The Agrio Formation was first described by Weaver in 1931 and its three members, from bottom to top: Pilmatué, Avilé and Agua de la Mula Members, were defined by Leanza and Hugo in 2001.[5][1][3] The formation
crops out in an approximately 50 kilometres (31 mi) wide band from north to south along
70° longitude west, from
37° to
39° south in the northern-central part of the Neuquén Basin.[6] The southern termination of the formation is the Huincul High, formed by the
Huincul Fault. Towards the east in the basin, the formation grades into the
Centenario Formation.[3]
Stratigraphy
The Agrio Formation is included in the
Mendoza Group, representing its youngest formation. In the east of the Neuquén Basin, the formation rests upon continental clastic deposits of the
Mulichinco Formation, with the contact between the two formations characterized by a regional
transgressive surface. Towards the west, the formation unconformably overlies the
Bajada Colorada Formation.[7][8][9] In its eastern part, the Agrio Formation is overlain by the clastic, carbonaceous and evaporitic deposits of the
Huitrín Formation and in the western area by the
La Amarga Formation. The total thickness of the Agrio Formation reaches up to 1,500 metres (4,900 ft), with the Pilmatué Member having a thickness of 718 metres (2,356 ft) and the Agua de la Mula Member reaching 501 metres (1,644 ft).[10]
Lithologies
The Agrio Formation is primarily composed of
pelitic rocks with intercalations of
limestones,
sandstones and rare fine
conglomerates. The Pilmatué and Agua de la Mula Members are characterized by thick successions of
black shales with intercalating limestones and sandstones. The Avilé Member comprises sandstones and
claystones with conglomerates.[11][10]
Depositional environment
The Agrio Formation was deposited in a post-rift setting of the Neuquén Basin, probably representing a tectonic regime of
thermal subsidence. The sediments of the lower and upper members of the formation are marine in character, interpreted as the combination of thermal subsidence and a
eustatic sea level rise.[12][13][6] The middle Avilé member was deposited in a
fluvialenvironment. Within the marine Pilmatué Member, a succession of approximately 130 metres (430 ft) thick, described as "San Eduardo Beds", is recognized as deposited in a wave-dominated
deltaic setting with hyperdense currents. This sequence is overlain by about 40 metres (130 ft) thick limestones deposits in a
reefal environment.[10] The presence of the newly described
gastropodEunerinea mendozana led researchers to estimate
tropical conditions for the Agrio Formation.[14]
In
2018,
ichthyosaur remains not determined to the genus level were described from the Agrio Formation, suggesting the possibility of
viviparity of these marine reptiles in the
epeiric sea of the Neuquén Basin. The finds were notable as well because of a relative lack of abundance of ichthyosaur fossils from the Valanginian to Hauterivian worldwide.[21] Fossil fish of Gyrodus huiliches, and Tranawuen agrioensis were described from the formation in
2019.[22]
The first known brittle stars in the
Southern Hemisphere and
Cretaceous age have been identified in Agrio Formation.[28] However, fossils are not complete enough to define species.[28]
Petroleum geology
The Agrio Formation is considered the third-most important
source rock of the hydrocarbon-rich Neuquén Basin, after the older
Vaca Muerta and
Los Molles Formations.[29] Two levels of organic-rich sediments exist in the formation, related to the marine transgressions of the late Valanginian and the late Hauterivian, in the Pilmatué and Agua de la Mula Members respectively. The marly shales of the Pilmatué Member reach up to 400 metres (1,300 ft) thickness in the western Neuquén Basin, while the same facies in the Agua de la Piedra Member is less than 100 metres (330 ft) thick. The organic properties of the formation are similar to the Vaca Muerta, with a
TOC value averaging 2.5%, with some levels up to 5%. The
kerogen types are II to II/III.[2]
^
abWeaver, C. E. (1931). Paleontology of the Jurassic and cretaceous of west central Argentina. In Memoires of the University of Washington (Vol. 1, pp. 1–599).
^
abArchuby, F. M. (2009). Taphonomy and palaeoecology of benthic macroinvertebrates from the Agua de la Mula Member of the Agrio Formation, Neuquén Basin (Neuquén province, Argentina): sequence stratigraphic significance [Würzburg University].
http://www.opus-bayern.de/uni-wuerzburg/volltexte/2009/3717/index.html
^Leanza, H. A., Hugo, C. A., & Repol, D. (2001). Hoja Geológica 3969-I, Zapala. Provincia del Neuquén. In Referencia bibliográfica LEANZA, H.A., C. A. HUGO y D. REPOL, 2001. Hoja Geológica 3969-I, Zapala. Provincia del Neuquén. Instituto de Geología y Recursos Minerales, Servicio Geoló- gico Minero Argentino. Boletín (Vol. 275).
^Archuby, F. M., & Fürsich, F. T. (2010). Facies analysis of a higly [sic?] cyclic sedimentary unit: the Late Hauterivian to Early Barremian Agua de la Mula Member of the Agrio Formation, Neuquén Basin, Argentina. Beringeria, 41, 53–106.
https://doi.org/https://doi.org/10.15517/rbt.v56i3.27147
^Archuby, F. M., Wilmsen, M., & Leanza, H. A. (2011). Integrated stratigraphy of the Upper Hauterivian to Lower Barremian Agua de la Mula Member of the Agrio Formation, Neuquén Basin, Argentina. Acta Geologica Polonica, 61(1), 1–26.
^Archuby, F. M., & Fürsich, F. T. (2010). Facies analysis of a higly cyclic sedimentary unit: the Late Hauterivian to Early Barremian Agua de la Mula Member of the Agrio Formation, Neuquén Basin, Argentina. Beringeria, 41, 53–106.
https://doi.org/https://doi.org/10.15517/rbt.v56i3.27147
^Ballent, S., Concheyro, A., Náñez, C., Pujana, I., Lescano, M., Carignano, A. P., Caramés, A., Angelozzi, G., & Ronchi, D. (2011). Microfósiles Mesozoicos Y Cenozoicos. Relatorio Del XVIII Congreso Geológico Argentino, September 2017, 489–528.
Balgord, Elizabeth A. 2017. Triassic to Neogene evolution of the south-central Andean arc determined by detrital zircon U-Pb and Hf analysis of Neuquén Basin strata, central Argentina (34°S–40°S). Lithosphere 9. 453–462. .
Howell, John A.; Ernesto Schwartz; Luis A. Spalletti, and Gonzalo D. Veiga. 2005.
The Neuquén Basin: An overview. Geological Society, London, Special Publications 252. 1–14. Accessed 2019-02-14.