Lee Of Obstacles Research Articles

Geoarchaeological importance of sub-Arctic bird perches dated by lee side turf mound accumulation and identified by lichen plumes

ABSTRACTOrganic-rich turf mounds, fertilised with bird droppings, may develop in the lee of obstacles, including either natural or human-made structures, boulders and rock cairns in exposed mountain and sub-Arctic and Arctic periglacial/proglacial environments. These boulder-leeside deposits consist mainly of organic materials intermixed with loess trapped in baffling agents (turf), and may sometimes contain archaeological features. Certain animal and bird species may also contribute to turf-mounds by frequenting these locations. Boulders and other natural and human-made obstructions are observed to have lichen plumes, principally Xanthoria spp?, lacing the tops and downwind side of the mound and fertilised by defecating birds and mammals. Turf mounds, consisting of organic soil, can be used for both relative and chronometric age dating, the buried peat providing 14C dates for archaeological context for specific mountain sites when artefacts are recovered. In addition, field and laboratory analyses of soils provide important palaeoenvironmental and geoarchaeological information about the sites. Examples are provided from a field site in the Norra Storfjället in the Swedish SubArctic.

Volcaniclastic aeolian deposits at Sunset Crater, Arizona: terrestrial analogs for Martian dune forms

ABSTRACTSunset Crater in north‐central Arizona (USA) is a 900‐year‐old scoria‐cone volcano. Wind action has redistributed its widespread tephra deposit into a variety of aeolian dune forms that serve as a terrestrial analog for similar landforms and aeolian processes on Mars. Fieldwork was conducted to collect essential geomorphological and sedimentological data, and to establish a baseline for the type and morphometry of dunes, physical properties, interactions with topography, and saltation pathways. Our analyses focused primarily on coppice dunes, falling dunes, wind ripples, and sand streaks. For all collected volcaniclastic aeolian sediment samples, the sand‐size fraction dominated, ranging from almost 100% sand to 74.6% sand. No sample contained more than 1.6% silt. The composition is overwhelmingly basaltic with non‐basaltic particles composing 2 to 6% of the total. Coppice (nebkha) dunes form where clumps of vegetation trap saltating particles and create small mounds or hummocks. Mean grain size for coppice dune samples is coarse sand. Measured dune height for 15 coppice dunes ranged from 0.3 to 3.3 m with a mean of 1 m. Mean length was 6.7 m and mean width was 4.8 m. Falling dunes identified in this study are poorly developed and thin, lacking a prominent ramp‐like structure. Mean wavelength for three sets of measured ripples ranged from 22 to 36 cm. Sand streaks extend downwind for more than a kilometer and are up to 200 m in width. They commonly occur on the lee side of mesas and similar landforms and are typically the downwind continuation of falling dunes. Falling dunes, wind ripples, and sand streaks have been identified on Mars, while coppice dunes are similar to Martian shadow or lee dunes in which sand accumulates in the lee of obstacles. Copyright © 2012 John Wiley & Sons, Ltd.

Sedimentology of cold-climate aeolian sandsheet deposits in the Askja region of northeast Iceland

Active aeolian sandsheets cover an area of 270 km 2 in the Askja region of NE Iceland. The sandsheet is divided into (i) a deflationary upwind part that is characterised by deflation lags, ventifacted boulder fields and restricted aeolian sand patches in the lee of obstacles, (ii) an accumulating central part that is characterised by continuous aeolian sand cover up to 10 m thick, where three scales of aeolian ripples are evident along with larger scale forms including longitudinal zibars, and (iii) a downwind part that is currently subject to aeolian bypass with only localised accumulation occurring in topographic hollows between basaltic lava fields. Trains of aeolian granule megaripples are a characteristic landform of the downwind part of the sandsheet. These sinuous-crested, oblique bedforms attain heights of 0.35 m and downwind spacings of 6 m. They are composed principally of low-density, pebble-grade pumice clasts and other reworked volcaniclastic detritus derived from the nearby Askja volcanic centre. The distribution of landforms across the sandsheet reflects a regional aeolian sediment budget that is controlled by dynamic interactions between glacial, ice-margin fluvial, aeolian and volcaniclastic processes under the influence of a distinctive climatic regime. Sediment supply at the upwind sandsheet margin, adjacent to the Dyngjujökull glacier, increases during summer periods when rates of glacial ablation are at a maximum. Longer term controls on sediment supply rate are determined by episodes of glacier surging and retreat, and by periodic eruptions of volcaniclastic debris from the Askja volcanic centre. The availability of sediment for aeolian transport is controlled by the level of the groundwater table, the depth and continuity of the permafrost layer in the proglacial area, the degree of development of armoured pebble lags and freezing of the sandsheet surface during winter months. The transport capacity of the wind varies on a seasonal basis with mean wind velocities and wind gustiness reaching a maximum during winter periods. Both winter and summer mean wind velocities have been steadily increasing since formal records began in 1969. The suite of aeolian landforms developed within the Askja region, as defined by sediment texture and composition, and bedform scale, morphology and spatial distribution, reflects a special set of controlling circumstances, which mean that the Askja Sandsheet deposits differ significantly from those described from elsewhere.

