Gary Kocurek

Curiosity at Gale Crater, Mars: Characterization and Analysis of the Rocknest Sand Shadow

The Rocknest aeolian deposit is similar to aeolian features analyzed by the Mars Exploration Rovers (MERs) Spirit and Opportunity. The fraction of sand <150 micrometers in size contains ~55% crystalline material consistent with a basaltic heritage and ~45% x-ray amorphous material. The amorphous component of Rocknest is iron-rich and silicon-poor and is the host of the volatiles (water, oxygen, sulfur dioxide, carbon dioxide, and chlorine) detected by the Sample Analysis at Mars instrument and of the fine-grained nanophase oxide component first described from basaltic soils analyzed by MERs. The similarity between soils and aeolian materials analyzed at Gusev Crater, Meridiani Planum, and Gale Crater implies locally sourced, globally similar basaltic materials or globally and regionally sourced basaltic components deposited locally at all three locations.

Late Pleistocene and Holocene dune activity and wind regimes in the western Sahara Desert of Mauritania

The western Sahara Desert in Mauritania is dominated by extensive sand seas consisting largely of linear dunes. Analyses of Landsat images, geomorphic and stratigraphic studies, and optically stimulated luminescence dating of dunes in the Azefal, Agneitir, and Akchar sand seas provide evidence that three main generations of dunes were formed during the periods 25–15 ka (centered around the Last Glacial Maximum), 10–13 ka (spanning the Younger Dryas event), and after 5 ka. The wind regimes that occurred during each of these periods were significantly different, leading to the formation of dunes on three distinct superimposed trends—northeast, north-northeast, and north—and the development of the sand seas as composite geomorphic features.

Distinctions and uses of stratification types in the interpretation of eolian sand

ABSTRACT Eolian and subaqueous cross-strata cannot be distinguished reliably by many commonly cited criteria. They can, however, generally be distinguished by the characteristics of their component types of stratification, which represent processes of sorting and transport of the grain population on dunes. Eolian dune stratification types consist of grainflow cross-strata, grainfall laminae, and wind ripple-generated climbing translatent strata. Some of these types, especially translatent strata, have characteristics unique to the eolian realm. These same stratification types are found to compose some ancient cross-strata, and their occurrence confirms the eolian interpretations of pans of the Entrada (Jurassic), Navajo (Triassic-Jurassic), and Galesville (Cambrian) Formations, a well as revealing emergent islands in the Curtis Formation (Jurassic), previously considered to be totally marine in origin. Stratification types show a characteristic distribution on modern eolian dunes and differ in their relative abundances and structure on dunes of differing size and kind. These same relations allow some estimates of the type, shape, and original height of ancient dune deposits, as well as influencing the occurrences of surface features such as tracks and tipple forms. The geological record of stratification types and other dune features is greatly affected, however, by the extent of the post-depositional truncation of dunes.

Synthesis of late Paleozoic and Mesozoic eolian deposits of the Western Interior of the United States

