Navajo Sandstone Research Articles

Morphologic Clues to the Origins of Iron Oxide–Cemented Spheroids, Boxworks, and Pipelike Concretions, Navajo Sandstone of South-Central Utah, U.S.A.

AbstractConcretions 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...

Conditions and implications for compaction band formation in the Navajo Sandstone, Utah

Observations from quartz-rich eolian Navajo Sandstone in the Buckskin Gulch site in southernmost Utah show that pure compaction bands only occur in sandstones where current porosity > 0.29 ± 3, permeability > 10 ± 7 darcy, and grain size > 0.4 mm – properties restricted to the lower and most coarse-grained and well-sorted parts of grain flow units within the dune units. Hence a direct correlation between stratigraphy and band occurrence has been established that can be used to predict deformation band occurrences in similar sandstone reservoirs. We show that the pure compaction bands formed perpendicular to a subhorizontal σ 1, bisecting conjugate sets of shear-enhanced compaction bands. The latter bands locally developed into shear-dominated bands that transect entire dune units, suggesting that an increase in the amount of simple shear promotes band propagation into less porous and permeable lithologies. Stress considerations indicate that, as a continuous and overlapping sequence of events, pure compaction bands in quartz-rich Navajo Sandstone initiated at 10–20 MPa (∼1 km depth), followed by shear-enhanced compaction bands that locally developed into more stratigraphically extensive shear-dominated bands. The rare combination of special lithologic and stress conditions may explain why pure compaction bands are rarely observed in naturally deformed sandstones.

Joint controlled fluid flow patterns and iron mass transfer in Jurassic Navajo Sandstone, Southern Utah, USA

The Jurassic Navajo Sandstone of Grand Staircase Escalante National Monument (GSENM), Utah, exhibits a broad range of iron oxide/oxyhydroxide concretionary geometries that record a complex paragenetic history of the reservoir. The concretionary geometries are as follows: (i) common macroconcretions (>5 mm in diameter), (ii) ubiquitous microconcretions (<5 mm in diameter), (iii) localized iron oxide/oxyhydroxide-lined northeast-striking joints, and (iv) loopy asymmetrical mineralization and banded precipitation patterns (Liesegang bands) associated with the joints. Spheroidal concretion geometries typically indicate diffusive mass transfer, whereas asymmetrical mineralization represents advective directional flow. Liesegang bands parallel to asymmetrical mineralization denote a diffusive mass transfer component perpendicular to the principal flow direction. Careful study of concretionary geometries and their cross-cutting relationships establishes a relative timing of precipitation and mobilization events. Macroconcretions formed post-Laramide (<55 Ma), prior to and independent of the Miocene joints. The joints provided conduits for later oxidizing fluids that precipitated iron oxide/oxyhydroxide lining on the joint faces. Advective mass transfer overprinted the area with preferentially cemented flow lines (asymmetrical mineralization) less than 10 Ma, coincident with development of a hydraulic low to the southeast of the region (Colorado River downcutting). Certain trace elements are genetically tied to concretion formation, and evaluation of trace elements establishes precursor mineral phases. Enriched uranium concentrations in concretions (relative to the host rock) and low/undetectable sulfur concentrations in both host rock and concretions suggest that iron oxyhydroxide was a primary precipitate rather than a reduced iron mineral (i.e., pyrite or siderite) that later oxidized. Enrichment of Ni and As in concretions functions as fingerprints for diagenetic concretion formation when determining genesis of ambiguous iron oxide/oxyhydroxide spherules such as the remotely sensed ‘blueberries’ on Mars. Similarities between Mars and Utah spherules such as geometry, in situ spacing and volumetric density suggest that Mars spherules precipitated via a geochemically self-organized nucleation pattern in diffusive chemical reaction fronts.

