent) reflects the appearance of the ecological dynamic that characterizes modern desert communities. The invertebrate ichnofacies for eolian dunes is re-named herein as the “Octopodichnus–Entradichnus Ichnofacies” honoring the seminal work of previous workers who simultaneously tackled the issue of eolian dune ichnofacies. The Chelichnus ichnofacies is retained for vertebrate trace-fossil assemblages in eolian settings. Both ichnofacies occur in mobile and temporary stabilized sandy substrates, subject to frequent erosion and deposition, and to strong seasonality. Desert settings consist of complex mosaics of habitats or physical units associated with organism activity. Trace-fossil distribution can be understood as reflecting the partitioning of desert settings in a mosaic of landscape units, characterized by water content and its temporal fluctuations, nutrient availability, nature of the substrate, and the dominant organisms present. In turn, desert systems are dynamic entities that change as a response to regional climate. Landscape units, such as eolian sand seas, salt flat and playa lake systems, ephemeral rivers and alluvial fans, interact in response to regional-scale climate variations in hyper-arid, arid, and semiarid climatic settings. Ancient deserts completely developed under hyper-arid climatic conditions rarely preserve trace fossils due to the absence of moisture near the surface. However, the alternation of wet periods may represent windows for life development and thus, preservation of biogenic structures. Arid deserts display complex patterns of dunes combined with dry, wet, and flooded interdunes. Dry desert elements (e.g. dunes, interdunes, sand sheets) typically record the Entradichnus–Octopodichnus and Chelichnus Ichnofacies. Slight rises in regional precipitation produce elevation of the water table and increase of fluvial discharges that provide water and sediment to the system. These processes may result in the local concentration of trace fossils in wet interdunes and ephemeral fluvial systems, illustrating the Scoyenia and Chelichnus Ichnofacies. In semiarid systems playa lakes expand by the addition of freshwater, evolving into freshwater lakes, and fluvial systems may become more common; lake margins and fluvial overbanks typically contain trace-fossil assemblages that may be ascribed to the Scoyenia Ichnofacies.

Unravelling aquifer-wetland interaction using CSAMT and gravity methods: the Mollina-Camorra aquifer and the Fuente de Piedra playa-lake, southern Spain

The hydrological regime of Fuente de Piedra playa-lake (Málaga, southern Spain) has been significantly affected by the intensive exploitation of groundwater in the area. The playa-lake is situated above clays, marls, and gypsum, and under unaltered conditions received surface-subsurface runoff within the watershed as well as groundwater discharge from two carbonate aquifers. We have analyzed the structure of the main one, the Mollina-Camorra carbonate aquifer, by combining controlled source audio magnetotellurics (CSAMT), gravity prospecting, and time-domain electromagnetic (TDEM) soundings. This geophysical information, together with new structural and hydrogeological data, was gathered to develop a new conceptual hydrogeological model. This model allows the hydrological linkage of the carbonate aquifer with the playa-lake system to be established. Moreover, the intensive exploitation in the carbonate aquifer, even outside the watershed of the playa-lake, has affected the hydrological regime of the system. This multidisciplinary work demonstrates the potential of geophysical methods for understanding wetland-aquifer interaction, having important groundwater management implications.

MICROBIALITES IN A HIGH-ALTITUDE ANDEAN LAKE: MULTIPLE CONTROLS ON CARBONATE PRECIPITATION AND LAMINA ACCRETION

Microbialites comprise the mineralized record of early life on Earth and preserve a spectrum of fabrics that reflect complex physical, chemical, and biological interactions. The relatively rarity of microbialites in modern environments, however, challenges our interpretation of ancient structures. Here we report the occurrence of microbial mats, mineral precipitates, and oncoids in the Laguna Negra, a high-altitude hypersaline Andean lake in Catamarca Province, Argentina. Laguna Negra is a Ca-Na-Cl brine where abundant carbonate precipitation takes place. Extreme environmental conditions, including high UV radiation, elevated salinity, and temperature extremes, restrict multicellular life so that mineralization reflects a combination of local hydrologic conditions, lake geochemistry, and microbial activity. The resulting carbonates consist of micritic laminae, botryoidal cement fans, and isopachous cement laminae that are strikingly similar to those observed in Proterozoic stromatolites, providing insight into mechanisms of mineralization. Here, increased saturation with respect to carbonate minerals reflects mixing of spring-fed inlets and lake waters, favoring microbialite formation and preservation. This highlights the importance of hydrological mixing zones in microbialite formation and as taphonomic windows to record microbial activity. Recent discoveries of minerals related to evaporating playa-lake systems on Mars further highlights the potential of Laguna Negra to provide critical insight into biosignature preservation in both terrestrial and extraterrestrial settings.

