TAPHONOMIC PALEOFLORISTICS STUDIES OF THE UPPER TRIASSIC ISCHIGUALASTO FORMATION, NW ARGENTINA

The Ischigualasto Formation was deposited in a meandering channel system and contains an Upper Triassic flora. This flora was interpreted as a riparian forest dominated by Rhexoxylon piatnitzkyi Archangelsky & Brett emend. Zamuner - Zuberia papillata (Townrow) Artabe with herbaceous and shrubby vegetation on the floodplain dominated by Xylopteris elongata (Carruthers) Frenguelli, and Lepidopteris stormbergensis (Seward) Townrow (Spalletti et al., 1999). This flora was aggregated in six taphofacies: A) autochthonous silicified shrub roots associated with low-sinuosity channels; B) allochthonous silicified burned tree trunks associated with high-sinuosity channels; C) parautochthonous cuticules and charcoal fragments in the channel margins; D) autochthonous coalified roots and root haloes in crevasse-splay and levee deposits; E) autochthonous silicified stumps and F) horizontal layers of cuticle, charcoal fragments, and palynomorphs in abandoned channels and marshes.

The Ischigualasto Formation is dominated by taphofacies A, C, and D associated with calcic soils in the lower part and with argillic soils in the upper part of the formation. The other taphofacies are concentrated in one area of 500 meters of lateral extension and 200 meters thick, located in the middle to upper portion of the formation. We interpret this concentration to be both paleofloristic and the result of geochemical influences on preservation in these particular strata. Two hypothesis could explain this kind of arrangement: 1) an increase in humidity leading to higher water tables and greater development of vegetation and increased preservation; or 2) change in the tectonic influence on the water table without changes in climatic conditions.