Search

Sheeting joints are ubiquitous in outcrops of the Navajo Sandstone on the west-central Colorado Plateau, USA. As in granitic terrains, these are opening-mode fractures and form parallel to land surfaces. In our study areas in south-central Utah, liquefaction during Jurassic seismic events destroyed stratification in large volumes of eolian sediment, and first-order sheeting joints are now preferentially forming in these structureless (isotropic) sandstones. Vertical cross-joints abut the land-surface-parallel sheeting joints, segmenting broad (tens of meters) rock sheets into equant, polygonal slabs ~5 m wide and 0.25 m thick. On steeper slopes, exposed polygonal slabs have domed surfaces; eroded slabs reveal an onion-like internal structure formed by 5-m-wide, second-order sheeting joints that terminate against the crossjoints, and may themselves be broken into polygons. In many structureless sandstone bodies, however, the lateral extent of first-order sheeting joints is severely limited by pre-existing, vertical tectonic joints. In this scenario, non-conjoined sheeting joints form extensive agglomerations of laterally contiguous, polygonal domes 3-6 m wide, exposing exhumed sheeting joints. These laterally confined sheeting joints are, in turn, segmented by short vertical cross-joints into numerous small (~0.5 m) polygonal rock masses. We hypothesize that the sheeting joints in the Navajo Sandstone form via contemporaneous, land-surface-parallel compressive stresses, and that vertical cross-joints that delineate polygonal masses (both large and small) form during compression-driven buckling of thin, convex-up rock slabs. Abrasion of friable sandstone during runoff events widens vertical tectonic joints into gullies, enhancing land-surface convexity. Polygonal rock slabs described here provide a potential model for interpretation of similar-appearing patterns developed on the surface of Mars. [ABSTRACT FROM AUTHOR]

The Kanab Wonderstone is sandstone (Shinarump Member, Chinle Formation) that is cemented and stained with iron oxide. The iron-oxide cementation and staining in these rocks have been considered examples of the Liesegang phenomenon, but we will show that they comprise a microbially induced structure. The spacing of bands of iron-oxide stain follow the Jablczynski spacing law (wherein the spacing between bands of iron-oxide stain increases as one traverses a series of bands) characteristic of Liesegang. Bands of iron-oxide cement exhibit more variable spacing and exhibit a weak but significant correlation between band thickness and distance between bands of cement. The pore-filling cement contains morphotypes that are similar in size and habit to those exhibited by microaerophilic iron-oxidizing bacteria. Other disseminated iron-oxide mineralization occurs as rhombohedra interpreted to be pseudomorphs after siderite. We interpret the cement to be produced by microbially mediated oxidation of siderite (a typical early diagenetic mineral in fluvial sandstones). Iron-oxidizing bacteria colonized the redox interface between siderite-cemented sand and porous sandstone. Microbes oxidized aqueous Fe(II), generating acid that caused siderite dissolution. The iron-oxide cement is the microbial product of a geochemical drive for organization; whereas the iron-oxide stain is true Liesegang. Together, they comprise a distinctive microbially induced structure with high preservation potential. Key Words: Biosignatures-Iron oxides-Diagenesis-Iron-oxidizing bacteria-Shinarump. Astrobiology 15, 616-636. [ABSTRACT FROM AUTHOR]

Iron-rich carbonates and the oxidized remains of former carbonates (iron-oxide concretions) underlie bleached Navajo Sandstone over large portions of southern Utah. Iron in the carbonates came from hematite rims on sand grains in the upper Navajo that were dissolved when small quantities of methane accumulated beneath the sealing Carmel Formation. As a second buoyant gas (CO2 derived from Oligocene-Miocene magmas) reached the seal and migrated up dip, it dissolved in the underlying water, enhancing the solution's density. This water carried the released ferrous iron and the methane downward. Carbonates precipitated when the descending, reducing water degassed along fractures. The distribution of a broad array of iron-rich features made recognition of the extent of the ancient flow systems possible. Although siderite is not preserved, dense, rhombic, mm-scale, iron-oxide pseudomorphs after ferrous carbonates are common. Distinctive patterns of iron oxide were also produced when large (cm-scale), poikilotopic carbonate crystals with multiple iron-rich zones dissolved in oxidizing waters. Rhombic pseudomorphs are found in the central cores of small spheroids and large (meter-scale), irregular concretions that are defined by thick, tightly cemented rinds of iron-oxide-cemented sandstone. The internal structure and distribution of these features reveal their origins as ironcarbonate concretions that formed within a large-scale flow system that was altered dramatically during Neogene uplift of the Colorado Plateau. With rise of the Plateau, the iron-carbonate concretions passed upward from reducing formation water to shallow, oxidizing groundwater flowing parallel to modern drainages. Finally they passed into the vadose zone. Absolute dating of different portions of these widespread concretions could thus reveal uplift rates for a large portion of the Plateau. Iron-rich masses in other sedimentary rocks may reveal flow systems with similar histories. [ABSTRACT FROM AUTHOR]

