The Ensenadan Stage/Age in southern South America. Special Volume of the Quaternary International.

TONNI, E.P., RABASSA, J. Y CARLINI, A. A. (EDS).

ELSEVIER

Año: 2009 p. 110

The Ensenadan Stage/Age is a very important period of the Quaternary history of southern South America. Though its time boundaries have not been defined with absolute precision, it probably extends from the earliest Early Pleistocene, ca. 2 Ma (Olduvai event), to the Early Middle Pleistocene, around 0.5 Ma. Its original name and definition come from the sedimentary beds and their paleontological content found in the excavations of the harbour of Ensenada, a town located very close to the city of La Plata, Buenos Aires Province, on the western shore of the Rı´o de la Plata estuary, which were done in the late 1800s. The Ensenadan Stage/Age in its type area is developed during the genesis of the Ensenada Formation, loess and loess-like sediments and fluvial units. Inter-bedded marine coastal and littoral deposits were originally described by Florentino Ameghino, but their occurrence has not been undoubtedly confirmed since then. The Ensenadan Stage/Age is based upon the Mesotherium cristatum Biozone, which is spatially coincident with the Ensenada Fm. It comprises the epoch when an outstanding paleontological content demonstrated the evolution of, in particular, a very rich mammal fauna, as well as birds and other fossil vertebrates. Evidence of these times is present not only in the fossil vertebrates, but in marine molluscs, pollen content and sedimentary records as well, all of which are timely coeval with the Ensenadan Stage/Age of the Pampas. The Ensenadan Stage/Age is a very important chronostratigraphic unit, particularly recognized in the Pampas of Argentina, along the provinces of Buenos Aires, Co´ rdoba, La Pampa, Santa Fe and Entre Rı´os. However, there is also physical evidence of coeval geological and paleoenvironmental events already described from Bolivia, Paraguay and southern Brazil to southern Patagonia and Tierra del Fuego. Even without considering the regions of Patagonia which were glaciated during this period, the non-glacial, Ensenadan continental units alone extend over a surface of more than 2 million km2, i.e. approximately equal to the areal extent of France, Germany, UK, and Italy altogether, which gives an idea of their regional significance. As 2009 was the international Darwinian year, it is worth noting that Charles Darwin carefully described Ensenadan (then known as ‘‘Pampean’’) fossils and sediments almost 150 years ago (Darwin, 1863). His data was based upon information gathered during his memorable trip through the Pampas, Uruguay and the Rı´o Parana´ valley, when field excursions from the city of Buenos Aires were organized in 1832 and 1833, whilewaiting for the ‘‘Beagle’’ vessel to depart southwards to Patagonia and Tierra del Fuego, then to the Pacific Ocean and around the world. His own field data were combined later with artesian well data sent to him from Buenos Aires (Figs. 1 and 2). Darwin found that the ‘‘Pampean’’ fossils were awesome, fascinating and astounding, and he sent them to Professor Richard Owen, in England, for further study and interpretation, which subsequently generated many other paleontological and paleoenvironmental papers. The scientific influence of the study of the ‘‘Pampean’’ fossil vertebrates in Darwin’s mind, on his celebrated pathway to the ‘‘Origin of Species’’, should not be ruled out and deserves further analysis and interpretation. Later, paleontological and geological studies by Florentino Ameghino, Carlos Ameghino, Francisco P. Moreno and Joaquı´n Frenguelli, among many other distinguished scientists of the early 20th century advanced in the definition and characterization of this unit. Thus, a long scientific history of the Pampas and southern South America is also related to the Ensenadan Stage/Age, which should be recognized as well. The papers included in this volume are a selection of those presented in the III Argentine Congress on Quaternary and Geomorphology, held at the Universidad de Co´ rdoba, Argentina, in October 2006. The editors have arranged the papers by firstly presenting the purely geological contributions, followed by the papers dealing with the biostratigraphic, paleontological and paleoenvironmental aspects. In the introductory paper, the editors (Tonni et al.) present the problem of the Ensenadan Stage/Age of southern South America, from the stratigraphic, biostratigraphic and chronological points of view, both on historical terms as well in the sense presently used of these units, providing the conceptual framework in which this volume is placed. These aspects have been a matter of endless discussion for decades and they dreadfully need to be clearly stated to the reader. The geological, pedological and paleomagnetic knowledge on the late Cenozoic sedimentary sequence in southwestern Uruguay is summarized by To´ falo et al., who analyze lithofacial and pedological aspects of Quaternary units found along the coastal cliffs of the Rı´o de la Plata estuary. They also discuss the nature, components and age of the Raigo´n and Libertad formations, of fluvial and loessic origin, respectively. The available data indicate fluctuations of climatic conditions and the existence of several cycles of erosion, sedimentation and pedogenesis in the Pleistocene.