Synapsid Burrows and Associated Trace Fossils in the Lower Jurassic Navajo Sandstone, Southeastern Utah, U.S.A., Indicates a Diverse Community Living in a Wet Desert Ecosystem

Abstract Enigmatic structures in the Lower Jurassic Navajo Sandstone near Moab, Utah, occur at three localities below carbonate beds that record interdune lake deposits. These structures are interpreted as mammal and therapsid burrows based on their architectural and surficial burrow morphologies: Type I (large diameter) and Type II (mega diameter) burrows. Type I burrows include sinuous tunnels, Y- and T-branched tunnels, sinuous ramps, and chambers, and weather into mounds averaging 33 m × 22 m and extend ~ 1m above the surface. Type I burrows are dorsoventrally flattened, in cross section averaging 9.3 cm wide and 4.2 cm high, and are sand filled and structureless. Type I burrows mostly have smooth walls, though some have scalloped walls. Type II burrows are simple, inclined tunnels ~ 35 cm wide and ~ 20 cm high, and exhibit well-preserved bilobate morphology along the underside of the tunnel. The walls preserve a series of 3 or 4 thin (~ 4–8 mm), inclined scratch marks from the upper part of the wall and along the floor. The great complexity and high density of Type I burrow systems is best explained by multiple individuals living together in social groups. These burrows are more complex than Early Triassic therapsid burrows from South Africa and are most similar to burrows of extant social (e.g., voles) and eusocial mammals (e.g., naked mole rats). Type II burrows were likely constructed by therapsids, based on burrow size and comparison to Permian and Triassic therapsid burrows from South Africa and Antarctica.

Relationship of compaction bands in Utah to Laramide fault-related folding

Multiple sets of compaction bands of southwestern Utah occur in Navajo Sandstone near a bend in the adjacent East Kaibab monocline. Forward mechanical models of blind-thrust fault deformation are used in this paper to investigate the spatial relationship between compaction bands and fault-related folding. Results are consistent with the East Kaibab monocline being underlain by a west-dipping blind high-angle reverse fault between ~ 1.5 and 5 km depth below the top of the Navajo Sandstone. Reverse slip along this reactivated fault during the Laramide orogeny is inferred to have produced values of compactional normal strain that were largest in the footwall near the concave map-view bend in the monocline. The calculated compactional normal strains and Coulomb stress increases are consistent with the field occurrence of pure and shear-enhanced compaction bands in this area. Bends, relays, stepovers, or other geometric complexities above blind reverse faults are thus inferred to be potential sites for compaction band localization in appropriate rock types.

Precipitation patterns formed by self-organizing processes in porous media

ABSTRACT We compare laboratory and field examples of self-organized mineral precipitates in porous media. Laboratorytests of silver chromate precipitation in glass beads and glass bead⁄gel mixtures produce structures such as peri-odic banding and mm-size spherules. These are morphologically similar to the varied forms of iron oxide precipi-tates in the Jurassic Navajo Sandstone, Utah USA, that preserve records of former fluid redox boundaries in aporous and permeable sandstone. Experimental studies of periodic precipitates in porous media can providevaluable insight for understanding the diagenetic history of similar precipitates in natural environments.Key words: concretions, Liesegang bands, self-organizationReceived 23 April 2010; accepted 10 December 2010Corresponding author: L. M. Barge, Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109, USA.Email: Laura.M.Barge@jpl.nasa.gov. Tel: +18183938209. Fax: +18183934445.Geofluids (2011) 11, 124–133 INTRODUCTION In this study, we examine precipitates that form viaself-organizing processes in porous and permeable media.Self-organizing patterns in diffusion-controlled chemicalsystems have been studied for over a century, since the dis-covery by R. E. Liesegang in 1896 that the interactionbetween inter-diffusing silver and chromate ions in a gela-tin gel can produce periodically spaced bands of precipitate(Liesegang 1896). These patterns, termed ‘Liesegangbands’, have since been produced using various combina-tions of reactants (Henisch 2005, and references therein)and in many different types of gels, and the pattern mor-phology can vary widely depending on initial experimentalconditions. Liesegang bands are typically described as ‘peri-odic patterns’, and we will retain this description, althoughthe spacing of bands may not be strictly periodic in thespatial sense.The formation of Liesegang patterns has been attributedto various mechanisms including supersaturation–nucle-ation–depletion processes (in which the first band depletesthe immediate area of the least abundant reactant, resultingin the second band being deposited some distance fromthe first (Ostwald 1897; Smith 1984) and competitiveparticle growth (in which larger particles are less solubleand grow at the expense of smaller ones – a phenomenonknown as Ostwald ripening (Mu¨ller & Ross 2003; Sultan& Ortoleva 1993; Ortoleva 1982; Lifshitz & Slyozov1961). Liesegang band patterns tend to obey several exper-imental laws, for example, that the distance between con-secutive bands increases approximately according to ageometric series (Jablczynski 1923). Liesegang band spac-ing is a function of many factors including reactant con-centrations and thresholds for particle formation, andprecipitation patterns can be partially controlled by alteringsuch factors. Periodic precipitation in systems where homo-geneity of the medium or coagulation threshold is variedin the direction of diffusion, rather than kept as a constantvalue, can give rise to regularly spaced bands or ‘revert’patterns (where band spacing decreases with distance) (An-tal et al. 2007; Molna´r et al. 2008; Jahnke & Kantelhardt2010). Local concentrations of ions can be controlled by,for example, an applied time-dependent electric field acrossthe gel, resulting in band formation in predetermined loca-tions (Bena et al. 2008). The physical and chemical prop-erties of the diffusion medium can also have effects on thepattern formation as well. Gradients of factors such as tem-perature or pH that affect precipitation can give rise to