Surface water hydrology and geomorphic characterization of a playa lake system: Implications for monitoring the effects of climate change

Playa lakes are sensitive recorders of subtle climatic perturbations because these ephemeral water bodies respond to the flux of diffuse and channelized flow from their watersheds as well as from direct precipitation. The Black Rock Playa in northwestern Nevada is one of the largest playas in North America and is noted for its extreme flatness, varying less than one meter across a surface area of 310 km2. Geo-referenced Landsat imagery was used to map surface-area fluctuations of ephemeral lakes on the playa from 1972 to 2013 to provide baseline data on surface water hydrology of this system to compare to future hydrologic conditions caused by climate change. The area measurements were transformed into depth and volumetric estimates using results of detailed topographic global positioning system (GPS) surveys and correlated with available surface hydrological and meteorological monitoring data. Playa lakes reach their maximum size (<350 km2) in spring, fed by melting snows from high mountains on the periphery of the drainage basin, and usually desiccate by early- to mid-summer. The combination of a shallow groundwater table, sediment deposition, and hydro-aeolian planation probably are largely responsible for the flatness of the playa. When lakes do not form for a period of several years, the clay- and silt-rich playa surface transforms from one that is hard and durable into one that is soft and puffy, probably from upward capillary movement of water and resultant evaporation. Subsequent flooding restores the hard and durable surface. The near-global availability of Landsat imagery for the last 41 years should allow the documentation of baseline surface hydrologic characteristics for a large number of widely-distributed playa lake systems that can be used to assess the hydrologic effects of future climate changes.

Late Holocene evolution of playa lakes in the central Ebro depression based on geophysical surveys and morpho-stratigraphic analysis of lacustrine terraces

The origin and morpho-stratigraphic evolution of the largest playa-lake system (La Playa–El Pueyo) in the Bujaraloz–Sástago endorheic area, located in the semiarid central sector of the Ebro Depression, are analysed. The enclosed depressions are developed on gypsiferous Tertiary bedrock and show a prevalent WNW–ESE orientation parallel to the direction of the prevalent strong local wind (Cierzo). Yardangs have been carved in bedrock and unconsolidated terrace deposits in the leeward sector of the largest lake basins. A sequence of three lacustrine terrace levels has been identified by detailed geomorphological mapping. The treads of the upper, middle and lower terrace levels are situated at +9m, +6m and +0.5m above the playa-lake floors, respectively. Seismic refraction and electrical resistivity profiles acquired in La Playa reveal a thin basin fill (~2m) with a planar base. These data allow ruling out the genetic hypothesis for the depressions involving the collapse of large bedrock cavities and support a mixed genesis of combined widespread dissolution and subsidence by groundwater discharge and eolian deflation during dry periods. The 5m thick deposit of the middle terrace was investigated in hand-dug and backhoe trenches. Six AMS radiocarbon ages from this terrace indicate an aggradation phase between 3.9ka and ca. 2ka. These numerical ages yield a maximum average aggradation rate of 2.6mm/yr and a minimum excavation rate by wind deflation of 3mm/yr subsequent to the accumulation of the middle terrace. The latter figure compares well with those calculated in several arid regions of the world using yardangs carved in palaeolake deposits. The aggradation phase between 4 and 2ka is coherent with other Iberian and Mediterranean records showing relatively more humid conditions after 4ka, including the Iron Ages and the Iberian–Roman Period.

