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

Resumen

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 Peñasco
(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, monocrystalline 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