Characterization of Navajo Sandstone concretions: Mars comparison and criteria for distinguishing diagenetic origins

The eolian Jurassic Navajo Sandstone spheroidal hydrous ferric oxide (HFO) concretions are divided into two size classes: macro-concretions of > 5 mm diameter and micro-concretions of < 5 mm diameter. Three internal structural end-members of macro-concretions are described as rind, layered, and solid. Two end-members of micro-concretions are rind and solid. Chemical and mineralogical gradients (μm- to mm-scale) are identified with QEMSCAN (Quantitative Elemental Mineralogy using a SCANning electron microscope) and visible to near infrared (VNIR) reflectance spectroscopy. Three HFO phases are identified using VNIR reflectance spectroscopy. i. An amorphous HFO phase is typically located in the rinds. ii. Goethite is present along interior edges of rinds and throughout the interiors of layered and solid concretions. iii. Hematite is present in the centers of rind concretions. A synthesis of petrographic, mineralogical and chemical analyses suggests that concretions grow pervasively (as opposed to radially expanding). Our model proposes that concretions precipitate initially as an amorphous HFO that sets the radius and retains some original porosity. Subsequent precipitation fills remaining pore space with younger mineral phases. Inward digitate cement crystal growth corroborates concretion growth from a set radius toward the centers. Internal structure is modified during late stage precipitation that diffuses reactants through semi-permeable rinds and overprints the interiors with younger cements. Physical characterization of textures and minerals provides diagnostic criteria for understanding how similar concretions (“blueberries”) form in Meridiani Planum, Mars. The analogous Navajo Sandstone concretions show similar characteristics of in situ self-organized spacing, spheroidal geometries, internal structures, conjoined forms, and precursor HFO phases that dehydrate to goethite or hematite. These characteristics indicate a common origin via groundwater diagenesis.

Fault formation in porous sedimentary rocks at high strain rates: First results from the Upheaval Dome impact structure, Utah, USA

Previous theoretical work has suggested that porous sedimentary rocks subjected to sufficiently high strain rates may not form deformation band damage zones (DBDZs). This hypothesis is evaluated by an examination of faults within the porous Navajo Sandstone at the Upheaval Dome impact structure in Canyonlands National Park, southeastern Utah, where high strain-rate conditions are known to have occurred. We found no evidence for DBDZ formation along the accessible fault planes at Upheaval Dome. Instead, a layer of pulverized quartz grains within the Navajo Sandstone occurs adjacent to the fault planes. Similar material has been observed in association with dynamic rupture in crystalline and sedimentary rocks along the San Andreas fault in southern California and in metamorphic rocks along the Bosman fault in South Africa. Measured grain sizes obtained from the pulverized material collected at Upheaval Dome are consistent with strain rates of ∼1–3 × 10 3 s −1 . Pulverized sedimentary rocks along the San Andreas fault imply strain rates of ∼10 −2 to 10 1 s −1 . Strain rates along the faults at Upheaval Dome are well above the average values associated with intraplate tectonics but are consistent with, or faster than, seismic slip rates along faults such as the San Andreas fault. Our results support the hypothesized rate dependence of deformation in porous rocks.

