Exploring the completeness of the aeolian record within synthetic stratigraphy


To be submitted soon! 

Abstract: A reduced complexity model aeolian dune stratification model is developed and applied to explore the role of dune morphodynamics in the creation of synthetic sections of aeolian stratigraphy and shredding of environmental signals originating from three sets of environmental forcing: 1) steady transport capacity, 2) steady bed aggradation and variable transport capacity, and 3) steady 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 merger. However, continued dune growth scours deeply, and shreds all records of early dunes. Afterward, dunes self-organize into groups of dunes. Forward motion of dune groups create, truncate, and amalgamate 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. Although the importance of self-organization on modeled aeolian stratification is clear in the few presented scenarios, self-organization maybe throttled via variability within environmental forcings. Therefore, additional work is warranted as this numerical experiment only begins to sample possible sets of environmental forcing, boundary conditions, and initial conditions, geomorphic responses, and consequential preservation.

Here’s a sneak peak of the simulations:

The videos below so the co-evolution of dune topography and stratigraphy for three different model scenarios. In each video, bedform stratigraphy is vertically exaggerated 100x. Additionally, bedform topography is reduced 20x.  η* and x* are non-dimensional vertical and horizontal scales, respectively. η* represents the fraction of equilibrium dune height, and similarly, x* represents the number of equilibrium dune wavelengths. Enjoy!

1) Steady transport capacity

2) Steady bed aggradation and time-varying transport capacity

3) Steady bed aggradation and transport capacity

Exploring the morphodynamic response of Texas’ coastal barriers to sea-level rise

agu2017For AGU Fall Meeting 2017 in New Orleans, I gave a talk on the application of a simple morphodynamic model to forward model the response of coastal barriers (islands and peninsulas) to spatially variable sea-level rise over centuries. Within the model, coastal barrier geomorphology is simplified to a suite of characteristic scales and surface processes are simplified to parameterized expressions that characterize geomorphic responses to relative sea level rise. The abstract for this presentation is in an earlier post (Getting ready for AGU 2017), and a PDF of the presentation is available here (opens in a new window)!

Getting ready for AGU 2017!

Exploring the morphodynamic response of coastal barriers to sea-level rise along the Texas Gulf Coast

Swanson, T.1, Lorenzo-Trueba, J.2, Anarde, K.1, Odezulu, C.1, Anderson, J.1, Nittrouer, J1.

1 Rice University
2 Montclair State University

The Texas portion of the Gulf Coast spans nearly 600 kilometers and is chiefly composed of barrier islands and peninsulas that shelter numerous landward communities from damaging storm surge and waves. Presently, this coastal barrier system is evolving at an unprecedented rate, as sediment that comprises these protective barriers is being depleted while sea-level rise is accelerating, reducing the resilience of coastal communities. To help explain the morphodynamic response of Texas’ coastal barrier system to anticipated accelerated sea-level rise, a reduced complexity morphodynamic model is constructed from a combination of extant models of barrier morphodynamics, alongshore sediment transport, and time-variable ravinement depth. The model is initialized using a simplified geometric depiction of the barrier system morphology obtained from regional bathymetric and topographic surveys, and sediment composition from best-available subsurface geodatabases. Simulation timesteps capture the morphodynamic response of coastal barriers to accelerated sea-level rise by tracking the motion of key geomorphic boundaries within the barrier system: ravinement depth, shoreline, and bay line. The motion of these boundaries is calculated via parameterized expressions of alongshore, cross-shore, and barrier over-wash sediment transport that represent the time-integrated effect of short-term coastal processes, such as day-to-day waves and storms, and longer-term processes such as sea-level rise, dynamic barrier morphology, and barrier sediment composition. Model results are comparable with historical records and geological interpretations of regional coastal change sampled over a broad range of time and spatial scales.

Time and location: Tuesday, 12 December 2017 14:10 – 14:25 New Orleans Ernest N. Morial Convention Center – 353-355

Please check out the innovative work presented by Ben Cardenas, which uses a surface model for aeolian dune topography, with newly developed routines that allow for aeolian dune climb, and preservation of dune stratification:

Coupling Aeolian Stratigraphic Architecture to Paleo-Boundary Conditions: The Scour-Fill Dominated Jurassic Page Sandstone

Cardenas, B.1, Kocurek, G.1, Mohrig, D.1, Swanson, T2.

