Bedrock Exposure Research Articles

Orographic Controls on Subdaily Rainfall Statistics and Flood Frequency in the Colorado Front Range, USA

AbstractGeneralizable relationships for how subdaily rainfall statistics imprint into runoff statistics are lacking. We use the Colorado Front Range, known for destructive rainfall‐triggered floods and landslides, to assess whether orographic patterns in runoff generation are a direct consequence of rainstorm climatology. Climatological analysis relies on a dense network of tipping‐bucket rain gauges and gridded precipitation frequency estimates from the National Oceanic and Atmospheric Administration to evaluate relationships among subdaily rainfall statistics, topography, and flood frequency throughout the South Platte River basin. We find that event‐scale rainfall statistics only weakly depend on elevation, suggesting that orographic gradients in runoff “extremes” are not simply a consequence of rainfall patterns. In contrast, bedrock exposure strongly varies with elevation in a way that plausibly explains enhanced runoff generation at lower elevations via reduced water storage capacity. These findings are suggestive of feedbacks between bedrock river evolution and hillslope hydrology not typically included in models of landscape evolution.

Geologic factors leadingly drawing the macroecological pattern of rocky desertification in southwest China

Rocky desertification (RD) is a special process of land deterioration in karst topography, with a view of bedrock exposure and an effect of ecological degradation. Among the three largest karst regions in the world, southwest China boasts the largest RD area and highest diversity of karst landscapes. However, inefficient field surveying tends to restrict earlier studies of RD to local areas, and the high complexity of karst geomorphology in southwest China further lead to the shortage of the knowledge about its macroecological pattern so far. To address this gap, this study innovatively took county as the unit to statistically explore the links between the 2008-censused distributions of county-level RD in southwest China and its potential impact factors of three kinds (geologic, climatic, and anthropogenic), all transformed into the same mapping frame. Spatial pattern analyses based on spatial statistics and artificial interpretation unveiled the macroscopic characteristics of RD spatial patterns, and attribution analyses based on correlation analysis and dominance analysis exposed the links of the impact factors to RD and their contributions in deciding the macroscopic pattern of RD. The results suggested that geologic factors play a first role in drawing the macroecological pattern of RD, also for the slight-, moderate-, and severe-level RD scenarios, in southwest China. Despite this inference somehow collides with the popular awareness that anthropogenic factors like human activities are leadingly responsible for the RD-relevant losses, the findings are of practical implications in guiding making the macroscopic policies for mitigating RD degradation and advancing its environmental restoration.

Cosmic spherules from Widerøefjellet, Sør Rondane Mountains (East Antarctica)

A newly discovered sedimentary accumulation of micrometeorites in the Sør Rondane Mountains of East Antarctica, close to the Widerøefjellet summit at ∼2750 m above sea level, is characterized in this work. The focus here lies on 2099 melted cosmic spherules larger than 200 μm, extracted from 3.2 kg of sampled sediment. Although the Widerøefjellet deposit shares similarities to the micrometeorite traps encountered in the Transantarctic Mountains, both subtle and more distinct differences in the physicochemical properties of the retrieved extraterrestrial particles and sedimentary host deposits are discernable (e.g., types of bedrock, degree of wind exposure, abundance of metal-rich particles). Unlike the Frontier Mountain and Miller Butte sedimentary traps, the size fraction below 240 μm indicates some degree of sorting at Widerøefjellet, potentially through the redistribution by wind, preferential alteration of smaller particles, or processing biases. However, the cosmic spherules larger than 300 μm appear largely unbiased following their size distribution, frequency by textural type, and bulk chemical compositions. Based on the available bedrock exposure ages for the Sør Rondane Mountains, extraterrestrial dust is estimated to have accumulated over a time span of ∼1–3 Ma at Widerøefjellet. Consequently, the Widerøefjellet collection reflects a substantial reservoir to sample the micrometeorite influx over this time interval. Petrographic observations and 3D microscopic CT imaging are combined with chemical and triple-oxygen isotopic analyses of silicate-rich cosmic spherules larger than 325 μm. The major element composition of 49 cosmic spherules confirms their principally chondritic parentage. For 18 glassy, 15 barred olivine, and 11 cryptocrystalline cosmic spherules, trace element concentrations are also reported on. Based on comparison with evaporation experiments reported in literature and accounting for siderophile and chalcophile element losses during high-density phase segregation and ejection, the observed compositional sequence largely reflects progressive heating and evaporation during atmospheric passage accompanied by significant redox shifts, although the influence of (refractory) chondrite mineral constituents and terrestrial alteration cannot be excluded in all cases. Twenty-eight cosmic spherules larger than 325 μm analyzed for triple-oxygen isotope ratios confirm inheritance from mostly carbonaceous chondritic precursor materials (∼55% of the particles). Yet, ∼30% of the measured cosmic spherules and ∼50% of all glassy cosmic spherules are characterized by oxygen isotope ratios above the terrestrial fractionation line, implying genetic links to ordinary chondrites and parent bodies currently unsampled by meteorites. The structural, textural, chemical, and isotopic characteristics of the cosmic spherules from the Sør Rondane Mountains, and particularly the high proportion of Mg-rich glass particles contained therein, imply a well-preserved and representative new sedimentary micrometeorite collection from a previously unstudied region in East Antarctica characterized by distinct geological and exposure histories.