Geologic settings of Martian gullies: Implications for their origins

Martian gullies are found on steep slopes of all origins, on all sorts of terrains of all ages, scattered across nearly all of Mars. Gullies are observed on all manner of substrates (layered, massive, shattered, rubble), with or without nearby mantling deposits. Gullies are most common in the southern midlatitudes but also occur in the northern hemisphere, in near polar terrain, on equatorial volcanoes, and on northern plains. Most gullies in the southern hemisphere are on south‐facing slopes, but they occur on slopes of all orientations. Gullies are among the youngest features on Mars but locally are overlain by eolian deposits and cut by faults. Old or eroded gullies are rare, and those found have been partially stripped from slopes, leaving no rock debris behind. Most gully deposits contain no detectable rocks. These data are inconsistent with published hypotheses of gully formation, including seeps and breakouts of water or brine, hydrothermal activity, cryovolcanism, and breakouts from liquid carbon dioxide. The data are consistent with gullies being dry flows of eolian material (dust and silt), comparable to climax snow avalanches on Earth. Eolian sedimentation should be correlated little with underlying geology: cause of slope, age of terrain, type of terrain, or the nature of the rocks. Eolian sedimentation should be correlated with wind deceleration (which will cause suspended sediment to drop), and areas with common gullies are those with strong wind deceleration (predicted by global circulation model). In such areas, sediment will be deposited preferentially in the lee of obstacles; for the gully‐rich areas of the southern midlatitudes, winds blow from the NNW, so that sediment is deposited on SSE‐facing slopes (i.e., poleward). These predictions are in accord with observations.

Trilobite Taphonomy of a Silurian Reef: Attawapiskat Formation, Northern Ontario

Trilobites of exhumed reefs of the Late Llandoverian Attawapiskat Formation of northern Ontario occur in accumulations that have undergone extensive taphonomic sorting. Most spectacular are monospecific to low-diversity pockets of the large scutelluine Ekwanoscutellum. These accumulations, which cover areas of up to a square meter, are nested stacks of mostly inverted (concave-up) large sclerites, the majority of which are pygidia. They may have formed by passive settling of sclerites in current-shadows in the lee of obstacles in the reef system. Excellent preservation of the sclerites, absence of epibionts, and minimal internal sediment with in the pockets suggest rapid accumulation. Aggregation of individuals for molting and reproduction may explain the availability of large numbers of Ekwanoscutellum sclerites as raw material for the accumulations. A second, less common assemblage is a more diverse association dominated by the scutelluine Meroperix and the illaenid Stenopareia, and is often associated with abundant calciate brachiopods. Although some sorting has taken place, more variable dorso-ventral orientations and lower sclerite abundances indicate a different origin from the Ekwanoscutellum Association. The data underscore the taphonomic complexity of patchy environments such as Silurian reefs and indicate that extensive sorting of skeletons is a significant source of bias in the paleoecologic analysis of non-framework components of reef biofacies.

