Roundness in quartz grains from inland and coastal dune sands, Altar Desert, Sonora, Mexico

摘要

A roundness analysis of quartz grains (n=54) from dunes from San Luis Río Colorado (SLRC), El Pinacate (EP), Golfo de Santa Clara (GSC) and Puerto Pe?asco (PP), Sonora, Mexico, was carried out in order to determine the provenance of quartz grains and their relationship to different quartz types (monocrystalline quartz with straight/undulose extinction, with inclusions and polycrystalline quartz of plutonic origin and with straight/undulose extinction). Aeolian transport and wind selectiveness produces fine-grained and moderately to well sorted sands in SLRC and EP dune sands whereas medium sand sizes and moderately well sorted sands are present at sites GSC and PP suggesting mixing of aeolian/marine processes that generate different grain sizes of different composition. Roundness of quartz grains at sites SLRC and EP suggests fluvial transport in the Colorado River Delta but also to long aeolian transport. Some quartz grains from the GSC and PP sites showed angular and rounded quartz due to a mix of aeolian/marine processes. Samples from sites SLRC and EP concentrate monocrystalline quartz with straight and undulose extinction derived from a near source sediment. Samples from sites GSC and PP are dominated by monocrystalline quartz with straight and undulose extinction derived from plutonic and low-rank metamorphic rocks. Polycrystalline quartz is derived from plutonic and low-rank metamorphic rocks. Correlations between Mz vs. SA/SR (grain-size, subangular/subrounded quartz), VA/A vs. MU (very angular/angular quartz, mono-crystalline quartz with undulose extinction) and Mz vs. SA/SR (grain size, subangular/subrounded quartz) indicate that coarse-sized grains are subangular probably due to little abrasion and rounding of grain edges trough transport, the presence of monocrystalline quartz with undulose extiction probably derived from low-rank metamorphic sources and an increase of grain size in relation to the angularity of the grains.