Refining the age, emplacement and alteration scenarios of the olivine-rich unit in the Nili Fossae region, Mars

The Nili Fossae region of Mars exhibits from remote sensing spectral data the largest exposures of olivine-rich materials on the planet. However, it is not clearly constrained how and when these terrains formed. Some of the proposed scenarios favor a mode of formation closely related to Isidis impact basin: either under intense effusive volcanism following the impact, from cooling of an immediate impact melt sheet or silicate impact vapor condensate. These deposits might also be pyroclastic products ejected from now eroded or buried vents. Recent studies also proposed that lag deposits could be responsible for enrichment in olivine after deposition. In this contribution, we mapped the olivine-rich unaltered and altered bedrock exposures using near infrared and thermal inertia data, investigated the geometry and key contacts of the olivine-rich unit, and determined its surface age using crater counts and stratigraphical relationships. We find that the olivine-rich bedrock extends over at least ~18,000 km2 in the Nili Fossae region, with a large part of it being unaltered. Olivine-rich material that overlaps the northern rim of Jezero crater corresponds to a primary deposit (i.e. rather than reworked material). Since this crater is younger than Isidis, we favor the hypothesis of a post-Isidis origin, rather than the impact melt (consistently with Bramble et al., 2017) and impact condensate origin. Based on our observations and crater counts, we estimate an emplacement age of 3.82 ± 0.07 Ga (Mid to Late Noachian). We discuss the origin of the unit, with the most likely scenarios being ash falls and/or pyroclastic surges. To explain the circum-Isidis distribution of these deposits, we favor the hypothesis of a thinned and weakened crust in the region subsequently to the giant impact of Isidis, as suggested by Tornabene et al. (2008). Finally, the distribution of altered bedrock conflicts with the contact metamorphism scenario. As the olivine-rich unit exposed at Jezero crater, future home of the Mars 2020 rover, is a regional stratigraphic marker, return samples for precise dating of this unit should be made one of the major mission targets.

Arable soil formation and erosion: a hillslope-based cosmogenic nuclide study in the United Kingdom

Abstract. Arable soils are critical resources that support multiple ecosystem services. They are frequently threatened, however, by accelerated erosion. Subsequently, policy to ensure their long-term security is an urgent societal priority. Although their long-term security relies upon a balance between the rates of soil loss and formation, there have been few investigations of the formation rates of soils supporting arable agriculture. This paper addresses this knowledge gap by presenting the first isotopically constrained soil formation rates for an arable (Nottinghamshire, UK) and coniferous woodland hillslope (Shropshire, UK). Rates ranged from 0.026 to 0.096 mm yr−1 across the two sites. These rates fall within the range of previously published rates for soils in temperate climates and on sandstone lithologies but significantly differed from those measured in the only other UK-based study. We suggest this is due to the parent material at our sites being more susceptible to weathering. Furthermore, soil formation rates were found to be greatest for aeolian-derived sandstone when compared with fluvially derived lithology raising questions about the extent to which the petrographic composition of the parent material governs rates of soil formation. On the hillslope currently supporting arable agriculture, we utilized cosmogenically derived rates of soil formation and erosion in a first-order lifespan model and found, in a worst-case scenario, that the backslope A horizon could be eroded in 138 years with bedrock exposure occurring in 212 years under the current management regime. These findings represent the first quantitative estimate of cultivated soil lifespans in the UK.

