Playa Sediments Research Articles

The correlation of North-Iranian Late-Pleistocene and Holocene playa sediments to basin geomorphology

The correlation of North-Iranian Late-Pleistocene and Holocene playa sediments to basin geomorphology

Sand sheets interaction with aeolian dune, alluvial and marginal playa beds in Late Permian Upper Rotliegend setting (western part of the Poznań Basin, Poland)

The Upper Rotliegend deposits of the western part of the Poznan Basin, Poland, represent three main sedimentary environments: aeolian desert (dunes and sand sheets), alluvial plain (alluvial covers with wadi deposits) and marginal playa. In arid periods, dunes prograded into areas dominated by alluvial sedimentation. During wet periods, alluvial deposits eroded the aeolian deposits and prograded onto dune fields. Playa sediments were deposited among aeolian or alluvial sediments during periodic expansions of the playa. These aeolian, alluvial and playa deposits are arranged in depositional sequences. Boundaries between sequences are marked by rapid changes in depositional facies; and by erosional bounding surfaces. Maximum Wetting Surfaces have been distinguished within some parts of the depositional sequences, which enable regional correlations. The sections studied are composed of thick sequences of sand and sand-gravel sheets. The sand sheets represent periods of stratigraphic condensation. Systems of sand sheets are typical of marginal areas of ergs. We distinguish nine depositional complexes (units) numbered 1–9 from base to top, respectively. These subdivisions are based on the interpretation of the sedimentological record in palaeoclimatic terms, and in terms of major erosional boundaries, which are probably of regional extent. In the aeolian deposits, major boundaries are associated with deflation surfaces related to the stabilization of groundwater levels. In the marginal playa deposits, the most important boundaries are correlated to transgressive surfaces that developed during playa expansion. Time gaps represented by the boundaries and the scale of erosion below them, are difficult to estimate. The depositional system of the Upper Rotliegend was dominated by sand and sand-gravel sheets, and is either a unique feature in the entire Southern Permian Basin or, as we assume, sand sheets occur more commonly in the Rotliegend than has been previously thought.

Reconstruction of Holocene paleohydrological and sedimentological characteristics of Umm Ar-Rimam basin (northern Kuwait)

The paleohydrological and sedimentological characteristics of a playa lake in northern Kuwait (Arabian Gulf) are reconstructed using sedimentological, geochemical, and isotopic techniques. The sequence consists of up to 8 cycles of S-poor, alluvial sediments capped by a thin organic soil interbedded with gravity-fall calcrete sediments. The succession is locally derived from mainly Quaternary sediments and is regressive with upsection filling of the subsiding basin by cycles of sheetwash flow in response to climatic change. There is no natural, open-water lake water as indicated by low total organic carbon (TOC) data, but the presence of incised calcrete yardangs suggests that more extensive open-water conditions were operative in the past. Stable isotope (δ18O‰ and δ13C‰) values of the authigenic carbonates indicate the following three distinct processes: evaporation, meteoric fluid infiltration, and rapid per-descensum flow (rapid downward movement of water and playa sediment through pipes) through a porous, clastic sequence. Because evaporites are scarce, other factors besides evaporation action control chemical and isotopic compositions of the per-decensum lake fluids. Consequently, the isotopic composition cannot be interpreted exclusively as an indicator of salinity or evaporation ratio. The degassing of CO2 during groundwater discharge may explain the enriched carbon isotope values for the authigenic carbonates precipitated in the sediments. Hydrologically closed lake water bodies tend to show low negative carbonate oxygen and carbon isotopic signatures. Isotopically negative δ13C values imply a strong input of soil-zone carbon to the groundwater of the top 60 cm of the sediment. Lakes that are hydrologically closed and evaporate or equilibrate with atmospheric CO2 will tend to have low negative δ18O and δ13C values in the carbonates as reported by Talbot (Chem Geol: Isotope Geosci Sect 80(4):261–279, 1990). Biologically active lakes will tend toward lower δ13C of dissolved carbon due to the photosynthetic effects of 12C withdrawal as reported by Dunagan and Driese (J Sed Res 69:772–783, 1999). Increased biological activity during sedimentation may account for low carbon isotope values where plants were abundant, but in shrinkage-dominated systems (those of clay-rich soil subjecting to wet-dry conditions), carbon isotopes will be largely inherited from the calcretic limestones in the land extending landward of the coast and not influenced by coastal processes (known as hinterland), such as Umm Ar-Rimam depression. This basin does not fit the classic shallow playa-type basins of the Arabian Peninsula but rather the recharge playas of the southwestern USA.

