A 54 M THICK LOESS PROFILE IN NORTH PAMPA, ARGENTINA

First results of a research borehole drilled in San Guillermo locality (30°15´ lat.S and 61°50´ long.W), North Pampa, are presented. This work is part of the “FLAGS Project” founded by the National Agency of Science Promotion, Argentina.

The borehole (101.50 m depth) comprises the entire Quaternary column of the region and part of Tertiary sedimentary rocks. A total of 91 sediment samples were collected continuously at 1 m depth intervals. 15 oriented undisturbed tube cores were recovered at intervals of 6 m. Also geoelectric and neutron profiles were made. Grain-size, mineralogic, geochemistry, soil micromorphology, paleomagnetic analyses and TL-OSL datings are in progress.

According to preliminary results, the Quaternary section is composed of three primary (not reworked) loess units, and an underlying plastic clay. The upper loess unit is the well known Tezanos Pinto Formation (OIS 2). The other two loessic units are composed of dominant silt with scarce fine sand and abundant segregations of CaCO3. Colour varies from 7,5 YR 6/3 (light brown) to 7,5 YR 7/3 (pink). X-ray diffractions of total samples indicate a dominance of quartz, with scarce feldspars and illite.

The section is representative of a flat area of more than 15,000 km2 covering the NW of Santa Fe province and the NE of Córdoba province. The borehole revealed 54 m of loess which begins at the surface. A Quaternary paludal clayey unit (between 54 and 60.3 m deep) separates the loess column from a Miocene littoral sedimentary unit (Paraná Formation).

Such a thick aeolian profile provides detailed information on the Pampean Aeolian System, mainly considering that outcropping loess-paleosols sequences from North Pampa hardly reach 10 m. The study of the sedimentary cores recovered from the borehole will generate new data for understanding the complex history of the Pampean System during the Quaternary, focusing on the identification and analysis of the source areas of sediments and ages of deposition. The results will bring advances in the knowledge of Quaternary climatic changes in the Pampa plain.