The Laguna del Hunco Formation or Laguna del Hunco Tuff () is a localized Early Eocene (Itaboraian in the SALMA classification) fossiliferous geological formation of the Cañadón Asfalto Basin in central Patagonia, Argentina. The 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,Wilf et al., 2005, p.635Wilf et al., 2005, p.636 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 thick formation forms part of the Middle Chubut River Volcanic Pyroclastic Complex of the western Cañadón Asfalto Basin.Figari et al., 2015, p.154 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.Tejedor et al., 2009, p.5
The Laguna del Hunco Formation was formerly included in the Huitrera Formation,Zamaloa et al., 2006, p.1280 overlies the Barda Colorada Ignimbrite,Figari et al., 2015, p.153 and is overlain by the Sarmiento Group.Figari et al., 2015, p.155 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.Carvalho et al., 2013, p.1834
Paleontological significance
The paleoflora of the formation is considered one of the most biodiverse Cenozoic fossil deposits worldwide.Gandolfo et al., 2011, p.1 The biota is composed of extraordinarily rich assemblages of ferns, conifers, and flowering plants, many of which have not yet been formally described.Laguna del Hunco - Cornell University 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,Zamaloa et al., 2006, p.1288 and the species Ceratopetalum edgardoromeroi of the family Cunoniaceae are the only members of these families found outside of Australasia.Gandolfo & Hermsen, 2017, p.9 The floral assemblage is thought to represent a lakeshore vegetation, deposited during the Early Eocene Climatic Optimum (EECO),Sarzetti et al., 2009, p.432 with estimated average yearly temperatures of and annual rainfall of .Dibán Karmy, 2018, p.26 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:
Group Group Species Images references Vertebrates AvesUeekenkcoracias tambussiae 2021Degrange et al., 2021, p.6AmphibiaShelania pascuali Báez & Trueb, 1997 Actinopterygii Bachmannia chubutensis Azpelicueta & Cione, 2011 InvertebratesInsects Austropanorpodes gennaken Petrulevičius, 2009Austroperilestes hunco Petrulevičius & Nel, 2005Chinchekoala qunita Petrulevičius, 2016, p.863Frenguellia iglesiasi Petrulevičius & Nel, 2013Frenguellia patagonica Petrulevičius & Nel, 2003Huncoaeshna corrugata Petrulevičius et al., 2010, p.273Satelitala soberana Petrulevičius, 2017, p.780Urocerus patagonicus Petrulevičius, 1999, p.96IchnofossilsPaleoovoidus arcuatum Sarzetti et al., 2009, p.437Paleoovoidus bifurcatus Paleoovoidus rectus FloraAraucariaceaeAraucaria pichileufensis Wilf et al., 2005, p.639Cycads Austrozamia stockeyi Wilf et al., 2016CunoniaceaeCeratopetalum edgardoromeroi Gandolfo & Hermsen, 2017, p.3EscalloniaceaeEscallonia Dibán Karmy, 2018, p.25 Myrtaceae Eucalyptus Gandolfo et al., 2011, p.4 Gandolfo et al., 2011, p.5GinkgoaceaeGinkgo patagonica Villar de Seoane et al., 2015, p.4 CasuarinaceaeGymnostoma archangelskyi Zamaloa et al., 2006, p.1283Gymnostoma argentinum Zamaloa et al., 2006, p.1284Gymnostoma patagonicum Zamaloa et al., 2006, p.1281 Proteaceae Lomatia occidentalis Lomatia preferruginea Osmundaceae Todea amissa Carvalho et al., 2013, p.1835Podocarpaceae Podocarpus andiniformis Ripogonaceae Ripogonum americanum Carpenter et al., 2014SolanaceaePhysalis infinemundiMalvaceaeUiher karuen Siegert et al., 2024
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