Terrestrial trace fossils and Ichnostratigraphy of the Sarmiento Formation: implications for the oldest grass-dominated ecosystem

BELLOSI E.S., J.F. GENISE, J. H. LAZA & M.V. SÁNCHEZ

La Plata

Congreso; XV Congreso Geológico Argentino; 2005

Asociación Geológica Argentina

The Sarmiento Formation yields a diverse ichnofauna dominated by coleopteran and hymenopteran traces. Abundance, diversity, and relative size of ichnofossils enable a high-resolution ichnostratigraphy, and evidence paleoenvironmental changes. Most levels correspond to Coprinisphaera (dung-beetle brood balls) ichnofacies, but changes in the ichnologic assemblages are recorded in some paleosols. In Gran Barranca Member, Coprinisphaera, Teisseirei, Celliforma, Beaconites, pan-shaped traces, rhizoliths, cocoons and undetermined burrows were identified. The Rosado Member shows a low-diversity ichnoassemblage, with abundant Celliforma and scarce Pallichnus. Puesto Almendra Member ichnofossils are similar to Gran Barranca assemblage, but more abundant. The relative ichnodiversity changes through Puesto Almendra Member along with variations in paleosols and lithofacies. Rhizoliths and Beaconites, Coprinisphaera, Teisseirei, Celliforma, pan-shaped traces and possible termite nests are also recorded. Vera Member loessites preserve no invertebrate ichnofossils. Only large burrows (mammal dens?), and thin rhizoliths were observed. Colhue-Huapi Member includes rhizoliths, thick-walled tubes, Coprinisphaera, Teisseirei and Celliforma. The abundance and diversity of Coprinisphaera, subsequent to the Middle-Eocene, along with the high-proportion of Poaceae phytoliths, the presence of grazing mammals, and paleosol types compatible with subhumid-semiarid conditions and formed in loessic paleoenvironments, together comprise evidence leading us to confirm that one of the oldest grassland ecosystems was established in Patagonia. Coprinisphaera (dung-beetle brood balls) ichnofacies, but changes in the ichnologic assemblages are recorded in some paleosols. In Gran Barranca Member, Coprinisphaera, Teisseirei, Celliforma, Beaconites, pan-shaped traces, rhizoliths, cocoons and undetermined burrows were identified. The Rosado Member shows a low-diversity ichnoassemblage, with abundant Celliforma and scarce Pallichnus. Puesto Almendra Member ichnofossils are similar to Gran Barranca assemblage, but more abundant. The relative ichnodiversity changes through Puesto Almendra Member along with variations in paleosols and lithofacies. Rhizoliths and Beaconites, Coprinisphaera, Teisseirei, Celliforma, pan-shaped traces and possible termite nests are also recorded. Vera Member loessites preserve no invertebrate ichnofossils. Only large burrows (mammal dens?), and thin rhizoliths were observed. Colhue-Huapi Member includes rhizoliths, thick-walled tubes, Coprinisphaera, Teisseirei and Celliforma. The abundance and diversity of Coprinisphaera, subsequent to the Middle-Eocene, along with the high-proportion of Poaceae phytoliths, the presence of grazing mammals, and paleosol types compatible with subhumid-semiarid conditions and formed in loessic paleoenvironments, together comprise evidence leading us to confirm that one of the oldest grassland ecosystems was established in Patagonia.