Largest wind ripples on Earth?

MILANA, J. P

GEOLOGY

GEOLOGICAL SOC AMER, INC

Lugar: Boulder; Año: 2009 vol. 37 p. 343 - 346

0091-7613

Uniquely large wind ripples are reported from the Argentine Puna Plateau at ~4000 m altitude. Gravelly ripples range from small-scale incipient ripples of a few centimeters height and decimeters in wavelength to large-scale ripples reaching up to 2.4 m height and 43 m wavelength, being the largest ripples reported on Earth, comparable only to Martian counterparts. The average grain size on the crests is ~6-8 mm, although the coarsest ripples show mean grain sizes over 1 cm. The crests are the coarsest part of the ripples, exclusively formed by well varnished, non-pumice clasts (2.43 g/cm³), whereas the ripple bodies show well developed foresets and have variable proportions of low-weight pumice clasts (0.91 g/cm³). Large-scale ripples profiles are partly elaborated on bedrock. These ripples result from the combination of elutriation of the finer fractions of pre-existing deposits along with wind drift of lagged fractions of the bedform. Their abnormally large size seems to be related to very strong winds, a highly concentrated saltation layer due to the pumice, and a long time for evolution. Additional features as 1) a smaller ripple size over obstacles, 2) the lack of correlation between clast size and wavelength, and 3) extreme ripple size in spite of the thin air, suggest that while small-scale ripples may form according to reptation models, their evolution into local large-scale types may be related to aerodynamic instabilities originating at the saltation curtain/air interface.