LATE QUATERNARY LOESS-PALEOSOL RECORD OF THE NORTHERN PAMPAS OF BUENOS AIRES PROVINCE, ARGENTINA

ZARATE MARCELO

lawrence Kansas

Otro; 41st Annual Meeting GSA; 2007

Geological Society of America

 The northeastern Pampas of Buenos Aires province known as Pampa Ondulada, is an aggradational plain composed of an upper fine-grained mantle, several meters thick, interpreted as a loess cover; the uppermost 1.5-2 meters are modified by the development of the present soil. It is underlain by more compact and moderately indurated sandy silts (loess-like or loessoid deposits). Two representative sections (Gorina and Hudson), situated 10 km away from each other and located at different geomorphological settings, were analyzed. In the Gorina section, located at around 17 m above sea-level, five pedological units and two erosional unconformities were differentiated on the basis of field-scale morphological properties. Detailed micromorphological analysis, however, suggested the presence of an 8 m thick pedocomplex and did not support previous assumptions of alternating discrete loess and paleosol units. OSL dates yielded ages between circa 9 and 193 ka for the upper 6 meters of the succession. The Hudson section is located at 9 m above sea-level and much closer to the Rio de la Plata coastal plain. The uppermost 1.5-2 meters are composed of typical loess, underlain by clayey silts subdivided into several units including a marine layer with fragments of mollusk shells. Evidences of paleosol development were found at the lower unit and the marine layer. OSL ages range from circa 8 to 128 ka. The pedosedimentary reconstruction and the numerical chronology at both sections suggest pedogenic activity during part or all of the last interglacial (MIS 5). At Gorina is represented by the upper part of the buried pedocomplex and at Hudson is marked by the accumulation of marine deposits (MIS5e) and the subsequent development of a paleosol. The equivalent soil-forming interval has recently been reported in other localities of the Pampean region. The OSL geochronology suggests that loess accumulation was dominant during MIS 2 forming an extensive mantle on the Northeastern Pampas landscape.