El loess de La Pamoa norte en el bloque de San Guillermo

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