Earthquakes along convergent plate boundaries commonly occur in sequences that are complete within 1 yr, and may include 8-10 events strong enough to generate sand blows. Dune crossbeds within the Jurassic Navajo Sandstone of Utah (western United States) enclose intact and truncated sand blows, and the intrusive structures that fed them. We mapped the distribution of more than 800 soft-sediment dikes and pipes at two small sites. All water- escape structures intersect a single paleo-surface, and are limited to the upper portion of the underlying set of cross-strata and the lower portion of the overlying set. A small portion of one set of crossbeds that represents ~1 yr of dune migration encloses eight generations of eruptive events. We interpret these superimposed depositional and deformational structures as the record of a single shock-aftershock earthquake sequence. The completeness and temporal detail of this paleoseismic record are unique, and were made possible when sand blows repeatedly erupted onto lee slopes of migrating dunes. Similar records should be sought in modern dunefields with shallow water tables. [ABSTRACT FROM AUTHOR]

Iron-bearing concretions are valuable records of oxidation states of subsurface waters, but the first concretions to form can be altered drastically during later diagenetic events. Distinctive concretions composed of heavy rinds of iron oxide that surround iron-poor, mud-rich cores are common along bases of fluvial cross-bed sets of the Cretaceous Dakota Formation, Nebraska, USA. Concretion rinds thicken inward and cores contain 46 to 89% void space. Millimetre-scale spherosiderites are abundant in palaeosols that developed in floodplain facies. Evolution of rinded concretions began when intraformational clasts were eroded from sideritic soils, transported, abraded and deposited in river channels. Alteration of siderite and formation of rinds occurred much later, perhaps in the Quaternary when sandstone pore waters became oxic. Dakota concretions are analogous to 'rattlestones' in Pleistocene fluvial channels of The Netherlands, and their rinded structure is analogous to that of iron-rich concretions in the aeolian Navajo Sandstone of Utah. In all three deposits, rinded concretions formed when pre-existing, siderite-cemented concretions were oxidized within a sand matrix. Unlike fluvial examples, siderite in the Navajo Sandstone was autochthonous and of late diagenetic origin, having precipitated from carbon dioxide and methane-enriched waters moving through folded and jointed strata. Iron-rich rinds formed in all these strata because concretion interiors remained anaerobic, even as oxygen accumulated in the pore waters of their surrounding, permeable matrix. Iron oxide first precipitated at redox boundaries at concretion perimeters and formed an inward-thickening rind. Acid generated by the oxidation reaction drove siderite dissolution to completion, creating the iron-poor core. Iron-oxide rinds are indicators of the former presence of siderite, a mineral that forms only under reducing conditions, during either early or late diagenesis. Siderite is vulnerable to complete oxidation upon exposure, so the distinctive rinded concretions are valuable clues that aid in deciphering diagenetic histories and for recognizing methanic floodplain palaeoenvironments and wet palaeoclimate. [ABSTRACT FROM AUTHOR]

Abstract: Downslope coarsening in grainflows has been observed on present-day dunes and generated in labs, but few previous studies have examined vertical sorting in ancient aeolian grainflows. We studied the grainflow strata of the Jurassic Navajo Sandstone in the southern Utah portion of its outcrop belt from Zion National Park (west) to Coyote Buttes and The Dive (east). At each study site, thick sets of grainflow-dominated cross-strata that were deposited by large transverse dunes comprise the bulk of the Navajo Sandstone. We studied three stratigraphic columns, one per site, composed almost exclusively of aeolian cross-strata. For each column, samples were obtained from one grainflow stratum in each consecutive set of the column, for a total of 139 samples from thirty-two sets of cross-strata. To investigate grading perpendicular to bedding within individual grainflows, we collected fourteen samples from four superimposed grainflow strata at The Dive. Samples were analyzed with a Malvern Mastersizer 2000 laser diffraction particle analyser. The median grain size of grainflow samples ranges from fine sand (164μm) to coarse sand (617μm). Using Folk and Ward criteria, samples are well-sorted to moderately-well-sorted. All but one of the twenty-eight sets showed at least slight downslope coarsening, but in general, downslope coarsening was not as well-developed or as consistent as that reported in laboratory subaqueous grainflows. Because coarse sand should be quickly sequestered within preserved cross-strata when bedforms climb, grain-size studies may help to test hypotheses for the stacking of sets of cross-strata. [Copyright &y& Elsevier]

