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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Petrology
Vol. 43 (1973)No. 4. (December), Pages 1128-1142

Internal Geometry and Origin of Vegetated Coastal Sand Dunes

Victor Goldsmith


Frequency distributions of dip angles, azimuths, and elevations of 209 eolian cross-bed sets from Monomoy Island, Cape Cod, Massachusetts, were analyzed and compared with similar data from the coastal sand dunes of Mustang Island, Texas (McBride and Hayes, 1963), Sapelo Island, Georgia (Land, 1964), Praia de Leste, Brazil (Bigarella and others, 1969), and Israel (Yaalon and Laronne, 1971). These analyses from five widely scattered localities show that vegetated coastal sand dunes have a distinctive internal dune geometry. At all five locations, azimuth distributions correlate with the prevailing winds, either directly or with a bimodal azimuth distribution bisected by the prevailing wind vectors. Also, the dip distributions center around two modes, 11°-15° and 25°-35°. The smaller of the two modes is present at each of the five locations; the dip distribution is bimodal at three (and perhaps four) of the locations. Whereas the smaller dip mode tends to have a unimodal azimuth distribution, the larger dip modes (25°-35°) have bimodal azimuth distributions. It is suggested that the high-angle dips form as pyramidal wind shadow dunes via slip-face accretion behind large vegetation hummocks, which results in the distinct bimodal azimuth distribution.

For the Monomoy dunes, the cross-bed dip angles are mostly low (x = 11.2°) in all azimuth directions, at all dune elevations, on all sides of the dunes, and at all sample localities. Pyramidal wind shadows are rare on Monomoy. The combined azimuths show a statistically significant correlation with the three prevailing northwest, southwest, and southeast wind directions rather than with the dominant northeast storm winds. This association is valid for cross-bed sets at any dune elevation. The azimuth distribution varies appreciably among sample localities, with the cross-bed sets tending to dip seaward on all sides (i.e., away from the center of the island).

Biweekly field observations and numerous aerial photographs of Monomoy coastal dunes made during a three-year period suggest that the Monomoy internal dune geometry is closely dependent on growth of dune vegetation, especially Marram grass (Ammophila arenaria). The grasses act as baffles, trapping sand moved by the prevailing winds, which results in low-angle cross-bed sets (i.e., lower dip mode) formed by the vertical accumulation of sand behind vegetation hummocks on the gently undulatory, nearly horizontal upper surface of the dunes.

Thus, vegetation plays a distinctive role in the formation and internal geometry of vegetated coastal sand dunes.

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