This paper aims to contribute to the stratigraphic and geochronological knowledge of the Neogene-Quaternary of the geologic province of Tandilia by doing lithostratigraphic, paleomagnetic and paleontological analyses. Four lithostratigraphic units have been recognized. The oldest (Late Miocene) constitutes a unit composed of colluvium deposits; it lays in erosional unconformity over the Neoproterozoic rocks. This unconformity is related to important events in the Andes during the Middle-Late Miocene. Over the oldest colluvial sediments, and placed in erosional unconformity, there are sediments assigned to the Barker Formation which are essentially fluvial deposits with a development of several paleosols. Owing to the fossil remains, this unit is assigned to the Montehermosan-Chapadmalalan Stage/Age, whereas the paleomagnetic record would correspond to C3A, Gilbert and Gauss chrons. So, the age is narrowed to the timespan 7.1-3.1 Ma (Late Miocene-Early Pliocene). The Vela Formation lays in erosional unconformity over the Barker Formation. It also has fluvial origin, while the youngest unit (Las Á nimas Formation) corresponds to loess. Both units provide normal polarity and were assigned to Brunhes chron (\0.78 Ma). The Vela Formation presents fossil remains from Bonaerian-Lujanian Stage/Age and belongs to the Middle Pleistocene. The hiatus between the Barker and Vela Formations seems to be considerable: it represents at least 2.6 million years. The reason for the lack of sedimentary records is still a matter of discussion. The deposition of Las Animas Formation seems to span the last 40,000 years. However, this could be linked to a major entrance of windcarried sediments during dry periods, including the Last Glacial Maximum. The hiatus between the Vela and Las Á nimas Formations may have been over 100,000 years.
This work aims to contribute to the interpretation of soil colors; mainly to identify the pigments that generate the greenish colors in hydromorphic conditions (gley colors) in vertisols of the coastal plain of the Río de la Plata estuary. These colors are of cold hues, generally greenish (olive-gray), and occur in poorly drained soils with prolonged water accumulation. For this purpose, samples of Bssg horizons of hydromorphic vertisols from the coastal plain of the Río de la Plata estuary were analyzed using routine and chemical analyses, Mössbauer spectroscopy, determination of rocks magnetic parameters, X-ray diffraction, thermogravimetric and differential thermal analysis and petrographic-electronic microscopy. The pigments associated with the gley colors of these soils are mainly due to ferric iron content, corresponding to minerals such as ferric iron-rich smectites (nontronite/Fe-rich beidellite) and oxy-hydroxides like goethite, which colors range from green to yellow. These minerals, combined with gray or black components, e.g., manganese compounds and/or organic matter, contribute to the generation of the usual gley colors in the analyzed soils of the Pampean plain. The obtained data allows us to state that it is a mistake to assing the olive color in hydromorphic soils to the presence of ferrous iron compounds, as it is traditionally done. The oxidizing condition of iron is dominant in all the analyzed samples, integrating the composition of ferric iron-rich clay minerals and oxy-hydroxides (goethite).
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