Dimensional Grain Orientation Studies of Recent Coastal Sands

Abstract

Sand-grain-orientation measurements have been made on 110 undisturbed samples of artificially lithified Recent coastal sands to demonstrate the preferred orientation direction of elongate grains. The data presented show that there is a strong tendency for elongate grains to align themselves with their long dimensions parallel with the direction of the depositing current of air or water. This direction is parallel with the backwash direction of waves on a beach and, therefore, averages approximately perpendicular to the beach trend, although some deviation may be caused by a longshore current. Characteristic patterns showing the local fluctuations in current direction are found in hooked spits and washover deltas. The preferred orientation direction of wind-deposited sands is approximately parallel with the wind direction, and for a composite of a large number of samples appears to approach the prevailing wind direction. The preferred orientation of randomly selected wind- and wave-deposited samples from Recent elongate coastal sand bodies isolated from the ocean by progradation averages approximately perpendicular to the trend of the sand bodies. Possible applications of grain orientation studies are to determine the direction of current of air or water which deposited an ancient sand, to predict trends of subsurface shoestring sands of known coastal origin by study of oriented cores, and to aid in the evaluation of other directional mass properties in sediments.