Sedimentology and age model of the late Cenozoic deposits of the Sauce Grande River Basin (Central Argentina)

During the mid-late Pliocene to late Pleistocene, the Pampa Interserrana (i.e., the plains of central Argentina) experienced cycles of erosion and deposition on a Mio-Pliocene substrate generating the valleys of the main rivers of the region. These processes controlled the Quaternary sedimentary deposits that constitute their stratigraphy. The deposits of the Sauce Grande River Basin (SGRB) in Buenos Aires Province of Argentina are a useful case study to analyze in detail the sedimentary dynamics which took place during the late Cenozoic. Additionally, the development of river terraces and their fossiliferous content provide a temporal framework for such interpretations. The basin fill has been described sedimentologically and stratigraphically by several authors, but there is still much that is unknown about its evolution and sedimentary dynamics as well as the age of these deposits. Furthermore, the stratigraphy and age of the Saldungaray and La Toma formations remain largely unexplored, adding complexity to understanding the region's geological history.

In this context, the objective of the present study is to characterize and determine the sedimentary evolution and paleoenvironmental dynamics of the SGRB during the Plio-Pleistocene. Additionally, we aim to establish an age model for the deposits and the vertebrate associations preserved in them. To achieve this, we conducted sedimentological, magnetostratigraphic, biostratigraphic, geomorphological, and paleontological studies in four localities in the Upper basin of the SGRB. This article is specifically reoriented towards understanding the age and environments of the Saldungaray and La Toma formations and their broader implications. For the first time, a detailed chronology of these two units will be provided.

The Neogene substrate at our study sites comprises the Saldungaray and La Toma formation. These two formations characterized by sandy and silty clayey fluvial and facies with pedogenetic features, which are interpreted as channel fill and overbank floodplain deposits. The younger late Pleistocene to Holocene sequence ranges from a coarse gravelly facies interpreted as channel fill deposits grading into fine floodplain and aeolian facies near the surface. The latter constitutes a characteristic extensive mantle, which is distributed throughout the “Pampa Interserrana” and throughout almost the entire Buenos Aires Province.

We described eight sedimentary facies and group them into three facies associations: channelized fluvial deposits; non-channelized deposits (floodplain/overbank deposits) and aeolian deposits. These facies and their associations are characteristic SGRB environments that responded to climatic and tectonic drivers.

A key stratigraphic section (La Toma) proved to be critical for locating the boundary between the Pliocene and Pleistocene (the Marplatan Stage/Age, Vorohuean Substage/Subage) allowing us to link magnetically and biostratigraphically constrained ages and related them to the formation of two prominent fluvial terraces in the study area.