ALBERT

Description: We use the stratigraphy preserved in a washover fan to reconstruct the timing or emplacement and environmental conditions along the Matagorda Peninsula, Texas, during Hurricane Ike in 2008. Washover fan stratigraphy preserves a topset-foreset break (TFB) that rises 0.42 m in elevation as the fan built landward. We constrain overwash flow depths to 0.1–0.32 m through deposit sedimentology, and tie the rising trajectory of the TFB to rising storm surge water levels measured in the back-barrier bay (0.03 m h –1 ) as the hurricane approached the coast. This relation allows us to estimate that the fan took 0.52–0.90 days to build, and was finished building before the storm surge peaked. This is 15–25% of the 3.5 days of hurricane-induced storm surge near the site. We show how washover stratigraphy can be used to constrain the timing and amount of sediment redistribution on a coast associated with a hurricane; information that is necessary to test and/or calibrate existing numerical models that predict shoreline change during hurricanes.

Description: 〈span〉〈div〉Abstract〈/div〉Aeolian dune-field patterns arise from interactions (collisions) between dunes within a set of boundary conditions. Over time, patterns evolve toward fewer, larger, and more-widely spaced dunes, allowing dune spacing to be used as a proxy for dune-field maturity. In this work, the spatial density of dune interactions was measured in dune fields with mean dune spacings of 25 m to 4 km in order to study how this parameter varies with scale. Results demonstrate that the density of interactions in fields of linear and crescentic ridge dunes follows statistically distinct, but parallel, trends, and the relationship between dune spacing and interaction density can be modeled as an inverse quadratic function. When parameterized in non-dimensional terms, these two curves collapse to a constant, and the relative spatial density of interactions (termed the interaction index) does not change with spacing. This implies that interactions are drivers of pattern development at all scales of pattern coarsening. Disparate interaction indices for crescentic and linear dunes derive from the dynamic behavior of the dune morphologies. These morphologies reflect wind regime and sediment supply, and are largely independent of other boundary conditions.〈/span〉

Description: Lenticular sand bodies in the Upper Jurassic Norphlet Sandstone in the northeastern Gulf of Mexico basin have been interpreted as preserved large linear dunes that subsided into the underlying Louann Salt layer. Due to imaging difficulties, the thickness of the underlying salt is largely unknown. We performed physical experiments and created a mathematical model to understand dune and salt deformation and to determine the controls on the final preserved dune topography. Nonuniform dune geometry results in unequal loading by sand that drives locally varying subsidence into the salt substrate, resulting in varying degrees of dune flattening and dune relief. Our results indicate that initial salt thickness relative to dune size governs the initial rate of subsidence, thus affecting the final dune topography. However, interdune spacing and dune geometry are also important because they set the pressure gradient and determine the maximum possible subsidence. We conclude that preserved dunes with high relief indicate areas of relatively thin salt thickness and/or close dune spacing, and that dunes with low relief indicate areas of relatively thick salt thickness and/or wide dune spacing. For the case of the Norphlet Formation, dune spacing and salt thickness likely played the major roles in controlling dune subsidence.

Description: 〈span〉〈div〉Abstract〈/div〉A reduced-complexity aeolian dune stratification model is developed and applied to explore the role of dune morphodynamics in the creation of synthetic sections of aeolian stratigraphy originating from three sets of environmental forcing: 1) steady wind transport capacity, 2) steady bed aggradation and variable wind transport capacity, and 3) steady wind transport capacity and bed aggradation. In each scenario, the forward motion of initial, highly disorganized dunes generates a significant record exclusively containing autogenic signals that arise from early dune growth, deformation, and merging. However, continued dune growth scours deeply, and shreds all records of early dunes. Afterward, dunes self-organize into quasi-stable groups. Forward motion of dune groups creates, truncates, and amalgamates sets and co-sets of cross-strata, quickly forming a second, significantly more robust stratigraphic record, which preserves a comingling of signals sourced from ongoing autogenic processes and each scenario's specific set of environmental forcings, the allogenic boundary conditions of the sand sea. Although the importance of self-organization on modeled aeolian stratification is clear in the few presented scenarios, self-organization may be throttled via variability in environmental forcings, as thoroughly documented in a companion paper (〈a href="https://pubs.geoscienceworld.org/jsedres#i1527-1404-89-8-728-Cardenas1"〉Cardenas et al., this issue〈/a〉). Therefore, additional work is warranted because this numerical experiment only begins to sample possible sets of environmental forcing, boundary conditions, and initial conditions, geomorphic responses, and consequential preservation possible in the presented surface-stratigraphic bedform modeling framework.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The stratigraphic architecture of aeolian sandstones is thought to record signals originating from both autogenic dune behavior and allogenic environmental boundary conditions within which the dune field evolves. Mapping of outcrop-scale surfaces and sets of cross-strata between these surfaces for the Jurassic Page Sandstone near Page, Arizona, USA, demonstrates that the stratigraphic signature of autogenic behavior is captured by variable scour depths and subsequent fillings, whereas the dominant signatures of allogenic boundary conditions are associated with antecedent surface topography and variable water-table elevations. At the study area, the Page Sandstone ranges from 55 to 65 m thick and is separated from the underlying Navajo Sandstone by the J-2 regional unconformity with meters of relief. Thin, climbing sets of cross-strata of the basal Page representing early dune-field accumulations fill J-2 depressions. In contrast, the overlying lower and middle Page consist of cross-strata ranging from less than 1 to 15 meters thick (average 2.44 m), and packaged between outcrop-scale bounding surfaces, though parts of the lower Page are bounded from beneath by the J-2. These bounding surfaces have been previously correlated to highstand deposits of the adjacent Carmel sea and at this site possess up to 13 meters of erosional relief produced by dune scour. Notably absent in packages of cross-strata bounded by these outcrop-scale surfaces are strata of early dune-field accumulations, any interdune deposits, and climbing-dune strata. Instead, these packages preserve a scour-and-fill architecture created by large dunes migrating in a dry, mature, dune field undergoing negligible bed aggradation. Any record of early phases of dune-field construction for the lower and middle Page are interpreted to have been cannibalized by the deepest scours of later, large dunes. Interpretations are independently supported by the relatively large coefficients of variation (〈span〉cv〈/span〉) in middle Page set thicknesses (〈span〉cv〈/span〉 = 0.90), which are consistent with set production by successive deepest trough scours, the relatively low coefficient of variation for the depression-filling basal Page and lower Page sets consistent with a significant component of bed aggradation in J-2 depressions (〈span〉cv〈/span〉 = 0.64 and 0.49), and the fit of set thickness distributions to established theory. Numerical modeling presented here and more completely in the companion paper demonstrates how this cannibalization of early-phase stratigraphy is an expected outcome of autogenic dune-growth processes, and that early-phase strata can be preserved within antecedent depressions. Relative rise of the inland water table from basin subsidence and changing Carmel sea level forced preservation of 5–6 stacked packages composed of scour-and-fill architecture. Without these allogenic forcings, the Page would be little more than an erosional surface.〈/span〉