Litología y Génesis de los depósitos del Cenozoico tardío del Bajo del Durazno, provincia de La Pampa, Argentina

bajo del durazno (65°10' - 65°30'o y 36°33' - 36°45's, loventué departament, figs. 1 and 2) is a transitional landscape area between two major phisiographic subregions: subregion of hills and meadows (subregión de las colinas y lomas) and subregion of plateaux and valleys (subregión de las mesetas y valles,) which dominate the central-northeastern and middle-eastern part of la pampa province (inta, 1980). at present bajo del durazno is a closed drainage system forming a minor geomorphic unit within a regional longitudinal depression, which together with several other depressions run across an extensive structural plain. these longitudinal depressions are known as valles transversales (calmels, 1996). this paper reports the sedimentary characteristics of the deposits outcropping in the western part of bajo del durazno (fig. 1). a lithofacies analysis approach was followed to describe and interpret the sedimentary processes involved in the accumulation of the sediments, the environment and the source area of the deposits. the general purpose is to shed light on the geological evolution of the depression. bajo del durazno is excavated in reddish brown sandy silts capped by a calcrete crust (cerro azul formation, late miocene) which outcrop along the southern and western margin of the depression (figs.3 and 4). the deposits of bajo del durazno were studied in three main elevation systems of the western sector and the depression floor (mehl y zárate, 2006). the deposits outcropping in the three elevations system (i, ii y iii) comprise a fluvial facies association composed of clast supported gravels with planar crossbedded (lithofacies gp); gravel with tangential crossbedded (lithofacies gt); massive gravels (lithofacies gcm); massive matrix supported gravels (lithofacies gmm); silty sands with dispersed carbonate clasts (lithofacies sm1); and massive sandy silts (lithofacies fm2). the deposits are believed to be accumulated by ephemeral fluvial streams transporting fine