Composition, nucleation, and growth of iron oxide concretions

Iron oxide concretions are formed from post depositional, paleogroundwater chemical interaction with iron minerals in porous sedimentary rocks. The concretions record a history of iron mobilization and precipitation caused by changes in pH, oxidation conditions, and activity of bacteria. Transport limited growth rates may be used to estimate the duration of fluid flow events. The Jurassic Navajo Sandstone, an important hydrocarbon reservoir and aquifer on the Colorado Plateau, USA, is an ideal stratum to study concretions because it is widely distributed, well exposed and is the host for a variety of iron oxide concretions. Many of the concretions are nearly spherical and some consist of a rind of goethite that nearly completely fills the sandstone porosity and surrounds a central sandstone core. The interior and exterior host-rock sandstones are similar in detrital minerals, but kaolinite and interstratified illite–smectite are less abundant in the interior. Lepidocrocite is present as sand-grain rims in the exterior sandstone, but not present in the interior of the concretions. Widespread sandstone bleaching resulted from dissolution of early diagenetic hematite grain coatings by chemically reducing water that gained access to the sandstone through fault conduits. The iron was transported in solution and precipitated as iron oxide concretions by oxidation and increasing pH. Iron diffusion and advection growth time models place limits on minimum duration of the diagenetic, fluid flow events that formed the concretions. Concretion rinds 2 mm thick and 25 mm in radius would take place in 2000 years from transport by diffusion and advection and in 3600 years if transport was by diffusion only. Solid concretions 10 mm in radius would grow in 3800 years by diffusion or 2800 years with diffusion and advection. Goethite (α-FeO (OH)) and lepidocrocite (γ-FeO (OH)) nucleated on K-feldspar grains, on illite coatings on sand grains, and on pore-filling illite, but not on clean quartz grains. Model results show that regions of detrital K-feldspar in the sandstone that consume H + more rapidly than diffusion to the reaction site determine concretion size, and spacing is related to diffusion and advection rates of supply of reactants Fe 2+, O 2, and H +.

Porosity and grain size controls on compaction band formation in Jurassic Navajo Sandstone

Determining the rock properties that permit or impede the growth of compaction bands in sedimentary sequences is a critical problem of importance to studies of strain localization and characterization of subsurface geologic reservoirs. We determine the porosity and average grain size of a sequence of stratigraphic layers of Navajo Sandstone that are then used in a critical state model to infer plastic yield envelopes for the layers. Pure compaction bands are formed in layers having the largest average grain sizes (0.42–0.45 mm) and porosities (28%), and correspondingly the smallest values of critical pressure (∼22 MPa) in the sequence. The results suggest that compaction bands formed in these layers after burial to ∼1.5 km depth in association with thrust faulting beneath the nearby East Kaibab monocline, and that hardening of the yield caps accompanied compactional deformation of the layers.

Follow the water: Connecting a CO2reservoir and bleached sandstone to iron-rich concretions in the Navajo Sandstone of south-central Utah, USA

Concretions with thick, iron oxide–cemented rinds and lightly cemented, iron-poor sandstone cores are abundant within the Navajo Sandstone near the southeast fl ank 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 fl ow path that spanned more than 100 km from the anticlinal crest to the Colorado River. Buoyant fl uids 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 fl owed through a CO 2 - and methane-charged reservoir within the Escalante anticline. Iron-rich carbonates precipitated during CO 2 degassing. After methane was fl ushed from the aquifer, microaerophiles oxidized Fe 2+ in concretions, dissolving the carbonates and forming the iron oxide– rich rinds and iron-poor cores. These concretions defi ne the fl ow direction and geochemical evolution of a paleoaquifer. This approach could be used to follow the water on other planets.

