Laguna del Hunco Formation

The Laguna del Hunco Formation or Laguna del Hunco Tuff (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 m 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.

Description
The Laguna del Hunco Formation, named after Laguna del Hunco ("Lake of Reeds"), a desert pond in Chubut Province, is a localized sedimentary unit comprising tuffaceous sandstones and mudstones with primary and reworked ashfall layers. The formation was deposited in a crater lake environment. The approximately 170 m thick formation forms part of the Middle Chubut River Volcanic Pyroclastic Complex of the western Cañadón Asfalto Basin. This complex comprises a variety of volcaniclastic, intrusive, pyroclastic, and extrusive rocks, deposited over several million years. The complex is characterized by a great variety of volcanogenic bodies, such as ignimbrites, domes, lava flows, necks, intrusives, tuffs, and volcaniclastic deposits (of predominantly lacustrine origin), all of them frequently interbedded.

The Laguna del Hunco Formation was formerly included in the Huitrera Formation, overlies the Barda Colorada Ignimbrite, and is overlain by the Sarmiento Group. The formation has been dated using 40Ar/39Ar analysis on sanidine crystals of the ash beds to 52.22 ± 0.22 Ma, placing the deposits in the Early Eocene, or Itaboraian in the SALMA classification.

Paleontological significance
The paleoflora of the formation is considered one of the most biodiverse Cenozoic fossil deposits worldwide. The biota is composed of extraordinarily rich assemblages of ferns, conifers, and flowering plants, many of which have not yet been formally described. The flora of the formation, studied since the 1920s, was formerly thought to be Miocene in age.

Three described species in the genus Gymnostoma of the family Casuarinaceae, and the species Ceratopetalum edgardoromeroi of the family Cunoniaceae are the only members of these families found outside of Australasia. The floral assemblage is thought to represent a lakeshore vegetation, deposited during the Early Eocene Climatic Optimum (EECO), with estimated average yearly temperatures of 17.2 ± and annual rainfall of 1673 ±. Periodic gas bursts in the crater lake of Laguna del Hunco probably led to the sudden deaths of the flora and fauna found in the formation.

Paleobiota
The following fossil plants and animals have been described from the formation: