Multidisciplinary analysis of the last glacial loess at the NE of the Pampean aeolian system.

KROHLING D.M.; E. PASSEGGI; A. F. ZUCOL; G. ERRA; M. L. AGUIRRE; S. MIQUEL; A. C. GONZALEZ; BREA M.

Mendoza

Congreso; 18th Internacional Sedimentological Congreso. Sedimentology at the Foot of the Andes; 2010

Loess deposits are the main component of the Late Quaternary sequences of Pampa plain, covering the largest loessic area of Southern Hemisphere. Pampean loess region lies downwind from source areas located to the S and SW, comprising a large sand sea and floodplains of systems that drain the eastern Andean piedmont. The loess unit of the last glacial maximum (LGM) at the leeward side of the Pampean aeolian system represents a primary loess, indicating the expansion of semiarid conditions to the NE. Stratigraphic, sedimentologic, mollusk and phytolith data, including OSL and 14C ages, are presented for a loessic area of 14,000 km² at the SW of Entre Ríos province, Argentina. LGM loess is a homogeneous unit, characterized by considerable spatial continuity in the North Pampa. It occurs as a mantle of 2-4 m thickness; only locally the loess has been fluvial reworked 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 rhizoliths and concretions. Well-exposed LGM loess sections were investigated and sampled along the left margin of the Paraná 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 (principal mode in 26.7 μm and secondary modes in 6.7 and 0.2 μm). Median and mean values have the highest frequency in the range 5-6 ϕ (positive asymmetry). The standard deviation is close to 2 ϕ (very poorly sorted). Two subpopulations of particles associated with transport by suspension (short-term suspension: 63-15 μm, 49% of the distribution and long-term suspension: 125 μm, 1%) are deduced from cumulative frequency curves. The subpopulation of suspension between 4.5 and 7 ϕ is the best sorted. Ca. 50% of the particles were mobilized as dust clouds composing by grain size fractions minor than medium silt. Mean particle size in a SE-NW transect across the loess region (170 km long) reflects a northwestward gradient in particle size from 19-23 μm (coarse silt) at the SE margin to 9-14 μm (medium silt) at the northwestern area. The very fine sand content of the loess decreases from 16-20% at the SE to 4-14% at the NW; the total clay increase 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 source areas. The modal sand fraction is predominantly composed of Andean volcaniclastic materials (mainly glass shards, plagioclases, polycrystalline quartz and lithoclasts). The secondary source of the materials is the upper reach of the Paraná River basin (heavy minerals and quartz). Illite (main Pampean source), smectite and kaolinite (respectively upper Paraná and Uruguay River basins sources) are the common clay minerals. Luminescence ages from the loess key profile are: 32,640 ± 2120 yr. BP and 24,040 ± 1570 yr. BP on samples taken respectively at 3.5 m (swamp facies) and 2 m (loess) below the surface. Phytolith assemblages found in the loess show a homogeneous composition. With the presence of around 40 morphotypes, and according to their variability in the study area, a NW-SE differentiation was defined, in the same sense that observed in the grain-size trend. The presence of panicoid, chloridoid, pooid and stipoid element, jointly with arecoid and cyperoid remain 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 s characterized by Succinea meridionalis (dominant; 14C age of 27,650 ± 250 ka BP) associated with Biomphallaria peregrine, taxa living today in the same area.