The middle to late Triassic Bänkerjoch and Klettgau formations of northern Switzerland

In the context of the harmonisation of the Swiss lithostratigraphic scheme, the Late Ladinian to Early Norian Bankerjoch Formation and the Norian to Rhaetian Klettgau Formation of northern Switzerland are formally defined. The Bankerjoch Formation replaces the «Gipskeuper». Delimitation generally follows the traditional Swiss scheme with the onset and offset of sulphate facies as basal and upper boundary. The Klettgau Formation includes six members (from base to top): The Ergolz Member (formerly «Schilfsandstein» and «Untere Bunte Mergel») consists of variegated silty dolomitic marl with dolocretes and channels filled by fine-grained sand of Scandinavian origin. The Gansingen Member (formerly «Gansinger Dolomit s.l.») starts with partly porous dolomite with Carnian bivalves and continues with an alternation of dolomite beds with thin dolomitic marl interlayers. At several places, the member has been eroded to great parts during the Late Triassic. Towards east and southeast, dolomitic sulphates, locally altered to dolomite or limestone, replace the marine carbonates. The Berlingen Member includes coarse alluvial and fluvial sandstones of Vindelician origin. It is restricted to the Lake Constance area. For practical reasons, the Gruhalde Member (formerly «Obere Bunte Mergel» and «Knollenmergel») encompasses all playa sediments above the Gansingen Member and Berlingen Member. Evidence in good outcrops suggests that it consists of several sedimentary cycles separated by long time spans of omission and erosion. The Seebi Member (formerly «Stubensandstein») includes layers of coarse-grained sandstones of Vindelician origin with calcitic, siliceous or clayey matrix. Locally, matrix fraction increases and becomes mostly dolomitic in the upper part. The Belchen Member (formerly «Rhat») consists of partly fossiliferous sandstone and greenish dark marl of estuarine to marine origin. Due to late Triassic and early Jurassic erosion, the Belchen Member is restricted to northwestern Switzerland and the Lake Constance area.

Rise and Fall of an Egyptian Oasis: Artesian Flow, Irrigation Soils, and Historical Agricultural Development in El‐Deir, Kharga Depression, Western Desert of Egypt

The present study examines the geoarchaeological history of an oasis in Kharga Depression in central Egypt. El‐Deir is renowned for its Ptolemaic temple and Roman fortress on the road from former Hibis (Kharga) to the Nile Valley. During the survey, spring mounds and irrigation soils belonging to an ancient agricultural zone were discovered, and further documented by ceramics found on the site. Our methodology combines the geomorphological interpretation of landforms (especially yardangs) with ceramics and 14C‐dated charcoal to distinguish and date former agricultural areas in El‐Deir. The results show that the oasis experienced several phases of soil accretion and destruction through time. Playa sediments were deposited in the humid early Holocene and severely eroded by deflation before the onset of irrigated agriculture between Pharaonic and Persian times. Very fast vertical soil accretion occurred in the Ptolemaic period, but irrigation soils were later destroyed during the Roman period by a combination of wind deflation and flash floods (second to fourth century A.D.), suggesting a period of climate instability. The case of El‐Deir invites reevaluation of constructive agencies for the development of irrigated land and destructive agencies as limiting factors for the sustainability of agricultural practices in late antiquity.

