Paso Flores Formation

The Paso Flores Formation is a latest Late Triassic (Rhaetian) geologic formation in the Neuquén Basin of Neuquén Province in northwestern Patagonia, Argentina. The brown to red-stained conglomerates, sandstones and shales of the formation represent the youngest and only latest Triassic edimentary unit in the country, overlying basement.

Paso Flores Formation
Stratigraphic range: Latest Late Triassic
~205–202 Ma
↓
TypeGeological formation
UnderliesSañicó, Nestares, Collón Curá, Caleufú & Cerro Petiso Formations
OverliesCushamen basement
ThicknessUp to 230 m (750 ft)
Lithology
PrimaryConglomerate, sandstone
OtherShale
Location
Coordinates40.5°S 70.5°W / -40.5; -70.5
Approximate paleocoordinates50.1°S 31.1°W / -50.1; -31.1
RegionNeuquén Province
Country Argentina
ExtentSouthern Neuquén Basin
Type section
Named forBalsa Paso Flores
Named byFossa Mancini
Year defined1937
Paso Flores Formation (Argentina)

The formation has provided a rich fossil flora of various groups, preserved in leaves and fossil wood in the finer lithologies in the Paso Flores Formation. The fossil flora is representative of the ecosystem of the latest Triassic, preceding the Triassic-Jurassic extinction event.

Description

The formation was first defined by Fossa Mancini in 1937 as the Formación Continental de Paso Flores. Later, Frenguelli (1948) called the formation Estratos de Paso Flores ("Paso Flores Beds") and Galli (1954, 1969) described it as Serie de Paso Flores. Finally, the formation was described as Paso Flores Formation (Nullo, 1979, González Díaz, 1982, Spalletti et al., 1988).[1] based on a type section exposed along the Limay River in the southern Neuquén Basin.[2]

The Paso Flores Formation comprises conglomerates, sandstones and shales.

Extent
See also: Neuquén Basin § Geology

The formation crops out in its largest extent around the Limay River and the Ranquel-Huao hill and north of Estancia Corral de Piedra. Other outcrops are found northeast of Estancia Collón Curá y on the right banks of the Collón Curá River and in the cañadones of Pancho and El Pedregoso. The type section is found in the eponymous Paso Flores, on the left (western) banks of the Limay River and around Cerro Mariana.[1] The national roads 40 and 237 cross-cut the formation.[3]

Stratigraphy

Separated by a marked unconformity, the formation overlies the Paleozoic crystalline basement of the Cushamen Formation. The Paso Flores Formation is unconformably overlain by the Jurassic Sañicó and Nestares Formations, and Neogene Collón Curá, Caleufú and Cerro Petiso Formations and by alluvium.[2]

The Sañicó Lineament puts the Paso Flores Formation in contact with Famatinian basement.[4]

Age and correlation
See also: Triassic-Jurassic extinction event and Category:Rhaetian Stage

González Díaz (1982) assigned a Late Triassic age to the formation, based on a paleofloristic analysis of the Paso Flores Formation executed west of the Collón Curá River. Ganuza et al. (1995) realized a paleobotanical study at Cañadón de Pancho and reached the same conclusion of a Late Triassic age. Zavattieri (1997) observed that the macro- and microflora indicated a Rhaetian age. Several taxa known from the Jurassic are present as well as typical Triassic taxa as Telemachus elongatus and Pagiophyllum. The formation is the only fossiliferous latest Triassic formation.[2]

The Paso Flores Formation is correlated with the Garamilla, Los Menucos and Sierra Colorada Formations elsewhere in Argentina.[5]

Dictyophyllum tenuifolium, whose type locality is the Paso Flores Formation, is also found in the Ladinian Cortaderita Formation of San Juan Province.[6]

Lithology

The Paso Flores Formation is a terrestrial unit with a high fossil content, composed of conglomerates, conglomeratic sandstones and lightbrown sandstones and shales with iron staining. The thickness of the formation varies from 30 metres (98 ft) in the Ranquel Huao hills, 90 metres (300 ft) in the Cañadón de Pancho and according to Frenguelli (1948) and Spalletti et al.(1988), 263 metres (863 ft) in Estancia Paso Flores and Cerro Mariana.[1]

