Sand stringers are subtle, aeolian landforms that reach hundreds to thousands of metres in length and lack a slipface. While hundreds of sand stringers exist beyond the Last Glacial Maximum margin in ...the Upper Midwest, USA, little is known about the timing and nature of their formation. This research characterizes the morphology and stratigraphy and provides geochronological data from two sand stringers in the upper Mississippi River basin in Minnesota and Wisconsin.
The sand stringers investigated have similar west–northwest to east–southeast orientations and are located ~100 km from each other on uplands west and east of the Mississippi River valley. The sand stringer in Goodhue County, Minnesota (GC site) is ~870 m long and 50–80 m wide. To the east, the sand stringer in Eau Claire County, Wisconsin (ECC site) is ~925 m long and 30–50 m wide. The main body of the GC sand stringer is characterized by an ~80–270 cm‐thick silt‐rich surface unit overlying a sandy unit that is underlain by a dark brown paleosol, while the main body of the ECC sand stringer is sandier/coarser, stratified, and overlies outwash.
14C ages, interpreted as minima, at GC indicate formation began prior to 25–20 cal yr BP, while OSL and 14C ages indicate ECC formed ~11.25–8.9 ka. We interpret the main body of GC as equivalent to Peoria Loess, with minor inputs of reworked outwash. We interpret ECC as composed of reworked outwash and local bedrock‐derived sands. Pedogenesis at GC and ECC indicates stabilization with possible minor modification of near‐surface sediments through the Holocene. Differences in stratigraphy and chronology between the two sites highlight the complexity of aeolian systems in the Upper Midwest, interpreted as indicative of the variety of sediment sources contributing to landform development via aeolian processes due to spatial and temporal variability in deglaciation and permafrost melting.
Sand stringers are common across the upper Mississippi River basin, USA. Sand stringers were deposited beyond the LGM margin and may be linked to a hypothesized sandy aeolian pulse contemporaneous with permafrost degradation. GC contains an upward coarsening in the main body of the stringer and ECC is predominantly sandy throughout. OSL ages (11.25–8.9 ka) from ECC are similar to regional sandy aeolian depositional ages in prior research. 14C ages from both locations suggest deposition at or post‐LGM.
Although mountains represent a barrier to the flow of liquid water across our planet and an Earth of impenetrable mountains would have produced a very different geography, many rivers do cross ...mountain ranges. These transverse drainages cross mountains through one of four general mechanisms: antecedence-the river maintains its course during mountain building (orogeny); superimposition-a river erodes across buried bedrock atop erodible sediment or sedimentary rock, providing a route across what later becomes an exhumed mountain range; piracy or capture-where a steeper gradient path captures a lower gradient drainage across a low relief interfluve; and overflow-a basin fills with sediment and water, ultimately breaching the lowest sill to create a new river. This article reviews research that aids in identifying the mechanism responsible for a transverse drainage, notes a major misconception about the power of headward eroding streams that has dogged scholarship, and examines the transverse drainage at the Grand Canyon in Arizona.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This paper provides support for the concept of geomorphic process domains developed by Montgomery (1999) by linking geomorphic processes to ecological variations seen in the Pima arid granitic ...watershed of the Sonoran Desert, Phoenix, Arizona. Closer joint spacing shows a statistically significant correlation with lower percentages of mineral grain attachment as measured by digital image processing of backscattered electron microscope imagery. Lower mineral grain attachment leads to more frequent spalling of rock surfaces, as measured by varnish microlamination (VML) ages of the last spalling event. In contrast, more distant joint spacing leads to in situ 10Be erosion rates of 3.4–8.5mm/ka and the emergence of low domes and kopje granitic landforms; these low domes also serve as knickpoints along ephemeral washes. Distant jointing thus plays a key role in generating the bare bedrock surfaces that funnel limited precipitation to bedrock margins — enhancing the canopy cover of perennial plants next to the bare bedrock. Joint-influenced geomorphic processes at Pima Wash generate four distinct process domains: (PD1) armored drainage divides; (PD2) slopes with different granite landforms; (PD3) mid- and upper basin channels