Grain-rimming kaolinite in Permian Rotliegend reservoir rocks

Upper Rotliegend sediments of Permian age from the northeast Netherlands show moderate to good reservoir qualities. The predominant control is by the presence of authigenic grain-rimming kaolinite, which has a negative, but in some parts also a positive, effect on reservoir quality. To better understand the formation and distribution of grain-rimming kaolinite, reservoir rocks were studied in terms of composition and diagenetic processes. Petrographic evidence, summarized as a paragenetic sequence, is integrated with geochemical modeling results to identify early mesodiagenetic water–rock interactions under the participation of gases, i.e., CO2 and H2S, released from underlying Carboniferous source rocks. The sediments investigated were deposited at varying distance from the southern flank of the Southern Permian Basin. Sediments near the basin margin are mainly attributed to a fluvial environment and comprise medium to coarse-grained sandstones and conglomerates. There, vermicular kaolinite occurs with a lath-like structure. Distal to the basin margin, mainly in sandstones intercalated with fine-grained playa sediments, comparatively high amounts of grain-rimming kaolinite occur. There, the presence of this mineral has a significant influence on the rock properties and the reservoir quality. Geochemical modeling suggests that the formation of such kaolinites cannot be explained exclusively by in situ feldspar dissolution. The modeling results support evidence that kaolinite can be formed from precursor clay minerals under the presence of CO2-rich formation waters. Such clay minerals could be corrensite, smectite–chlorite mixed-layer minerals, or chlorite that is potentially present in Rotliegend sediments during early diagenesis. Furthermore, the geochemical modeling can reflect several mineral reactions that were identified from petrographic analysis such as the formation of illite and kaolinite at the expense of feldspar dissolution and consequent silica precipitation. Under the presence of sulfate (SO42−), partial hematite dissolution and the reduction of Fe3+ to Fe2+ occur, where the latter is further incorporated in Fe–Mg-rich carbonates. The sulfate is bonded in anhydrite. The results of this study are relevant for reservoir quality predictions in kaolinite-dominated systems, as well as for CO2 storage projects and the prediction of long-term fluid–rock interactions under the participation of carbon dioxide and/or other gases.

Seismometer Detection of Dust Devil Vortices by Ground Tilt

We report seismic signals on a desert playa caused by convective vortices and dust devils. The long-period (10-100s) signatures, with tilts of ~10$^{-7}$ radians, are correlated with the presence of vortices, detected with nearby sensors as sharp temporary pressure drops (0.2-1 mbar) and solar obscuration by dust. We show that the shape and amplitude of the signals, manifesting primarily as horizontal accelerations, can be modeled approximately with a simple quasi-static point-load model of the negative pressure field associated with the vortices acting on the ground as an elastic half space. We suggest the load imposed by a dust devil of diameter D and core pressure {\Delta}Po is ~({\pi}/2){\Delta}PoD$^2$, or for a typical terrestrial devil of 5 m diameter and 2 mbar, about the weight of a small car. The tilt depends on the inverse square of distance, and on the elastic properties of the ground, and the large signals we observe are in part due to the relatively soft playa sediment and the shallow installation of the instrument. Ground tilt may be a particularly sensitive means of detecting dust devils. The simple point-load model fails for large dust devils at short ranges, but more elaborate models incorporating the work of Sorrells (1971) may explain some of the more complex features in such cases, taking the vortex winds and ground velocity into account. We discuss some implications for the InSight mission to Mars.

Neoproterozoic continental arc volcanism at the northern edge of the Arabian Plate, SE Turkey

New geochemical, Sr/Nd isotope and zircon U–Pb LA-ICP-MS data from the Derik Volcanics in the Southeast Anatolian Autochthone Belt of Turkey are consistent with an Andean-type Cadomian arc that developed along the northern edge of the Arabian Plate during the Late Neoproterozoic. The Derik Volcanics represent a volcanic complex including andesites, rhyolites and basalts, with volcanoclastic and fluvial sediments. They are unconformably overlain by playa sediments with Early Cambrian ichno-fossils, followed by Middle Cambrian-Silurian shallow marine deposits.Geochemically, the Derik Volcanics (DV) display transitional-calcalkaline affinities. On Th/Nb–Ti/Zr diagram, DV display negative trends from rhyolites to late to early-stage andesites and mafic dykes, respectively. The REE patterns are highly to moderately fractionated, with [(La/Yb)N=5.20–6.77 in late-stage andesites, 4.38–10.51 in rhyolites, 2.58–4.65 in the early-stage andesites, and 2.51–4.21 in mafic dykes]. Normalized trace element and REE diagrams display Th, La, Ce, Sm enrichment and depletion of Nb, Ti and Eu as is typical for Andean-type active continental margin igneous rocks. The enrichment of LILE and LREE, combined with depletion of HREE also suggests that the Derik Volcanic rocks were formed in relation with a subduction zone. Negative Eu anomalies (Eu/Eu)N, range from 0.50 to 1.81, indicates fractional crystallization of feldspar. The DV have positive ɛNd (+0.15 to +4.20) and mean model ages of 1.28Ga for the early-stage andesites, 1.34Ga for rhyolites, 1.35 for late-stage andesites, and 1.36Ga for mafic dykes. The positive ɛNd(T) data show that DV are product of mantle-derived magmas. Lower 143Nd/144Nd isotope ratios than the depleted MORB mantle (DMM) source indicate that the DV may have been contaminated by crustal material during magma genesis. Geochemical modeling shows that all four volcanic rock types may have been formed by different percentages of batch melting of DMM and subducting slab sources.Zircon LA-ICP-MS data give crystallization ages of 581.4±3.5Ma (n=7) and 559.2±3.2Ma (n=3) for the early and late-stage andesitic rocks, as well as ages of 569.6±1.6Ma (n=17), 571.6±1.9Ma (n=18), 575.4±4.3Ma (n=6) for the rhyolites.The geological and geochemical features together with the new age data suggest that the Derik Volcanics formed along a continental arc in the course of the southward subduction of Prototethys oceanic lithosphere along the northern margin of the Arabian Plate, which is attributed to a late-stage phase of the Cadomian Orogeny of the Pan-African cycle.

