Dune-field pattern of blowouts and sinuous ridges for a vegetated aeolian system, western Pampas, South America

TRIPALDI, ALFONSINA; FORMAN, STEVEN L.; DÍAZ BALOCCHI, LUIS; MARÍN, LILIANA

Dublin

Congreso; 20th INQUA Congress; 2019

International Union for Quaternary Research

The role of vegetation in active and non-active dune fields has been amatter of attention in aeolian research, particularly in regards of landscapereactivation, stabilization and related dune mobility. It is well known theexistence of vegetated dunes with simultaneous sand accumulation, such as parabolicdunes coastal foredunes and sand sheets. Stabilized dune fields are common insemi-arid to semi-temperate environments and reflect a shift from the dominanceof aeolian processes to ecologic succession resulting in landscapestabilization in response to climate change and/or anthropic impacts. In thewestern Pampas of southern South America (~34°´S), the SanLuis paleo?dune field is a vegetated savannah-like environment (grassland with scattered woodlands)of a stabilized aeoliansystem with a rich sedimentary record spanning the last ~50 ka. This aeoliansystem shows dunes of different morphology, size and orientation, with cross-cuttingrelationships, indicating dune formation at different times, consistent withthe stratigraphic record. Uncertainty remains on driving forces for theevolution of this landscape, particularly with recent landscape stability ca.1000 years ago and partial reactivation in the 1930s.  Specifically, the relation between dunedepositional/erosional events and the formation of compound aeolian bedforms oftendefies known process-based reasoning. Here we present observations on compoundand complex blowout dunes (variable mean length of 835-1700 m and mean width of 500-1130 m) and unusual, multiple-setsof sinuous ridges (mean length, amplitude and slopes of ~620 m, ~2.5 m and ~3°,respectively) (Fig.1). Our aim is to contribute to the understanding of aeoliangeomorphology and to improve the use of dunes and aeolian sand sheets as ageoproxy of landscape response to past environmental and climate changes. Theworking hypotheses is that the compound and complex blowout dunes were inherentemergent elements of this aeolian landscape, and the two different orientations(NW and SW transport directions) correspond to distinct and successive aeolianphases during the late Pleistocene. Three possible explanations are evaluatedand discussed for formation of sinuous ridges: 1) The forms were initiallylinear dunes, later modified to the present morphology, with stabilization bygrassland succession, 2) Sinuous ridges were originally a primary dune form,such as barchanoid-ridges and stoss and lee slopes were extensively modified withstabilization and changes in wind direction and, 3) These sinuous ridges aresome type of zibar form that originally grows in conflict with vegetation andreflect long wavelength and low amplitude aeolian transport.