Climate and relief‐induced controls on the temporal variability of denudation rates in a granitic upland

AbstractHow soil erosion rates evolved over the last about 100 ka and how they relate to environmental and climate variability is largely unknown. This is due to a lack of suitable archives that help to trace this evolution. We determined in situ cosmogenic beryllium‐10 (10Be) along vertical landforms (tors, boulders and scarps) on the Sila Massif to unravel their local exhumation patterns to develop a surface denudation model over millennia.Due to the physical resistance of tors, their rate of exhumation may be used to derive surface and, thus, soil denudation rates over time. We derived soil denudation rates that varied in the range 0–0.40 mm yr‐1. The investigated boulders, however, appear to have experienced repositioning processes about ~20–25 ka bp and were therefore a less reliable archive. The scarps of the Sila upland showed a rapid bedrock exposure within the last 8–15 ka. Overall, the denudation rates increased steadily after 75 ka bp but remained low until about 17 ka bp. The exhumation rates indicate a denudation pulse that occurred about 17–5 ka bp. Since then the rates have continuously decreased.We identify three key factors for these developments – climate, topography and vegetation. Between 75 and 17 ka bp, climate was colder and drier than today. The rapid changes towards warmer and humid conditions at the Pleistocene–Holocene transition apparently increased denudation rates. A denser vegetation cover with time counteracted denudation.Topography also determined the extent of denudation rates in the upland regime. On slopes, denudation rates were generally higher than on planar surfaces. By determining the exhumation rates of tors and scarps, soil erosion rates could be determined over long timescales and be related to topography and particularly to climate. This is key for understanding geomorphic dynamics under current environmental settings and future climate change. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

Cryptic regional magmatism in the southern Saharan Metacraton at 580 Ma

The Saharan Metacraton is largely covered by the Sahel and Sahara with only sparse bedrock exposure making it one of the most poorly studied cratons in the World. Consequently, detrital zircon geochronology is invaluable to discern the lithotectonic development of the Saharan Metacraton in the absence of outcrop. Detrital zircons from the northern Chari River, the largest river system of North-Central Africa, have ages between ~700 Ma and ~500 Ma with a small Cambrian to Ordovician (486 ± 11 Ma and 422 ± 11) population, a few Devonian to Middle Jurassic zircons (394 ± 9 Ma–159 ± 4) and a single Mesoproterozoic (959 ± 19 Ma) zircon. The absence of ancient detrital zircons suggests that the crust in the vicinity of Lake Chad is primarily juvenile. A significant (~17%) proportion of zircons yielded ages of ~580 Ma that are broadly coeval with nappe and wrench faulting, and granulite exhumation in the Central African and Trans-Saharan orogenic belts. Moreover, we identified a gabbro from the Guéra Massif of south-central Chad that yielded an in situ zircon weighted-mean 238U/206Pb age of 580 ± 6 Ma. The age of the gabbro is similar to the Kekem gabbro-norite complex (576 ± 4 Ma) in west Cameroon, inherited zircons from silicic rocks near Lake Fitri and Lake Chad, and the detrital zircons from the Chari River. Our results unequivocally demonstrate that there was magmatism at ~580 Ma and that it was contemporaneous with deformation along the Tcholliré-Banyo and Central Cameroon shear zones indicating there was a major regional tectonomagmatic episode at that time. We suggest the mafic magmatism was related to post-orogenic mantle upwelling that may have been responsible for or contributed to lithospheric delamination of the southern Saharan Metacraton.