Modelling the spatial variation of processes in the redistribution of soil: digital terrain models and 137Cs in southwest Niger

An interpolated grid-based digital elevation model (DEM) was created from survey data to derive two compound topographic indices, a stream power index to provide a surrogate measure of water erosion and an index of susceptibility to aeolian processes. Maps of both indices were compared with a map of interpolated percentage 137Cs residuals, used as a surrogate for net soil redistribution over the last ca. 30 years. Results suggest that wind erosion is pervasive throughout the study area, whereas water erosion is concentrated on the steeper slopes. Areas of potential aeolian deposition were identified on the lee-side of obstacles, but were validated in only a few cases, by the map of 137Cs residuals. This anomaly probably results from considerable reworking of aeolian material deposited in the lee of obstacles, on slopes highly susceptible to water erosion. Although the digital terrain models (DTMS) could not explain the pattern of 137Cs residuals in some locations, the inclusion of the percentage of vegetation cover further elucidated the patterns of soil redistribution. Vegetation cover was found to explain a considerable amount of variation in the patterns of soil redistribution over the medium-time scale. Further research into the modelling of these processes is necessary to elucidate the pattern of vegetation cover removal and its link with soil redistribution.

Turbulence at a roughness transition in a depth limited flow over a gravel bed

Turbulence statistics derived from velocity measurements along a longitudinal transect characterized by a roughness transition are presented. The bed morphology along the transect is dominated by composite scales of roughness ranging from an armoured bed in the upstream section to the presence of large, protuberant clasts superimposed on the bed in the downstream section. An array of four electromagnetic current meters operating at a frequency of 20 Hz was used to measure the longitudinal and vertical velocities at various heights above the bed surface and at different positions along the transect. Emphasis is put on the longitudinal and vertical distributions of turbulent flow statistics such as Reynolds shear stress, uv cross-correlation, and quadrant analysis of velocity fluctuations. The results indicate that the presence of protuberant clasts modifies the turbulence structure, increases turbulence intensity and appears to dominate turbulence generation. Distinct flow zones are observed. In particular, a clear distinction is made between (i) a sublayer below the top of the major roughness elements identified by fluid motions predominantly towards the bed surface and characterized by intense eddying, with some vortices or rollers possibly being trapped in this near-bed flow region and (ii) an inner layer, located immediately above the major protuberant clasts, identified by positive vertical velocities and hence upward-directed fluid motion. The dominant generative mechanism of flow structure appears to be the shedding of vortices from the lee of obstacles. Interaction of vortices stretched by shear into the main body of flow downstream from major obstacles dominates the turbulence structure in depth-limited flows over coarse and irregular bed surfaces.

Obstacle marks produced by flow around stranded ice blocks during a glacier outburst flood (jökulhlaup) in west Greenland

ABSTRACTThe effects of glacier ice block grounding on the morphology and sedimentology of proglacial fluvial outwash were examined during a glacier outburst flood or jökulhlaup, near Søndre Strømfjord, west Greenland. Observations made during and after the 1987 jökulhlaup both on the surface of an ice contact delta and within a confined valley sandur plain provided information about the formation of ice block obstacle marks and the significance of these bedforms for sandur morphology and sedimentology. Flow directions determined from obstacle mark morphology have been used successfully to chart flow direction changes on the falling limb of the jökulhlaup. Maximum flow depths for scour around stranded ice blocks may be given by 0·5–0·9 times the diameter of the ice block, as estimated from the depth of scour, the height of the obstacle shadow or the extent of ice block meltout sediments. Minimum flow depths can be represented by the height of the obstacle shadow above the mean bed level. The internal composition of the shadow indicates the ability of the flow to transport various sizes of material into the lee of obstacles. Ice block obstacle marks within the distal portion of the sandur initiated waning stage channel change. Proximal and lateral erosion around stranded ice blocks extended downstream from the ice block, forming chute channels which then captured waning stage flows, resulting in significant bar incision with associated deposition of lobate or deltaic deposits. It is suggested that ice block obstacle marks are important in terms of channel morphology, channel morphological change and their usefulness as palaeohydrological indicators.

