Eolian landforms are the natural result of the action of strong winds on an exposed land surface. Certain characteristics of the wind can be interpreted from these landforms. The spacing of sand dunes and draas (dune groups) is one parameter that can be used to estimate wind velocity and available wind power. LANDSAT imagery or aerial photography may be used to identify the eolian landforms, and measurements of spacings can be made directly from these datum. Fourier analysis of these measurements reveals the predominant spacings. Assuming that shear velocity is the dominant factor determining the wave length of bedforms, the spacing (λ) can be used to compute the threshold shear velocity (V *T ) using mathematical relationships derived by Wilson (1972): V ′ * T = 2.06 λ 1 / 3 cm / sec ( dunes ) V ′ * T = 0.59 λ 1 / 3 cm / sec ( draas ) These values may then be used to estimate wind velocity at any other height above the surface or to estimate available wind power. The Killpecker and Seminoe Dune Fields in south-central Wyoming were used as test areas in which to demonstrate this technique.

Abstract A review of the data gathered over the last five decades on the NW African continental shelf within numerous research programmes shows that the sedimentary processes along the shelf are driven by long-term factors such as Quaternary glacial–interglacial periods and shelf morphology, and by short-term factors such as fluvial and aeolian sediment supply, local climate (temperature, rainfall and wind) and hydrodynamic conditions (tides, swell, longshore current, the Canary Current and upwelling). Based on the sedimentary characteristics, the margin has been subdivided into four segments: northern Morocco (30–36°N) and the Grande Côte of Senegal (15–16°30′N) show long mid-shelf mud belts controlled by fluvial input and low hydrodynamic energy; southern Morocco–Mauritania (16°30′N–30°N) and the Petite Côte south of Cape Vert, Gambia and Casamance (12–15°N) are dominated by biogenic and aeolian sands or no sediment input. In this chapter, we present a synopsis of the state of knowledge on the NW African shelf sediments and processes based on various published and unpublished documents.

Abstract In this study, based on a sedimentary analysis, an interpretation in terms of sequence stratigraphy is proposed for the ‘Rich’ group of the Early Devonian Drâa Basin (of Pragian to late Eifelian age). This group is composed of four depositional sequences (third order) containing a transgressive systems tract and a highstand systems tract, both deposited on a storm- and tide-dominated shelf. The depositional sequences are related to global sea-level changes but their geometric architecture is controlled by tectonic subsidence and by sediment supply. The basin forms an ENE–WSW channel with two internal high blocks acting on the depocentre shifts. The channel is surrounded by high points and by marine distal condensed series, and bordered by inherited various regional faulted axes. The regional setting corresponds to an extensional passive margin on the northern edge of the West African craton.

Abstract Auk field is located in the central North Sea and produces oil from Permian Zechstein carbonates and the underlying Rotliegend sandstones. The Rotliegend is 150 to 500 m thick and can be divided into five discrete episodes of desert sedimentation. Deposits of the upper three episodes are oil bearing, with eolian slipface sands forming the main producing intervals. The earliest deposits (unit 5) are localized waterlain conglomerates, which possibly infill topography on the unconformity with the Devonian. They are overlain by a large wedge-shaped mass of eolian slipface sands (unit 4) that onlaps the Devonian. Unit 3 represents a change in eolian deposition with a marked increase in wind-ripple laminated strata. Their abundance probably indicates more variable winds, and thin conglomerates of interdunal dolomite clasts suggest periodically wet conditions. This depositional unit varies in thickness by about 100%, with thick areas corresponding to the stacked deposits of slipfaceless draa and thin areas to stacked interdunes. Dune slipface sands within the draa accumulations are orientated toward the east and cannot be correlated over hundreds of meters between wells in a cross-wind direction. Unit 2 contains a still greater proportion of wind-ripple laminated sands and waterlain deposits. Its geometry is dominated by a large depositional mound -7 km across wind, 87 m thick, and thinning to 10 m in its adjacent interdunes. The mound is located above an interdunal thin of unit 3. Several intervals of dune and probably draa slipface sands occur where the mound is thickest, and a distinctive body of fine-grained eolian sands mantles its northern flank. The uppermost unit (1; Weissliegend) comprises waterlain mass-flow sands that partially infill interdunal lows and appear to represent reworking of a largely abandoned erg by rain water. Intercalated organic-rich shales and dolomites contain indications of evaporitic conditions, desiccation, and deflation and yield no evidence of marine faunas. The succeeding marine Zechstein gently inundated remaining topography.

Abstract In order to understand how an eolian sequence was generated, a 45-m-high, 54,000-m 2 outcrop of Middle Jurassic Page Sandstone near Page, Arizona, was mapped and reconstructed within the context of a 9.5-km-long lateral traverse. Recognition of prominent bounding surfaces and distinctive sequences of cross-strata between surfaces allows the identification of nine complexes. The 'A' complex consists primarily of coarser grained, low- to moderate-angle wind-ripple laminae arranged in sets, but also shows a single set of high-angle grainflow strata. The complex is situated in a depression on the 3-2 unconformity at the top of the Navajo Sandstone, and is interpreted to represent deposits of isolated barchan dunes. slipfaceless bedforms, and ‘wedges’ of sand filling the depression. The ‘A’ complex may be part of the J-2 ‘event’, predating the Page per se and representing local deposition in an overall deflationary period, or it may herald widespread renewed deposition immediately preceding the Page system. The B' and 'C complexes are separated by a prominent surface, but both consist of large sets of trough cross-strata in which the directional spreads of the foresets define crescentic shapes, and the dominance of grainflow strata indicates deposition by transverse dunes. Complex ‘B’ also contains sets that are interpreted as the result of secondary air flow along flanks of crescentic draas. A wavy-laminated, silty, fine-grained, red standstone rests on a prominent surface that truncates the ‘C’ complex. Cross-strata above this surface are distinctly different from those of the underlying complexes. Complexes ‘D’ to T, each of which is separated by a surface with polygonal fractures, are similar to one another and consist of sets dominated by wind-ripple laminae with strongly developed cyclic stratifications, and are interpreted as oblique-dune deposits. The complexes differ, however, in interpreted dune size and whether the bedforms were simple dunes or draas. All complexes on the outcrop show a mean transport direction to the south, which corresponds to predominant northerly summer winds. Dunes represented by complexes ‘B’ and ‘C’ appear to have been little affected by winter winds from the northeast, but dunes in the overlying complexes were moderately modified by more variable winter winds. Radically different interpretations of the complexes and prominent surfaces can be reasonably proposed. Interpretation of the surfaces as first-order surfaces envisions that the vertical and lateral Page sequence results from uninterrupted accumulation of a widespread erg, albeit under changing conditions with evolving dune types. Interpretations of the surfaces as super surfaces and the complexes as deposits of separate dune fields and ergs forces the Page sequence to be viewed as a complex amalgamation of overlapping segments of ergs and dune fields formed in a dynamic-basin setting with varying times, sites, and degrees of deposition and erosion. These alternate views are fundamental to the understanding of the genesis of eolian sequences at the basin level, and are presently best addressed at the regional and not outcrop scale.