Abstract Late Paleozoic and Mesozoic eolian deposits include rock units that were deposited in ergs (eolian sand seas), erg margins and dune fields. They form an important part of Middle Pennsylvanian through Upper Jurassic sedimentary rocks across the Western Interior of the United States. These sedimentary rock units comprise approximately three dozen major eolian-bearing sequences and several smaller ones. Isopach and facies maps and accompanying cross sections indicate that most eolian units display varied geometry and complex facies relations to adjacent non-eolian rocks. Paleozoic erg deposits are widespread from Montana to Arizona and include Pennsylvanian formations (Weber, Tensleep, Casper and Quadrant Sandstones) chiefly in the Northern and Central Rocky Mountains with some deposits (Hermosa and Supai Groups) on the Colorado Plateau. Lower Permian (Wolfcampian) erg deposits (Weber, Tensleep, Casper, Minnelusa, Ingleside, Cedar Mesa, Elephant Canyon, Queantoweap and Esplanade Formations) are more widespread and thicken into the central Colorado Plateau. Middle Permian (Leonardian I) erg deposits (De Chelly and Schnebly Hill Formations) are distributed across the southern Colorado Plateau on the north edge of the Holbrook basin. Leonardian II erg deposits (Coconino and Glorieta Sandstones) are slightly more widespread on the southern Colorado Plateau. Leonardian III erg deposits formed adjacent to the Toroweap-Kaibab sea in Utah and Arizona (Coconino and White Rim Sandstones) and in north-central Colorado (Lyons Sandstone). Recognized Triassic eolian deposits include major erg deposits in the Jelm Formation of central Colorado-Wyoming and smaller eolian deposits in the Rock Point Member of the Wingate Sandstone and upper Dolores Formation, both of the Four Corners region. None of these have as yet received a modern or thorough study. Jurassic deposits of eolian origin extend from the Black Hills to the southern Cordilleran arc terrain. Lower Jurassic intervals include the Jurassic part of the Wingate Sandstone and the Navajo-Aztec-Nugget complex and coeval deposits in the arc terrain to the south and west of the Colorado Plateau. Major Middle Jurassic deposits include the Page Sandstone on the Colorado Plateau and the widespread Entrada Sandstone, Sundance Formation, and coeval deposits. Less extensive eolian deposits occur in the Carmel Formation, Temple Cap Sandstone, Romana Sandstone and Moab Tongue of the Entrada Sandstone, mostly on the central and western Colorado Plateau. Upper Jurassic eolian deposits include the Bluff Sandstone Member and Recapture Member of the Morrison Formation and Junction Creek Sandstone, all of the Four Corners region, and smaller eolian deposits in the Morrison Formation of central Wyoming and apparently coeval Unkpapa Sandstone of the Black Hills. Late Paleozoic and Mesozoic eolian deposits responded to changing climatic, tectonic and eustatic controls that are documented elsewhere in this volume. All of the eolian deposits are intricately interbedded with non-eolian deposits, including units of fluvial, lacustrine and shallow-marine origin, clearly dispelling the myth that eolian sandstones are simple sheet-like bodies. Rather, these units form some of the most complex bodies in the stratigraphic record.

First-order and super bounding surfaces in eolian sequences—Bounding surfaces revisited

Abstract Extensive bounding surfaces in eolian sequences form with bedform migration (first-order bounding surfaces) and hiatuses in erg development (super bounding surfaces). First-order bounding surfaces represent the floors of interdune areas between simple or compound-complex dunes (draas), and form by ordinary migration of bedforms and interdune areas within a single erg. In contrast, super surfaces represent the truncation of ergs, or portions of ergs, and are a measure of how ergs respond to changing conditions and external events. Ergs are dynamic systems whose initiation, location, evolution and termination are a function of climate, sea level, basin configuration, sediment supply, and sandflow patterns; the latter are in turn determined by regional and local winds, pressure gradients and topography. As the overall conditions for erg development in an area change from favorable to unfavorable, ergs respond relatively rapidly or with a very long lag time. This response is evidenced by three broad categories of super surfaces. These categories are: (1) surfaces originated by regional termination of ergs because of climatic causes; (2) surfaces formed by erg contraction because of changes in sea level or tectonic setting; and (3) surfaces formed by the migration of ergs. Super surfaces can represent deflation of ergs and erg deposits to the water table, to an armored lag, to a point where a protective mantle of vegetation occurs, or to a point where renewed deposition of any sort begins. Features associated with super surfaces include root structures, paleosols, evaporites, polygonal fractures, preferential cementation, lag surfaces, zibars and granule ripples, and the surface may be planar, irregular, or show degraded dune topography. Super bounding surfaces allow the recognition of genetic units within thick and laterally extensive eolian sequences, and represent tools for correlating within (and possibly between) eolian bodies. The potential exists for relating the cause of super-surface development to specific geological events. Work thus far has recognized super surfaces largely in Pennsylvanian-Permian eolian units that are coastal and associated with glacial cycles. This in part may reflect ease in recognition, and identification of less obvious super surfaces in thick, inland erg sequences may ultimately show that these sequences also consist of diachronous, unrelated erg deposits.

Bedform spacing from defect dynamics

Spacing is a time-varying characteristic of bedform fields deriving from the behavior of defects (ends of crest lines) in the bedform pattern. In a model based on this hypothesis, crest line length is lost and spacing increases because bedform defects, which are smaller in height and faster migrating than surrounding bedform crest lines, merge with larger bedforms as defects migrate through bedform fields. Spacing in large bedform fields asymptotically increases with the logarithm of time as pairs of oppositely facing defects meet and annihilate. Spacing in small bedform fields, such as flumes, exponentially approaches a fixed value as defects are eliminated at the boundaries of the field. Model predictions are compatible with observed spacing of transverse bedforms, ripples formed under waves and linear dunes, calling into question the widespread assumption that bedform spacing approaches a steady-state value characteristic of fluid flow and sediment transport over two-dimensional bedforms.