that mix knickzones, strath floodplains, and sandy alluvial sections; and (PD4) the main ephemeral channel transitioning to the piedmont. Distant jointing promotes bedrock exposure and rock armoring along drainage divides in PD1 that then concentrates runoff and promotes perennial plant growth. More distant joint spacing on slopes in PD2 promotes exposure of granitic bedrock forms that shed overland flow to their margin and promotes flora and fauna growths along the margins of low granitic domes and kopjes. Similarly, wider joint spacing along ephemeral washes in PD3 leads to knickpoints, which in turn act to concentrate moisture immediately downstream. The stream terraces in PD4 influence the ecology through xeric desert pavements on terrace treads and roofs for coyotes (Canis latrans) and gray fox (Urocyon Cinereoargenteus) dens on terrace scarps via stage 3 pedogenic carbonate. These four process domains occur in six other randomly selected granitic watersheds with drainage areas <5km2 in the Mojave and Sonoran Deserts. Results on rates of geomorphic processes in the Pima Wash watershed provide new insight in the desert geomorphology of small granitic watersheds. Catchment-wide denudation rates (CWDRs) recorded by 10Be sampled along the main ephemeral wash vary between 15 and 23mm/ka and do not appear to be influenced by knickpoint or knickzone occurrence; instead slightly lower CWDRs appear to be associated with sediment contributions by subbasins with more abundance of bare bedrock forms. Resampling for CWDR after a 500-year flood event from hurricane moisture at two sites along the main ephemeral channel revealed no detectable changes; this finding confirms the average representativeness of CWDR as a long-term denudation proxy and also means that sediment transport on these arid granitic hillslopes must be incremental and without rapid crest to wash transport. The first reported measurements of incision rates into a small granitic Sonoran Desert watershed, using 10Be and VML, reveal rates on the order of 70–180mm/ka in the lower quarter of Pima Wash for the last 60ka — producing a narrow and deep trench. As this base-level fall propagates upstream, erosion focuses on weaker material with higher joint densities; this facilitates the emergence of domes and kopje landforms with more widely spaced jointing.
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•Ecological patterns are influenced by desert geomorphic processes.•Unique data on jointing, denudation rates, surface stability in an arid granitic range.•Bedrock granitic forms from higher joint spacing shed runoff and promote biotic growth.•Closer joint spacing leads to enhanced mineral dissolution and spalling.
Talus flatirons (TFs) are morphostratigraphic markers of prior talus deposition that are now disconnected from the active hillslope. Three generations of TFs (TF1, TF2, TF3) exist flanking a Sonoran ...Desert inselberg, Rock Peak, in a welded tuff caprocks-controlled landscape bounded by pediments. TFs at Rock Peak enable estimation of slope retreat rates through the application of cosmogenic
10
Be, optically stimulated luminescence dating, and catchment-wide denudation rates (CWDR). We estimate disconnection of TF1 on Rock Peak at 88.9 ± 7.8 ka (northern slope) and 29.1 ± 2.5 ka (southern slope). Rates of hillslope retreat measure between 311.6 mm·ka
−1
(northern slope) and 728.5 mm·ka
−1
(southern slope). Asymmetry in retreat rates is consistent with CWDR, with southern slopes denuding ∼1.5 times faster. The asymmetry is interpreted as the result of the southward structural dip of strata present (>10°). Denudation rates on the summit of Rock Peak (54.3 ± 19.4 mm·ka
−1
welded tuff; 111.2 ± 15.3 mm·ka
−1
sandstone conglomerate) support interpretation that removal of welded tuff caprock accelerates denudation of this landscape and amplifies the impact of the structural dip. Given this, we interpret that Rock Peak will evolve into a rounded residual hill as pediments flanking the inselberg lengthen through time, similar to landforms observed in the surrounding landscape where the welded tuff and underlying sedimentary caprocks are no longer present. Using the range of slope retreat rates from Rock Peak, we provide a first estimate for the length of time necessary for pediments to form via hillslope retreat in the Sonoran Desert. Key Words: caprock, landscape evolution, pediment association, talus flatiron,
10
