Geological, pedological and paleomagnetic study of the late Cenozoic sedimentary sequence in southwestern Uruguay, South America

摘要

The results of interdisciplinary studies carried out on the coastal cliffs of southern Uruguay are presented here. The Raigon Formation (late Pliocene-middle Pleistocene) has a cratonic provenance and comprises two sections limited by a regional discontinuity. Each section is composed of an association of lithofacies Ss, Sp, Sr, rare St, and Fm. The last one represents floodplain deposits and the others are interpreted as channel-filled deposits, in the upper part, a well-structured paleosol with clay coatings and abundant rhizoliths is observed in a sharp contact with silty deposits of the Libertad Fm. (early-middle Pleistocene); these sediments have a mixed provenance (magmatic arc and cratonic), sheet-like geometry, thick stratification and vertical walls, and are composed of lithofacies Fr. These characteristics suggest a loessic origin. A paleosol with illuvial B horizons and silty loam texture is recognized at the bottom of this unit. The recent soil (Argiudoll) is developed at the top of the Libertad Fm. The characteristics of the sequence indicate climatic fluctuations and several cycles of erosion, sedimentation and pedogenesis. According to magnetostratigraphy, the chronology is roughly compatible with that one based on the fossil contents and is in agreement with magnetic ages determined in Argentine fossiliferous sequences.