Browsing by Subject "Dunes"
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Item Aeolian dune dynamics and the stratigraphic record(2016-12) Brothers, Sarah Christian; Kocurek, Gary; Byrne, Shane; Hern, Caroline; Holt, John; Mohrig, DavidDunes and dune fields are dynamic features that respond to the unique boundary conditions under which they evolve by changing migration habits, types and frequencies of interactions, and morphologies. As such, dunes convey information about changing atmospheric and surface conditions on planetary bodies including Earth, Mars, Venus, and Titan, and the stratigraphic record left by these bedforms is used to interpret the evolution of paleoclimate and surface environments. In four parts this work examines how the dynamics of individual dunes, dune fields, and aeolian systems influence stratigraphic architectures across multiple spatial scales. At White Sands Dune Field, New Mexico, the decadal histories of defect and bedform repulsion-type interactions are traced through a time-series of images. The resultant stratigraphic architectures are derived from plan-view exposures and ground penetrating radar sensing of dune interiors. This is the first documentation of the unique stratigraphic architectures that result from interactions between dunes. Within Chasma Boreale, Planum Boreum, Mars, boundary conditions controlling the discrete geomorphic expressions of the Hyperboreae Undae dune fields are evaluated using morphologic distributions. The distributions reveal that dune field expressions change over short horizontal scales and are dependent upon wind regime and sediment state, which operate under the background condition of dune induration by ice. Hyperboreae Undae form only part of the Chasma Boreale sediment system, however. Perennial layered ice deposits also are present on the floor of the reentrant. The changing balance of ice- and sand-dominated processes in space and time have resulted in an accumulation that transitions between ancient aeolian cavi unit-style accumulation and modern layered ice deposit-style accumulation. These accumulations can be interpreted through aeolian sequence stratigraphy. Findings from the Chasma Boreale transitional sediment system are applied to nearby outcrops of a transitional section of ancient cavi unit, which formed in an environment analogous to the modern Chasma Boreale sediment system. A sequence interpretation of the transitional cavi unit stratigraphic succession demonstrates how the aeolian system transitioned into overlying water ice deposits without a gap in the geologic record, verifying that the cavi unit and NPLD should be unified under a sequence stratigraphic framework.Item Aeolian dune-field boundary conditions and dune interactions related to dune-field pattern formation on Earth and Mars(2009-12) Ewing, Ryan Cotter; Kocurek, GaryAeolian dune fields form some of the most striking patterns on Earth and Mars. These patterns reflect the internal dune dynamics of self-organization within boundary conditions, which are the unique set of environmental variables within which each dune field evolves. Dune-field pattern self-organization occurs because of interactions between the dunes themselves and the rich diversity of dune-field patterns arises because boundary conditions alter the type and frequency of dune interactions. These hypotheses are explored in three parts. First, source-area geometry and areal limits are two newly recognized boundary conditions. Measurements of crest length and spacing from satellite images of dune patterns with point and line source-area geometries show an increase in crest length and spacing over distance, whereas crest length and spacing in plane-sourced patterns emerge equally across the dune field. The areal limit boundary condition is the size and shape of the dune field itself. Empirical measurements from ten dune fields ranging over four orders of magnitude in area show that spacing increases and defect density decreases as the area of the dune field increases. A simple analytical model indicates that dune fields that are five times longer in the dune migration direction can achieve the greatest spacing for a given area. Second, time-series aerial photographs and airborne LiDAR show that fully developed, crescentic aeolian dunes at White Sands, New Mexico, interact and the dune pattern organizes in systematically similar ways as wind ripples and subaqueous dunes and ripples. Interaction type, classified as constructive, regenerative or neutral in terms of pattern development, changes spatially with the pattern because of the imposition of the line-source area and sediment availability boundary conditions. Upwind dominance by constructive interactions at the field line-source yields to neutral and regenerative interactions in the sediment availability-limited field center. Third, the dune-field pattern in the Olympia Undae Dune Field on Mars is comprised of two generations of dunes. This scenario of pattern reformation with a new wind regime shows that the emergence of the younger pattern is controlled