Regional transport of a chemically distinctive dust: Gypsum from White Sands, New Mexico (USA)

Abstract The White Sands complex, a National Monument and adjoining Missile Range in southern New Mexico, occupies the dry bed of an ice-age lake where an active gypsum dunefield abuts erodible playa sediments. Aerosols entrained from White Sands are sometimes visible on satellite images as distinct, light-colored plumes crossing the Sacramento Mountains to the east and northeast. The IMPROVE network (Interagency Monitoring of PROtected Visual Environments) operates long-term aerosol samplers at two sites east of the Sacramento range. In recent years a spring pulse of sulfate aerosol has appeared at these sites, eclipsing the regional summer peak resulting from atmospheric reactions of sulfur dioxide emissions. A significant fraction of this spring sulfate is contributed by gypsum and other salts from White Sands, with much of the sulfur in coarse particles and concentrations of calcium and strontium above regional levels. The increase in these gypsiferous species coincides with a drought following a period of above-average precipitation. White Sands and the IMPROVE samplers together provide a natural laboratory: a climatically sensitive dust source that is both well characterized and chemically distinct from its surroundings, with a signature that remains identifiable at long-term observatories 100–200 km downwind.

Detecting patterns of aeolian transport direction

The direction of sediment transport is of direct interest to those engaged in the study of aeolian processes. Here, a technique was employed that provides information regarding aeolian transport direction using measurements of wind direction and aeolian activity. This technique involves detecting one-second periods of blowing sediment, called saltation seconds, with a piezoelectric saltation sensor and assigning active periods to one of sixteen principal wind direction sectors. Saltation seconds are then summed for each wind direction sector to compute the relative fraction associated with a specific wind direction. Measurements of aeolian activity collected over a fifteen-year period at a large saline playa on the Llano Estacado of North America were used to compute the relative frequency that sediments are transported by winds blowing from a specific direction. Results of this study suggest that winds with a westerly component contribute significantly to the transport of playa sediments at this site and this result is consistent with the location of massive fringing dunes along the eastern margins of the playa. Results also reveal a significant contribution of strong northerly winds associated with dry continental polar cold fronts that often pass through the area, especially during the winter, late fall, and early spring.

Petrography and genesis of Quaternary coastal gypcrete in North Kuwait, Arabian Gulf

Gypcrete crusts cover Late Quaternary playa deposits within the northern coastal flat of Kuwait Bay, Arabian Gulf. This study provides insight into the genesis of the gypcrete crust associated with the fluvial playa in an arid coastal area. The playa sediments are mostly muddy sand rich in diagenetic gypsum. The latter is concentrated at the top part of this playa where the gypcretic crust is developed. Field investigation, megascopic, microscopic and nanoscopic examination, and XRD analysis revealed the occurrence of several types of gypcretes, namely massive hard crust, hard clustered gypsum roses, gypseous rhyzocrete, and powdery crust. With the exception of the last type, all gypcretes are formed largely of gypsum with subordinate amounts of bassanite. Two genetic types of bassanite are recognised; these are replacive and precipitative. Replacive bassanite is the most common and is mainly represented by topotactic alteration of gypsum, whereas the precipitative mode is mostly restricted to the powdery crumbled crust and root cavity-fill. The micromorphology of both types of bassanite is discussed. It is suggested that the stability of bassanite is attributed to the elevated ambient temperatures in the presence of highly saline brine and organic acids.

