THE TAPHONOMIC MODES IN THE FLUVIAL SYSTEM OF THE UPPER TRIASSIC ISCHIGUALASTO FORMATION, NW ARGENTINA

The Upper Triassic Ischigualasto Formation was deposited in a fluvial system during one of the last filling stages of the Ischigualasto Basin. This succession preserves one of the most important collections of paleovertebrates, including most of the oldest dinosaurs already known. Besides, it also maintains a complete range of paleofloristic remains that include tree trunks, roots, cuticles and palynomorphs. Finally, conchostracans have also been found in the succession.

Detailed taphonomic studies in the vertebrate and floristic assemblages have identified the taphonomic modes that characterized each of the sub-environments. These are: A) low sinuosity channels, characterized by silicified roots of woody plants, uncommon silicified allochthonous tree trunks, and rare paleovertebrates, mostly highly altered isolated remains mineralized by calcite; B) high sinuosity channels; typified by more abundant allochthonous silicified tree trunks together with troughs having concentrated parautochthonous leaf cuticles, and similar taphonomic patterns in respect to the palevertebrates; C) levees, typified by abundant herbaceous carbonized roots and root halos and the most significant collection of paleovertebrates from fresh, articulated skeletons to isolated highly weathered remains, mineralized by calcite and/or hematite; D) distal well-drained floodplains with calcisols, with abundant root halos and similar taphonomic features of paleovertebrates, although with a dominance of isolated hematite mineralized remains; E) distal well-drained floodplains with argillisols, distinguished by an additional abundance of carbonized herbaceous roots and isolated paleovertebrate remains mineralized by hematite in the highest weathering stage; F) crevasses, characterized by carbonized herbaceous and silicified woody roots and an entire range of paleovertebrate types of preservation, dominated by semi-articulated mineralized with calcite remains; G) abandoned channels; with the particular preservation of authochtonous leaf cuticles, palynomorphs, some carbonized herbaceous roots, very rare burned remains of paleovertebrates and molds of cochostracons; finally the mode H) burrows, is characterized by a completely articulated fresh skeleton, sometimes associated to more than one.