Los Adobes Formation

Los Adobes Formation is an Early Cretaceous (Aptian) geologic formation in Chubut Province, in the Cañadón Asfalto Basin of central Patagonia, Argentina. The formation belongs to the Chubut Group and represents the Early Cretaceous K1 megasequence in the basin, unconformably overlying the Late Jurassic Cañadón Calcáreo Formation and is overlain by the Albian Cerro Barcino Formation.

Los Adobes Formation Stratigraphic range: Aptian ~118–114 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Type	Geological formation
Unit of	Chubut Group
Sub-units	Bardas Coloradas member
Underlies	Cerro Barcino Formation
Overlies	Cañadón Calcáreo Formation (generally) Paso del Sapo Formation (Sierra de Traquetrén)
Thickness	Up to 700 m (2,300 ft)
Lithology
Primary	Mudstone, sandstone
Other	Conglomerate
Location
Coordinates	43.2°S 69.1°W
Approximate paleocoordinates	44.1°S 35.6°W
Region	Chubut Province
Country	Argentina
Extent	Cañadón Asfalto Basin
Type section
Named by	Tasch & Volkheimer
Year defined	1970
Los Adobes Formation (Argentina)

Los Adobes Formation was deposited in an alluvial to fluvial environment and the mudstones of the overall sandy amd conglomeratic unit have provided fossils of a chelid turtle whose indeterminate remains were described by Sterli et al. in 2020.

Description

Paleogeographic evolution with Barremian-Aptian (C)

Los Adobes Formation, the lower unit in the Chubut Group was first described in 1970 by Tasch and Volkheimer. The formation underlies the Cerro Barcino Formation and unconformably overlies the Late Jurassic Cañadón Calcáreo Formation. The formation crops out in the southern Cañadón Asfalto Basin, south of the North Patagonian Massif until the Meseta de Canquel.[1]

In the Sierra de Traquetrén, in the extreme northwest of the Cañadón Asfalto Basin, Los Adobes Formation has been thrusted onto the Maastrichtian Paso del Sapo Formation.[2]

The formation is absent in the Cañadón del Zaino around Cerro Baya and Gastre, where the Early Jurassic Las Leoneras and Lonco Trapial Formations as well as the Middle Jurassic Cañadón Asfalto Formation crop out. The oldest overlying formations in this area are the Neogene La Pava and Cráter Formations.[3]

Geologic framework

During the late Early Cretaceous, the Cañadón Asfalto Basin was a back-arc basin with thinned underlying crust. The subduction of the Chasca and Catequil Plates, forerunners of the Farallones Plate, caused volcanic activity in the present Andean terrain represented by the Divisadero Group in Chile.[4] Los Adobes Formation represents the K1 megasequence in the basin history, following the Jurassic megasequences when the depocenter of the basin was deeper.[1]

Lithology and depositional environment

The formation comprises mudstones, sandstones and conglomerates, deposited in a terrestrial alluvial to fluvial environment. Paleocurrent analysis of the sediments suggest a transport direction towards the south and east. Los Adobes Formation is correlated with the Pozo D-129 Formation of the Golfo San Jorge Basin to the south.[5] The basal part of Los Adobes Formation is irregular, covering the Jurassic sediments in small depocenters. The maximum noted thickness is 700 metres (2,300 ft).[6] The formation shows progressive unconformities that show up in seismic sections as onlap structures.[7]

Age

U-Pb dating of the formation has provided ages ranging from 118 to 114 Ma.[8] At this time, during the Aptian extinction events (~117-116 Ma) and the opening of the South Atlantic, the Neuquén Basin to the northwest was experiencing the Middle Mirano unconformity, dated at 117 Ma and separating the contemporaneous Huitrín Formation.[9]

Fossil content

The formation has provided fossils of an inderminate chelid turtle.[10][11]

See also

• Aptian formations
• Paja Formation, contemporaneous fossiliferous formation of the Altiplano Cundiboyacense, Colombia
• Caiuá Formation, contemporaneous fossiliferous formation of the Paraná Basin, Brazil
• Crato Formation, contemporaneous fossiliferous formation of the Araripe Basin, Brazil
• Agrio and La Amarga and Lohan Cura Formations, contemporaneous fossiliferous formations of the Neuquén Basin
• Río Belgrano Formations, contemporaneous fossiliferous formation of the Magallanes or Austral Basin

References

1. Figari et al., 2015, p.150
2. Echaurren González, 2017, p.107
3. Di Pietro, 2016, p.26
4. Echaurren González, 2017, p.120
5. Figari et al., 2015, p.151
6. Figari et al., 2015, p.152
7. Echaurren González, 2017, p.106
8. Echaurren González, 2017, p.95
9. Leanza et al., 2005, p.150
10. Sterli et al., 2020
11. Los Adobes Formation at Fossilworks.org

Bibliography

Geology

• Echaurren González, Andrés. 2017. Evolución tectónica del sistema orogénico Andino en la Patagonia norte (42-44° S) (PhD thesis), 1–170. Universidad de Buenos Aires. Accessed 2019-02-27.
• Di Pietro, Pablo Federico. 2016. Geología de la zona del Cerro Bayo, Bajo de Gastre, Provincia de Chubut (B.S. thesis), 1–107. Universidad de Buenos Aires. Accessed 2019-02-27.
• Figari, Eduardo G.; Roberto A. Scasso; Rubén N. Cúneo, and Ignacio Escapa. 2015. Estratigrafía y evolución geológica de la Cuenca de Cañadón Asfalto, Provincia del Chubut, Argentina. Latin American Journal of Sedimentology and Basin Analysis 22. 135–169. Accessed 2018-09-10.
• Leanza, Héctor A. 2005. Las principales discordancias del Jurásico Superior y el Cretácico de la Cuenca Neuquina. Anales de la Academia Nacional de Ciencias Exactas, Físicas y Naturales 57. 147–155. Accessed 2019-02-14.

Paleontology

• Sterli, J.; E. Vlachos; P. Puerta; C. Oriozabala, and M. Krause. 2020. The diversity of the fossil record of turtles (Testudinidata) from Chubut province (Argentina) and its significance in understanding the evolution of turtles in Argentina and South America. in prep in press. x.