Internal Structure of an Aeolian Dune using Ground‐Penetrating Radar
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Abstract:
A ground-penetrating radar survey was made on a large complex aeolian dune along the margin of Great Sand Dunes National Monument, Colorado, to delineate the internal structures formed by dune migration in a complex wind regime. Radar waves were partially reflected from sediment interfaces that had differing densities or moisture contents. In this way bounding surfaces between sets could be interpreted from changes in the attitude of sets of reflectors. The radar reflectors were recorded to depths of 15 m, but the best resolution of bounding surfaces was obtained in the upper 5 m of the dune sand. Bounding surfaces interpreted from reflectors define a main dune set 5–8m thick, with foresets up to 23 m long. Thicknesses of other wedge-shaped and tabular planar sets range from 0.75 to 1.5 m, averaging 1 m; set lengths range from 6 to 12 m, averaging 8.5 m. Trough-shaped sets range in thickness from 0.5 to 3m, averaging 1.1m, and range in width from 5 to 22 m, averaging 10 m. These trough structures may have been caused by the migration of scour pits associated with small superimposed dunes, or may be the result of scour fills formed during reversing winds. Reversing winds also formed numerous subtle bounding surfaces (reactivation surfaces) along the leading edge of the dune as it migrated, defining sets ranging in thickness from 0.5 to 2 m, averaging 1 m, and with foreset lengths ranging from 15 to 23 m, averaging 20 m. This study demonstrates the usefulness of ground-penetrating radar in resolving the internal structures of damp to dry, clay-free, aeolian dune sand. The numerous sets and bounding surfaces resolved with radar indicate a relationship between the complexity of internal structure and the multiple directions of sand-carrying or sand-scouring winds. In a tectonically active basin such as the San Luis Valley, these complex aeolian dune sand bodies have excellent preservation potential.Keywords:
Ground-Penetrating Radar
Sand dune stabilization
Trough (economics)
Bounding overwatch
Ground-Penetrating Radar
Katabatic wind
Sand dune stabilization
Prevailing winds
Bedrock
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This paper presents ground-penetrating radar (GPR) profiles obtained from an aeolian dune of the Botucatu Formation. A 50 MHz antenna was used and an excellent resolution was achieved down to 10 m deep. GPR surveys have allowed a detailed reconstruction of the bed morphology. In the parallel to the foreset dip GPR section the reflectors that delineate the cross bedding are straight and steep whereas in the transversal section they are subhorizontal. This configuration indicates a bedform with straight to slightly wavy crestline (2D). Third- and first-order bounding surfaces were interpreted from the termination of the reflectors' sets. The presence of several third-order-bounding surfaces suggests either an aeolian dune subject to cyclical fluctuations of its height or reverse dune. The identification of a first-order bounding surface indicates climbing and accumulation of dunes in the studied area
Ground-Penetrating Radar
Sand dune stabilization
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Ground-Penetrating Radar
Outcrop
Sand dune stabilization
Reflection
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Abstract Meltwater flows emanating from the Pyrenees during the Pleistocene constructed a braided outwash plain in the Ebro Basin and led to the karstification of the Neogene gypsum bedrock. Synsedimentary evaporite dissolution locally increased subsidence rates and generated dolines and collapses that enabled the accumulation and preservation of outwash gravels and associated windblown deposits that were protected from erosion by later meltwater flows. In these localized depocentres, maximum rates of wind deceleration resulted from airflow expansion, enabling the accumulation of cross‐stratified sets of aeolian strata climbing at steep angles and thereby preserving up to 5 m thick sets. The outwash plain was characterized by longitudinal and transverse fluvial gravel bars, channels and windblown facies organized into aeolian sand sheets, transverse and complex aeolian dunes, and loess accumulations. Flat‐lying aeolian deposits merge laterally to partly deformed aeolian deposits encased in dolines and collapses. Synsedimentary evaporite dissolution caused gravels and aeolian sand deposits to subside, such that formerly near‐horizontal strata became inclined and generated multiple internal angular unconformities. During episodes when the wind was undersaturated with respect to its potential sand transporting capacity, deflation occurred over the outwash plain and coarse‐grained lags with ventifacts developed. Subsequent high‐energy flows episodically reached the aeolian dune field, leading to dune destruction and the generation of hyperconcentrated flow deposits composed in part of reworked aeolian sands. Lacustrine deposits in the distal part of the outwash plain preserve rhythmically laminated lutites and associated Gilbert‐type gravel deltas, which developed when fluvial streams reached proglacial lakes. This study documents the first evidence of an extensive Pleistocene proglacial aeolian dune field located in the Ebro Basin (41˙50° N), south of what has hitherto been considered to be the southern boundary of Pleistocene aeolian deposits in Europe. A non‐conventional mechanism (evaporite karst‐related subsidence) for the preservation of aeolian sands in the stratigraphic record is proposed.
Outwash plain
Meltwater
Red beds
Bedform
Bedrock
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Interaction between wind regimes and topography can give rise to complex suites of aeolian landforms. This paper considers aeolian sediment associated wit11 troughs on Mars and identifies a wider range of deposit types than has previously been documented. These include wind streaks, falling dunes, lateral dunes, barchan dunes, linear dunes, transverse ridges, sand ramps, climbing dunes, sand streamers, and sand patches. The sediment incorporated into these deposits is supplied by wind streaks and ambient Planitia sources as well as originating within the trough itself, notably from the trough walls and floor. There is also transmission of sediment between d~~neTsh. e flow dynamics which account for the distribution of aeolian sediment have been modeled using two-dimensional computational fluid dynamics. The model predicts flow separation on the upwind side of the trough followed by reattachment and acceleration at the downwind margin. The inferred patterns of sediment transport compare well with the distribution of aeolian forms. Model data indicate an increase of wind velocity by approx. 30 % at the downwind trough margin. This suggests that the threshold wind speed necessary for sand mobilization on Mars will be more freq~~entmlye t in these inclined locations.
Trough (economics)
Bedform
Landform
Sand dune stabilization
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Interaction between wind regimes and topography can give rise to complex suites of aeolian landforms. This paper considers aeolian sediment associated with troughs on Mars and identifies a wider range of deposit types than has previously been documented. These include wind streaks, falling dunes, “lateral” dunes, barchan dunes, linear dunes, transverse ridges, sand ramps, climbing dunes, sand streamers, and sand patches. The sediment incorporated into these deposits is supplied by wind streaks and ambient Planitia sources as well as originating within the trough itself, notably from the trough walls and floor. There is also transmission of sediment between dunes. The flow dynamics which account for the distribution of aeolian sediment have been modeled using two‐dimensional computational fluid dynamics. The model predicts flow separation on the upwind side of the trough followed by reattachment and acceleration at the downwind margin. The inferred patterns of sediment transport compare well with the distribution of aeolian forms. Model data indicate an increase of wind velocity by ∼30% at the downwind trough margin. This suggests that the threshold wind speed necessary for sand mobilization on Mars will be more frequently met in these inclined locations.
Trough (economics)
Bedform
Landform
Sand dune stabilization
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Ground-Penetrating Radar
Landform
Deposition
Bed
Sand dune stabilization
Bedding
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