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

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

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

Cambridge University Press

Lugar: New York; Año: 2010; p. 302 - 312

The ichnofacies analysis of the Sarmiento Formation at Gran Barranca, along with older and younger adjacent Patagonian units, permits an assessment of the early Eocene to middle Miocene evolution of terrestrial ecosystems. Most ichnofossils found in Sarmiento Formation paleosols were made by insects: Coprinisphaera ispp. (dung beetles), Celliforma ispp. (solitary bees), Teisseirei barattinia and Pallichnus dakotensis (beetles), Feoichnus challa (cycads), and cocoons. Thick-walled vertical tubes (Loloichnus baqueroensis) were produced by crayfishes (Parastacidae). The nature of other traces is uncertain: Lazaichnus fistulosus, septate burrows, Beaconites coronus, and large burrows. Different types of root traces are also present. The lower middle Eocene Koluel-Kaike Formation contains Feoichnus, Skolithos, Taenidium, and root traces. In the middle Eocene Gran Barranca Member, Feoichnus is abundant, Coprinisphaera, Teisseirei, Beaconites, Celliforma, and cocoons are less common. The late middle Eocene Rosado Member, a pedogenic calcrete, only contains Celliforma and Pallichnus. The Lower Puesto Almendra Member (late Eocene) shows the greatest density and diversity of Coprinisphaera, and includes Lazaichnus and Teisseirei. Large burrows attributed to mammals and very few Coprinisphaera are found in the late Eocene to early Oligocene Vera Member. The Oligocene Upper Puesto Almendra Member records the greatest ichnodiversity in the whole succession, with relative high number of nearly all ichnofossils. The early Miocene Colhue-Huapi Member has two distinctive associations. Ichnodiversity of the lower and middle sections of the member is similar to that of the underlying member; but diversity and abundance decline in the upper section, where a calcrete contains only Celliforma. Elsewhere in west–central Patagonia, the early Miocene Pinturas Formation includes Coprinisphaera, Syntermesichnus, and Palmiraichnus. In the early to middle Miocene Santa Cruz Formation only Celliforma is recognized. The Sarmiento Formation at Gran Barranca represents the oldest and one of the best examples worldwide of the Coprinisphaera ichnofacies. This ichnofacies is present in Gran Barranca, Puesto Almendra, and Colhue-Huapi Members, and elsewhere in the Pinturas Formation. The record of the Coprinisphaera ichnofacies, and particularly the high diversity and abundance of Coprinisphaera since the late Eocene, is interpreted as being the result of the appearance of grass-dominated warm–temperate environments with abundant dung produced by mammalian herbivores. The Rosado Member and the upper section of the Colhue-Huapi Member record the tentative Celliforma ichnofacies, probably indicating drier and/or colder environments with poor vegetation coverage. The Koluel-Kaike and Santa Cruz Formations and the Vera Member would have unfavorable conditions for the development of either Coprinisphaera or Celliforma ichnofacies.