| Laguna del Hunco Formation | |
|---|---|
| Stratigraphic range: Ypresian (Itaboraian) ~ | |
| Type | Geological formation |
| Unit of | Middle Chubut River Volcanic Pyroclastic Complex |
| Underlies | Sarmiento Group |
| Overlies | Barda Colorada Ignimbrite |
| Thickness | 170 m (560 ft) |
| Lithology | |
| Primary | Tuff, mudstone, sandstone |
| Location | |
| Coordinates | 42°18′S 70°00′W / 42.3°S 70.0°W |
| Approximate paleocoordinates | 44°48′N 60°00′W / 44.8°N 60.0°W |
| Region | Chubut Province |
| Country | Argentina |
| Extent | Cañadón Asfalto Basin |
| Type section | |
| Named for | Laguna del Hunco |
| Named by | Aragón & Mazzoni |
| Location | Languiñeo Department |
| Year defined | 1997 |
| Coordinates | 42°18′S 70°00′W / 42.3°S 70.0°W |
| Approximate paleocoordinates | 44°48′N 60°00′W / 44.8°N 60.0°W |
| Region | Chubut Province |
| Country | Argentina |
| Thickness at type section | 170 m (560 ft) |
  Laguna del Hunco Formation (Argentina) | |
The Laguna del Hunco Formation or Laguna del Hunco Tuff (Spanish: Formación Laguna del Hunco, Tufolitas Laguna del Hunco) is a localized Early Eocene (Itaboraian in the SALMA classification) fossiliferous geological formation of the Cañadón Asfalto Basin in central Patagonia, Argentina. The 170 metres (560 ft) thick formation comprises tuffaceous mudstones and sandstones deposited in a crater lake environment and crops out at Laguna del Hunco in the northwestern Chubut Province.
The formation has been precisely dated to 52.22 ± 0.22 Ma on the basis of sanidine crystals in the tuffs of the formation. The Laguna del Hunco formation overlies the Barda Colorada Ignimbrite and is covered by the Sarmiento Group. The unit is renowned for the preservation of an extraordinarily rich fossil flora assemblage of mixed South American families and presently uniquely Australasian flora, among which the oldest Eucalyptus fossils found worldwide. The formation also has provided many fossil insects, including insect eggs, fossil fish of Bachmannia chubutensis and the frog Shelania pascuali. Periodic bursts of gas in the volcanic crater lake are thought to have produced the sudden death and preservation of the floral and faunal assemblage.