Concretions cemented by iron oxide are abundant and diverse in the Jurassic Navajo Sandstone of southern Utah. Some of these structures are considered terrestrial analogs for concretions imaged on Mars. The Navajo concretions can be spheroidal, pipelike, or tabular with multicompartmented boxworks. These iron oxide concretions typically display a rinded structure: dense sandstone rinds cemented by iron oxide surround pale, iron-poor sandstone cores. Within a single structure, the iron-rich rinds may be single or multiple. Pseudomorphs of siderite are present in local residual, iron-rich cores of boxworks. Workers in the late nineteenth through mid-twentieth centuries, many of whom found evidence for siderite precusors, concluded that many spherical, rinded, iron oxide-cemented concretions were formed by centripetal precipitation of iron oxide inward from the perimeter of the concretion; in contrast, the walls of pipelike concretions of iron oxide grew centrifugally outward. We interpret the Navajo spheroids and boxworks as centripetal products of the oxidation of siderite-cemented (precursor) concretions that were very similar in both size and shape to the current concretions: rinds grew (thickened) inward toward the internal source of Fe(II). Siderite pseudomorphs appear to be absent from spheroids and many boxworks because all siderite was dissolved. In the cores of the larger boxworks some siderite was oxidized in situ; the Fe(II) did not migrate away from the original siderite crystals. The oxidation process was mediated by iron-oxidizing microbes and began at concretion perimeters when oxidizing groundwater started to displace the CO2- and methane-bearing water that had precipitated the siderite. In contrast, pipelike concretions are centrifugal-rinds formed around a cylindrical reaction front and thickened outward toward Fe(II) and away from the oxygenated water flowing within the cylinders. The cylindrical shape of the reaction front was produced by self-organizing feedbacks between dissolution of dispersed siderite cement and focused flow through a heterogeneous sandstone matrix. [ABSTRACT FROM AUTHOR]

Concretions with thick, iron oxide-cemented rinds and lightly cemented, iron-poor sandstone cores are abundant within the Navajo Sandstone near the southeast flank of the Escalante anticline (Utah, United States). Previous workers suggested the spheroidal concretions as analogs for Martian "blueberries" (hematite concretions), and linked the origin of concretions and bleaching of the Navajo Sandstone to the buoyant rise of hydrocarbons toward anticlinal crests. We measured azimuths of 163 pipe-like (pipy), joint-associated concretions and those of 58 "comet tails" that extend from some spherical concretions. Both data sets have vector magnitudes >90%, and subparallel, southeast-directed resultant vectors. Our data indicate that groundwater transported and deposited iron over a flow path that spanned more than 100 km from the anticlinal crest to the Colorado River. Buoyant fluids could not have transported iron from bleached rock to sites of concretion growth, however, because iron moved downdip. We propose that iron was transported southeastward by reducing groundwater that flowed through a CO2- and methane-charged reservoir within the Escalante anticline. Iron-rich carbonates precipitated during CO2 degassing. After methane was flushed from the aquifer, microaerophiles oxidized Fe2+ in concretions, dissolving the carbonates and forming the iron oxide-rich rinds and iron-poor cores. These concretions define the flow direction and geochemical evolution of a paleoaquifer. This approach could be used to follow the water on other planets. [ABSTRACT FROM AUTHOR]