by the boundary condition of the antecedent dune topography imposed upon the interaction between the younger and older patterns.Item Architecture of deposits formed in a tectonically generated tidal strait, upper Baronia Fm., Ager Basin, South Central Pyrenees, Spain(2011-05) Bens, Ashley Elizabeth; Steel, Ronald J.; Olariu, Cornel; Kerans, CharlieThe upper Baronia Fm. of the Ager Basin, Spain, is composed of a hierarchy of prominently stacked sets of primarily unidirectional cross-strata in units up to 40m thick. These large sets of cross-strata are interpreted as deposits of migrating subaqueous tidal simple dunes, compound dunes, and compound dune complexes within an approximately 10km wide north-east to south-west oriented seaway with water depths of a calculated 60-90m. These interpretations are opposed to prior interpretations of the upper Baronia Fm. which suggests deposits were formed by tidal bars within a deltaic environment (Mutti et al., 1985). Dunes developed due to dominantly north-east directed tidal currents driven through the strait by tidal phase differences between the two bodies of water (Mediterranean and Atlantic basins) connected by the seaway. Evidence for syn-tectonic deposition further constrains timing of movement of the northern basin bounding Montsec thrust to the early Eocene. Indicators for movement on the Montsec thrust include the development of the Ager Basin elongate to the thrust front, and syn-tectonic signals in the fill of the basin such as local conglomerate wedges and emplacement of olistoliths. Individual cross-stratified successions are interpreted to have formed with variable flow velocity and orientation, resulting in a basin wide stacking of compound dune complexes. These compound dune complexes form cross stratified successions which are distributed throughout the basin according to the variable current speeds, dune size which impacts migration, and sediment availability during deposition. This results in the observed distributions of muddy and sandy sediments, where finer grained materials accumulate preferentially in the low energy troughs of the hierarchy of compound dunes.Item Bedform interaction and preservation(2015-08) Swanson, Travis Eric; Mohrig, David; Kocurek, GaryThe shape and motion of bedforms are envisioned to arise as a consequence of a hierarchical cascade of interactions that occur within a bedform field. Interactions within the field are driven by conditions external to the bedform field, called boundary conditions. One such boundary condition is transport regime, the annual cycle of sediment transporting winds that drive uneven movement and deformation of bedforms. Paradoxically, uneven bedform movement results in geometric organization, as bedform crests align to wind regime. High resolution mapping of bedform crest movement reveals stochastic crest motion that acts to maintain the geometric organization of the bedform field. Stochastic motion of bedforms is driven by the interaction of bedform topography with sediment transporting flows. One such interaction occurs when oblique bedform crests create a wake containing trailing helical vortices. Each trailing helical vortex may preferentially route sediment to or away from a downstream bedform. This routing causes deformation or ablation of the downstream bedform. Either way, a section of the downstream bedform may deform to an oblique orientation. The oblique section may also trail a helical wake and further propagate stochastic bedform motion downstream. Bedform field boundary conditions provide an external influence on stochastic bedform motion. A numerical model of dynamic bedform topography is fueled by four sets of boundary conditions: a uni-modal and bi-modal wind regime each investigated with and without the restriction of a linear bedform source area. The chosen boundary conditions are demonstrated to influence the shape and stochastic motion of bedforms within the field the rate at which the bedforms grow and mature. Fields of bedforms, driven by boundary conditions may leave a stratigraphic record of their stochastic motion. Given sufficient time, boundary conditions are transient. A numerical experiment is performed to investigate the preservation of signals from both stochastic dune motion and a transient boundary condition, the strengthening and weakening of sediment transport with time. The numerical experiment demonstrates that stochastic dune motion and transient boundary conditions influence the preservation of bedform topography in the resulting stratigraphic record.Item Dune behavior in a multidirectional wind regime : White Sands Dune Field, New Mexico(2014-08) Pederson, Anine Oehlenschlaeger; Kocurek, GaryAs with most dune fields, the White Sands Dune Field in New Mexico forms in a wind regime that is not unimodal. In this study, dune behavior at White Sands was documented from a time series of five lidar-derived digital elevation models (DEM) and compared to a record of wind direction and speed during the same period. For the study period of June 2007 - June 2010, 244 sand-transporting