Geomorphic and geophysical evidence of a collapse origin for large sandstone depressions atop the western Cumberland Plateau in Tennessee

For over 70 years, closed topographic depressions developed in siliciclastic rocks of the Cumberland Plateau have been hypothesized to form via collapse of rock into caverns formed in the carbonate-rich rocks below. We made observations of surface characteristics and applied geophysical methods across two of these depressions on the western part of the plateau to collect subsurface information and test the cave collapse hypothesis. Electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) were conducted at both Sites 1 and 2 and two-dimensional (2D) multi-channel analysis of surface waves (MASW) was conducted at Site 2. Three zones of material are interpreted at both sites based on field observations, and variations in density (velocity) and resistivity characteristics; a relatively thin cover of organic matter and colluvium, sandstone bedrock primarily observed at the rims and upper slopes of the depression, and a possible boulder/rubble zone identified towards the centers of the depressions. Details derived from LiDAR data, field observations and collected geophysical data for Site 2 allow for the projection of a circular breccia pipe in the subsurface. A 3-stage model of development is proposed for these features: A Youthful Stage occurs after initial collapse of surface material into a subsurface cavity in which the depression has a cylindrical form. In the Mature Stage, the slopes have shallowed, and slope adjustments are made as the breccia pipe fills and settles. Bedrock cliffs are mechanically weathered and influenced by gravity, forming talus that covers slopes and the floor with colluvium mixed with organic matter. The Degraded Stage is entered when the breccia pipe has settled, the bedrock exposures are covered, and the depression begins to fill. The depressions studied for this research are classified as being in the Mature stage. Future geophysical surveys and additional geologic mapping and field work at these and other depressions on the plateau will add to our understanding of their development and evolution over time.

A survey of Sierra Nevada magmatism using Great Valley detrital zircon trace-element geochemistry: View from the forearc

AbstractThe well-characterized Sierra Nevada magmatic arc offers an unparalleled opportunity to improve our understanding of continental arc magmatism, but present bedrock exposure provides an incomplete record that is dominated by Cretaceous plutons, making it challenging to decipher details of older magmatism and the dynamic interplay between plutonism and volcanism. Moreover, the forearc detrital record includes abundant zircon formed during apparent magmatic lulls, suggesting that understanding the long-term history of arc magmatism requires integrating plutonic, volcanic, and detrital records. We present trace-element geochemistry of detrital zircon grains from the Great Valley forearc basin to survey Sierra Nevadan arc magmatism through Mesozoic time. We analyzed 257 previously dated detrital zircon grains from seven sandstone samples of volcanogenic, arkosic, and mixed compositions deposited ca. 145–80 Ma along the length of the forearc basin. Detrital zircon trace-element geochemistry is largely consistent with continental arc derivation and shows similar geochemical ranges between samples, regardless of location along strike of the forearc basin, depositional age, or sandstone composition. Comparison of zircon trace-element data from the forearc, arc, and retroarc regions revealed geochemical asymmetry across the arc that was persistent through time and demonstrated that forearc and retroarc basins sampled different parts of the arc and therefore recorded different magmatic histories. In addition, we identified a minor group of Jurassic detrital zircon grains with oceanic geochemical signatures that may have provenance in the Coast Range ophiolite. Taken together, these results suggest that the forearc detrital zircon data set reveals information different from that gleaned from the arc itself and that zircon compositions can help to identify and differentiate geochemically distinct parts of continental arc systems. Our results highlight the importance of integrating multiple proxies to fully document arc magmatism, demonstrating that detrital zircon geochemical data can enhance understanding of a well-characterized arc, and these data may prove an effective means by which to survey an arc that is inaccessible and therefore poorly characterized.

The small inner satellites of Saturn: Shapes, structures and some implications

Properties of the small inner satellites of Saturn are updated using final Cassini image data. The equatorial ridges on Pan and Atlas are morphologically, structurally, and gravitationally distinct from their cores and represent distinct additions to, rather than modifications of, the precursor bodies. Exposures of lineated substrates on Pandora and Epimetheus show a relationship between regolith grooves and structures in underlying bedrock. Structures exposed on Epimetheus suggest formation at depth including filling or annealing of fractures. These structures, and sharp bedrock-regolith boundaries indicate a history far more complex than simple rubble-pile assembly. These exposures of bedrock also indicate past removal of the expected regolith from large fractions of these object's surfaces late in their cratering records.