Abstract: The Great Plains of the central United States contain numerous presently stabilized dune fields that were active during the late Holocene. The most recent period of reactivation occurred about 700–1000 years ago during the Medieval Climatic Anomaly, a period of frequent, severe drought recognized throughout the North American West. Dune orientations and internal bedding preserve information about paleowinds from this time period. We compared paleowind information with modern winds and potential sand transport analyzed from meteorological data. While both reconstructed paleowinds and modern winds include a strong northwesterly component over the northern to central portions of the Great Plains, a significant difference exists to the south. During the late Holocene, southwesterly winds had a strong influence on dune forms over the central and southern Great Plains, while modern potential sand transport is dominated by southerly to southeasterly winds that bring moisture to the region from the Gulf of Mexico. Hypothetical dune types forming under modern winds would be different and have different crestal orientations than those preserved from the late Holocene. In order for such a shift in southerly winds to take place it is likely that: 1) the typical position of the Bermuda High in summer was shifted either to the east or to the south resulting in weaker low-level winds from the south and southeast and greater extent of southwesterly winds on the Great Plains, and/or 2) drought, likely enhanced by evapotranspiration and soil moisture feedbacks, brought the normally higher-level southwesterly flow down to the land surface. Given our understanding of megadroughts over the Great Plains as well as recent climate predictions for increased warmth and drought over this area, future reactivation of the dunes seems likely. [Copyright &y& Elsevier]

Outcrops of the Early Jurassic Navajo Sandstone in southern Utah and northern Arizona, south-western USA are being actively eroded by sand-laden, south-westerly winds. Small-scale stepped topography with risers facing into the wind develops even on steep canyon walls when wind-swept grains strike the rock at a low angle. Photosynthetic, endolithic microbes directly underlie most outcrop surfaces; the crusts formed by these organisms are essential to formation of the small-scale steps. Wind erosion of highlands also forms troughs and pits that are tens of metres across. The pits have deeply scalloped, overhanging walls, and contain central domes surrounded by ‘moats’ filled with dune sand. Wind erosion of aeolian sandstone is favoured by a positive feedback mechanism in which grains that are liberated from outcrops by impacting particles become a fresh supply of pre-sorted abrasive particles for further attack. [ABSTRACT FROM AUTHOR]

At the Wave, a photogenic landform on the Utah-Arizona border, modern, southwesterly, sand-carrying winds abrade the Navajo Sandstone. Abundant trains of centimeter-scale, transverse, upwind-facing treads and risers cut sedimentary structures at a high angle. Central to the formation of these erosional steps are crusts produced by microbes lying just beneath exposed sandstone surfaces. Treads and risers are present on the walls of smoothly curved troughs at the Wave, on the walls of nearby circular scour pits, and on bedrock domes found at the center of scour pits. Because of their locations and orientations, the large-scale troughs and scour pits could not have been formed by flowing water or groundwater sapping; treads and risers indicate sculpting by the wind. [ABSTRACT FROM AUTHOR]

The Middle Jurassic Entrada Sandstone, exposed near the town of Escalante, southern Utah, consists of large-scale cross-bedded eolian deposits that are interbedded with horizontally laminated sand sheets and thin sets of eolian cross-strata, representing periods with a moister climate. The flat-bedded units contain numerous tracks and trackways from small to large-sized theropod dinosaurs. These tracks are today exposed in several distinct erosional states, allowing detailed studies of track and undertrack formation in eolian deposits. Tracks that originally were emplaced on sloping surfaces show, in their present-day erosional state, a morphology distinct from those originally emplaced on horizontal surfaces. Further, the range of eroded track morphologies can help identify badly eroded tracks from nonbiogenic structures in similar deposits. [ABSTRACT FROM AUTHOR]

Historic, sulphur-rich volcanic eruptions have altered global climate for as much as five years, and much larger events are known from the geologic record. At Scotts Bluff, Nebraska, Early Oligocene strata of the lower Arikaree Group contain a tephra bed with abundant calcite pseudomorphs after gypsum. Previous work has shown sulphate from the pseudomorphs in this tephra bears a high17O anomaly indicative of oxidation of sulphur gases by ozone or hydrogen peroxide in the atmosphere. Possible sources of the tephra were caldera eruptions at about 28 Ma in the San Juan volcanic field of south-western Colorado (∼500 km SW of the study site) and the eastern Great Basin (∼1000 km WSW). The present sedimentological study shows that tephra and volcanogenic sulphate were deposited and preserved within a small, surface-discharging playa that developed on the irregular upper surface of aeolian siltstones of the subjacent White River Group. Sulphate solutions (including perhaps sulphuric acid) percolated downward within the vadose zone, dissolving early formed smectite cement within underlying volcaniclastic sandstones, reddening these rocks along an irregular alteration front. Preserved fine-scale stratification within the sandstones precludes the possibility that reddening took place during pedogenesis. Displacive growth of gypsum at the playa centre folded tephra beds and forced tephra into underlying sandstones, forming elongate cones. The large mass fraction of gypsum (now replaced by calcite) in the playa sediments suggests a huge, long-distance delivery of sulphate aerosols. Some of the sulphate and tephra may have come from the same eruption, or the fine-grained tephra may simply have aided preservation of dry-fog sulphate derived from an unrelated, effusive eruption of lava. [ABSTRACT FROM AUTHOR]