Be exposure dating.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A paucity of research exists regarding the millennial-scale response of inland alluvial streams to abrupt base-level fall. Studies of modern systems indicate that, over short time scales, the ...response is a diffusion-like process of upstream-propagating incision. In contrast, evidence from the lower Chippewa River (LCR), located in the upper Midwest of the USA, suggests that autogenic controls operating over time scales of several millennia can overwhelm diffusion, resulting in incision that is prolonged and episodic. During the Last Glacial Maximum, the LCR drained the Chippewa Lobe of the Laurentide Ice Sheet to the glacial upper Mississippi River (UMR). As a meltwater stream, it aggraded and filled its valley with glacial outwash, as did its largest tributaries, which were also meltwater streams. Its nonglacial tributaries aggraded, too, filling their valleys with locally derived sediment. During deglaciation, the UMR incised at least twice, abruptly lowering the LCR's base level — ~15m at 16ka or earlier and an additional 40m at ca. 13.4ka. Each of these base-level falls initiated incision of the LCR, led by upstream migrating knickpoints. The propagation of incision has, however, been a lengthy process. The optically stimulated luminescence (OSL) ages of terrace alluvium indicate that, by 13.5ka, incision had advanced up the LCR only 15km, and by 9ka, only 55km. The process has also been episodic, resulting in the formation of fill-cut terraces (inferred from GPR surveys and exposures of terrace alluvium) that are younger and more numerous in the upstream direction. Autogenic increases in sediment load and autogenic bed armoring, the result of periodic tributary-stream rejuvenation and preferential winnowing of fines by the incising river, may have periodically caused knickpoint migration and incision to slow and possibly stop, allowing lateral erosion and floodplain formation to dominate. A decline in sediment flux from stabilizing incised tributary stream systems would have led to renewed knickpoint migration and incision when floods of sufficient magnitude to breach the channel armor occurred. Minimal floodplain development along the upper section of the present-day LCR, along with the channel morphology of an unstable wandering gravel-bed river immediately downstream from it, suggest that the river is still responding to the base-level falls that happened many millennia ago. The autogenic controls on the LCR's response to UMR incision are a direct consequence of the thick fills of noncohesive sediment that accumulated in its valley and the valleys of its tributary streams during the Late Wisconsinan, making the LCR a prime example of a former proglacial river that remains a paraglacial fluvial system.
•The response of former proglacial alluvial streams to base-level fall can be complex.•Autogenic variations in sediment supply and bed armoring are likely key controls.•The result is upstream propagating incision that is episodic and prolonged.•Abundant glacial-age alluvial fill likely affects the autogenic incision process.•Former proglacial streams can be characterized as paraglacial fluvial systems.
Pediments of the Sonoran Desert in the United States have intrigued physical geographers and geomorphologists for nearly a century. These gently sloping bedrock landforms are a staple of the desert ...landscape that millions visit each year. Despite the long-lived scientific curiosity, an understanding of the processes operating on the pediment has remained elusive. In this study we revisit the extensive history of pediment research. We then apply geospatial, field, and laboratory cosmogenic
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Be nuclide dating and back-scattered electron microscopy methods to assess the pace and processes of landscape change on pediment systems abutting the Salt River in Arizona. Our study focuses on the Usery pediments linked to base-level fluctuations (river terraces) of the Salt River. Relict pediment surfaces were reconstructed with dGPS data and kriging methodologies utilized in ArcGIS-based on preserved evidence of ancient pediment surfaces.
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Be ages of Salt River terraces established a chronology of incision events, where calculating the volume between the reconstructed relict pediment and modern surface topography established minimum erosion rates (∼41 mm/ka to ∼415 mm/ka). Pediment area and length appear to have a positive correlation to erosion rate and development of planar pediment surfaces. Field and laboratory observations reveal that pediment systems adjust and stabilize at each Salt River terrace. Relief reduction across the pediment begins with pediment channel incision via headward erosion. Next, tributary drainage capture begins and collapses interfluves. Lateral stream erosion promotes planation where the porosity of decayed granite along channel banks exceeds the bedrock underneath ephemeral channels.
