Alfonsina Tripaldi

The upper carboniferous postglacial transgression in the Paganzo and Rio Blanco basins (northwestern Argentina): facies and stratigraphic significance

Abstract During the early Late Carboniferous (Namurian–early Westphalian), an important postglacial transgression took place in the basins of western Argentina. The regional distribution, lithological characteristics, facies arrangement, and age of the transgression are analyzed herein. The postglacial transgressive event was studied in the Rio Blanco and Paganzo basins. Seven regionally extensive facies were recognized. Laminated mudstones were deposited during the maximum flooding stage, including some thin marls and black limestone beds interpreted as condensed levels. Laminated mudstones with dropstones facies, due to iceberg melting, point out to deglaciation processes. Bouldery and pebbly diamictites mainly represent gravity flow deposits, which, in some cases, are associated with interbedded sandstones and mudstones sequences interpreted as turbidites. Sandstones with large-scale cross-bedding represent high constructive Gilbert-type deltas. Finally, coarsening and thickening upward sequences result from the progradation of mouth bars deposited in delta-front environments. On the basis of the facies arrangement, three major postglacial facies associations were recognized: open marine, transitional, and continental-dominated. Palynological assemblages suggest a Namurian to early Westphalian age for the postglacial transgression.

Geological evidence for a drought episode in the western Pampas (Argentina, South America) during the early–mid 20th century

Drought episodes during the early–mid 20th century were recognized and described in several places around the world, with extreme dry conditions and widespread landscape denudation, like during the famous ‘Dust Bowl’ in North America. However, there is scant documentation of droughts in southern South America, particularly from the Pampas, and none based on the geological record. In this article, we provide clear evidence of aeolian reactivation and sand deposition in some areas of La Pampa and San Luis provinces, western Pampas (Argentina), during early–mid 20th century in response to drier conditions, probably amplified, like historic droughts in North America, by anthropogenic factors (e.g. significant population increase and agriculture expansion into a fragile environment). Evidence includes widespread bare sand blowouts, extensive surfaces with active sand migration, steep dune lee slopes, and sharp crests covered by weak soil development (A/C profile), accompanied by historical documents. Optically stimulated luminescence (OSL) ages on aeolian beds confirm mobilization and sedimentation by wind processes c. 95–60 yr BP. Considering the dominant (over 70%) austral spring–summer precipitation, it is possible the rainfall deficit in western Pampas was linked to positive sea surface temperature (SST) anomalies in the western subtropical South Atlantic Ocean (20–30°S and 30–50° W), according to significant canonical correlation between the precipitation field in subtropical South America and the Atlantic Ocean SST anomalies.

The Continental Record of Marine Isotope Stage 3 (MIS 3; ~60–25 ka) in Central Argentina: Evidence from Fluvial and Aeolian Sequences

Marine Isotope Stage 3 (MIS 3) is characterized by high climatic variability resulting from numerous centennial to millennial scale events. The environmental and climatic reconstruction of this interval is restricted by the sparsity of high-resolution (centennial-scale) terrestrial records in most of South America. This contribution is an attempt to reconstruct the general environmental and climatic conditions of southern South America during MIS 3 by means of continental records located in central Argentina; this is an extensive and heterogeneous region made up of diverse geomorphological settings under different climatic conditions. Therefore, the main features of several aeolian and fluvial records situated in different geomorphological settings across the region are overviewed. The results indicate the predominance of regional aggradation during MIS 3 with differences in the accumulation rates and dominance of either aeolian or fluvial deposits depending on the geomorphological setting. The aggradation process was interrupted by stability intervals evidenced by paleosols in the San Rafael plain, the San Luis paleo-dunefield, the eastern Sierras Pampeanas piedmont and the eastern Pampean plain. The paleosols might represent lapses of decreasing aeolian input and perhaps more humid conditions. In addition, paleobiological indicators from alluvial sequences suggest higher temperatures and water availability between 35 and 31 ka in the Andean piedmont, while dry subhumid or strongly seasonal conditions with alternating subhumid-humid phases were inferred in the eastern Pampean plain during MIS 3. These intervals tend to cluster during the second part of MIS 3, and might reflect the environmental responses to some of the climatic oscillations that occurred during MIS 3. Detailed analysis and a more adjusted chronology are needed to correlate the aeolian and fluvial episodes along with the stability intervals at regional and continental scales.