Evolution of staggered Oligocene-Miocene transgressions and molluskan faunas in eastern Patagonian basins

Abstract

Among the Cenozoic marine incursions in southern South America, the Chattian-Langhian Patagoniense episode is particularly significant due to its extensive spatial distribution, abundant fossil record, and comprehensive research history. It flooded several Atlantic basins, between S 40o - 53o , each with different tectonic context and infill history. In this work we present new geochronologic, stratigraphic, sedimentologic and paleontological data of the Patagoniense successions to evaluate basin evolution, sedimentation and their molluskan faunas; and to establish the regional architecture based on new correlations. The results confirm that the Patagoniense consisted of three separated transgressive episodes: Juliense, Leonense and Superpatagoniense, whose ages are here determined using Sr-isotope stratigraphy. Although largely overlooked before now as a separate transgression, we demonstrate that the Superpatagoniense interval is clearly distinguishable as a unique transgression, with an extensive record in basins of central and northern Patagonia between 15.9 and 15.0 Ma. The biostratigraphic reappraisal of the Patagoniense Molluskan Assemblages indicates that the youngest (NVG) fauna is preserved in the uppermost Leonense and Superpatagoniense beds, and can be calibrated between 17.0 and 15.0 Ma. This fauna was dominated by tropical genera, and ranged up to S 47o during the Miocene Climatic Optimum, whilst also showing latitudinal changes in composition that are related to temperature. The three transgressions and the subsequent regressions exhibit a staggered pattern, being younger northwards. Such large-scale, orderly architecture would have responded to the independent or combined effects of several geological and climatic factors, but to date, none of them can be clearly established. They were not uniform or instantaneous, but rather propagated over ten million years across the marginal southeastern Atlantic basins. This tiered pattern suggests a common link between processes that controlled the transgressions, which must be considered in future works.