Controls on marine–erg margin cycle variability: aeolian–marine interaction in the mid‐Cretaceous Iberian Desert System, Spain

AbstractThe interaction between aeolian dunes of the Iberian erg and Tethys waters during the mid‐Cretaceous led to a variety of sedimentary facies associations such as subtidal deposits, aeolian dunes, playa lakes, coastal lakes with tidal creeks, and marshes and lagoonal embayments with tide‐influenced delta deposits. Facies associations are organized in several stacked cycles. Every cycle is defined by a sand‐drift surface separating playa lake deposits below from aeolian dune deposits above, followed by a transgressive surface separating aeolian deposits below from shallow marine deposits above. The latter are covered by playa lake deposits and finally topped by the next sand‐drift surface. Similar marine–erg margin cycles have been explained previously in terms of high‐order relative sea‐level variations (normally controlled by glacioeustasy). The studied erg margin developed during the mid‐Cretaceous which is considered to be the archetypal example of a polar ice‐free greenhouse period. This manuscript presents an alternative explanation for the cycles as caused by climate‐induced variations in the aeolian sediment supply. In this scenario, orbitally induced latitudinal shifts of the boundary between climate belts led to alternating periods of increased and decreased precipitation in the highland catchments of the back‐erg system (Variscan Iberian Massif). When climate belts shifted to lower latitudes, back‐erg highlands were under the influence of the Northern Warm Humid Belt. While arid conditions prevailed in the lowland central‐erg, increased precipitation in the highlands and the consequent recharge of ground water led to a rise of the phreatic level in the erg system; that diminished wind erosion and windblown sediment input to the fore‐erg, so that the desert margin contracted. With ongoing basin subsidence this favoured the transgression. The increased desert‐ground water flux favoured permanent coastal lakes in the fore‐erg margin and the local development of vegetation. In places where such lakes (lagoons) were connected to the sea, tidal channels were active, as seen nowadays along the desert coast of Qatar (Persian Gulf). When climate belts moved to higher latitudes, the catchment areas in the back‐erg Iberian Massif were under the influence of the Northern Hot Arid belt and a decrease of precipitation led to a drop of the phreatic level, allowing deflation of dunes and exposed wadi channel floors, and the formation of desert pavements and deflation lags in the back‐erg area; this favoured the import of large volumes of windblown sand and dust, forcing progradation of the erg and consequently a retreat of the coastline. The sand fraction accumulated in climbing aeolian dunes and dust was trapped in extensive playa lake systems. Another long‐term allocyclic control is formed by active extensional tectonics that enhanced the creation of accommodation space, especially in the first cycle. A high ground water table sustained by subsurface water supply from the Variscan Iberian Massif generated coastal fresh/brackish lakes in which tide‐influenced meandering channels and salt marshes developed. Erg margin cycles thin upward in response to a decrease of the rate of basin subsidence, suggesting that transgressions led to deep erosion of the underlying aeolian dune sands.

Palaeogroundwater evolution in playa–lake environments: Sedimentary facies and stable isotope record (Palaeogene, Almazán basin, Spain)

The Palaeogene history of the Almazán basin is that of a non-marine basin sediments derived from fluvial distributary systems and lateral playa–lakes. The coeval uplifting of the Cameros Massif and the Aragonian Branch of the Iberian Chain during Alpine compression exposed Triassic evaporites, which became the main source of sulphates to the playa–lake system, as recorded by the Bordalba Formation. This unit exhibits the classical facies distribution of gypsum-precipitating playa–lakes, which can be grouped into two main settings: 1) distal alluvial/mud flat and 2) saline mud flat/ephemeral lake. The stable isotope data for gypsum from the distal alluvial/mud flat areas (12.2 < δ 18O SMOW < 16.5; 10.8 < δ 34S CDT < 13.0‰) lie within the range of values of neighbouring Triassic gypsum (or are slightly lower for δ 34S CDT). This agrees with palaeogeographical interpretations suggesting that the Triassic evaporites were the source of gypsum for the Almazán basin. The slightly lower values could be explained in terms of a minor contribution through the oxidation and dissolution of Palaeozoic-derived sulphides. The isotopic values for gypsum of the saline mud flat/ephemeral lake environments (19.8 < δ 18O SMOW < 23.82; 15.6 < δ 34S CDT < 19.9‰) are higher than those of the distal alluvial/mud flat. The higher δ 18O SMOW in the saline mud flat/ephemeral lake environments is interpreted as being a consequence of the progressive evaporation of shallow groundwaters from proximal to distal areas. The heavier values for the δ 34S CDT in the saline mud flat/ephemeral lake environments are interpreted as reflecting the activity of sulphate-reducing bacteria, which is evidenced by spheroidal dolomite aggregates on gypsum crystals and by the grey colours of the gypsum facies.