Cross-equatorial, westerly winds are key features of tropical circulation in monsoonal regions. Although prominent in numerical climate models of Pangaea (the supercontinent straddling earth's equator, Late Palaeozoic to Early Mesozoic), such flow has not been confirmed previously by migration directions of ancient dunes. Wind-blown sandstones that span 100 million years of earth history are widely exposed in south-western USA. If recent palaeomagnetic data from the Colorado Plateau are used to correct Mesozoic palaeogeographic maps, the Plateau is placed about 10° further south than previously assumed, and the prevailing north-westerly surface winds recorded by dune-deposited sandstones are explicable as cross-equatorial westerlies – the hallmark of modern monsoon circulation. Permian to Early Jurassic dunes were driven by north-westerlies produced by a steep pressure gradient spanning the supercontinent during December–January–February. Although winds are light in most modern, near-equatorial settings, the East African Jet accounts for more than half the cross-equatorial flow in June–July–August. The thicknesses of annual depositional cycles within the Navajo Sandstone indicate that the near-equatorial, north-westerly winds that drove these particular dunes were stronger than the modern East African Jet. The Early Jurassic dunes that deposited the thick cycles were positioned west of the dominant (southern hemisphere) thermal low and against highlands to the west – a setting very similar to the East African Jet. The mountains along the western coast of Pangaea not only enhanced wind strength, but also cast a rain shadow that allowed active dunes to extend very close to the palaeoequator. [ABSTRACT FROM AUTHOR]

The Navajo Sandstone of the American Southwest was deposited at approximately 190 Ma in a giant, subtropical dune field near the western margin of Pangea. From this unit, we report thick intervals of dune cross-strata that were churned by insects and trampled by reptiles. Although dunes continued to migrate freely, the distribution of trace fossils shows that plant life in wet interdune areas sustained high levels of animal activity on the dunes for many thousands of years. We interpret this suite of structures as the record of a pluvial episode climatologically similar to the period of "greening" in the Sahara 4000-10,000 yr ago. A high percentage of the rainfall on the Navajo erg recharged the water table and led to the development of highly dilute, local groundwater flow systems that discharged into interdune areas. [ABSTRACT FROM AUTHOR]

Examines causative factors influencing sandslides in eolian dunes in the Upper Cretaceous Djadokhta Formation of Southern Mongolia. Contribution of rainfall events on the mass wasting of eolian dunes; Impact of rainwater evaporation on calcite accumulation; Generation of translational slides through the formation of calcitic zones.

Discusses intrinsic versus climatic controls on the accumulation of lake and marsh sediments in the Nebraska Sand Hills. Regional setting and previous work; Buried paleovalleys and dune-dammed lakes in western Sand Hills; Depositional controls at swan lake.

Describes Jurassic aeolian oolite on a palaeohigh in the Sundance area in Bighorn Basin, Wyoming. Abundance of aeolian limestones in the Quaternary record; Interpretation of the limestones as a low latitude signal of glacio-eustatic sea level fluctuations; Subaerial exposure on the crest and north flank of the Sheridan arch as evidenced by dunes.

Examines ridges as wind-sculpted desert landforms that developed on the lithified upper surface of the White Rim Sandstone during an extended period of hyperaridity towards the end of the Permian. Angular blocks of sandstone within the lag and san-filled fissures; Other types of wind-eroded landforms.

Discusses the origin of extensive bedding planes in aeolian sandstones. Interpretation on the bedding planes that separate cross-bed sets and cosets; Origin of the first-order bounding surfaces in aeolian sandstones; Alternation between periods of deflation with periods of dune climbing.