The well-consolidated conglomerates comprise rounded polymict granitic and volcanic clasts of up to 20 centimetres (7.9 in) in a sandy matrix. Intercalated in the conglomerates are lenses of fine-to-medium grained sandstones and shales preserving fossil leaves and fossilized treetrunks.[1]

Fossil content

The Paso Flores has provided a variety of flora fossils, reported from three outcrops:[7] In 2003, more specific descriptions were carried out of Cladophlebis grahami, C. denticulata, Dictyophyllum chihuiuensis, D. tenuiserratum and D. rothi, Rhexoxylon brunoi, Dicroidium crassum, D. odontopteroides, D. lancifolium, Xylopteris argentina and X. elongata. Abundant other flora are Scleropteris grandis, Linguifolium arctum, L. lilleanum, L. tenisonwoodsii and L. steinmannii, Cycadocarpidium and Heidiphyllum, Telemachus and Protocircoporoxylon marianaensis.[2]

In 2017, several flora were described; Lutanthus ornatus, Rissikistrobus plenus, Rissikistrobus reductus, Rissikia media, Umkomasia sp., Sphenobaiera argentinae, Pseudoctenis spatulata, Taeniopteris crassinervis and Yabeiella brackebuschiana.[8]

Additionally, Baiera triassica, in replacement of the homonymous species previously identified in Argentina as Baiera taeniata, Ginkgo taeniata and Sphenobaiera taeniata, were described as well as new species previously described from other locations than the analyzed Quemquemtreu area of the Cañadón Pancho locality. These species are: Asterotheca rigbyana, Marattiopsis muensteri, Cladophlebis kurtzii, C. indica, Dictyophyllum (Dictyophyllum) tenuifolium, Goeppertella stipanicicii, Dicroidium incisum, D. odontopteroides, D. lancifolium, Pachydermophyllum praecordillerae, Heidiphyllum elongatum, Baiera furcata, Sphenobaiera robusta, Pseudoctenis carteriana and P. falconeriana.[8]

Asterotheca sp.
Baiera
Cladophlebis
Dicroidium

Paso Flores
PGAP 2402 - Spalletti et al. (1988)[9]

Cañadón del Ranquel Huao
PGAP 2403 - idem[9]

Cañadón de Pancho
PGAP 2404 - Artabe et al. (1994)[10]
  • Ginkgoopsida
    • Cycadocarpidium sp.
    • Czekanowskia sp.
    • Johnstonia sp.
    • Nilsonia sp.
    • Pseudoctenis sp.
    • Scleropteris sp.
    • Taeniopteris sp.
    • Zuberia sp.

Additionally, several spores and pollen were described from the formation by Zavattieri and Mego in 2008:[11]

See also

References
  1. Escosteguy, 2013, p.14
  2. Escosteguy, 2013, p.15
  3. Zavattieri & Mego, 2008, p.484
  4. Benedini, 2015, p.135
  5. Benedini, 2015, p.13
  6. Bodnar et al., 2018, p.403
  7. Paso Flores Formation at Fossilworks.org
  8. Gnaedinger & Zavattieri, 2017
  9. Spalletti et al., 1988
  10. Artabe et al., 1994
  11. Zavattieri & Mego, 2008, pp.485-487

Bibliography

Further reading
  • Herbst, R. 1993. Dipteridaceae (Filicales) del Triásico del Arroyo Llantenes (provincia de Mendoza) y de Paso Flores (provincia del Neuquén), Argentina. Ameghiniana 30. 155–162.
  • Bonaparte, J.F. 1978. El Mesozóico de América de Sur y sus Tetrapodos - The Mesozoic of South America and its tetrapods. Opera Lilloana 26. 1–596.
  • Bonetti, M.I.R., and R. Herbst. 1964. Dos especies de Dictyophyllum del Triásico de Paso Flores. Provincia del Neuquén, Argentina. Ameghiniana 3. 273–279.
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