Analyses of ephemeral granitic drainages of <5 km at South Mountain metamorphic core complex, central Arizona, reveal a previously undocumented process of bedrock strath formation in this setting. ...Granitic channel banks experience a higher degree of mineral decay than that of granitic channel floors. Electron microscope observations show that grussification along the granitic channel banks occurs through abiotic processes of biotite oxidation and biotic processes associated with mycorrhizal fungi and roots of plants preferentially growing along channel banks. Digital image processing of backscattered electron microscope (BSE) images measured: (a) an enhancement of porosity along channel banks 2x to 5x greater than mid-channel positions; and (b) the gradual separation of grains over a 13-year period caused by the roots of Paloverde (Parkinsonia microphylla) trees. Ongoing mineral decay along banks facilitates differential erosion similar to Montgomery's (2004) hypothesis. Ephemeral washes migrate laterally into the decayed granite of their banks and erode the distal end of bounding pediments, expanding beveled bedrock straths. Direct observations of strath widening in six drainages during three distinct flash floods reveal a range from 4 to 23 millimeters of lateral bank erosion and <1 mm of channel bed abrasion. The widening of straths is likely limited by long-term rates of in situ physical separation of granitic minerals.
CGILS—the CFMIP‐GASS Intercomparison of Large Eddy Models (LESs) and single column models (SCMs)—investigates the mechanisms of cloud feedback in SCMs and LESs under idealized climate change ...perturbation. This paper describes the CGILS results from 15 SCMs and 8 LES models. Three cloud regimes over the subtropical oceans are studied: shallow cumulus, cumulus under stratocumulus, and well‐mixed coastal stratus/stratocumulus. In the stratocumulus and coastal stratus regimes, SCMs without activated shallow convection generally simulated negative cloud feedbacks, while models with active shallow convection generally simulated positive cloud feedbacks. In the shallow cumulus alone regime, this relationship is less clear, likely due to the changes in cloud depth, lateral mixing, and precipitation or a combination of them. The majority of LES models simulated negative cloud feedback in the well‐mixed coastal stratus/stratocumulus regime, and positive feedback in the shallow cumulus and stratocumulus regime. A general framework is provided to interpret SCM results: in a warmer climate, the moistening rate of the cloudy layer associated with the surface‐based turbulence parameterization is enhanced; together with weaker large‐scale subsidence, it causes negative cloud feedback. In contrast, in the warmer climate, the drying rate associated with the shallow convection scheme is enhanced. This causes positive cloud feedback. These mechanisms are summarized as the “NESTS” negative cloud feedback and the “SCOPE” positive cloud feedback (Negative feedback from Surface Turbulence under weaker Subsidence—Shallow Convection PositivE feedback) with the net cloud feedback depending on how the two opposing effects counteract each other. The LES results are consistent with these interpretations.
Key Points
Reasons of negative and positive cloud feedbacks in SCMs are explained
A framework is provided to interpret cloud feedbacks in models
SCM results are compared with LES simulations
Toe-cut terraces Larson, Phillip H; Dorn, Ronald I; Faulkner, Douglas J ...
Progress in physical geography,
08/2015, Letnik:
39, Številka:
4
Journal Article
Recenzirano
Alluvial fans and fluvial terraces occur in nearly all climatic settings and often coexist within the same drainage basin. These landforms play an important role in understanding the geomorphic, ...hydrologic, sedimentologic and erosional histories of a basin. The juxtaposition of fans and fluvial terraces, in some instances, can lead to misinterpretation in distinguishing traditional fluvial terraces from the truncated toe of tributary alluvial fans. This becomes particularly troublesome for those attempting to interpret results from published field studies where fan-cut terrace, truncated alluvial fan, toe-cut alluvial fan, alluvial terrace, and incision of the lower end of a fan piedmont all refer to the same genetic landform. We call for use of the term "toe-cut terrace" to represent this landform. We also present criteria to aid in the identification of toe-cut terraces, defined as an abandoned alluvial surface, formed by the truncation of the distal portion of tributary alluvial fans by streams flowing obliquely or perpendicular to the fan surface. Truncation occurs through lateral erosion ("toe-cutting") or through vertical incision by the trunk drainage lowering the base-level of the alluvial fan. This results in incision into the fan surface abandoning the fan's depositional surface at a higher level above the modern floodplain - a form that often resembles a fluvial terrace. A case study from the Sonoran Desert in central Arizona illustrates a sequence of abandoned alluvial surfaces that resemble fluvial terraces, but use of the proposed criteria reveal the presence of both toe-cut terraces and traditional fluvial terraces formed by the abandonment of the rivers former floodplain.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, ODKLJ, OILJ, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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