The extent of permafrost during the Last Permafrost Maximum (LPM) on the Ordos Plateau, north China

This paper presents new field observations that assist in further defining the extent of Last Permafrost Maximum (LPM) permafrost and frozen ground in north China. In that respect, this region is specific by its arid environment and continental position. Bedrock exposures and sand and gravel pits were examined along a south-north transect from southern Ordos as far as the vicinity of Dongsheng District, Ordos City on the Ordos Plateau (North China). Attention focussed upon the recognition of sand-wedge remains and possible cryoturbations. Two types of wedge structures were recognized: type A wedges extend to depths in excess of 3.0 m and are thought to have formed under conditions of continuous permafrost. Type B wedges are restricted to the upper 1.5 m and are thought to reflect discontinuous permafrost and/or deep seasonal frost. OSL dating of wedge infills indicates that type A wedges formed between 23–19 ka and younger type B wedges formed until the early Holocene. It is now possible to delineate a more exact position of the boundary between continuous permafrost and discontinuous permafrost and/or deep seasonal frost on the Ordos Plateau during the LPM. This boundaries occur between Dongsheng and Jinbian at 37°-39°N.

An orbit-based remote sensing geological assessment of the CanMars Mars Sample Return Analogue Deployment (MSRAD) landing site situated in the Henry Mountains Basin, near Hanksville, Utah

Mars-analogous and relevant terrestrial- and orbit-based remote sensing datasets were gathered and used for both an in and out of simulation geological assessment of a pre-selected “landing site” for the CanMars, Mars Sample Return Analogue Deployment (MSRAD), and Mars Utah Rover Field Investigation (MURFI) analogue missions. The analogue mission site is situated in the Henry Mountains Basin just outside of Hanksville, Utah. The results of a comprehensive multispectral analysis based on Landsat 8 and ASTER of the landing site, landing ellipse area, and surrounding region, were synthesized with other datasets providing information on the morphology and basic physical properties of the spectrally-defined surface units. The synthesis yielded insights into the geologic materials present, the sampling opportunities for the analogue rover available at the site and the geologic history of the site. The datasets provided to the in-simulation science team were determined to be sufficient to generally identify ∼88% of the geologic units previous mapped in the field. The area was determined to be dominated by clastic sedimentary rocks (a high-silica/quartz- and Fe-oxide-bearing geologic materials), but also include a variety of aqueous alteration phases consistent with a complex climate, environmental, and water-rock history. Positive identification of additional phases from orbit include: Al-smectite clay-, gypsum- and carbonate-bearing rocks or deposits. An association between these components with fine-scale layered bedrock exposures and sinuous ridges, consistent with inverted channels, provide evidence of a viably Mars-analogous “landing site”, which yeilds diverse samples for in-situ analysis and sampling by the CanMars and MURFI analogue missions.

The Missouri High-Conductivity Belt, revealed by magnetotelluric imaging: Evidence of a trans-lithospheric shear zone beneath the Ozark Plateau, Midcontinent USA?

Crust in the central Midcontinent of the USA, part of North America's intracratonic platform, consists of Precambrian crystalline basement overlain by a cover of Phanerozoic sedimentary strata. Analysis of EarthScope long-period magnetotelluric (MT) data provides an opportunity to characterize lithosphere-scale structures of this region. Inversion of these data reveals a distinct belt of high electrical conductivity (≤30 Ωm) traversing Missouri. This belt strikes northwest-southeast, dips steeply to the northeast, and extends at least from the mid-crust into the lithospheric mantle. Significantly, this “Missouri high-conductivity belt” (MHCB) spatially coincides with the Missouri Gravity Low (a distinctive feature on Bouguer anomaly maps), lies adjacent to the southwestern edge of Precambrian bedrock exposure in the Ozark Plateau, trends parallel to both magnetic-anomaly lineaments and mapped fault traces, and aligns with lateral steps (transfer or transform faults) in the Midcontinent rift and the Reelfoot rift. We suggest that the MHCB initiated as a lithosphere-scale transtensional shear zone, which linked rift segments that were active during Proterozoic extensional events. Conceivably, pull-aparts within this shear zone localized felsic igneous activity and produced depressions that filled with sediment in association with the development of the 1.5–1.3 Ga Eastern Granite-Rhyolite Province, yielding a region of less-dense crust that could account for the Missouri Gravity Low. The shear zones likely also provided pathways for fluid migration. High conductivity in the MHCB may reflect deep crustal and lithospheric mantle alteration in response to the passage of fluids during dilatational shearing. We postulate that fluid movement accompanied by shear resulted in graphite and perhaps sulfide concentration along the shear zone in the lower crust, and hydration and/or grain diminution and crystallographic preferred orientation in the lithospheric mantle. This alteration also may have weakened lithosphere along the MHCB, explaining the spatial association of the belt with Paleozoic reactivated faults and with contemporary seismicity.