wind events occurred and define a dominant wind mode from the SW and lesser modes from the NNW and SSE. Based upon difference maps and tracing of dune brinklines, overall dune behavior consists of migration to the NE, but with along-crest migration of dune sinuosity to the SE. Permutations of the DEMs allow matching specific dune behavior with wind modes. The SW winds are transverse to dune orientations and cause most forward migration. The NNW winds cause along-crest migration of dune sinuosity and low stoss bedforms, as well as SE migration of NE-trending dune terminations. The SSE winds cause ephemeral dune deformation, especially crestal slipface reversals. Dune deformation occurs because of unequal deposition along the lee face as a function of the incidence angle formed between the wind and the local brinkline orientation. Incidence-angle control on dune deformation and types of lee-face surface processes allows for an idealized model for White Sands dunes. The dunes behave as complex systems in which each wind event deforms the dune shape, this new shape then serves as the configuration for the next wind event.Item Formation and organization of aeolian bedforms(2004) Valentine, Daniel Shane; Kocurek, GaryAeolian bedforms represent some of the most intricate land forms on the planet encompassing a variety of scales from wind ripples on the order of centimeters to desert sand seas comprising thousands of square kilometers. Hypotheses focusing on the formation and organization of these bedforms have been nearly as complex and varied as the dunes themselves. Three distinct topics were chosen for the focus of this study: (1) the formation and organization of wind ripples; (2) morphology and sampling effects from measurement of dune lee side airflow; and (3) the formation of star dunes based on wind tunnel studies of small scale star ripples. Each of these topics comprises a chapter of this thesis, which will be published individually at a later date. Wind ripple formation has been a topic of discussion for nearly a century. For this study ripples were formed in a wind tunnel and recorded using time-lapse photography. Data reveal that ripples form in a pattern that closely matches the formation of computer simulated bedforms using only basic rules of sand transport, but neglecting any sort of aerodynamic control of bedform morphology. These findings support the hypothesis that aeolian bedforms are self-organizing and that no predisposed template of formation exists within the fluid flow. Aeolian dune lee side airflow comprises a complex subject of study. The airflow is the result of a complex interaction between the wind and the sand surface. Using a new technique of wind velocity data collection, a more accurate picture of the lee side airflow morphology is now available. This data set provides the most comprehensive look at this environment to date. Of particular interest are the data outlining the Internal Boundary Layer, which is closest to the sediment surface and controls sediment transport. The data collected imply that no point of dune nucleation, caused by a drop in shear velocity, exists in the interdune corridor, rather, the shear increases downwind until another dune is reached. Conclusions based on the data support the theory of self organization when applied to desert dunes. Star dunes are the largest and most complex single bedforms found on Earth. Due to their size and long development time, observations of star dune formation are scant. Using a rotating table mounted in a wind tunnel, a series of complex wind patterns were simulated. Small scale star ripples were formed and the process recorded using time-lapse photography. Based on these findings at least two distinct types of star dunes are possible in nature. Because of the relationship between the wind regime and the bedforms produced, a causal link between aerodynamic control and bedform formation is dubious. More likely, however, is the self organization theory which has proven useful in all scales of bedforms examined in this studyItem Pattern formation and preservation in aeolian systems(2017-05-02) Day, Mackenzie Denali; Kocurek , Gary; Mohrig, David; Holt, John; Kim, Wonsuck; Anderson , WilliamAeolian sediment transport forms natural patterns common on Earth and other planetary bodies. The self-organization of sand in transport results in dune fields with dune morphologies determined by wind regime. Patterning in dune fields is known to arise from the autogenic process of dune interactions, but the evolution of dune patterns over time remains poorly constrained. In this work dune fields were parameterized in terms of dune interactions to quantify dune-field pattern stability. Interactions are fundamental to dune-field development, but studies of interactions have focused on their surface expression, and how interactions are expressed in the ancient record has yet to be documented. This problem is addressed with five examples of interaction-generated stratigraphy identified in well-known Jurassic aeolian sandstones using criteria based on recent