A GPR 3D study of aeolian gravelly dunes of Puna and implications to the Holocene wind regimes of NW Argentina

MILANA, J., KROHLING, D., PERALTA, C, VIRAMONTE, J.

Mendoza

Congreso; International Sedimentological Congress 2010; 2010

International Association of Sedimentologists

The wind in the Southern Puna of Argentina moves medium-sized gravels forming gravelly megaripples that can reach 2.3 m high and a spacing of up to 43 m. Also, the gravel is transported to the deflation corridors, where it forms a continuous sheet of clasts abraded by wind. However today remains under discussion if the wind can form gravel dunes. In this contribution, we show unequivocally that during the Holocene, after the deposition of the complex of Purulla ignimbrite dated in 12 Ka. BP and the deposition of an alluvial gravel blanket possible at the end of the Pleistocene, there was a major alteration of the landscape by the wind and as a result, a special type of dune was developed. It has an atypical morphology and abnormal grain size, which involves a significant amount of gravel of different densities. The dunes shown in this study were informally called "aerodynamic dunes" because its profile is opposite to that of crescentic dunes and it is composed of a windward face of steep gradient (up to 27º) respect to the lee face (2-5º). Basically, the longitudinal profile is similar to a yardang, but in this case the landforms are partly constructive as shown in this study, and not produced by aeolian erosion. They show a crescentic plain shape, resembling the morphology of a barchan but in this case, the concavity looks windward. These inverted morphology and longitudinal profile are explained for the action of very strong winds in relation to typical sand dunes. The frontal concavity is reminiscent of that of parabolic dunes, where the concentration of erosion at the center of the dune occurs. This atypical morphology and coarseness led the investigation of the internal structure by ground-penetrating radar (GPR). As was suggested earlier,GPR study corroborated that the uppermost part of both profiled dunes is composed entirely of megaripple translatent strata. This type of aeolian stratification is not very well known as megaripples are usually bedforms whose deposition/erosion balance is negative, so their preservation potential is low and there are not fossil examples described. This stratification is represented by a radiofacies defined by layers with a high heterogeneity of reflectance, which can be: (a) laterally continuous but showing lateral changes in the intensity of the reflector or (b) showing apparent lateral continuity but in detail subtle endings are observed. Because of the low index of megaripples near dune crests, there are no foresets, but in the lower parts of the dunes it is sometimes possible to recognize a low angle crude crossbedding due to preservation of these non-avalanche foresets. Although the aggradation of megaripples is basically represented by light-weight pumice grains, layers richer in lithics may occur. Their higher density makes them more reflective, generating heterogeneous reflectivity into a pseudo-continuous laminated context. Thus, the stratification of megaripples defines a readily identifiable radiofacies that allow interpretation of how dune was constructed. The crest of the dune body is made up entirely of megaripples advancing against to the basin regional slope, since the present advancing direction coincides with a progressive downlap of these megaripple radiofacies to the west and to the flanks of the dune. Three main constructive intervals were recognized, the first one associated with much stronger winds than the present day and formed simultaneously to the main deflation stage, generating a dune higher than what is observed today. A second intervals suggest a stage of wind reversion and deposition of a thick unit over the previously winward face. The last stage indicates a return to the normal E-directed winds, although in a more moderate scale. In this stage, a minor dune accretion occurs; and some infilling on the front of the dune with materials more finer in grain-sizes also occurs, with smaller megaripples, probably corresponding to modern conditions.