Laguna del Hunco Formation

The Laguna del Hunco Formation or Laguna del Hunco Tuff (Spanish: Formación Laguna del Hunco, Tufolitas Laguna del Hunco) is a localized Early Eocene (Casamayoran 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.

Laguna del Hunco Formation
Stratigraphic range: Ypresian
(Casamayoran)
~52.44–52 Ma
↓
TypeGeological formation
Unit ofMiddle Chubut River Volcanic Pyroclastic Complex
UnderliesSarmiento Group
OverliesBarda Colorada Ignimbrite
Thickness170 m (560 ft)
Lithology
PrimaryTuff, mudstone, sandstone
Location
Coordinates42.3°S 70.0°W / -42.3; -70.0
Approximate paleocoordinates44.8°N 60.0°W / 44.8; -60.0
RegionChubut Province
Country Argentina
ExtentCañadón Asfalto Basin
Type section
Named forLaguna del Hunco
Named byAragón & Mazzoni
LocationLanguiñeo Department
Year defined1997
Coordinates42.3°S 70.0°W / -42.3; -70.0
Approximate paleocoordinates44.8°N 60.0°W / 44.8; -60.0
RegionChubut Province
Country Argentina
Thickness at type section170 m (560 ft)
Laguna del Hunco Formation (Argentina)

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
Early Eocene (50 Ma)

The Laguna del Hunco Formation, named after Laguna del Hunco ("Lake of Reeds"), a desert pond in Chubut Province,[1][2] 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 metres (560 ft) thick formation forms part of the Middle Chubut River Volcanic Pyroclastic Complex of the western Cañadón Asfalto Basin.[3] 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.[4]

The Laguna del Hunco Formation was formerly included in the Huitrera Formation,[5] overlies the Barda Colorada Ignimbrite,[6] and is overlain by the Sarmiento Group.[7] 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 Casamayoran in the SALMA classification.[8]

Paleontological significance
The Laguna del Hunco Formation was deposited during the Early Eocene Climatic Optimum

The paleoflora of the formation is considered one of the most biodiverse Cenozoic fossil deposits worldwide.[9] The biota is composed of extraordinarily rich assemblages of ferns, conifers, and flowering plants, many of which have not yet been formally described.[10] The flora of the formation, studied since the 1920s, was formerly thought to be Miocene in age.[8]

Three described species in the genus Gymnostoma of the family Casuarinaceae,[11] and the species Ceratopetalum edgardoromeroi of the family Cunoniaceae are the only members of these families found outside of Australasia.[12] The floral assemblage is though to represent a lakeshore vegetation,[2] deposited during the Early Eocene Climatic Optimum (EECO),[13][14] with estimated average yearly temperatures of 17.2 ± 2.3 °C (63.0 ± 4.1 °F) and annual rainfall of 1,673 ± 426 millimetres (65.9 ± 16.8 in).[15] 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.[13]

Fossil content

The following fossils have been described from the formation:

ClassGroupFossilsImagesNotes
VertebratesFrogsShelania pascuali[16]
FishBachmannia chubutensis[13]
InvertebratesInsectsAustropanorpodes gennaken[17]
Austroperilestes hunco[18]
Chinchekoala qunita[19]
Frenguellia iglesiasi[20]
Frenguellia patagonica[21]
Huncoaeshna corrugata[22]
Satelitala soberana[23]
Urocerus patagonicus[24]
IchnofossilsInsect eggsPaleoovoidus rectus[25]
FloraAraucariaceaeAraucaria pichileufensis[26]
CycadsAustrozamia stockeyi[27]
CunoniaceaeCeratopetalum edgardoromeroi[28]
EscalloniaceaeEscallonia[29]
MyrtaceaeEucalyptus[30]
[31]
GinkgoaceaeGinkgo patagonica[32]
CasuarinaceaeGymnostoma archangelskyi[33]
Gymnostoma argentinum[34]
Gymnostoma patagonicum[35]
ProteaceaeLomatia occidentalis[26]
Lomatia preferruginea[26]
PodocarpaceaePodocarpus andiniformis[26]
RipogonaceaeRipogonum americanum[36]
OsmundaceaeTodea amissa[37]
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See also

References
  1. Wilf et al., 2005, p.635
  2. Wilf et al., 2005, p.636
  3. Figari et al., 2015, p.154
  4. Tejedor et al., 2009, p.5
  5. Zamaloa et al., 2006, p.1280
  6. Figari et al., 2015, p.153
  7. Figari et al., 2015, p.155
  8. Carvalho et al., 2013, p.1834
  9. Gandolfo et al., 2011, p.1
  10. Laguna del Hunco - Cornell University
  11. Zamaloa et al., 2006, p.1288
  12. Gandolfo & Hermsen, 2017, p.9
  13. Azpelicueta & Cione, 2011
  14. Sarzetti et al., 2009, p.432
  15. Dibán Karmy, 2018, p.26
  16. Báez & Trueb, 1997
  17. Petrulevičius, 2009
  18. Petrulevičius & Nel, 2005
  19. Petrulevičius, 2016, p.863
  20. Petrulevičius & Nel, 2013
  21. Petrulevičius & Nel, 2003
  22. Petrulevičius et al., 2010, p.273
  23. Petrulevičius, 2017, p.780
  24. Petrulevičius, 1999, p.96
  25. Sarzetti et al., 2009, p.437
  26. Wilf et al., 2005, p.639
  27. Wilf et al., 2016
  28. Gandolfo & Hermsen, 2017, p.3
  29. Dibán Karmy, 2018, p.25
  30. Gandolfo et al., 2011, p.4
  31. Gandolfo et al., 2011, p.5
  32. Villar de Seoane et al., 2015, p.4
  33. Zamaloa et al., 2006, p.1283
  34. Zamaloa et al., 2006, p.1284
  35. Zamaloa et al., 2006, p.1281
  36. Carpenter et al., 2014
  37. Carvalho et al., 2013, p.1835

Bibliography
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