Unique gravelly dunes at the windiest place of the world: a natural laboratory for wind-driven catastrophes

MILANA, J. P

Mar Chiquita, Cordoba, Argentina

Congreso; Holocene environmental catastrophes in South America: From the lowlands to the Andes; 2005

IGCP 490 and ICSU/IUGS

            How strong can the wind blow and affect the environment? An aeolian system dominated by moving gravel, surveyed at the Southern Puna plateau, is shown here. Impressive wind-speeds produce a wide variety of active depositional bedforms, generating a completely different landscape. Besides the generation of unique gravelly dunes, minor bedforms as ripples, that grow according to wind-strength, show the largest size surveyed to date.             Aerial photographs and field surveys showed a crescentic inverted gravelly dune type (opposed o normal barchans) reaching up to 50m high, 200-300m long and 80-100m wide. The stoss-face is steep (27°) and a coarse grained transverse crest separates it from a gentle lee face (5-10°) covered by megaripples. Tnother dome-shaped dune type, is elongated along the wind, with a less steep stoss-face and lacking of the coarse grained crest. Their shapes and profiles seems to reduce wind-drag. Megaripples attain 1.2 m high and up to 18 m spacing; composed by two types of clasts with average densities of 2.42 and 0.92 g/cm³, and crests show only the heavy-clast population. Giant megaripples reaching up to 3 m high and 50 m wavelength are only composed by the light-weight clasts. Coarsest megaripples crests show clasts up to 27mm (mean of 12.3mm), although most of them show mean grain sizes of 7mm. Clasts on dunes and at ripple troughs may reach 5 cm; and up to 20cm near field margins. Largest megaripples reported reach 0.6m high and 10m spacing, while only the flat Antarctic megaripples (~10cm high) show similar coarse-sized grains (mean =< 9.6mm).             Wind velocity measurements from 300 km southward, reported a wind-speed peak of 443 km/h (log-normal population), indicating supporting the present-day movement of these bedforms. While the onset of the Holocene caused a global tendency to slow down wind speed, changes at the southern Puna suggest that windier conditions start prevailing there, being now a natural laboratory to test extreme wind conditions that may occur on Earth.