The supercontinent Pangea dominated our planet from the Permian into the Jurassic. Paleomagnetic reconstructions have been used to estimate the latitudinal position of Pangea during this 100-million-year period. Atmospheric circulation, recorded by eolian sandstones in the southwestern United States, shows a broad sweep of northeasterly winds over their northernmost extent, curving to become northwesterly in the south: This evidence is consistent with paleomagnetic reconstructions of the region straddling the equator in the Early Permian but is at odds with its northward movement to about 20°N by the Early Jurassic. At least one of the following scenarios must be true: The latitude based on paleomagnetism is incorrect; the interpretation of how winds shaped the dunes is mistaken; the basic climate controls in the Jurassic were different from those of today; or the paleogeographic reconstructions available are insufficient to adequately reproduce the wind fields responsible for dune formation. [ABSTRACT FROM AUTHOR]

Spring-summer winds from the south move moist air from the Gulf of Mexico to the Great Plains. Rainfall in the growing season sustains prairie grasses that keep large dunes in the Nebraska Sand Hills immobile. Longitudinal dunes built during the Medieval Warm Period (800 to 1000 years before the present) record the last major period of sand mobility. These dunes are oriented NW-SE and are composed of cross-strata with bipolar dip directions. The trend and structure of the dunes record a drought that was initiated and sustained by a historically unprecedented shift of spring-summer atmospheric circulation over the Plains: Moist southerly flow was replaced by dry southwesterly flow. [ABSTRACT FROM AUTHOR]

Presents information on a study that analyzed slump masses in the annual depositional cycles within the deposits of wind-blown sand accumulated in dunes of a vast, low-latitude desert at Pangaea. Two distinct types of mass movement; What determine not only the frequency of slumping events; What the early Jurassic climate model shows.

A response to a letter to the editor is presented by Richard M. Kettler, David B. Loope and Karrie A. Weber, authors of an article on the geology in the Jurassic period of Navajo sandstone located in Utah.

The article presents an author's reply to the discussion regarding the burrow dug by large vertebrates in Utah which was declared by George P. Oldier as rain-moistened middle Jurassic sand dunes. He stated that Odier's article in 2007 is concerned with his mistakes in interpreting the structures maintained in the Entrada Sandstone as burrows and the uncited work of Odier in its 2006 article on the burrows in the Navajo Sandstone. He said that Odier's article in 2007 interpreted the burrows as wells formed by heavy rain falling on dune sand. He reasoned out that rain-wetted blocks of cohesive sand occasionally move down steep lee faces of dunes.

A dinosaur trackway in the Middle Jurassic eolian Entrada Sandstone of southern Utah, U.S.A., exposes three undertracks that we have modeled as isolated tectonic regimes showing the development of fold-thrust ramp systems induced by the dinosaur's feet. The faulted and folded sequence is comparable to crustal-scale tectonics associated with plate tectonics and foreland fold-thrust belts. A structural analysis of the dinosaur tracks shows the timing and direction of the forces exercised on the substrate by the animal's foot during the stride. Based on the structural analysis, we establish a scenario for foot movements and weight distribution in the feet. During the end of the weightbearing phase of the stride, the weight of the animal was transferred to the front of the digits, creating a rotated disk below the foot that was bounded by an extensional fault at the front and a thrust ramp toward the back. As the body accelerated, the foot was forced backward. The rotated disk was forced backward along a detachment fault that was bounded by lateral ramps. The interramp segment matches the width of the dinosaur's foot, which created an imbricate fan thrust system that extended to the far end of the undertrack. The total length of the tectonic disturbance created by the dinosaur is up to three times that of the original footprint. Early, near-surface cementation gave the substrate the rheological properties necessary for development of the observed structures. [ABSTRACT FROM AUTHOR]

Optical ages for eolian sands from the Nebraska Sand Hills indicate periods of extensive eolian activity at ca 115±25, 840±70, 2300±240, and 3560±340 a. Activity was also noted at single sampling locations at ca 6180±370, 8430±510 and 13110±800 a. Many of these ages are similar to those noted by earlier authors. Optical ages from samples collected within paleosols indicate shorter and possibly less extensive periods of eolian activity at approximately 1220±150, 1590±110, and possibly 1950±150 a, during which the paleosol sands accumulated. What was originally interpreted as a single 1.2 m thick paleosol is shown by optical dating to consist of three or more welded soils developed within eolian sands with optical ages of ca 3800±240, 2740±240, 1560±110, and possibly 1930±140 a, each of which match eolian pulses recognized elsewhere. Scatter in some optical ages is attributable to intersection of sand-filled rodent burrows extending in outcrop 1.5 m below the contact between paleosol and overlying topset beds. A 5310±360 a optical age for one probable intersected burrow provides evidence for upward or lateral transport of older sands. [Copyright &y& Elsevier]