Topographic eddies in temporally varying oceanic flows

Abstract In this paper we examine the behaviour of oceanic unsteady flow impinging on isolated topography by means of numerical simulation. The ocean model is quasigeostrophic and forced by an oscillatory mean flow. The fluid domain is of the channel type and open-boundary numerical conditions are used to represent downstream and upstream flow. In certain cases, vortex shedding, either cyclonic or anticyclonic, is observed in the lee of obstacles. Such shedding can be explained as the consequence of both an enhanced process of vorticity dissipation over the topography which locally affects the balance of potential vorticity on the advective timescale, and a periodic dominance of advective effects which sweep the fluid particles trapped on the seamount. For refined resolution and smallest viscosity the model will predict flows in which the shed eddies are coherent structures with closed streamlines. The model suggests a mechanism by which topographically generated eddies may be swept away from a seamount i...

The Origin of Glacially Fluted Surfaces-Observations and Theory

AbstractStudies of fluted surfaces beyond the margins of glaciers in Spitsbergen, Iceland, Norway and the Alps show that almost all emanate from rigid obstructions, commonly boulders in till. Field relations of flutes are described and it is shown that a relationship exists between flute height and the height of the initiating obstruction. Subglacial observations indicate that flutes form when till is intruded into tunnels which tend to open up in the lee of obstacles. The pattern of strain implied by this process is shown to be reflected by micro- and macrofabrics in the till. The commonly found occurrence of an average spacing between flutes does not arise because of some rhythmic or periodic mechanism in the glacier, but is produced by the random seeding of boulders which themselves generate flutes.It is suggested that the term flute be used as a genetic rather than a descriptive term, and be restricted to long parallel-sided ridges which reflect accurately the direction of ice movement and which form when deformable subglacial materials are intruded into tunnels which tend to open up on the lee sides of single rigid obstructions on the glacier bed.

The Origin of Glacially Fluted Surfaces-Observations and Theory

Abstract Studies of fluted surfaces beyond the margins of glaciers in Spitsbergen, Iceland, Norway and the Alps show that almost all emanate from rigid obstructions, commonly boulders in till. Field relations of flutes are described and it is shown that a relationship exists between flute height and the height of the initiating obstruction. Subglacial observations indicate that flutes form when till is intruded into tunnels which tend to open up in the lee of obstacles. The pattern of strain implied by this process is shown to be reflected by micro- and macrofabrics in the till. The commonly found occurrence of an average spacing between flutes does not arise because of some rhythmic or periodic mechanism in the glacier, but is produced by the random seeding of boulders which themselves generate flutes. It is suggested that the term flute be used as a genetic rather than a descriptive term, and be restricted to long parallel-sided ridges which reflect accurately the direction of ice movement and which form when deformable subglacial materials are intruded into tunnels which tend to open up on the lee sides of single rigid obstructions on the glacier bed.

The Debris Content of Surging Glaciers in Svalbard and Iceland

In Svalbard and Iceland there appears to be much more debris entrained in glaciers that surge than in those which do not. Conditions particularly favourable for the basal incorporation of debris develop as a consequence of the high flow velocities attained by a surge. These are increased cavitation in the lee of obstacles and an increased supply of basal melt water resulting from frictional heat and from the trigger zone. Layers of regelation ice incorporating debris can thus develop to a much greater vertical and horizontal extent than in non-surging glaciers. Excessive shearing, and the distortion of foliation structures in the terminal zone of compressive flow, enhance the vertical development of the debris-rich regelation layers. Glaciers that surge over outwash and/or fjord-bottom sediments become particularly rich in debris.

The Debris Content of Surging Glaciers in Svalbard and Iceland

In Svalbard and Iceland there appears to be much more debris entrained in glaciers that surge than in those which do not. Conditions particularly favourable for the basal incorporation of debris develop as a consequence of the high flow velocities attained by a surge. These are increased cavitation in the lee of obstacles and an increased supply of basal melt water resulting from frictional heat and from the trigger zone. Layers of regelation ice incorporating debris can thus develop to a much greater vertical and horizontal extent than in non-surging glaciers. Excessive shearing, and the distortion of foliation structures in the terminal zone of compressive flow, enhance the vertical development of the debris-rich regelation layers. Glaciers that surge over outwash and/or fjord-bottom sediments become particularly rich in debris.