Abstract

KROHLING, Daniela M  and  IRIONDO, Martin H. The loess of North Pampa in the San Guillermo Elevated Block. Rev. Asoc. Argent. Sedimentol. [online]. 2003, vol.10, n.2, pp.137-150. ISSN 1853-6360.

An investigation on the regional distribution of loess in the North Pampa resulted in the identification of the San Guillermo Elevated Block. The pivotal work of the project was the drilling of a research borehole at the town of San Guillermo (30°15´ lat.S and 61°50´ long.W), North Pampa. 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 gamma profiles were made. The borehole revealed 54 m of loess which begins at the surface. A Quaternary paludal unit (between 54 and 60.3 m deep) separates the loess column from a Miocene littoral sedimentary unit (Paraná Formation). The Quaternary section is composed of three primary (not reworked) loess units, and an underlying clayey silt. The upper loess unit is the well known Tezanos Pinto Formation (OIS 2). The other two loessic units are composed of dominant silt (average textural composition: 65% silt), with subordinated clay and scarce fine sand; the segregations of CaCO3 are abundant. Colour varies from 7,5 YR 6/3 (light brown) to 7,5 YR 7/3 (pink). The mineralogic composition of the very fine sand fraction is dominated by three forms of quartz: polycrystalline, forming microagglomerates and monocrystalline quartz. X-ray diffractions of total samples indicate a dominance of quartz, with scarce feldspars and illite. The source of the loessic units is probably an old loess located at the south. Such a thick aeolian profile of the borehole provides detailed information on the Pampean Eolian System (PES), mainly considering that outcropping loess-paleosols sequences from North Pampa hardly reach 10 m. The section is representative of a relatively elevated flat area with a surface of 17,800 km² covering the NW of Santa Fe Province and the NE belt of Córdoba Province. That area is limited by fractures and has a marked stratigraphic, sedimentological and geomorphological identity, different to the surrounding region. It is identified here as the San Guillermo Elevated Block, which during the Quaternary underwent only eolian influence, in contrast to the rest of the North Pampa, which was subject mainly to fluvial and alluvial influence of currents arriving from the west and the north. According to the complementary information obtained from more than 100 previous boreholes covering an area of 50,000 km², the stratigraphic column of the San Guillermo Elevated Block is clearly different to the stratigraphic column of the area located to the east. The stratigraphic correlation evidences that the San Guillermo Block is the western limit of the Pliocene- Lower Pleistocene Ituzaingó Fm deposited by the Paraná River.

Keywords : Research borehole; Loess; Pleistocene; Sedimentology; North Pampa.

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