Fault fluid composition from fluid inclusion measurements, Laramide age Uinta thrust fault, Utah

The Laramide age (Late Cretaceous to Early Oligocene) Uinta thrust fault exposed on the northeast flank of the Uinta Mountains provides a fluid conduit for hydrocarbons that enter the Jurassic–Triassic age Nugget Sandstone, chemically reduce the red ferric oxide coloring minerals and bleach the sandstone. Mass spectrometric analyses of gases released from fluid inclusions during crushing of fault-associated calcite veins show fault fluids trapped in inclusions contain substantial CH 4 in addition to CO 2, N 2, Ar, H 2S, and C 2–C 6 hydrocarbons. The abundance of CO 2, CH 4, and other hydrocarbons is used to establish fault fluid redox conditions. Fluid inclusion temperature measurements together with gas analysis show fluids were trapped in inclusions over a range of temperatures and pressures. Temperature and salinity of the Uinta fault fluids determined by fluid inclusion homogenization temperatures corrected for hydrostatic pressure (350–850 bar) and ice melting are 95 to 217 °C and 0 to 11.8 eq. wt.% NaCl (average 3.8%). Vitrinite reflectance temperature estimates are 87 to 152 °C and temperature estimated from overburden thickness and thermal gradient are 142 to 202 °C. Log a H2(aq) ranges from − 5.3 to−5.7 estimated from CO 2/CH 4 equilibria at 160 °C. N 2/Ar ratios in excess of air-saturated water are consistent with derivation of N 2 from thermal decomposition of organic matter, and carbon and oxygen isotopic compositions of vein calcite suggest involvement of formation water. The Nugget Sandstone exposed in the fault footwall on the flanks of the Uinta Mountain uplift has been bleached from the moderate reddish brown color produced by hematite that coated the sand grains during early diagenesis to cream and nearly white colors, similar to bleached outcrops that have been observed in the correlative Navajo Sandstone on the Colorado Plateau. Geochemical modeling suggests that ample CH 4 and other hydrocarbons are available to reduce ferric oxide, bleach the sandstone, and precipitate disseminated pyrite and siderite.

Characterization of deformation bands associated with normal and reverse stress states in the Navajo Sandstone, Utah

Deformation-band networks at Buckskin Gulch, Utah, and the Big Hole fault, Utah, both formed in the Navajo Sandstone with similar initial porosity and permeability, at similar burial depths, and result in similar reductions in effective permeability. However, the band networks at Buckskin Gulch, which formed in a contractional tectonic setting, appear to be much more areally extensive and are not associated with any discrete faults having displacements greater than at most a few meters and more likely only a few tens of centimeters. In contrast, the bands at Big Hole fault are generally limited to the damage zone of a about 25-m (82-ft) displacement normal fault formed in a locally extensional environment. These results suggest that deformation bands in well core from extensional settings may be indicative of discrete damage zones associated with normal faults, whereas deformation bands in well core from contractional settings may be indicative of much more areally extensive deformation-band networks. The band networks in both cases will affect similar reductions in reservoir effective permeability, but only in the latter case will the affected area be sufficiently large to affect well performance.

Trench infiltration for managed aquifer recharge to permeable bedrock

AbstractManaged aquifer recharge to permeable bedrock is increasingly being utilized to enhance resources and maintain sustainable groundwater development practices. One such target is the Navajo Sandstone, an extensive regional aquifer located throughout the Colorado Plateau of the western United States. Spreading‐basin and bank‐filtration projects along the sandstone outcrop's western edge in southwestern Utah have recently been implemented to meet growth‐related water demands. This paper reports on a new cost‐effective surface‐infiltration technique utilizing trenches for enhancing managed aquifer recharge to permeable bedrock. A 48‐day infiltration trench experiment on outcropping Navajo Sandstone was conducted to evaluate this alternative surface‐spreading artificial recharge method. Final infiltration rates through the bottom of the trench were about 0·5 m/day. These infiltration rates were an order of magnitude higher than rates from a previous surface‐spreading experiment at the same site. The higher rates were likely caused by a combination of factors including the removal of lower permeability soil and surficial caliche deposits, access to open vertical sandstone fractures, a reduction in physical clogging associated with silt and biofilm layers, minimizing viscosity effects by maintaining isothermal conditions, minimizing chemical clogging caused by carbonate mineral precipitation associated with algal photosynthesis, and diminished gas clogging associated with trapped air and biogenic gases. This pilot study illustrates the viability of trench infiltration for enhancing surface spreading of managed aquifer recharge to permeable bedrock. Published in 2010 by John Wiley &amp; Sons, Ltd.

