IMPORTANCE OF PSEUDO-GRANULOMETRY IN THE SEDIMENTARY RECORD OF ANCIENT RIVER PLAIN SYSTEMS ASSOCIATED WITH THE LAST MARINE TRANSGRESSION IN SOUTH AMERICA

The aggregation process in current fluvial system depends on the type and concentration of the sediment, the conditions, the flow and the ion concentration in the water column, mainly salinity. These characteristics could be defined in the Holocene age sedimentary record, where it is feasible to differentiate the natural aggregation of fine particles associated with low slope river transport in warm climate, from post-depositional phenomena. This contribution analyzes of the river transport of fine particles with a focus on the clay and silt fractions forming aggregates of greater granulometry.

In the regional frame, the main Holocene geological record is linked to the tributary basins of the Paraná River. This is represented by the deposits of the La Picada Formation, which constitutes the low river terrace of the current tributaries streams of the Paraná River in Entre Ríos, Argentina.

Three stratigraphic sections on paleochannel outcrops corresponding to the same river system were analyzed with sampling in the different depositional facies in the current Arroyo Doll stream. In order to obtain information on binding effects by organic matter and salt concentration on the relative distribution of each grain-size class, the samples were treated in two forms prior to granulometric quantification: without prior dispersion treatments and elimination of organic matter and clay dispersion .

To achieve the analysis of grain size distribution, at each of the stages, classical granulometric analyses were used. At each stage the samples were observed in Electronic Scanning Microscope (SEM), in order to directly verify the possible presence of sedimentary aggregates.

In an appropriate geological context, photographs taken under SEM of sediments collected from the stratigraphic profile can be fast and efficient tool for the detection of pseudogranulometries in the sequence before its preparation for granulometric analysis.

It is concluded that in the holocene river sedimentary record from flat environments under warm climates, it is important to take into account the possible presence of pseudogranulometries. In this case, it is not recommended classical sediment disaggregation techniques, as these treatments could lead to misinterpretation when the objective is to evaluate the paleodynamics of past river systems.