COMPARISON OF MORPHOLOGY, DYNAMICS, AND STRATIFICATION OF EOLIAN AND SUBAQUEOUS DUNES. D. M. Rubin, Department of Earth & Planetary Sciences, UC Santa Cruz, Santa Cruz, CA (drubin@ucsc.edu)

Overview: Eolian and subaqueous dunes have

more similarities than differences. Here we compare morphology, dynamics, and internal stratification of dunes in air and water.

Morphology: Size. Dunes water can be smaller than those in air,

although the overlap is greater than the differences. In water, dune wavelengths range from ~1m to ~1km [1]; in air, dune wavelengths range from ~100 m to several km [2].

Cross-sectional profile. Dunes in unidirectional flows are asymmetrical regardless of fluid. In tidal flows that reverse direction with equal transport in the ebb and flood directions, tidal dunes—like some linear dunes in air—are symmetrical in cross-section.

Planform shape. Dunes in both air and water have barchan or barchanoid morphology in unidirectional flows. In bidirectional reversing flows, dunes tend to have a straighter and more linear morphology (Fig. 1) [3]. In more complex flows, dunes in both fluids can have more complex morphology. Directional proper-ties of the flow are generally more important than fluid medium in controlling planform shape.

Superimposed dunes. As dunes in both fluids be-come larger, they can support superimposed dunes [4]. The superimposed dunes can migrate in the same direction as the main dunes or toward other directions [5].

Dynamics: Generating mechanism. Dunes in both fluids arise

from the same process. Flow separates at or near the crest of one dune; where the flow reattaches to the bed downstream, shear stress increases locally, the rate of sand flux increases in the reattachment area, scour occurs there, and then downstream from the reattach-ment area deposition occurs as the shear stress decreases [7]. The pattern perpetuates downstream forming a train of bedforms [8].

Orientation. On a bed that is covered with sand, orientation of dunes and ripples in air and water is con-trolled by the same process: the bedforms take an ori-entation that is as transverse as possible to all transport (orientation having the maximum gross across-crest transport) [9, 10, 11]. The situation differs for dunes on a partially starved bed, where dunes take a differing orientation [12].

Role of bypassing sediment. In air, transverse dunes are extremely effective traps for sand blown over their

crests, and sand moves with the dunes. In water, sand in suspension commonly bypasses dune troughs, and has been suggested to cause the transition to upper plane bed.

Dynamics of superimposed bedforms. In air and water, superimposed dunes arise when the large dune generates a boundary layer in which superimposed dunes can arise [4]. This requires that the flank of the dune be extensive enough to accommodate the super-imposed dunes. In both fluids, size of superimposed dunes can increase from trough to crest on the main dunes.

Figure 1. Linear dunes in reversing flows in air and water; both examples include superimposed dunes. A. Tidal dunes in San Francisco Bay [4]; wavelength is 60 m. B. Linear dunes from Namib Desert; wavelength is 2 km. Landsat Earth as Art series; NASA and USGS Eros Data Center.

3009.pdfFifth Intl Planetary Dunes Workshop 2017 (LPI Contrib. No. 1961)

Dune interactions. In both air and water, dunes interact in response to a variety of mechanisms [13]. Small dunes move faster than larger ones and merge or interfere with those that they overtake. In bidirectional flows where sand transport toward the two directions equal and the flows diverge by ~90°, two intersecting sets of dunes arise in air and water and water [11, 14, 15].

Stratification: Grain size. Subaqueous dunes commonly contain

gravel and/or mud drapes. Both of these size fractions are rare in eolian dunes.

Cross-bedding geometry. In both air and water, the thickness of a set of cross-beds deposited by a dune is generally smaller than the dune height [16]. In suba-queous flows, however, the proportion of sediment that bypasses dune troughs can be substantial, and this by-passing sediment can contribute to deposition; this enables subaqueous dunes to preserve a greater propor-tion of their height. In some cases, subaqueous dunes can be preserved completely, which is extremely rare for eolian dunes.

Because their of similar morphology and dynamics, dunes in air and water deposit cross-beds with similar geometries. For example, compound cross-bedding deposited by small dunes migrating along the length of larger dunes occurs in both air and water [6, 17]. Simi-larly, stratification produced by reversing tidal flows can resemble (geometrically) stratification produced by eolian dunes that reverse seasonally (Fig. 2) [17, 18].

References:

[1] Ashley G.M. (1990), J. Sed. Res., 60, 160-172. [2] Lancaster N. (1988), Geology, 16, 972-975. [3] Rubin D.M. (2012), Earth-Science Reviews, 113, 176-185. [4] Barnard P.L., Erikson L.H., and Kvitek (2011) Geo-Marine Letters, 13, 227-236. [5] Rubin D.M. and McCulloch D.S. (1980) Sedimentary Geology, 26, 207-231. [6] Rubin D.M. and Hunter R.E. (1983), Devel-opments in Sedimentology, 38, 407-427. [7] Raudkivi A.J. (1963), Proc. Am. Soc. Civil Engrs., J. Hydraulics Div., 89, 15-33. [8] Southard J.B. and Dingler J.R. (1971) Sedimentology, 16, 251-263. [9] Rubin D.M. and Hunter R.E. (1987) Science, 237, 276-278. [10] Rubin D.M. and Ikeda H. (1990) Sedimentology, 37, 673-684.

Figure 2. Cyclic cross-bedding deposited by dunes in reversing flows. A. Subaqueous deposit of a small dune in reversing tidal currents. Vertical set thickness is ~1 m. Cyclic “tidal bundles” record ~1 m of net mi-gration from right to left per tidal cycle. Photo by Joost Terwindt [19]. B. Cyclic eolian cross-beddding in Navajo Sandstone. Set thickness is ~10 m. Each cycle records a year of dune migration from left to right [18]. [11] Reffet E. Courrech du Pont S., Hersen P., and Douady S. (2010) Geology, 38, 491-494. [12] Cour-rech du Pont S., Narteau C., and Gao X. (2014) Geolo-gy, 43, 1027-1030. [13] Werner B.T. and Kocurek G. (1999) Geology, 27, 727-730. [14] Dalrymple R.W. and Rhodes R.N. (1995) Developments in Sedimentol-ogy, 53, 359-422. [15] Werner B.T. and Kocurek G. (1997) Geology, 25, 771-774. [16] Rubin D.M and Hunter R.E. (1982) Sedimentology, 29, 121-138. [17] Rubin D.M., (1987), SEPM Concepts in Sed., 1, 1-187. [18] Hunter R.E. and Rubin D.M. (1983), Develop-ments in Sedimentology, 38, 429-454. [19] Terwindt J.H.J. (1981) Special Publ. of Int. Assoc. Sedimentolo-gists, 5, 4-26.

3009.pdfFifth Intl Planetary Dunes Workshop 2017 (LPI Contrib. No. 1961)