Grain-shape sorting of medium-sized quartz sand by eolian action.

摘要

Previous wind tunnel studies of eolian shape-sorting have documented a change in preferential transport, from non-spherical grains at low wind velocities, to spherical grains at higher wind velocities (Pye, 1994). However, most field oriented studies have documented preferential transport of only spherical grains (Pye, 1994).; To investigate these differences in eolian shape-sorting, Fourier grain-shape analysis (FGSA) was used to measure the quartz grains obtained from 305 samples, collected as three experimental sets from the Santa Maria and Kelso Dune Fields.; FGSA results from the first sample set demonstrated that eolian shape-sorting of elongate grains observed in wind tunnel testing were also observable in the natural environment. Thus, future field-oriented studies of eolian shape-sorting must consider the prevailing wind conditions. Shape-sorting was found to occur throughout the dune field, and was not limited to the backbeach-foredune region. Grain asperity was not as important as particle form in influencing eolian transport of medium-sized sand.; FGSA results from the second sample set demonstrated that, at friction velocities between 25 cm/sec and 44 cm/sec, quartz sand had greater average elongation and higher grain-shape variation in the lower-most 10 cm of the sediment column. Average grain elongation within this 10 cm zone decreases with increasing wind velocity and demonstrates that grains with greater elongation have higher drag and greater lift than spherical grains, at near-threshold friction velocities. The mechanism of grain saltation changed from predominately wind dislodgement, to predominately collision dislodgement, above 34.4 cm/sec, as spherical grains were more effectively transported by collision dislodgement.; A linear relationship was developed to express shape variation as a change in the short to long axial ration, as a function of wind friction velocity:{dollar}{dollar}s{lcub}:{rcub}l Axial Ratio (Delta%) = 93.272 usp*sb{lcub}cm/sec{rcub} - 32.755 eqno (1){dollar}{dollar}; FGSA results from the third sample set demonstrated that subsurface eolian shape-sorting trends can be used to infer the historic average "transport" wind velocities for modern dune deposits. However, significant variation in the preserved grain elongation is caused by numerous sampling problems, which include the inability to sample single time-horizons, and single sand-types.; Additional work is needed to determine the applicability of modern eolian shape-sorting trends to those preserved in ancient dune deposits.