The discovery of spherical accumulations of iron oxide mineralization on Mars has prompted renewed interest in widespread but enigmatic accumulations of iron oxide on Earth. The rinded iron oxide concretions found in the Navajo Sandstone (Utah, USA) have been among the most extensively studied of these occurrences. At least four different models have been proposed for the formation of iron oxide concretions in the Navajo Sandstone. These include 1) mixing of reduced Fe(II)-bearing and oxygenated groundwaters, 2) neutralization of oxidized, acidic waters by calcite concretions, 3) neutralization of reduced, acidic waters by calcite concretions, and 4) oxidation of siderite by microbes during invasion of the Navajo aquifer by oxygenated groundwaters. We measured δ56Fe values of iron-bearing minerals from whole-rock Navajo Sandstone, concretionary iron oxide cements, iron oxide stains that are oriented in the direction of paleo-groundwater flow, iron oxide stains on the interior of rinded concretions, and ferroan carbonate cements. We have then combined these data with published measurements to evaluate each of the four published models. Whole rock Navajo Sandstone from the Kolob Plateau has been interpreted to be unaltered by bleaching fluids and has a δ56Fe value of 0.21‰. Iron in the Kolob Plateau sample comprises a mixture of hematite grain coatings (δ56Fe = 0.12‰) and a magnetic mineral fraction (interpreted to comprise largely detrital magnetite) that has a higher δ56Fe value (0.45‰). Iron oxide cements have δ56Fe values that are typically negative and as low as −0.74‰. In contrast, iron oxide-stained Navajo Sandstone in close proximity to a rinded iron oxide concretion has δ56Fe values that are 0.9 to 1‰ greater than the δ56Fe value of the associated iron oxide concretion. Finally, ferroan carbonates from the Navajo Sandstone yield δ56Fe values as low as −0.86‰. These values are in good agreement with measurements published previously for lithologies for which comparable measurements have been made. The presence of ferroan carbonates with negative δ56Fe values is evidence that aqueous Fe(II) was transported through the Navajo Sandstone aquifer and produced carbonate mineralization with negative δ56Fe values. The difference in δ56Fe values between the iron oxide concretions and associated iron oxide staining is more consistent with equilibrium fractionation between aqueous Fe(II) and Fe(II) adsorbed on quartz grains than it is with equilibrium fractionation between aqueous Fe(II) and any reduced or oxidized iron oxide mineral species. The negative δ56Fe values of the iron oxide concretions are best explained by oxidation of a reduced iron phase in a closed system from which no aqueous Fe(II) could escape. We interpret the iron isotope data as evidence that iron was stripped from Navajo sand grains by reducing fluids, reprecipitated in the Navajo aquifer as siderite, and oxidized by groundwaters under closed conditions during incision of the Colorado Plateau. • Iron oxide concretions with negative δ56Fe values occur in Navajo Sandstone • Ferroan carbonate in the Navajo Sandstone also has low δ56Fe values • The Navajo Sandstone aquifer contained reduced iron and carbonate-bearing waters • Low δ56Fe values of Navajo concretions unlikely if primary precipitate was iron oxide • Low δ56Fe values are evidence that iron oxide concretions formed from siderite [ABSTRACT FROM AUTHOR]

The Nebraska Sand Hills is a stabilized dune field on the Great Plains of North America. Although it is well known that this dune field, like several others on the Great Plains, last experienced widespread activity during the Medieval Climatic Anomaly (MCA, ~AD 900–1300), spatial variation in the timing and nature of drought development is poorly constrained. To elucidate spatial trends in dune reactivation, samples potentially representing MCA activity across the Sand Hills were collected and dated using optically stimulated luminescence (OSL). Ages from the older part of the MCA were obtained from eolian sediments in the northwestern Sand Hills, while ages from later in the episode were obtained to the southeast, suggesting a geographic trend in the timing of revegetation of the dunes near the end of the drought. Revegetation likely occurred to the northwest initially as a result of renewed moisture availability from a rising water table in the interdunes, which serve as refugia for vegetation during times of drought. Vegetation then gradually spread to the southeastern Sand Hills. An additional spatial trend in ages is apparent in the chronology of linear dune mobilization across the Sand Hills. Linear dunes in the northwest are superimposed on megadunes and originated during the last reactivation, while linear dunes in the southeast are built around older cores of dunes and formed during several reactivations. Our geochronology reveals three episodes of eolian transport, including the MCA, in the formation of linear dunes in the southeast. [ABSTRACT FROM PUBLISHER]

