Holocene sand sheet depositional record San Luis province, western Argentina

FORMAN, S.L.; TRIPALDI, A.

Denver

Congreso; 125th GSA Annual Meeting; 2013

Geologica Society of America

Stabilized dune fields are common in western Argentina and reflect potentially drought variability in the late Quaternary. Sedimentologic and geochronologic studies focused on the San Luis dune field, in the semi-arid western Pampas Dune exposures reveal a complex eolian history with multiple buried soils and intercalated eolian sand. Optical dating focused on the 100-150 micron quartz grains using single aliquot regeneration (SAR) protocols. These eolian sands are immature mineralogically with quartz percentages between 40 and 50%. Unlike other Andean-sourced quartz Pampean sediments show a clear dominance of fast luminescence component with medium and slow components near background levels. Geochronometric sensitivity of Pampean quartz reflects full solar resetting, the dominance of fast luminescence components and high light output, possibly reflecting multiple dose and solar reset cycles for this quartz. There are thick sequences (5-6 m) of eolian cover sands initially deposited ca. 11.6 to 10 ka ago and continuing to at least 1 ka reflecting deposition of a sand sheet. Post ca. 1 ka the Rio Quinto degraded by 30 m, isolating many upland surface from further eolian additions. Sand sheet accretion is associated with drier conditions than today with predominance of grassland versus savannah-type environments (Espinal) that were common prior to European settlement. This dry condition is associated with a weakened South American Monsoon and less import of Amazon-sourced moisture from orographic jets that are confined to the Andean Front. Near the top of many sections is a distinct ash that is probably associated with the 1932 Cerro Azul phreatic eruption, which in many places is immediately covered by eolian sand. The ash may have increased land surface hydrophobicity during a multi-year, particularly dry period, enhancing sediment availability. In turn over grazing and expansion of acreage to wheat, like the 1930?s Dust Bowl on the Great Plains, expanded the footprint of eolian activity. https://gsa.confex.com/gsa/2013AM/webprogram/Paper228532.html