Diverse products of near‐surface sediment mobilization in an ancient eolianite: outcrop features of the early Jurassic Navajo Sandstone

ABSTRACTThe Navajo Sandstone of the Colorado Plateau, USA, displays a wide range of soft‐sediment deformation (SSD) features, including decametre‐scale features that have not been found in any modern desert environment. Laboratory simulations and partial analogues from other depositional environments suggest that these features derived from episodic liquefaction and fluidization of unconsolidated dune deposits. Outcrop details at many locations preserve the effects of fluid‐escape dynamics through porous, permeable, well‐sorted sand, which was partitioned by subtle textural changes at depositional boundaries between successive dune deposits and, less commonly, by distinct lithofacies changes marking the interface between wet and dry depositional environments. Extreme deformation and turbulent sediment flow have effaced primary structures in some zones of deformation; but other sites preserve the ductile modification of primary structures. Some outcrops preserve evidence of dramatic alterations in topography and sedimentation patterns due to localized compaction and large, subsurface, sediment displacements. Particularly notable among these extraordinary features are those representing the foundering of active dunes, sediment eruptions, and the subsidence of interdune surfaces in the ancient erg. The distinctive patterns of deformation in the Navajo Sandstone, interpreted in the context of its primary sedimentary architecture, provide unique insights into environmental conditions in the region during the Early Jurassic. Widespread ductile deformation in the Navajo indicates extraordinarily wet conditions for an active dune environment. The truncation of successive deformation features by deflation surfaces establishes the episodic nature of deformation in the unit and also suggests climatic variability. The widespread, episodic occurrence of SSD features, viewed within the seismic context provided by palaeotectonic reconstructions, strongly suggests that earthquake triggering of localized liquefaction controlled the distribution of deformation in this unit.

Imaging spectroscopy of jarosite cement in the Jurassic Navajo Sandstone

Imaging spectroscopy is a powerful tool for mapping surface mineralogy. Interactions of energy and surface materials on atomic and molecular levels result in specific absorption features that are diagnostic of mineralogy. Hyperspectral airborne sensors such as HyMap have sufficient spectral resolution to identify subtle features over narrow wavelength ranges. An anomalous zone of jarosite cement within the Jurassic Navajo Sandstone in southern Utah was identified with airborne hyperspectral data. To date no other locations of sulfate-rich cements such as this have been documented in the Navajo Sandstone. Here, we use multiple spectral analysis techniques over a micro to macro spatial continuum in order to focus our evaluation on the distribution and relative abundance of jarosite in this area. In this study, imaging spectroscopy is used to help determine the extent of the distribution of jarosite [KFe3(SO4)2(OH)6], at “Mollie's Nipple” (MN), a significant geomorphic landmark located within the Grand Staircase-Escalante National Monument, in the Jurassic Navajo Sandstone of south central Utah. The extent of the jarosite across the butte is identified in this study by mapping two diagnostic absorption features, referred to as the ∼2.26μm and near-infrared ferric iron absorption features (∼0.70–1.15μm), independently. This analysis shows that there is a single circular zone of abundant jarosite ∼1km in diameter that includes both in-situ and weathered out accumulations. Jarosite formation requires acidic and sulfate-rich fluids, which are unusual for the Navajo Sandstone. Imaging and field spectroscopy provides the spectral resolution needed to map and analyze the mineralogic characteristics of this area; characteristics that may help constrain the conditions under which this atypical butte formed.