Concretions, preferentially cemented masses within sediments and sedimentary rocks, are records of sediment diagenesis and tracers of pore water chemistry. For over a century, rinded spheroidal structures that exhibit an Fe(III) oxide--rich exterior and Fe-poor core have been described as oxidation products of Fe(II) carbonate concretions. However, mechanisms governing Fe(III) oxide precipitation within these structures remain an enigma. Here we present chemical and morphological evidence of microbial biosignatures in association with Fe(III) oxides in the Fe(III) oxide--rich rind of spheroidal concretions collected from the Jurassic Navajo Sandstone (southwest United States), implicating a microbial role in Fe biomineralization. The amount of total organic carbon in the exterior Fe(III) oxides exceeded measured values in the friable interior. The mean δ13 C value of organic carbon from the Fe(III) oxide--cemented exterior, δ13C of -20.55‰, is consistent with a biogenic signature from autotrophic bacteria. Scanning electron micrographs reveal microstructures consistent with bacterial size and morphology, including a twisted-stalk morphotype that resembled an Fe(II)-oxidizing microorganism, Gallionella sp. Nanoscale associations of Fe, O, C, and N with bacterial morphotypes demonstrate microorganisms associated with Fe(III) oxides. Together these results indicate that autotrophic microorganisms were present during Fe(III) oxide precipitation and present microbial catalysis as a mechanism of Fe(III) oxide concretion formation. Microbial biosignatures in rinded Fe(III) oxide--rich concretions within an exhumed, Quaternary aquifer has broad implications for detection of life within the geological record on Earth as well as other Fe-rich rocky planets such as Mars, where both Fe(II) carbonate and Fe(III) oxide--rich concretions have been identified. [ABSTRACT FROM AUTHOR]

Dune fields and loess deposits of the Great Plains of North America contain stratigraphic records of eolian activity that can be used to extend the short observational record of drought. We present a 10,000 yr reconstruction of dune activity and dust production in the central Great Plains region, based on 95 optically stimulated luminescence ages. The integration of data from both eolian sand and loess is an important new aspect of this record. Clusters of ages define episodes of extensive eolian activity, which we interpret as a response to frequent severe drought, at 1.0-0.7 ka and 2.3-4.5 ka (with peaks centered on 2.5 and 3.8 ka); sustained eolian activity occurred from 9.6 to 6.5 ka. Parts of this record may be consistent with hypotheses linking Hobocene drought to sea surface temperature anomalies in the Pacific or Atlantic oceans, or to the El Niño-Southern Oscillation phenomenon, but the record as a whole is difficult to reconcile with any of these hypotheses. [ABSTRACT FROM AUTHOR]

Dunefields of the Great Plains contain stratigraphic records of episodic Holocene aeolian activity, potentially providing a valuable record of climatic change. It has been difficult to establish unambiguous links between activity at specific sites and regional palaeoclimate, however. Here we demonstrate that widespread late-Holocene aeolian activity in the Nebraska Sand Hills, the largest dunefield on the Great Plains of North America, occurred during a period of hydrological drought. Aeolian sand sheets are interbedded with peat that accumulated in interdunes throughout much of the Holocene. The youngest of these sand sheets was deposited between 950 and 650 cal. BP, about the same time as the most recent major episode of aeolian activity at upland sites up to 120 km apart. Sand sheets could only have advanced across the interdunes if the local groundwater flow systems that maintain interdune wetlands were temporarily reduced or eliminated by a reduction in recharge. Both reduced recharge and upland aeolian activity are best explained by regional megadrought. [ABSTRACT FROM AUTHOR]

Presents a study of the oxygen-isotope values of two massive sulphate mineral deposits which show large isotopic anomalies despite being formed in surface environments. Origination of deposits in the central Namib desert and the sulphate-bearing Miocene volcanic ash-beds in North America; Questions regarding the source of the oxygen isotope; Possibility of the chemical origin of variable isotope anomalies on planets other than Earth.