near-surface interpretations of interaction stratigraphy form White Sands Dune Field, New Mexico. Interactions control the autogenic development of dune fields, but allogenic factors including basin subsidence, water table rise, and sediment supply largely control the accumulation and preservation of aeolian strata. In a case study of a section of the Jurassic Entrada Sandstone, this work addresses the interplay between allogenic and autogenic controls on what is actually preserved in the rock record, and demonstrates how long stretches of time can be collapsed into surfaces between geologic units that represent relatively short-lived events. The competition between allogenic and autogenic influences on aeolian pattern formation is not unique to Earth, and Mars also hosts patterned landscapes thought to be generated by aeolian sediment transport. Such landscapes include intra-crater layered mounds such as Aeolis Mons in Gale crater, the landing site of the Mars Science Laboratory rover. Competing hypotheses about whether these mounds formed by aeolian erosion of crater-filling deposits, or by aeolian deposition were addressed with wind tunnel and large eddy simulation experiments. The results are compatible with an erosional origin of the mounds. Additional analyses of wind-formed landscapes within and around Gale crater further supported the wind-erosion hypothesis of the central mound.Item Patterns and paleoshorelines of White Sands Dune Field, New Mexico(2011-05) Baitis, Elke Elise; Kocurek, Gary; Mohrig, David; Helper, MarkThe dune field at White Sands, New Mexico, shows a well-defined pattern of dunes and interdune areas, as well as spatial variations in this pattern. The purpose of this research is to determine which measured pattern parameters are most consistent across the dune field and to determine the cause of depositional spatial variability. This was accomplished using an airborne LiDAR generated digital-elevation model (DEM) collected in June 2007 and covering 39 km² of the dune field. Properties of the dune field are defined by measurements from three dune populations: 1) 110 randomly selected dunes, 2) 247 dunes along transects oriented in the net transport direction, and 3) 171 dunes from three zones within the field where differences in pattern are visible. Measurements of eight common dune parameters show that the lowest coefficients of variation occur with dune orientation and crestline sinuosity, which largely define the field pattern. Cross-plotting of parameters shows generally poor correlations, which is thought to reflect variation around field-scale means that are comparable to other dune fields globally. Removing the dunes from the LiDAR DEM reveals a depositional substrate with breaks in slope interpreted as three paleoshorelines associated with Pleistocene Lake Otero. The paleoshorelines are antecedent boundary conditions that exert the primary control on spatial variability within the dune pattern.Item Pyroclastic flows and surges(2009-03) Barker, Daniel S.Item Tidal signatures in sand prone, tectonically generated Jurassic straits, Scotland(2006) Blackwood, Stuart, 1972-; Steel, R. J.The Mid-Jurassic Bearreraig Formation is superbly well exposed on the Isles of Skye and Raasay in NW Scotland. In southern Skye the Bearreraig displays a spectacular, cyclical hierarchy of sand rich, tide-generated bedforms: 1. Upward thickening to thinning compound and compound-compound trough cross-sets (3D dunes) up to 2.5m thick 2. Compound trough cross-sets (3D dunes) up to 10m thick 3. Upward thinning, simple and compound planer cross-sets (2D dunes) up to 50cm thick Tectonically-generated sequences (up to 140m thick) consisting of these three dune types can be recognized within the formation. Sedimentation occurred in NNE-SSW-aligned, North Skye/Raasay and South Skye half-grabens, defined by Aalenian-Bajocian faulting associated with the initial opening of the North Atlantic. SSW oriented (S. Skye) and NNE oriented (N. Skye/Raasay) fault-parallel paleocurrent directions (indicative of basin separation) suggest that the faults were active and had enough topographic expression to enhance the tidal regime in these very narrow, block-tilted zones. Previous depositional interpretation of the Bearreraig is one of alternating tide-dominated deltaic and macro-tidal estuarine cycles with high-frequency regressions and transgressions. Facies cropping out at Trotternish in N. Skye, and Raasay display trends consistent with this regional interpretation. However, in southern Skye the overall thickness of the cross-stratified succession, anomalously large size of the bedforms (implying relatively deep water), intensely sand-rich character of the succession, together with the high bioclastic content of broken shell debris is indicative of a more complex, higher-energy depositional environment. The character and scale of the bedforms, and the absence of pro-delta, lagoonal or delta-plain deposits suggests a narrow seaway or tidal strait generated by active tilting of narrow fault blocksItem Wind Work(2009-04) Barker, Daniel S.