Influence of hydrogeology, microbiology and landscape history on the geochemistry of acid hypersaline waters, N.W. Victoria

Abstract There is now evidence that naturally occurring acid–water is more abundant than previously thought and that it has been important in the geologic past. Understanding the processes leading to the formation of such systems is required to appreciate the role of acid systems in geologic processes and to develop indicators for recognizing the geologic/environmental importance of these systems in the past. This paper characterizes the hydrogeology, hydrogeochemistry, microbial biogeochemistry and landscape attributes of acid–groundwater surface water systems in Australia with an emphasis on a well studied playa-lake system, Lake Tyrrell, Murray Valley. A model for the origin of these acid brines is presented and the importance of acid-brine producing systems is speculated upon. Data include porewater and groundwater geochemical measurements (collected during a 10 day field campaign) and results from geochemical modeling and graphics (e.g., Piper diagrams and x–y plots of seawater evaporation trajectories). Key characteristics of the system are (1) aquifer materials have low acid buffering capacities, (2) saline groundwater flowing onto playa surfaces is an oxic, H2SO4 solution, (3) authigenic minerals include combinations of jarosite [KFe3(SO4)2(OH)6], alunite [KAl3(SO4)2(OH)6] and Fe oxides that can form as evaporite minerals, (4) a source for solutes can be marine aerosols and (5) the formation of ironstones. Groundwater acidification by various processes including sulfide oxidation and ferrolysis, and at many different times, are the unique aspects for evolution of these acid brines and they can be considered another end member of the Eugster–Jones–Hardie model for the evolution of brines in closed basins. Acid–hypersaline groundwater and playa systems such as Lake Tyrrell may be an example of expected changes in the hydrogeochemistry of terrestrial water during late-stage continental denudation under arid conditions. Historically these systems may have been integral to the formation of opal, bauxite, some low temperature ore deposits, of authigenic K-feldspars, and continental redbeds. Natural acid saline systems, such as those in Australia, may also be representative of acid saline systems on Mars.

Experimental evaporation of superficial brines from continental playa–lake systems located in Central Ebro Basin (northeast Spain)

Abstract Solutions coming from two natural playa–lake saline systems located in Central Ebro Basin (NE Spain) have been evaporated in the laboratory, in order to obtain the precise path of chemical evolution followed until high concentration stages. The lakes belong to two chemically different neutral brines: La Playa (with Na–Cl solutions) and La Salada (Na–Mg–SO 4 type). Experimental evaporation has been carried out at 25 °C until total dryness, and samples collected along the experiment have been analysed for their major components. Application of geochemical modelling techniques allowed calculation of the saturation indexes for the main saline minerals using the PHRQPITZ program, which incorporates Pitzer's model. The mineral precipitation sequence for La Playa brines following the saturation data is: gypsum, halite, thenardite and epsomite. Brines reach saturation almost simultaneously with respect to both halite and thenardite, but halite precipitates more massively and hence the solid samples collected after total desiccation were composed mainly of halite. In the case of La Salada brines, the order of precipitation is somewhat different and is as follows: gypsum, mirabilite, thenardite and bloedite. Solid samples here consisted of bloedite and thenardite, this latter formed after subaquatic dehydration of mirabilite when brines attained a peritectic point. The evolution of saturation indexes is in good agreement with mineral determinations carried out on the solid experimental samples, and it allowed us to interpret the evaporative evolution of both La Playa and La Salada brines.