Changes in soil nitrogen stocks following vegetation restoration in a typical karst catchment

AbstractSoil nitrogen (N) accumulation enhances carbon (C) sequestration and subsequently the restoration of degraded ecosystems, but the changes in soil N properties after vegetation restoration in such ecosystems are poorly understood. We collected data from 358 fixed points in the karst catchment in China to determine the trends, magnitude, and mechanisms of soil N sequestration after 10 years of revegetation. The revegetation types were tillage fields that were abandoned or were converted to pasture, planted forest, or converted to mixed pasture and forest and pasture lands that were abandoned (P_Aban) or converted to mixed pasture and forest (P_Mix). The changes in soil N density (g m−2) over time were insignificant, and only P_Aban, P_ Mix, and the overall catchment experienced significant increases in soil N concentration (g kg−1). Structural equation modeling indicated that bedrock exposure affected the change in soil N concentration (∆NC) and the soil N sequestration value (∆ND) via affecting the change in soil C concentration, by directly affecting bulk density, soil depth, and Ca2+. The soil C:N ratio increased significantly, and the increase in soil N was slower than that of C, suggesting a decoupling of soil C and N accumulation in the initial stage of revegetation. ∆ND was significantly related to the change in soil C:N ratio, indicating that the coupling between soil C and N may be constrained by insufficient soil N sequestration during revegetation. These findings suggest that the restoration of degraded karst ecosystems requires an increase in soil N input early during vegetation restoration.

Distinct Geologic Settings of Opal‐A and More Crystalline Hydrated Silica on Mars

AbstractOpaline silica deposits have been identified on Mars from orbital near‐infrared (NIR) reflectance spectra, but opal types (e.g., −A,/CT) have not yet been conclusively determined due to insufficient laboratory spectra acquired at Mars‐relevant conditions. NIR reflectance spectra of siliceous deposits acquired by the Compact Reconnaissance Imaging Spectrometer for Mars instrument are compared with new laboratory spectra of terrestrial opal samples that we measured under Mars‐relevant conditions. Opal‐A occurrences on Mars are commonly associated with bedrock exposures, whereas more crystalline hydrated silica (opal‐CT and quartz/chalcedony) is primarily observed in unconsolidated sediments. These differences suggest some opal‐bearing deposits may have experienced prolonged interaction with near‐surface water or diagenetic fluids, and bedrock exposures dominated by opal‐A may have experienced less post‐formation diagenesis compared with those containing more crystalline hydrated silica. NIR data thus provide a useful tool to assess opal maturity and relative potential for biosignature preservation for different siliceous deposits on Mars.

The Dynamics of Channel Slope, Width, and Sediment in Actively Eroding Bedrock River Systems

