Phytolith analysis in fluvial sediments of El Palmar Formation (Late Pleistocene) in the eastern Argentina

PATTERER, N. I.; BREA, M.

Colorado

Encuentro; 8th International Meeting on Phytolith Research; 2011

PaleoResearch Institute, Golden, Colorado

The El Palmar Formation is a sedimentological term deposited during the Late Pleistocene, which was developed during the OIS 5a, which corresponds to the Last Interglacial. This lapse is considered to have been the warmest and humid interval of the Late Pleistocene. This Formation begins in Pleistocene upper terraces of the Uruguay River and is distributed in a coastal strip at the southernmost of the Basin in eastern Argentina. In general sense this sedimentological unit has 3 to 12 m thick, lies at the surface and has not been buried since its deposition; and is mainly composed of medium, reddish and yellowish ochre sands. Lenses of gravel and pebbles up to 90 m long and up to 2 m thick are interspersed in quartzose sand masses. Sand strata and gravel lenses represent channel facies and fine sediments from facies of inundation. This formation was dated 80,670 ± 13,420 years BP by TL (thermoluminescense dating) at Federación city, and 88,370 ± 35,680 years BP by TL obtained at Salto city (Uruguay), in the upper levels of the sedimentary sequence.  For this study, the silica microremains of different profiles composed by facies characteristics of a high-energy fluvial environment, as well as floodplain environments, were analyzed.  Described microremain assemblages are composed by abundant phytoliths (isolated and articulated forms) and whole and fragmented sponges’ spicules. The main phytoliths types present in these assemblages are fan-shaped, point-shaped, polyhedrical and  elongated among the largest, whereas among the smaller ones were abundant bilobate, boat-shaped, truncated cone, crescent, square, oblong and crenate and globular spherical and ellipsoidal of spiny surface phytoliths. Phytolith that mostly had graminoid (panicoid, pooid and chloridoid), arecoid affinities and less abundant ciperoid, podostemoid and dicot of affinities.  The larger phytoliths showed different degrees of wear, which in many cases cannot be observed clearly contours, while the smallest were unchanged. Comparative analysis of the various studied facies possible to establish differences in their compositions, while on the other hand settled of wears versus not wear phytolith abundances were compared in order to establish allochthonous/autochthonous ratios thereof.