1 The University of Texas at Austin
2 Rice University

The stratigraphic architecture of aeolian sandstones is thought to encode signals originating from both autogenic dune behavior and allogenic boundary conditions within which the dune field evolves. Mapping of outcrop-scale bounding surfaces and sets of cross-strata between these surfaces for the Jurassic Page Sandstone near Page, AZ, USA, demonstrates that dune autogenic behavior manifested in variable dune scour depth, whereas the dominant boundary conditions were antecedent topography and water-table elevation. At the study area, the Page Sandstone is ~ 60 m thick and is separated from the underlying Navajo Sandstone by the J-2 regional unconformity, which shows meters of relief. Filling J-2 depressions are thin, climbing sets of cross-strata. In contrast, the overlying Page consists of packages of one to a few, meter-scale sets of cross-strata between the outcrop-scale bounding surfaces. These surfaces, marked by polygonal fractures and local overlying sabkha deposits, are regional in scale and correlated to high stands of the adjacent Carmel sea. Over the km-scale outcrop, the surfaces show erosional relief and packages of cross-strata are locally truncated. Notably absent within these cross-strata packages are early dune-field accumulations, interdune deposits, and apparent dune-climbing. These strata are interpreted to represent a scour-fill architecture created by migrating large dunes within a mature dry aeolian sand sea, in which early phases of dune-field construction have been cannibalized and dune fill of the deepest scours is recorded. At low angles of climb, set thickness is dominated by the component of scour-depth variation over the component resulting from the angle of climb. After filling of J-2 depressions, the Page consists of scour-fill accumulations formed during low stands. Carmel transgressions limited sediment availability, causing deflation to the water table and development of the regional bounding surfaces. Each subsequent fall of the water table with Carmel regressions renewed sediment availability, including local breaching of the resistant surfaces and cannibalization of Page accumulations. The Page record exists because of preservation associated with Carmel transgressions and subsidence, without which the Page would be represented by an erosional surface.

Time and location: Wednesday, 13 December 2017 13:40 – 18:00 New Orleans Ernest N. Morial Convention Center – Poster Hall D-F

The dynamics of spur-bearing bedforms

sedimentologyPaper2017Bedform spurs are formed by helical vortices  that trail from the lee surface of oblique segments of bedform crest lines. Trailing helical vortices quickly route sediment away from the lee surface of their parent bedform, scouring troughs and placing this bed material into the body of the spur. Here’s a video of a single bedform spur:

When present, spur-bearing bedforms and their associated trailing helical wakes exert tremendous control on bedform morphology by routing enhanced sediment transport between adjacent bedforms. Field measurements collected at the North Loup River, Nebraska, and flume experiments described in previous studies demonstrate that this trailing helical vortex-mediated sediment transport is a mechanism for bedform deformation, interactions and transitions between two-dimensional and three-dimensional bedforms. Below is a time lapse image of many spur bearing bedforms. Watch as they pause and surge due to spur-routing of sediment transport.

Click on the picture of the manuscript heading to visit the publisher’s webpage and access more information about spur bearing bedform dynamics, including more videos!

A new surface model for aeolian dune topography

MatGeoPaper2016Aeolian dune topography arises from a highly non-linear interaction between sediment transport,  topography, and boundary shear stress.  To explore the growth of aeolian dunes under a variety of boundary conditions, a new surface model for aeolian bedform topography is adapted from a surface model of subaqueous bedform topography (Jerolmack and Mohrig, 2005)[1]. The resulting modeling framework approximates the dynamic motions of aeolian bedform topography driven by bedform field boundary conditions; namely, different distributions of sediment transport direction and investigating bedform growth with and without the constraint of a fixed sediment source area (modeled as a fixed elevation boundary). The rates at which modeled aeolian bedforms grow and morphologically mature are found to be highly sensitive to the chosen boundary conditions. Click on the image of the manuscript header to visit the journal’s website and read more about this study.

The videos below show four permutations of two boundary conditions: uni- and bi-modal distributions of sediment transport direction are used to grow bedform topography with and without the constraint of a sediment source area. In these videos hot colors indicate higher topography and cooler colors indicate lower topography.

Uni-modal distribution of sediment transport direction with periodic boundaries

Uni-modal distribution of sediment transport moving sediment from a fixed source

Bi-modal distribution of sediment transport direction with periodic boundaries

Bi-modally distributed sediment transport moving sediment from a fixed source

The aeolian bedform surface model code is malleable and readily modified for exploratory study of bedform topography that inherits morphological traits from aeolian bedform field boundary conditions. A version of the source code for these simulations is available from MATGEO. However, a newer version of this software will be made available via a public repository on GitHub, shortly.

[1] Jerolmack DJ, Mohrig D (2005) A unified model for subaqueous bed form dynamics. Water Resour Res 41(12):W12421. doi:10.1029/2005WR004329 (PDF link)

Self organization of aeolian dunes

sedimentologyPaperTitle2016Aeolian dune motion is thought to be driven by an annual cycle of sediment-transporting wind events. Each wind event drives uneven motion of dune crestlines, yet dune crestlines align as a trend to an annual cycle of wind . Understanding the variability in dune motion over such a cycle aids the interpretation of aeolian cross-stratification, often available only in the limiting exposure of core and outcrop.

Digital elevation models obtained by light detection and ranging (lidar, Fig. 1) are used to estimate dune brink motion and sediment flux along the sinuous crestlines of crescentic dunes at White Sands gypsum dune field (south-central New Mexico, USA) over an annual cycle of wind.

Fig. 1 Time lapse animation of dune elevation of study area within White Sands, NM. Duration is approximately 3 yrs.

By using an edge detection algorithm, dune brink motion  (Fig. 1) can be used to estimate local values of sediment flux. These estimations reveal that dune motion and sediment flux are very well described by a circular normal distribution when sampled using a spatial window of approximately the size of six average dunes. At this scale, the distribution of erratic dune motion is symmetrically distributed around the average lee surface dip direction. Therefore, uneven motion of dune crest lines offset, and the geometric self-organization of dune crests as a trend line is maintained.

Fig. 2 Dune brink movement occurring over slightly more than a year is shown by the colormapped circles. The elevation of the aeolian dunes is shown by the grayscale.