Bed-form dynamics: Does the tail wag the dog?

Bed-form patterns reflect the dynamics of the defects in the pattern. From this new hypothesis, the differential migration of defects within a field of bed forms determines how crest-line orientation responds to changing transport conditions. Predictions from a model for defect dynamics compare well to field measurements and computer simulations of bed-form orientation. This model permits quantification of the time for bed-form reorientation across a broad range of scales, from hours in the rapidly changing nearshore environment to thousands of years for linear dune fields.

Airflow up the stoss slope of sand dunes: limitations of current understanding

The windward (stoss) side of a sand dune acts as a streamlined obstacle in the path of the wind. Continuity principles necessitate compression of the flow field up the stoss slope of a dune, and shear stress must progressively increase as the flow accelerates. Measurements in transverse flow over thirteen dunes at Padre Island, Texas, the Algodones, California, and White Sands, New Mexico, confirm that velocity profiles on the stoss slope are not log-linear, and that flow acceleration occurs very close to the surface within an internal boundary layer. As a consequence, in the overlying flow where measurements have historically been made, an overall decrease in shear stress occurs up the slope. The actual shape of the velocity profiles, and the identification of the appropriate segment of the profile from which to derive the shear stress that drives saltation represent major problems not approachable by traditional means.

Dune and Dune-Field Development on Padre Island, Texas, with Implications for Interdune Deposition and Water-Table-Controlled Accumulation

ABSTRACT Portions of the back-island dune fields on Padre Island, Texas, undergo seasonal destructive and constructive phases in which they are reduced to a nearly planar surface during the winter and then reform during the spring and summer. Initial depositional sites for dry sand that we observed were associated with roughness elements that caused a lowering of the transport capacity of the flow. Bedforms develop through a series of morphologic and dynamic stages: 1) irregular patches of dry sand, 2) wind-ripple protodunes with leeward flow expansion, 3) grainfall protodunes with a range from leeward flow expansion to flow separation, 4) barchan dunes with grainflow, and 5) crescentic ridges that characterize the mature field. The survival and further development of protodunes and dunes is f vored by increased size, a more advanced stage of development, unidirectional and moderate wind, and a plentiful sand supply. These factors can be viewed as the result of the interrelationship of aspects of eolian transport, the nature of leeward secondary airflow patterns, and sand supply. Dune-field ordering occurs by repeated merging, splitting, cannibalization, and lateral linking of dunes. On Padre Island and other areas of dunes that we studied, flow conditions across portions of interdune flats favor the transport of dry, loose sand from interdune flats onto the dunes, so that dunes grow at the expense of interdune flats. Dry-surface deposits are, therefore, unlikely to accumulate on interdune flats, but rather to characterize interdune depressions that may represent the minimum d stance to which an interdune flat can be closed. Otherwise, accumulations on interdune flats most commonly should be formed on wet or damp surfaces or occur associated with some mechanism that protects the deposits from deflation. Padre Island dune fields represent a type of eolian system where accumulations of dunes and interdune flats occur during periods of a water-table rise, with bypassing and deflation occurring as the water table becomes static or falls, respectively.

Dune field pattern formation and recent transporting winds in the Olympia Undae Dune Field, north polar region of Mars

High-Resolution Imaging Science Experiment (HiRISE) imagery of the central Olympia Undae Dune Field in the north polar region of Mars shows a reticulate dune pattern consisting of two sets of nearly orthogonal dune crestlines, with apparent slipfaces on the primary crests, ubiquitous wind ripples, areas of coarse-grained wind ripples, and deflated interdune areas. Geomorphic evidence and dune field pattern analysis of dune crest length, spacing, defect density, and orientation indicates that the pattern is complex, representing two constructional generations of dunes. The oldest and best-organized generation forms the primary crestlines and is transverse to circumpolar easterly winds. Gross bed form-normal analysis of the younger pattern of crestlines indicates that it emerged with both circumpolar easterly winds and NE winds and is reworking the older pattern. Mapping of secondary flow fields over the dunes indicates that the most recent transporting winds were from the NE. The younger pattern appears to represent an influx of sediment to the dune field associated with the development of the Olympia Cavi reentrant, with NE katabatic winds channeling through the reentrant. A model of the pattern reformation based upon the reconstructed primary winds and resulting secondary flow fields shows that the development of the secondary pattern is controlled by the boundary condition of the older dune topography.