Sliding stones of Racetrack Playa, Death Valley, USA: The roles of rock thermal conductivity and fluctuating water levels

On occasion, Racetrack playa in Death Valley National Park becomes flooded and temperatures then drop appreciably below freezing. The thermal conductivity of rock is greater than that of water, so heat is conducted from a partially-submerged rock faster than from water. Consequently, a collar of thicker ice forms at the water surface, a layer of ice forms on more deeply-submerged parts of the rock, and playa sediment beneath the rock may even become frozen to it. While this occurs, only a surface layer of ice forms on water away from the rock. Once the ice becomes thick enough, perhaps only 5–10mm, either the buoyancy of the ice or additions of water to the playa by rain, snow-melt, or groundwater seepage then reduce the normal force between the rock and the playa to the point where wind shear can move the ice sheet with its entrained rocks, making trails (Stanley, 1955; Reid et al., 1995). After the ice melts, rocks are left at the ends of the trails, sometimes atop a pedestal of silt. A renewed increase in water level on the playa before the ice melts may lift a rock completely free of the playa surface and whisk it away, leaving a rockless trail. During a movement event, changes in rock orientation or water depth may result in changes in width along the track. Rock speeds are likely tens to hundreds of millimeters per second.

Wind Erosion at a Saline Playa Environment, Ebinur Lake, Xinjiang, China - A Case Study on the Source of Saline Dust Storm

In many arid and semiarid lands, dry lake beds (saline playa) represent a tremendous source of unconsolidated salt-rich sediments that are available for aeolian transport. Severe salt-dust storms caused by the erosion of such landforms have become very harmful natural phenomena. Base on texture and appearance characteristic, five principal undisturbed playa surfaces for sampling to investigate the deflation rate and the vertical distribution of material abraded using a wind-tunnel experiment in this study. Two additional field deflation monitoring transect were aslo established to examine vertical deflation by wind from measurements of erosion pins at the Ebinur (dry) Lake. The results indicate that winds greater than 8 m/s is the main factor for inducing the erosion of the playa sediments. Soft salt, aeloian sediment and alluvial deposit are the main sources of the saline dust storms in Ebinur region. The near-surface vertical distribution of material abraded concentrated in 0 -10 cm height. The annual wind erosion rate ranged from 0.48 cm to 5.6 cm in the northwest portion of the lake and from 0.24 cm to 0.96 cm in the southeast portion.

The Ca isotopic composition of dust-producing regions: Measurements of surface sediments in the Black Rock Desert, Nevada

Dust is a relatively unconstrained flux in the geochemical cycle of Ca. The isotopic composition of dust-derived Ca has not been studied, though it is an important part of any attempt at deciphering Ca isotope-based proxy records. Accordingly, this study reports the elemental and calcium isotope geochemistry of 30 surface (upper ∼0.5cm) sediments from an arid dust producer, the Black Rock Desert in northwestern Nevada. Geochemical data for sequential water and 0.5N HCl leaches, meant to sample mobile Ca, and selected leached residues are presented, along with X-ray diffraction (XRD) determinations of major mineralogy. Bulk playa sediments have Ca concentrations between 0.28 and 40wt.% (median: 6.8wt.%) and calcite concentrations of 2–32%. Isotopically, Ca sampled by water leaches (<1% of total Ca) are, on average, +0.33±0.16‰ (1SD) heavier than acid leaches (>60% of total Ca), though the degree of fractionation (Δw-a) varies between 0‰ and 0.6‰. Acid leaches, which are the primary component of mobile Ca in the sediments, have δ44CaSRM-915a values of 0.78±0.08‰, similar to the δ44Ca of modern nannofossil ooze and modern rivers. This means that dust produced in closed continental basins likely has little isotopic leverage to change the ocean’s isotopic composition, and suggests that the Ca isotopic composition of dust is tied to rivers/weathering. In addition, while the Ca concentration data in the water leach suggest that evaporative evolution controls the amount of Ca in this reservoir, the isotope data are inconsistent with this conclusion. Instead, we hypothesize that adsorption of Ca on clays controls the Ca isotope systematics in the water leach. This hypothesis requires that there is no significant isotopic fractionation during evaporite mineral precipitation and may suggest that sorption at rates appropriate for natural systems might fractionate differently than sorption at laboratory rates.

