CONICET | Buscador de Institutos y Recursos Humanos

Loess deposits are the main component of the Late Quaternary sequences of Pampa plain, covering thelargest loessic area of Southern Hemisphere. Pampean loess region lies downwind from source areas located tothe S and SW, comprising a large sand sea and alluvial systems that drain the eastern Andean piedmont. Theloess unit of the last glacial maximum (LGM) at the leeward side of the Pampean aeolian system represents aprimary loess, indicating the expansion of semiarid conditions to the NE. Stratigraphic, sedimentologic, molluskand phytolith data, including OSL and 14C ages, are presented for a loessic area of 14,000 km² at the SW ofEntre Ríos province, Argentina. LGM loess is a homogeneous unit, characterized by considerable spatial continuityin the North Pampa. It occurs as a mantle of 2-4 m thickness; only locally the loess has been fluvially re -worked or evolved under non- permanent swamp conditions. Typical loess is a well-sorted brown (7.5YR 5/4),massive and porous silty sediment. Secondary structures are represented by burrows, fissures, CO3Ca rhizolithsand concretions. Well-exposed LGM loess sections were investigated and sampled along the left margin of theParaná River cliff and in other selected profiles. Particle size data reveal that the silt fraction of the loess (4-8 ϕ,63-4 μm) is dominant (60-80%). The grain-size frequency distribution of the key section is polymodal (principalmode in 26.7 μm and secondary modes in 6.7 and 0.2 μm). Median and mean values have the highest frequencyin the range 5-6 ϕ (positive asymmetry). The standard deviation is close to 2 ϕ (very poorly sorted). Two subpopulationsof particles associated with transport by suspension (short-term suspension: 63-15 μm, 49% of the distributionand long-term suspension: <15 μm, 47%) and two subpopulations transported by saltation (moderated saltation:63-125 μm, 3% and pure saltation: >125 μm, 1%) are deduced from cumulative frequency curves. Thesubpopulation of suspension between 4.5 and 7 ϕ is the best sorted. Ca. 50% of the particles were mobilized asdust clouds composing by grain-size fractions minor than medium silt. Mean particle size in a SE- NW transectacross the loess region (170 km long) reflects a northwestward gradient in particle size from 19-23 μm (coarsesilt) at the SE margin to 9-14 μm (medium silt) at the NW area. The very fine sand content of the loess decreasesfrom 16-20% at the SE to 4-14% at the NW; the total clay increases from 4-10% (˂2% colloids) at the SE to 11-24% (3-6% colloids) at the NW area. Mineralogical studies support the participation of multiple loess sourceareas. The modal sand fraction is predominantly composed of Andean volcaniclastic materials (mainly glassshards, plagioclases, polycrystalline quartz and lithoclasts). The secondary source of the materials is the upperreach of the Paraná River basin (heavy minerals and quartz). Illite (Pampean source) is the common clay miner -al; smectites and kaolinite (respectively upper Paraná and Uruguay River basins sources) are subordinated. Luminescenceages from the loess key profile are: 32,640 ± 2120 yr. BP and 24,040 ± 1570 yr. BP on samplestaken respectively at 3.5 m (swamp facies) and 2 m (loess) below the surface. Phytolith assemblages found in theloess show a homogeneous composition. With the presence of around 40 morphotypes, and according to theirvariability in the study area, a NW- SE differentiation was defined, in the same sense that observed in the grainsizetrend. The presence of panicoid, chloridoid, pooid and stipoid elements, jointly with arecoid and cyperoidremains reference to a megathermic grassland with patches of palms trees and swamp communities with sedges.The molluscan fauna preserved exhibits considerable abundance but low diversity. It is characterized by Succineameridionalis (14C age of 27,650 ± 250 yr. BP) associated with Biomphalaria peregrina (14C age of 24,390 ±170 yr. BP), taxa living today in the same area.