Phytolithic composition of the Tezanos Pinto Formation (Late Pleistocene loess) at the Southwest of the Entre Ríos province, Argentina

G. ERRA, A. F. ZUCOL AND D. KRÖHLING

Mar del Plata

Congreso; 7mo. Encuentro Internacional de Investigaciones Fitolíticas y 4to. Encuentro Sudamericano de Investigaciones Fitolíticas.; 2008

The Tezanos Pinto Formation (Last Glacial Maximum in age) appears as a continuous mantle in the Southwest of Entre Ríos province (14.000 km²; North Pampa) forming the Crespo Loessic Hills Geomorphological Unit. This unit forms the northeastern border of the Peripherical Loessic Belt of the Pampean Aeolian System, defined by Iriondo and Kröhling in 1996. The Quaternary stratigraphic sequence of the study area comprises two defined cycles of aeolian deposition. The Tezanos Pinto Fm corresponds to the most recent cycle, accumulated during the Oxygen Isotopic Stage 2 (OIS 2), in a range spanning from 36,000 to 8,500 years BP. The aeolian facies of the unit formed by primary loess is dominant in the area, with an areal thickness ranging between 2 - 5 meters. It corresponds to a friable and permeable brown (7.5 YR 5/4) massive deposit, with prevalence of silt (63-4 μm; 60,7-80,3%), with subordinate clay and scarce very fine sand, that forms vertical cliffs. In order to investigate its phytolith composition, representative profiles from the Southwestern Entre Ríos province, were relevated by continuous sampling (equidistant points 10 cm) following a NW-SE transect. Samples were processed according to the methodology of Zucol and Osterrieth (2002) . The studied profiles of the Tezanos Pinto Fm showed high abundance of phytolith, with dominance of prismatic, globular and truncated cone morphotypes. The flattened prismatic morphotypes, with side edges smooth (the most frequent type), scalloped, serrated or wavy, mainly with graminoid affinities, were associated with others of probable cyperoid affinities. Other phytoliths with graminoid affinities, such truncated cones, mainly of arundinoid or more rarely pooid type were also present. Less frequent forms included phytoliths originated by prickles or trichomes, symmetrical or asymmetrical fan-shaped, saddle forms of chloridoid affinity, polyhedral and bilobate or dumbbell-shaped with panicoid affinity. The assemblage also includes abundant sponge spicules, primarily smooth macroscleres from freshwater sponges, as well as stomatocysts from Chrysostomataceae, and in lower proportions, different types of diatoms. No compositional changes were observed along the sampled sequence. The paleocommunity would have been formed by gramineans (primarily arundinoid types, arid warm climates, and pooid, and to lesser extent chloridoid and panicoid) with associated palm trees and sedges. The presence of freshwater sponge spicules with scarce degree of alteration, joined with the occurrence of diatoms, indicate the existence of water bodies. The low percentage of cyperacean phytolits and chrysostomatacean cysts indicates scarcity of lentic water bodies. This scenario agrees with geological interpretations that indicate arid conditions for the first cycle (36–16 ka), and semiarid characteristics for the second cycle (14-8.5 ka) of depositation of this formation in Santa Fe province.