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

FORMAN, S.L.; DÍAZ BALOCCHI, L.; TRIPALDI, A.; MARÍN, L.

Dublín

Congreso; 20th International Union for Quaternary Research Congress; 2019

International Union for Quaternary Research Congress (INQUA)

The role of vegetaon inacve and non-acve dune fields has been a maer of aenon in aeolian research,parcularly in regards of landscape reacvaon, stabilizaon and related dunemobility. It is well known the existence of vegetated dunes with simultaneoussand accumulaon, such as parabolic dunes coastal foredunes and sand sheets.Stabilized dune fields are common in semi-arid to semitemperate environmentsand reflect a shi from the dominance of aeolian processes to ecologic successionresulng in landscape stabilizaon in response to climate change and/or anthropicimpacts. In the western Pampas of southern South America (~34°´S), the San Luispaleo?dune field is a vegetated savannah-like environment (grassland withscaered woodlands) of a stabilized aeolian system with a rich sedimentaryrecord spanning the last ~50 ka. This aeolian system shows dunes of differentmorphology, size and orientaon, with cross-cung relaonships, indicang duneformaon at different mes, consistent with the stragraphic record. Uncertaintyremains on driving forces for the evoluon of this landscape, parcularly withrecent landscape stability ca. 1000 years ago and paral reacvaon in the 1930s.Specifically, the relaon between dune deposional/erosional events and theformaon of compound aeolian bedforms oen defies known process-based reasoning.Here we present observaons on compound and complex blowout dunes (variable meanlength of 835-1700 m and mean width of 500-1130 m) and unusual, mulple-sets ofsinuous ridges (mean length, amplitude and slopes of ~620 m, ~2.5 m and ~3°,respecvely) (Fig.1). Our aim is to contribute to the understanding of aeoliangeomorphology and to improve the use of dunes and aeolian sand sheets as a geoproxyof landscape response to past environmental and climate changes. The workinghypotheses is that the compound and complex blowout dunes were inherentemergent elements of this aeolian landscape, and the two different orientaons(NW and SW transport direcons) correspond to disnct and successive aeolianphases during the late Pleistocene. Three possible explanaons are evaluated anddiscussed for formaon of sinuous ridges: 1) The forms were inially lineardunes, later modified to the present morphology, with stabilizaon by grassland succession,2) Sinuous ridges were originally a primary dune form, such asbarchanoid-ridges and stoss and lee slopes were extensively modified withstabilizaon and changes in wind direcon and, 3) These sinuous ridges are some typeof zibar form that originally grows in conflict with vegetaon and reflect longwavelength and low amplitude aeolian transport.