Optically stimulated luminescence and radiocarbon dating of sediments from Lop Nur (Lop Nor), China

Lop Nur is a playa lake occupying the lowest part of the Tarim Basin, northwestern China, and is now a desolate and barren region. In the past decades, the ages of the lacustrine sediments from the lake were determined mainly by radiocarbon dating on bulk sediment. In this study, both optically stimulated luminescence (OSL) and radiocarbon methods were used to date the sediments from a pit in the central part of the lake. The OSL ages obtained for ten samples range from 0.5 to 9.4 ka, and are in stratigraphic order except for one sample. The 14C ages obtained for twenty-two bulk sediment samples range from 5.8 to 30.2 cal ka BP with erratic distribution. Based on the comparison of 14C with OSL ages and their age-depth models, we argue that the OSL ages are relatively reliable. The disequilibria in the U decay chain for some samples are deduced from the comparison of the NAA and TSAC results. We suggest that the OSL dating technique should preferably be applied to the playa sediments from Lop Nur, but the disequilibria in the U decay chain should be considered in evaluating dose rates. Additionally, radiocarbon reservoir effects in lakes in western China are reviewed.

Hydrochemical characteristics and brine evolution paths of Lop Nor Basin, Xinjiang Province, Western China

Abstract This study was carried out in the Lop Nor basin, a large arid closed drainage basin in Western China. The objective was to contribute to the understanding of the hydrological and hydrochemical processes of the Lop Nor basin by analysis of the chemical composition of different water sources and associated mineralogical characteristics of the playa sediments. The dominant river inflow waters to the Lop Nor basin are of the Na–Mg–Ca–SO 4 –Cl–HCO 3 type. Spring inflow is dominated by Na + and Cl − . Present-day concentrated groundwater brines vary little in the study area and are consistently rich in Na + and Cl − and poor in Ca 2+ and HCO 3 - + CO 3 2 - , but also contain a considerable amount of Mg 2+ and SO 4 2 - . EQL/EVP (equilibrium/evaporation), a brine equilibrium model, simulated evaporation of inflow water and groundwater brines in an open system and showed good agreement between theoretically predicted and observed minerals in the Lop Nor basin. Brine chemical modeling cannot however explain the massive amounts of glauberite (Na 2 SO 4 ·CaSO 4 ) and polyhalite (K 2 SO 4 ·MgSO 4 ·2CaSO 4 ·2H 2 O) deposits found in a 230 m deep core ZK1200B from the Lop Nor basin. EQL/EVP simulations under a closed system allowed brine reactions with previously formed minerals and indicate that glauberite forms by back reaction between brine, gypsum and anhydrite and polyhalite forms by reaction between brine and glauberite. Diagenetic textures related to recrystallization and secondary replacement were commonly observed in core ZK1200B, indicating significant mineral–brine interaction during crystallization of glauberite and polyhalite. Mineral assemblages predicted from the evaporation of Tarim river water match closely with natural assemblages and abundances, which can explain the unusual glauberite deposits in the Lop Nor basin. It is suggested that the Tarim river inflow is the dominant source over the lake’s history. The distribution of minerals in the cored sediments documents the history of inflow water response to wet and dry periods in the Lop Nor basin.

