Los Rastros Formation

Los Rastros Formation
Stratigraphic range: Carnian 234.91–234.03 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Type	Geological formation
Unit of	Agua de la Peña Group
Underlies	Ischigualasto Formation
Overlies	Chañares Formation
Thickness	Up to 1,000 m (3,300 ft)
Lithology
Primary	Claystone, sandstone
Other	Mudstone
Location
Coordinates	29°06′S 69°06′W / 29.1°S 69.1°W / -29.1; -69.1
Approximate paleocoordinates	45°48′S 34°54′W / 45.8°S 34.9°W / -45.8; -34.9
Region	La Rioja Province San Juan Province
Country	Argentina
Extent	Ischigualasto-Villa Unión Basin
Los Rastros Formation (Argentina)

The Los Rastros Formation is a mid-Carnian fossiliferous formation of the Agua de la Peña Group, in the Ischigualasto-Villa Unión Basin of northwestern Argentina. Fossil archosaur tracks,^[1] as well as many insects,^[2] fish,^[3] bivalves, crustaceans and plants have been reported from the formation.^[4]

The formation overlies the Chañares Formation (and/or the Ischicuca Formation), and is overlain by the Ischigualasto Formation.^[5] The formation belongs to the Agua de la Peña Group of the Ischigualasto-Villa Unión Basin, where it is exposed in the Ischigualasto Provincial Park, a World Heritage Site in Argentina.^[6] The up to 1,000 metres (3,300 ft) thick formation of claystones, mudstones and sandstones was deposited in a deltaic to lacustrine environment during the first post-rift sequence in the basin.^[5][4][7][8]

A CA-TIMS U-Pb zircon age of 234.47 ± 0.44 Ma from the lower Los Rastros Formation was published in 2020. The acquired age demonstrates that most of the overlying lacustrine strata within the basin were deposited during or after the Carnian Pluvial Event (CPE).^[9]

The only tetrapod body fossil reported from the Los Rastros Formation is a poorly-preserved temnospondyl amphibian.^[10][4]

Tetrapod footprints are a conspicuous type of ichnofossils (trace fossils) found in the Los Rastros Formation, particularly at the Quebrada de Ischichuca locality near Cerro Bola.^[1][11][12] Most of the trackways are symmetrical prints created by animals with a narrow bipedal stance, three toes contacting the ground, and a raised fourth toe. They were presumably early dinosaurs or dinosauromorphs,^[12] with a hip height exceeding 75 cm. These prints are found in deltaic sandbars, parallel to the lakeshore.^[11] While some sources tentatively interpret these as theropod footprints,^[13] the generalized nature of Triassic dinosaurs and their relatives prevent a precise referral.^[12] Some of the tridactyl trackways also preserve hand prints, with quadrupedal pseudosuchians or other archosaurs as likely trackmakers.^[12] Named reptile ichnotaxa from the Los Rastros Formation include Rhynchosauroides sp. (sphenodonts or other small reptiles) and Rigalites ischigualastianus (large quadrupedal pseudosuchians).^[1][14]

Another form of trackway consists of numerous non-overlapping sets of broad rounded depressions on a heavily trampled wackestone surface. They may have been left by a herd of dicynodonts walking down to the lakeshore. Palaeophycus tubularis invertebrate burrows are also abundant.^[11]

The Los Rastros Formation has produced many insect fossils, with most new species belonging to blattopterans (cockroaches and kin), coleopterans (beetles and kin), or hemipterans (true bugs).^[2] Among Triassic formations of South America, the Los Rastros Formation's insect diversity is only rivaled by the Potrerillos Formation in the Cuyo Basin. Other invertebrate fossils from the Los Rastros Formation include Lioestheria ("conchostracans" or clam shrimp) and Paleomutella (freshwater mussels).^[1]

Indeterminate corixids (water boatmen) are also known from the Gualo locality.^[18]

A wide variety of plant fossils are known from the Los Rastros Formation, including both microfossils^[4][8] and macrofossils.^{[24][25][26][8]}

Most species from the formation have been listed by Lutz et al. (2011)^[24] and Pedernera et al. (2020).^[25]

• List of dinosaur-bearing rock formations
  • List of stratigraphic units with theropod tracks
• Chañares Formation
• Santa Maria Formation
• Omingonde Formation

• Aceituno Cieri, P.; Zeballos, M.E.; Rocca, R.J.; Martino, R.D.; Carignano, C.; Guereschi, A.; Giambastiani, M. (2015), "Condicionantes geológicos en el cruce de la sierra de Valle Fértil. San Juan - Geological constraints at the crossing of sierra Valle Fertil. San Juan", Revista de Geología Aplicada a la Ingeniería y al Ambiente, 35: 57–69
• Leonardi, Giuseppe (1994), Annotated Atlas of South America Tetrapod Footprints (Devonian to Holocene) with an appendix on Mexico and Central America, Ministerio de Minas e Energia - Companhia de Pesquisa de Recursos Minerais, Geological Service of Brazil, pp. 1–248, retrieved 2019-03-25
• Weishampel, David B.; Dodson, Peter; Osmólska, Halszka (2004), The Dinosauria, 2nd edition, Berkeley: University of California Press, pp. 1–880, ISBN 0-520-24209-2, retrieved 2019-02-21