New data on the origin of the unique gravelly aeolian dunes and giant megaripples of the Puna Plateau, Argentina

MILANA, J.P., KRÖHLING, D., PERALTA, C., VIRAMONTE, J.

Berna

Congreso; XVIII INQUA BERN 2011; 2011

INQUA

A unique gravelly aeolian dune system is found at Purulla (Argentine Puna). Lightweight dust would create an abnormally dense transport layer that helps of large clast transport. Additionally, widespread low-density sand and gravel would fostered the formation of these wind dunes. Although they share aspects with yardangs (a steep wind-face and a gently sloping lee face), the material actively eroded at the dune front could not be transported past the highest point where wind flow lines start to be decompressed. As a result, a mixed landform is formed by the direct erosion of both unconsolidated Late Pleistocene alluvial cover (normal density grains) and the low-consolidated Purulla Ignimbrite (low density fragments). A recent contribution using GPR showed that there is a constructional part of the landform that elevates 15 m over the previous alluvial plain. Radiograms show perfectly well, how this yardang-like dune has been built by megaripple translatent strata and how these beds truncate at the windward face and downlap to both lateral margins and downwind, onto the alluvial cover (‹12 ka. BP). The initial work on dating both megaripples and dunes suggest this dune system started being active at the onset of the Holocene, then undergo a period of wind reversal in which dune top degraded and sediment was moved in opposed direction (westward) and returning to the today active eastward winds, since ca. 3 ka. As the W-directed wind systems could not move the material of these dunes today a major wind-regime change would be expected within the Holocene of the region, a change that may explain other Holocene climate shifts. The second conclusion is that these landforms are so unique due to the distorted physical conditions for sediment transport, as the dominant constitutive sediment of the dunes is lightweight fragments. This system is therefore quite well suited to study extraterrestrial sediment transport by wind, at gravity fields lower than the terrestrial.