Ichnofacial analysis of the Sarmiento Formation (Middle Eocene-Lower Miocene) at Gran Barranca, Central Patagonia.

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

The Paleontology of Gran Barranca: Evolution and Environmental Change through the Middle Cenozoic of Patagonia

Cambridge University Press

Lugar: Londres; Año: 2010; p. 302 - 313

The ichnofacial analysis of the Sarmiento Formation at Gran Barranca, along with olderand younger closer Patagonian units, allows to assess the Early Eocene-Middle Mioceneevolution of terrestrial ecosystems. Most ichnofossils in paleosols correspond to insects:Coprinisphaera ispp (dung-beetles), Celliforma ispp. (solitary bees), Teisseirei barattinia,Pallichnus dakotensis, pan-shaped traces, and cocoons. Affinities of other traces are uncertain:Lazaichnus fistulosus, septate burrows, thick-walled tubes, Beaconites coronus, and largeburrows. Different types of root traces are also present. The Lower-Middle Eocene Koluel-Kaike Formation contains pan-shaped traces, large root traces, and burrows. In the MiddleEocene Gran Barranca Member, abundant pan-shaped traces, scarce Coprinisphaera, Teisseirei,Beaconites, Celliforma, and cocoons were recognized. The late Middle Eocene RosadoMember, a pedogenic calcrete, only bears Celliforma and Pallichnus. The Upper Eocene LowerPuesto Almendra Member shows the greatest density and diversity of Coprinisphaera, andincludes Lazaichnus and Teisseirei. In the Uppermost Eocene-Lower Oligocene Vera Memberburrows attributed to mammals, and very few Coprinisphaera occur. The Oligocene UpperPuesto Almendra Member records the greater ichnodiversity of the whole succession, withrelative high proportion of nearly all ichnofossils. The Lower Miocene Colhue-Huapi Memberhas two distinctive associations. Ichnodiversity of the lower-middle sections is similar to theunderlying member; but diversity and abundance decrease in the upper section, where a calcreteonly bears Celliforma. The Lower Miocene Pinturas Formation includes Coprinisphaera,Syntermesichnus, and Palmiraichnus. In the Middle Miocene Santa Cruz Formation onlyCelliforma was recognized. The Sarmiento Formation at Gran Barranca represents the oldestand one of the best examples worldwide of the Coprinisphaera ichnofacies. This ichnofacies ispresent in Gran Barranca, Puesto Almendra and Colhue-Huapi Members, and PinturasFormation. The record of the Coprinisphaera ichnofacies and particularly the high diversity andabundance of Coprinisphaera since the Late Eocene, is interpreted as being the result of theappearance of grass-dominated warm-temperate environments with abundant herbivore dung.The Rosado Member and the upper section of the Colhue-Huapi Member record the tentativeCelliforma ichnofacies, probably representing drier and/or colder environments with poorvegetation coverage. The Koluel-Kaike and Santa Cruz Formations, and the Vera Memberwould have unfavorable conditions for the development of either Coprinisphaera or Celliformaichnofacies.