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

ERRA G; ZUCOL A. ; D. KRÖHLING.

Mar del Plata

Exposición; 7th IMPR – 4th SMPR; 2010

Society for Phytolith Research GEFACS

The Tezanos Pinto Formation (Last GlacialMaximum in age) appears as a continuous mantle inthe Southwest of Entre Ríos province (14.000 km²;North Pampa) forming the Crespo Loessic HillsGeomorphological Unit. This unit forms thenortheastern border of the Peripherical Loessic Beltof the Pampean Aeolian System, defined by Iriondoand Kröhling in 1996. The Quaternary stratigraphicsequence of the study area comprises two definedcycles of aeolian deposition. The Tezanos Pinto Fmcorresponds to the most recent cycle, accumulatedduring the Oxygen Isotopic Stage 2 (OIS 2), in arange spanning from 36,000 to 8,500 years BP. Theaeolian facies of the unit formed by primary loess isdominant in the area, with an areal thicknessranging between 2 - 5 meters. It corresponds to afriable and permeable brown (7.5 YR 5/4) massivedeposit, with prevalence of silt (63-4 μm; 60,7-80,3%), with subordinate clay and scarce very finesand, that forms vertical cliffs. In order toinvestigate its phytolith composition, representativeprofiles from the Southwestern Entre Ríos province,were relevated by continuous sampling (equidistantpoints 10 cm) following a NW-SE transect. Sampleswere processed according to the methodology ofZucol and Osterrieth (2002). The studied profiles ofthe Tezanos Pinto Fm showed high abundance ofphytolith, with dominance of prismatic, globular andtruncated cone morphotypes. The flattened prismaticmorphotypes, with side edges smooth (the mostfrequent type), scalloped, serrated or wavy, mainlywith graminoid affinities, were associated withothers of probable cyperoid affinities. Otherphytoliths with graminoid affinities, such truncatedcones, mainly of arundinoid or more rarely pooidtype were also present. Less frequent forms includedphytoliths originated by prickles or trichomes,symmetrical or asymmetrical fan-shaped, saddleforms of chloridoid affinity, polyhedral and bilobateor dumbbell-shaped with panicoid affinity. Theassemblage also includes abundant sponge spicules,primarily smooth macroscleres from freshwatersponges, as well as stomatocysts fromChrysostomataceae, and in lower proportions,different types of diatoms. No compositionalchanges were observed along the sampled sequence.The paleocommunity would have been formed bygramineans (primarily arundinoid types, arid warmclimates, and pooid, and to lesser extent chloridoidand panicoid) with associated palm trees and sedges.The presence of freshwater sponge spicules withscarce degree of alteration, joined with theoccurrence of diatoms, indicate the existence ofwater bodies. The low percentage of cyperaceanphytoliths and chrysostomatacean cysts indicatesscarcity of lentic water bodies. This scenario agreeswith geological interpretations that indicate aridconditions for the first cycle (36–16 ka), andsemiarid characteristics for the second cycle (14-8.5ka) of depositation of this formation in the provinceof Santa Fe.