Wind erosion of saline playa sediments and its ecological effects in Ebinur Lake, Xinjiang, China

In many arid and semiarid areas, dry lake beds (saline playa) represent a tremendous source of unconsolidated salt-rich sediments that are available for aeolian transport. Severe salt-dust storms caused by the erosion of such landforms have become very harmful natural phenomena. In this study, sample analysis and field erosion monitoring of Ebinur Lake was conducted to investigate the salt content, chemical composition, and wind erosion intensity of surface salt-rich sediments. The effects of salt-dust rising from the playa on the growth and physiological health of plants were also evaluated in this study through a leaf dustfall test. The results indicate that water-soluble salts assemble densely on the dry lake bed surface. At a depth of 0–2 cm, the highest salt contents can exceed 40%, with sulfate and chloride being the main anions present and Na+, Ca2+, and Mg2+ being the primary cations. The annual wind erosion rate ranged from 0.48 to 5.6 cm in the northwest portion of the lake and from 0.24 to 0.96 cm in the southeast portion. Salt-dust storms caused by wind erosion of saline playa sediments seriously influenced the normal absorption of minerals by plant leaves. Under the influence of salt-dust storms, plant leaves absorb more Na+, but far less K+.

Controls on the stratigraphic development of the Triassic Fundy Basin, Nova Scotia: implications for the tectonostratigraphic evolution of Triassic Atlantic rift basins

Abstract: The Late Triassic synrift succession of the Minas sub-basin (Bay of Fundy, Nova Scotia) includes a fluvio-alluvial fan-dominated unit (lower and middle Wolfville Formation), a transitional fluvio-playa unit (upper Wolfville Formation) and an upper playa-dominated package (Blomidon Formation). We describe the facies development in the 240 m thick upper Wolfville Formation, examine vertical facies stacking patterns and develop a depositional model for this succession. The unit comprises channelized and unconfined fluvial deposits interbedded with aeolian and playa sediments that grade upward into playa deposits. The succession comprises progradational and retrogradational fluvial packages and thin aggradational playa claystones. Comparison with 11 other Atlantic margin continental Triassic rift basins allows an assessment of the controls that govern rift basin development. The transition from fluvial to playa or lacustrine conditions occurs in all basins at different times. The lack of synchronicity indicates that global climate change was not a controlling factor. We suggest that the fluvial to lacustrine or playa transition was due to a decrease in source area relief related to a decline in regional tectonic activity, and that the basin was hydrologically closed throughout sedimentation. This tectonostratigraphic model may be appropriate for other Triassic basins developed along the Atlantic margin.

Paleosols in Late Moscovian (Carboniferous) Marine Carbonates of the East European Craton Revealing "Great Calcimagnesian Plain" Paleolandscapes

Abstract Late Paleozoic strata commonly include lithologic cycles (“cyclothems”), driven by sea-level fluctuations that repeatedly flooded and exposed broad expanses of epeiric shelves. Such successions, commonly with multiple unconformities, are well documented in Permo-Pennsylvanian sedimentary successions, but remain less understood in the East European Craton (EEC), part of the Euramerican continental shelf rim. This study documents unconformities and paleosols from the Podolskian to Myachkovian (middle Pennsylvanian) epeiric-carbonate succession of the EEC and explores the non-actualistic soil types, landscapes, and the character and density of vegetation in this Pennsylvanian calcimagnesian dryland environment. Thin (mostly < 1 m) paleosols developed at seven successive unconformities consist of an upper terrestrial clay layer or “topclay”, a crust of beta calcrete beneath the topclay, and a weakly karsted substrate limestone with rare rhizocretions. Solution pits and small (< 3 m deep) karst sinkholes are rare. Nontruncated topclays commonly consist of two layers: in situ clayey epipedons and resedimented terrestrial clays. The latter are interpreted as playa sediments. The Myachkovian unconformities of the northernmost localities studied, near Arkhangelsk, lack beta calcrete and root traces, and the limestone beneath these unconformities is sepiolitized. In Podolskian paleosols of the southern Moscow Basin, the topclays are predominantly palygorskitic. The Myachkovian paleosol clays are smectitic–illitic with occasional chlorite admixture. The majority of late Moscovian paleosols surveyed formally conform to lithic (rendollic) haplocalcids within the aridisol class or rendzic calcic leptosols. A detailed study of a representative Podolskian paleo-pedon reveals development of shallow soil carbonate, low alumina/bases and Ba/Sr ratios, enhanced Mn and Sr, presence of soil gypsum and opal, and a characteristic peak in magnetic susceptibility, all suggesting a semiarid to arid pedogenic environment. The palygorskite clay of this paleo-pedon retains 1.1–1.5% of connate organic matter in the form of covalently bound organomineral complexes, and is fulvate-dominated resembling organic matter from extant dryland soils. The palygorskitic composition of Podolskian topclays and shallow (< 30 cm) pedogenic carbonates is interpreted to reflect hot, well-drained semidesert conditions with precipitation less than 300 mm/yr. The transition from palygorskitic to smectitic soil clays across the Podolskian–Myachkovian boundary, apparently deeper soil carbonate, and an increase in the degree of karstification record a general increase in humidity such that annual precipitation may have exceeded 300–400 mm. The available data reconstruct the “great calcimagnesian plain” landscapes formed during low sea level, supposedly glacial phases, across tens of thousands of square kilometers of the EEC. This non-actualistic system apparently lacked a fluvial network and was covered by uniform soils and playas. The primary productivity of this early (310–305 Ma old) dryland ecosystem inferred from high TOC and rhizogenic beta calcretes was not markedly different from present-day steppe or savanna grasslands and semideserts. In the northernmost outcrops surveyed, near Arkhangelsk, drier conditions probably limited colonization by higher plants. The available data open a “hidden chapter” in the Paleozoic evolution of terrestrial ecosystems which is not recorded by paleobotany and urges modifications in the concept of low-productivity character of pre-Miocene dry landscapes.

