Ancient Landscapes of Uruguay

Abstract

In this chapter, based on the available geological information, a model for the genesis and evolution of the Uruguayan landscape is proposed. A structural framework of the landscape evolution is provided and the record of such evolution in the most representative geological units is considered. A brief summary of the Uruguayan geology and its location in the regional context is performed, from Precambrian to Cenozoic times.From the analysis of the geological record, it may be observed that the climate was very arid during part of the Jurassic and the Early Cretaceous. Together with the lava flows of the Arapey Formation, the climate became less arid as the Gondwana continents were becoming apart from each other. However, the geological record suggests that semiarid climates were still prevailing. In the Middle Cretaceous, semiarid and wetter climates progressively alternated, until the Early Tertiary, when very wet and warm conditions were established, in coincidence with the ?Paleocene Eocene Thermal Maximum (PETM)?, followed by semiarid climates in the Oligocene, wetter conditions in the Miocene and semiarid again in the Pliocene, with alternating semiarid and humid conditions during the entire Quaternary.On the basis of the paleoclimatic evolution, the development of relief is discussed, considering as bases for the analysis the different morphostructural units in which the country is divided. Due to their size, shape and location (passive margin) of Uruguay, climate uniformity is assumed for each period throughout the entire territory. It is also assumed that the surfaces around elevations of 500 meters correspond to relicts of probably pre-Cretaceous etchplains, strongly denudated, which are observed only in the surroundings of Aiguá.The landforms situated below the oldest surfaces, for instance those below 320 m a.s.l. in the Easthern Hills Regions (Sierra del Este), correspond to a new generation of geomorphological surfaces that may be considered of Cretaceous age, according to the information presently available. This surface may be correlated with the oldest surface developed on top of the lava flows of the Arapey Formation.The extremely warm and wet climate of the Eocene prepared the conditions for the planation processes that covered most of the Uruguayan territory during the Oligocene, generating pediplains which were later reworked during the Late Cenozoic, up to the Quaternary, generating a landscape of smooth hills.The morphogenetic potential of each morphostructural region determined the available energy of the resulting landscape, being this at a minimum in the Santa Lucía Basin, which continued to be under subsidence condition until the Tertiary, and almost non-existant in the Laguna Merín Basin, where subsidence remains active until the Holocene.