Los Bastos Formation

Los Bastos Formation is a geologic formation of the Neuquén Basin in the northern Patagonian provinces of Mendoza and Neuquén. The formation dates to the Late Cretaceous, early to middle Coniacian, and belongs to the Río Neuquén Subgroup of the Neuquén Group. The formation overlies the Portezuelo Formation, in which it was formerly included and is overlain by the Sierra Barrosa Formation. Los Bastos Formation comprises mudstones and sandstones deposited in a fluvial environment. The dinosaur Malarguesaurus florenciae and a meridiolestid mammal were recovered from the formation.

Los Bastos Formation Stratigraphic range: Early-mid Coniacian ~89.8–88 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Type	Geological formation
Unit of	Neuquén Group  Río Neuquén Subgroup
Underlies	Sierra Barrosa Formation
Overlies	Portezuelo Formation
Thickness	Up to 35 m (115 ft)
Lithology
Primary	Mudstone
Other	Sandstone
Location
Coordinates	38°50′20″S 68°47′05″W
Approximate paleocoordinates	42.3°S 49.0°W / -42.3; -49.0
Region	Mendoza & Neuquén Provinces
Country	Argentina
Extent	Neuquén Basin
Type section
Named for	Los Bastos Field
Named by	2010
Year defined	Garrido
Los Bastos Formation (Argentina)

Description

The formation was named by Garrido in 2010 as shaly unit conformably and transitionally overlying the Portezuelo Formation, to which the layers were formerly defined and in the same manner underlying the Sierra Barrosa Formation, all belonging to the Río Neuquén Subgroup of the Neuquén Group in the Neuquén Basin. The unit now known as Los Bastos Formation was included in the original definition by Herrero Ducloux (1938, 1939) as "Portezuelo Medio", as part of the "Portezuelo Beds" he described.[1]

The type locality of the formation is located at the foot of the outcrop of the eponymous Los Bastos Field, between the localities of Sierra Barrosa and Cerro Senillosa. The formation at its type section reaches a thickness of 35 metres (115 ft). The formation comprises red mudstones intercalated by thin levels of siltstones and fine well-sorted yellowish to greenish grey sandstones. The formation has similar lithological characteristics as the Lisandro Formation and was deposited in a fluvial environment characterized by sinuous channels. Based on the stratigraphic relations with the overlying and underlying units, the age has been estimated to be early to middle Coniacian.[2]

Fossil content

The formation has provided fossils of:[3]

• Saurischia
  • Malarguesaurus florenciae[4]
• Mammals
  • Meridiolestida indet.[5]

See also

• List of dinosaur bearing rock formations

References

1. Garrido, 2011, p.236
2. Garrido, 2011, p.237
3. Los Bastos Formation at Fossilworks.org
4. González Riga et al., 2008
5. Forasiepi et al., 2012

Bibliography

• Forasiepi, A.M.; R.A. Coria; J. Hurum, and P.J. Currie. 2012. First dryolestoid (Mammalia, Dryolestoidea, Meridiolestida) from the Coniacian of Patagonia and new evidence on their early radiation in South America. Ameghiniana 49. 497-504.
• Garrido, Alberto C. 2011. El Grupo Neuquén (Cretácico Tardío) en la Cuenca Neuquina, 231–244. XVIII Congreso Geológico Argentino. Accessed 2020-07-07.
• González Riga, Bernardo J.; Elena Previtera, and Cecilia A. Pirrone. 2008. Malarguesaurus florenciae gen. et sp. nov., a new titanosauriform (Dinosauria, Sauropoda) from the Upper Cretaceous of Mendoza, Argentina. Cretaceous Research 30. 135–148. doi:10.1016/j.cretres.2008.06.006