internal structure and surface geometry of longitudinal seif dunes

two depositional types of eolian sand: 1) "accretion deposits," which are laminae deposited when the wind velocity falls and some deposition occurs, and 2) "avalanche deposits," which are laminae deposited as a result of sand avalanches on the slip face and which generally have a dip of more than 30°. Hunter (1977) and Kocurek and Dott (1981) differentiated "sand (grain) flow cross-strata, grainfall laminae, and climbing translatent strata." In this differentiation "sand (grain) flow crossstrata" are the "avalanche laminae," and the rest are actually the "accretion laminae."
ABSTP.ACT: A mutual relationshipexists between the morphologyof longitudinal(self) dunes and their dynamics and internalstructure. The basic mechanismof longitudinaldune movementresults from the deflectionof obliquewindflow intoa longitudinalflow on the lee flank. The magnitudeof the longitudinal flow dependson the angleof the wind incidencewith the crest line. Depositionoccurs when this angle of incidenceincreasesthrough meanderingof the longitudinaldune crest line. After heavy rainexposed the internalstructureof the dune, two kinds of depositionaluniห้องสมุดไป่ตู้ were found: the mainunit forms in the meanderingarea on both sidesof the dune;the other, secondaryunitis a narrow belt on both sides and parallel to the crest line. The two unitshave differentstrikes. Because the laminae deposited on the dune are of the three primarystratificationtypes--"climbingtranslatentstrata." "sand flow cross-strata," and "grainfalllaminae"--the range of dips is wide (10° to 30°). These dips accord with topographicprofiles across the dune. A model of the internalstructure of a longitudinaldune has been developed from the field data. The model shows the erosionaland delx~sitionalareas along this lineardune. INTRODUCTION The structure and geometry of sand dunes are controlled by wind directions and erosional and depositional processes occurring on the dune. Knowledge of these dynamic processes enables analysis and identification of the internal structure and the geometry of the dune. Longitudinal (self) dunes are very common in deserts, but their dynamics and morphology are poorly understood. The diverse explanations of the dynamics and morphology of longitudinal dunes can be grouped in two main theories: the first is the " w i n d drift" theory which sees these dunes as a product of erosion rather than deposition (Melton, 1940; King, 1960); the other theory sees the longitudinal dunes as depositional forms. T h e " w i n d drift" theory can be dismissed because there is some evidence of depositional internal structure in longitudinal dunes (Bagnold, 1941; McKee and Tibbitts, 1964). Among those who support the depositional theory, there is debate as to the nature of the prevailing winds that affect the dune. According to one view the dune results from a bidirectional wind regime and elongates parallel to the resultant wind (Bagnold, 1941; Cooper, 1958; McKee and Tibbitts, 1964; Wopfner and Twidale, 1967). The most recent view is that longitudinal dunes are formed by a strong unidirectional wind that blows parallel to the dune and develops helicoidal flow movement (Hanna, 1969; Glennie, 1970; Folk, 1971a, 1971b, 1976). In both cases the wind or the helocoidal eddies encounter the longitudinal dune on both sides, thus producing a slip-face effect on both flanks of the dune. It therefore appears to most researchers (Bagnold, 1941; McKee and Tibbitts, 1964; Glennie, 1970; Bigarella, 1972) that the ideal internal structure of longitudinal dunes must consist of cross-laminae dipping in opposite directions (Fig. 1). Figure 1 is a speculative model. The only comprehensive research on this subject has been done by McKee and Tibbitts (1964) on a Libyan self dune devoid of vegetation. They showed that all the laminae dip in opposite directions perpendicular to the crest line and at angles to 10° to 33 °, except close to the dune base, where the dip is less. This was interpreted as resulting from winds from two opposing directions and supports the ideal structure of Figure 1. Following Bagnold (1941), McKee and Tibbitts (1964) and Sharp (1966) distinguished
~Manuscript received November 23, 1981; revised March 8, 1982.
Jotn~qALOFSFED~EN'rARY PETROLOGY,VOL. 52, NO. 3, SEPTEMBER,1982, P. 0823-0831 Copyright© 1982,The Societyof EconomicPaleontologistsand Mineralogists 0022-4472/82/0052-0823/$03.00
RESEARCH METHOD AND APPROACH
Within the framework of study of the dynamics of longitudinal dunes (Tsoar, 1978), it was important to ascertain where sand is deposited or eroded from the dune. Analysis of the internal structure of the dune could provide such information. The research was carfled out on a typical longitudinal dune in the northern Sinai Desert (Fig. 2). The amount of rain in the area averages about 60 to 70 mm a year. The dune is similar in its morphology to that which McKee and Tibbitts (1964) examined in Libya and to a great many longitudinal dunes all over the world (see, e.g., Peel, 1966, Plate IX; Madigan, 1936, Figs. 4 and6; Holm, 1960, Fig. 4; Warren, 1972, Fig. 3). The dune is meandering and its height ranges from 12 to 14 m. Studies of internal structures of active dunes are hampered by ripples and avalanche beds which hide the internal structure. This obstacle can be overcome by digging deeply, locating the laminae, and measuring their orientations. This is a very difficult process, especially in arid areas where the sand is dry to a great depth and a considerable amount of water is needed to wet it in order to preserve the vertical walls. Moreover, it is impossible to dig down deeper than 1 or 2 m, especially in the steep sections of the dune, because of the dan-
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HAIM TSOAR
FIG. 1.--Cross section showing the ideal internal structure of a longitudinal dune (according to Bagnold, 1941; McKee and Tibbitts, 1964, and Glennie, 1970).
I N T E R N A L S T R U C T U R E AND S U R F A C E G E O M E T R Y O F L O N G I T U D I N A L (SEIF) DUNES 1 HAIM TSOAR
Department of Geography Ben Gurion University of the Negev Beer Sheva 84120, Israel