Paleosols of the Middle Cenozoic Sarmiento Formation, Central Patagonia

E. BELLOSI AND M. GONZÁLEZ

The Paleontology of Gran Barranca: Evolution and Environmental Change through the Middle Cenozoic of Patagonia. Madden, R., A. Carlini & M.G. Vucetich (eds.)

Cambridge Univ. Press

Año: 2007;

In the loessic and fluvial pyroclastic deposits of the mammal-bearing Sarmiento Formation (Middle Eocene-Early Miocene) of the Gran Barranca (Central Patagonia) several types of alkaline and oxidized paleosols are recognized. These are characterized by a relative uniform parent material, diverse insect trace fossils (Coprinisphaera and Celliforma ichnofacies) and abundant opal phytoliths of grasses and palms. The paleosol succession preserves, with relative minor gaps, changes in soil moisture, biogenic activity, chemical weathering and sediment influx. Such factors determined two maturation trends of paleosols, identified on the basis of field, micromorphologic and geochemical analyses. Both trends alternated several times during the Sarmiento time. Trend A developed exclusively from thepric loessites, and includes calcic Entisols, calcic Andisols and Aridisols. It corresponds to periods of a semiarid-arid climate represented in the Gran Barranca (Middle Eocene), Vera (Uppermost Eocene-Lowermost Oligocene) and upper Colhue-Huapi (Lower Miocene) members. Most of paleosols are very weakly- to weakly-developed, indicating continuous or faster and steady eolian sediment influx. They probably formed in grasslands, shrubby grasslands and palm savannahs. A moderately- to strongly-developed Aridisol, identified in the Rosado Member (late Middle Eocene), represents the driest environmental conditions. Trend B is associated to fluvial facies (intraformational conglomerates and tufoarenites) and subordinately to loessic deposits, and includes non-calcic Andisols, Alfisols and intermediate Alfisols-Ultisols. It records periods of seasonal, subhumid to humid climate, and woodland or wooded-grassland vegetation, corresponding to the Gran Barranca (partially), Lower (Late Eocene) and Upper (Oligocene) Puesto Almendra, and lower Colhue-Huapi (Lower Miocene) members. The stronger development of these paleosols evidence a more discontinuous or slower fluvial-eolian accumulation.