The Middle Carnian Wet Intermezzo of the Stuttgart Formation (Schilfsandstein), Germanic Basin

The Middle Carnian Wet Intermezzo (MCWI) of the Stuttgart Formation (Schilfsandstein) and age-equivalent strata of the northwestern Tethys occurred entirely within equivalents of the upper subzone of the Austrotrachyceras austriacum ammonoid zone of the late Julian. Its duration is estimated to be only about 0.7–0.8 myr. In both the Germanic Basin and the northwestern Tethys, the warm climate during the MCWI was characterized by a rate of precipitation that exceeded somewhat the evaporation but was not so great as to be pluvial. The MCWI was related to the atmospheric circulation of a megamonsoonal system that was characterized by strong, moisture-laden, northwesterly flowing trade winds that rose as they reached the estimated 2000–3000 m high eastern shoulder uplift of a huge rift causing them to drop an extraordinary amount of rain. This eastern shoulder uplift lay within the Caledonides of modern day western Scandinavia. This region only, between 30 and 50°N palaeolatitude, had a truly pluvial climate, and the huge amounts of fresh water dropped there transported large amounts of siliciclastics from this rift-shoulder uplift southward into the Germanic Basin. Before deposition of the siliciclastics an early Julian eustatic sea-level fall caused widespread erosion in the Germanic Basin. In the later part of late Julian, a transgression from the eastern gates with the concurrent strong fresh water influx from the north flooded the centre of the northern Germanic Basin with a shallow brackish sea in which the Osterhagen Horizon (Basisschichten) of the lower Stuttgart Formation was deposited. In the upper Osterhagen Horizon the salinity rapidly decreased from mesohaline through mio- and oligohaline to fresh water levels. In southern Germany the Basisschichten formed entirely within fresh water or very low salinity brackish environments . Later, a slight subsidence of the southwestern Germanic Basin shifted the main outflow of fresh water toward the southwestern end of the basin, creating local brackish conditions in the northern marginal part of the northwestern Tethys (e.g. in the Lunz Beds). During this time interval, tidal influence also can be found in the Stuttgart Formation deposited in palaeoestuaries of the southwestern Germanic Basin adjacent to the Tethys. The very strong fresh water influx from the north, however, prevented these estuaries from developing a strongly elevated salt content, so that only fresh water to oligohaline brackish faunas are found there such as the Eberstadt bivalve–conchostracan fauna. At the base of the Tuvalian, the megamonsoonal system either disintegrated or else the trade winds shifted their principal flow direction. This caused the climate within the Germanic Basin and in the nearby northwestern Tethys Sea to become arid again as it had been before the deposition of the Stuttgart Formation. These changes in the atmospheric circulation patterns also terminated the pluvial climatic regime to the north along the Scandinavian eastern rift-shoulder uplift. This in turn ended the transport of huge amounts of siliciclastics from this region southward into the Germanic Basin and ended the deposition of the Schilfsandstein. After this, the hypersaline sabkha and playa sedimentation of the Weser Formation began, which was accompanied by some minor marine ingressions in the southwestern Germanic Basin.