Contrasting bedrock incision rates from snowmelt and flash floods in the Henry Mountains, Utah

Hydrograph variability and channel morphology influence rates of fluvial bedrock incision, but little data exist on these controls in natural channels. Through field monitoring we demonstrate that (1) short-term bedrock channel incision can be rapid, (2) sustained floods with smaller peak discharges can be more erosive than flash floods with higher peak discharges, due to changes in bed alluviation, and (3) bedrock channel morphology varies with local bed slope and controls the spatial distribution of erosion. We present a three-year record of flow depths and bedrock erosion for a human-perturbed channel reach that drains the Henry Mountains of Utah, USA. Starting from a small and steep (∼30% slope), engineered knickpoint in Navajo sandstone, erosion has cut a narrow, deep, and tortuous inner channel in ∼35–40 years. Along the inner channel, we measured up to 1/2 m of vertical incision into Navajo sandstone over ∼23 days, caused by the 2005 season of exceptional snowmelt flow. In contrast, flash floods caused little bedrock incision even when peak discharges were much higher than the peak snowmelt flow. Flash floods were net depositors of coarse sediment while snowmelt flow cleared alluvial cover. We document the formation of a pothole and interpret that it was abraded by bedload rather than fine suspended sediment. Finally, several slot canyons (Peek-a-boo, Spooky, and Coyote Gulch narrows) in the nearby Escalante River drainage basin have erosional morphologies similar to the monitored channel reach. Feedbacks between flow, sediment transport, and transient erosion provide a plausible explanation for the evolution of channel slope, width, and bed roughness of these natural bedrock channels.

A new Basal sauropodomorph dinosaur from the Lower Jurassic Navajo sandstone of Southern Utah.

BackgroundBasal sauropodomorphs, or ‘prosauropods,’ are a globally widespread paraphyletic assemblage of terrestrial herbivorous dinosaurs from the Late Triassic and Early Jurassic. In contrast to several other landmasses, the North American record of sauropodomorphs during this time interval remains sparse, limited to Early Jurassic occurrences of a single well-known taxon from eastern North America and several fragmentary specimens from western North America.Methodology/Principal FindingsOn the basis of a partial skeleton, we describe here a new basal sauropodomorph dinosaur from the Lower Jurassic Navajo Sandstone of southern Utah, Seitaad ruessi gen. et sp. nov. The partially articulated skeleton of Seitaad was likely buried post-mortem in the base of a collapsed dune foreset. The new taxon is characterized by a plate-like medial process of the scapula, a prominent proximal expansion of the deltopectoral crest of the humerus, a strongly inclined distal articular surface of the radius, and a proximally and laterally hypertrophied proximal metacarpal I.Conclusions/SignificancePhylogenetic analysis recovers Seitaad as a derived basal sauropodomorph closely related to plateosaurid or massospondylid ‘prosauropods’ and its presence in western North America is not unexpected for a member of this highly cosmopolitan clade. This occurrence represents one of the most complete vertebrate body fossil specimens yet recovered from the Navajo Sandstone and one of the few basal sauropodomorph taxa currently known from North America.

Navajo Sandstone–brine–CO2 interaction: implications for geological carbon sequestration

The injection of CO2 into deep saline aquifers is being considered as an option for greenhouse gas mitigation. However, the response of an aquifer to the injected CO2 is largely unknown. Experiments involving the reaction of Navajo Sandstone with acidic brine were conducted at 200°C and 25 or 30 MPa to evaluate the extent of fluid–rock interactions. The first experiment examined sandstone interaction with CO2-impregnated brine; the second experiment examined sandstone dissolution in CO2-free acidic brine; the third one is carried out in a mixed-flow reactor and designed to measure sandstone dissolution rates based on time-series Si concentrations. The solution chemistry data indicate that the SiO2(aq) increases gradually and pH increases slowly with reaction progress. Silicate minerals in the sandstone display textures (dissolution features, secondary mineralization), indicating that these phases are reacting strongly with the fluid. Dissolution of feldspars and conversion of smectite to illite are likely to be the two reactions that contribute to the release of SiO2(aq). The product minerals present at the end of the experiments are illite, illite/smectite, allophane, and carbonate minerals (for the CO2-charged system). Dissolved CO2 is likely to acidify the brine and to provide a source of carbon for the precipitation of carbonate minerals. Mineral trapping through the precipitation of carbonate minerals is favored thermodynamically and was observed in the experiments. The chemical reactions likely increase the bulk porosity of the sandstone due to dissolution of silicate minerals. However, allophane and illite/smectite fill voids in sandstone grains. There is no evidence for the removal of clay coatings due to chemical reactions. It is uncertain whether the mechanical forces near an injection well would mobilize the smectite and allophane and clog pore throats. Trace amounts of metals, including Cu, Zn, and Ba, were mobilized.