AbstractThe evolution of rivers in eroding landscapes plays a key role in determining landscape relief and modulating climate‐tectonic interactions. A common approach to quantifying river system evolution uses a one‐dimensional, detachment‐limited stream power equation. One potential drawback of this model is that it does not incorporate the effects of changes in channel width or the role of sediment transport dynamics. Here I present a new method for modeling the influence of channel width on river dynamics to explore how variable width and sediment transport impact river profile evolution. With this approach, vertical river erosion can operate based on any number of river erosion models, such as a simple shear stress model (e.g., detachment limited), sediment cover‐shear stress hybrid models, or mechanistic saltation‐abrasion models. I explore the sensitivity of these three models to increases in rock‐uplift rate (i.e., 2, 3, 5, 10, and 20× increase). Generally, the results show that incorporating channel width adjustment or sediment transport dynamics lowers the sensitivity of a river profile to rock‐uplift rate. For the sediment transport‐dependent models, the degree of sensitivity depends on whether the system is limited by bedrock exposure or erosion potential (i.e., detachment potential). The approach produces transient responses that reveal distinct patterns of width and slope, which may provide valuable insight into the limiting physical mechanisms of bedrock erosion in a region. The implications of the work are broad and include the potential to distinguish underlying erosion controls from field observations of width and slope as well as understanding climate‐tectonic interactions.

Revised Quaternary glacial succession and post-LGM recession, southern Wind River Range, Wyoming, USA

We present here a more complete cosmogenic chronology of Pleistocene glacial deposits for the Wind River Range, Wyoming, USA. Fifty-one new and thirty-nine re-calculated 10Be and 26Al exposure ages from Sinks and North Fork canyons, Stough Basin, Cirque of the Towers and the Temple Lake valley allow us to more tightly constrain the timing and sequence of glacial alloformations in the southern portion of the range.Moraines, diamicts and bedrock exposures here have previously been correlated with as many as five Pleistocene and four Holocene glacial events. Exposure ages from Pleistocene alloformations associated with trunk glaciers in Sinks Canyon and North Fork Canyon generally confirm earlier age estimates. Cosmogenic radionuclide (CRN, 10Be and 26Al) ages from moraines and striated bedrock surfaces previously mapped as Pinedale correspond to MIS2, while boulder exposure ages from moraines mapped as Bull Lake correspond generally to MIS5-MIS6. Geomorphic data from a moraine previously mapped as Younger pre-Sacagawea Ridge appears to correspond most closely to the Sacagawea Ridge glacial episode (MIS-16), but the uncertainty of a single 10Be exposure age suggests the unit could be as young as MIS-10 or as old as MIS-18. Boulders from a diamict on Table Mountain previously reported as Older pre-Sacagawea Ridge yield two 10Be exposure ages that suggest the presence of Early Pleistocene glacial activity here possibly older than 1–2 Ma (>MIS-30).Bedrock exposure ages within Sinks Canyon suggest the Pinedale valley glacier had retreated from the floor of Sinks Canyon to above PopoAgie Falls by ca. 15.3 ka. Cirque glaciers in Stough Basin appear to have retreated behind their riegels by ca. 16 ka, which suggests the cirque glaciers were decoupling across their riegels from the valley glaciers below at this time, prior to their readvance to form Lateglacial moraines.New 10Be boulder exposure ages from moraines previously correlated to the Temple Lake and Alice Lake allostratigraphic units in the cirques of Stough Basin and Cirque of the Towers show general equivalence to the stadial event just prior to the onset of the Bølling interstadial (17.5–14.7 ka) and to the Intra-Allerød Cold Period-Younger Dryas stadial phase (13.9–11.7 ka), respectively. From this evidence, the Temple Lake Alloformation of the Wind River Mountains now should correspond to the INTIMATE GS-2.1a (Oldest Dryas) stadial event while the Alice Lake Alloformation should correspond to the INTIMATE GS-2 stadial (IACP-Younger Dryas). Thus, we consider that evidence no longer exists for early-to mid-Holocene glacial events in the southern Wind River Range.

Spatial Identification of Key Alteration Minerals Using ASTER and Landsat 8 Data in a Heavily Vegetated Tropical Area

The Central Gold Belt (CGB) of Malaysia is a major host to gold deposits. Penjom, Raub, Selising and Buffalo reef are major gold mines in CGB. The study area, Selinsing gold mine, is located at the northwest of Pahang province on the lineament known as the Raub Bentong Suture. Presence of dense vegetation and cloud cover in tropical regions are main obstacles in alteration mapping using satellite imageries. In this study, Landsat 8 and ASTER level 1B images were used to map clay minerals and quartz rich zones at Selinsing gold mine and surrounding areas. Direct principal component analysis (DPCA), matched filtering (MF) and band ratio were the effective methods used in this study. High concentration of clay minerals was detected using band ratio 6/7, DPC2 and MF and ratio 14/12 was carried out to highlight quartz rich zones. The results of image processing methods were verified by in situ inspection and X-ray diffraction analyses. The results show that, in spite of limited bedrock exposure, the known gold prospects and potential areas of mineralization can be recognized by the methods employed in this study.

