The environmental changes of eolian, alluvial and lacustrine systems of Central-Western Argentina at the onset of the Holocene.

MILANA, J. P

Mar Chiquita, Cordoba, Argentina

Congreso; Holocene environmental catastrophes in South America: From the lowlands to the Andes; 2005

IGCP 490 and ICSU/IUGS

            The analysis of the recent sediments and the present-day sedimentary process occurring at the lowlands of central-western Argentina show that a large environmental change occurred at the onset of the Holocene. Three important sedimentary systems were deeply affected by that climate change and reported here: Perennial fluvial streams sourced at the Cordillera, ephemeral or saline lakes and playas on closed basins and aeolian sand seas.             Fluvial streams sourced from Cordillera show a pre-Holocene aggradational tendency within valleys due to a low transport efficiency. Datings suggest that this tendency reverted into erosive at the onset of the Holocene, generating a short period of river downcutting and tributary alluvial fan toe erosion, as a result of increased discharges that allowed sediment transport. Valley-deposits incision extends several kilometers into depositional basin. This is replaced by a relative sedimentary stability during the Holocene and minor accumulation or sediment transport due to an important discharge decrease.             Closed hydrological basins suggest that perennial lakes turned into the present-day basin-center dry playas and salt-pans. Basins like Salinas Grandes and Mascasín, show morphologic and sedimentologic evidences indicating a highstand epoch during the previous climate stage. Sediment supply during lake highstand generated a coastal  slope break. Regrading of ephemeral streams brings their load directly to the former lake floor, and produced a kinematic wave of alluvial mud-plain rill-erosion that is now reaching the upper alluvial plane. A comparable evolution was observed at the Bebedero basin. Highstand ends at the Holocene onset, and this regional desiccation of lakes is explained by changes in wind patterns.             Aeolian sediments from central-western Argentina, indicates that present-day winds are weaker and different from those responsible for the sand sea growth and activity until the upper Pleistocene. Dunes and draas are now inactive and fixed by vegetation, while deflation of sand ridges and deposition of dust & sand at interdune hollows are the dominant present-day eolian process at these former ergs. Dune network analysis shows paleowinds different to present regimes: they formed under a bimodal wind regime. The almost complete deactivation of the main wind regime (from the NEE), the growth of the SSE wind and the appearance of  the W-wind (Zonda), inhibited further sigfnificant eolian sedimentary activity.             The alteration of regional wind-patterns explains both the deactivation of major aeolian sand seas and the existence and further desiccation of perennial lakes, as the NEE wind is today carrier of most the humidity to south-western Pampean Ranges. Fluvial evolution would instead respond to the variable release of melt water at the Andes.