Thick and Thin

Thick and Thin Gregory Morris (bio) I am not a mystical guy. I have never known, or been especially comfortable with, the spiritual and the unseen. I have little confidence or interest in the immaterial. I place my faith in solid substance, in the tangible. I like to tap my toe on Samuel Johnson's rock. I am, for the most part, a material boy. I have also generally resisted the temptation to let loose of the body, to occasionally take its leave. None of that transparent eyeball business for me, thanks. Transcendence is just too impermanent, ephemeral. I like to stay in place, and I find the body generally a good place to be. I am also one who had, until fairly recently, never known deserts. My life has been lived almost entirely near waters: large rivers, great lakes, even for a short while in a seam in the American landscape between the Caribbean Sea and the Pacific Ocean. To be more precise, though, I have known deserts only through language, through the words of those who have made the desert an intimate part of their lives. What I understood about deserts—American deserts, in particular—I took from people like Ed Abbey, Terry Tempest Williams, Charles Bowden, Mary Austin. I have been, for the most part, a guy most comfortable with waters. But it was a desert place that got me into trouble and out of my body. Six summers ago, my wife and I traveled to southern Utah, drawn by [End Page 23] the slickrock and by the power of landscapes with fabled names: Zion, Bryce, Grand Staircase-Escalante, Capitol Reef. We wanted to get to know desert, to learn some of its language. And so we walked, covering trails, absorbing heat, craving waters, and painting ourselves with the red dust of Navajo sandstone. We desired desert, and the desert gave itself to us in dry waves of feverish heat. By the time of that June's summer solstice, we found ourselves at Arches National Park. Solstice is a time of imbalance between light and dark, but we were in a place that seemed to be created from balance itself. Mammoth rust-red arches and long, narrow fins of sandstone rose out of the dust-dry earth of this desert-place. Prickly pear cactus bloomed in bright, yellow bursts. Wyoming paintbrush and golden columbine and cliff rose threw out brilliant dashes of color amid the sere browns of grasses exhausted by heat and sun. As the day waned and the night sky made its color shift from blue to pink, the near-full moon rose, its yellow perfection framed by one of the two Window Arches. Like everyone else there, we grew silent, capable only of watching the spectacle as it unfolded. We had seen the desert—or this desert—through its day. We were weary, footsore, dressed in the red dust that is the signature of this place. We had gained a kind of ground truth—minor and minuscule, to be sure—but still a palpable knowledge. We had taken the diurnal impress of this place upon us. But what of the desert at night? What of a desert blackened into quiet, unmeasured space? What are we and the desert left with when the colors and the landmarks—the hoodoos, the arches—are erased? Near the midnight of that day, my wife and I drove outward, leaving the park to itself. Before we had made the park gate, though, we stopped at a trailhead. I wanted to take one last, brief hike in this place and in its dark. This want was really more of a compulsion, a felt and undeniable need to move down and into that darkness, and to do so on my own, without intention and intentionality. My wife accommodated my desire, walking with me only partway before hiking back to the car, letting me traverse the Park Avenue Trail alone. I had the trail, this place, to myself. The farther I descended, picking my way with some care, the darker the desert-world became. That night's sky came fuller with stars. The just-past-full [End Page 24] moon illuminated my...