The Skipjack Island fault zone: An active transcurrent structure within the upper plate of the Cascadia subduction complex

The Skipjack Island fault zone has been mapped in the San Juan Islands between Vancouver Island, Canada, and the Washington State mainland, USA. A decade ago, interpretation of multibeam sonar seafloor imagery revealed that Skipjack Island, an east–west striking sedimentary bedrock outcrop, was a fault-controlled structural feature. A major fault separates Skipjack Island from a deformed sedimentary bedrock outcrop on the seafloor to the north. Recently the Skipjack Island fault zone's morphology and extent have been explored both to the west and east of the island using multibeam echosounder bathymetry, seismic reflection profiles, and sediment cores. The character of the Skipjack Island fault zone is well defined locally by the interpreted seismic-reflection profiles, which show active faults that displace sediments deposited since the Last Glacial Maximum. The central part of the fault zone, near Skipjack Island, appears as a near-vertical structure in the shallow subsurface that has been subjected to left-lateral motion as evidenced by a bedrock exposure on the seabed north of the island where folded strata bend eastward against the fault, the result of drag from fault motion. Interpretations of recent geophysical data suggest an extension of the Skipjack Island fault zone farther to the west, where it trends towards, and possibly connects with, the Fulford Fault on Vancouver Island, and to the east, where it either terminates north of Lummi Island, or continues eastward onto the mainland. The Skipjack Island fault zone is interpreted to be the northern boundary of the San Juan Archipelago with the Devils Mountain fault zone being the southern boundary. Both fault zones represent the longest continuous faults of the San Juan Archipelago. This newly mapped fault zone and its zone of deformation to the north may represent the northern boundary of the Cascadia forearc. Potential sesimicity of the Skipjack Island fault zone and its proximity to the steep unstable northeastern side of Orcas Island and the southern front of the Fraser River Delta poses a geohazads problem in the form of earthquakes, landslides, submarine slides, and tsunamis.

Young, small-scale surface features in Meridiani Planum, Mars: A possible signature of recent transient liquid and gas emissions

Enigmatic small-scale (<1 m) depositional and erosional features found in basaltic sands partly covering bedrock exposures, imaged at several locations in the equatorial Meridiani Planum region by the Mars Exploration Rover Opportunity, may be evidence of previously unrecognized, geologically young (or even contemporary) Martian surface processes. Leveed fissures appear to have formed by venting from beneath; possible explanations include aeolian blowholes near crater margins, volcanic fumarole activity, or gas/vapour escape resulting from the decomposition of small pockets of ground ice, methane clathrates or hydrated sulphate minerals. Some leveed fissures cross-cut and are therefore younger than aeolian ripples thought to have last been active c. 50,000 years ago. Erosional gutters are sharply defined and fresh-looking, internally terraced, sometimes are deeper near one end, and in one case seem to give way to small depositional fans downslope; they have the appearance of having been formed by liquid flow rather than by wind erosion. There is evidence elsewhere that contemporary ground-ice thaw and consequent transient surface run-off may occur occasionally under present conditions at low, near-equatorial latitudes on Mars; short-lived (even for just a few minutes) meltwater emission and flow at the surface could form gutters before evaporating. Further possibilities are the decomposition of buried pockets of methane clathrates (which theoretical considerations suggest might be present and stable even in equatorial regions) giving rise to both methane gas venting and transient surface water, or the release of liquid brines by decomposition of hydrated magnesium sulphate minerals or deliquescence of perchlorates. Dry granular flow mechanisms proposed as explanations for Recurring Slope Lineae seem inadequate to explain the morphologies of leveed fissures and gutters.