Chañares Formation


The Chañares Formation is a geologic formation of the Ischigualasto-Villa Unión Basin, located in La Rioja Provence, Argentina. The claystones and tuffs of the formation date to the Carnian stage of the Late Triassic and were deposited in a fluvial to lacustrine environment.
The formation represents the onset of the first syn-rift phase in the Triassic rift basin and is the lowermost stratigraphic unit of the Agua de la Peña Group, unconformably overlying the Tarjados Formation of the Paganzo Group. The Chañares Formation is overlain by the Ischichuca Formation and both formations have a combined maximum thickness of.
The Chañares Formation has provided a rich faunal assemblage, including many of the earliest crocodylomorph fossils, as Tropidosuchus, Chanaresuchus, and Gualosuchus, as well as other archosaurs; Lewisuchus admixtus, Lagerpeton, Marasuchus lilloensis, Gracilisuchus, Luperosuchus and Pseudolagosuchus major. Cynodonts are represented by Probainognathus and Massetognathus and other therapsids include Dinodontosaurus.

Description

The Chañares Formation is the lowermost unit of the Agua de la Peña Group, representing the onset of the first syn-rift phase of the basin. The combined thickness of the overlying Ischichuca Formation and the Chañares Formation is.
It was originally thought to be formed during the Ladinian age of the Middle Triassic epoch, in the Triassic Period of the Mesozoic Era; Marsicano et al. dated it to early Carnian, Late Triassic. 2020 U-Pb dating of the lower Los Rastros Formation overlying the Ischichuca Formation yielded an age of 234.47 ± 0.44 Ma, making the Chañares Formation lowermost Carnian.
The formation overlies the Tarjados Formation and is overlain by the Ischichuca Formation. The formation is exposed in the Ischigualasto Provincial Park, a World Heritage site in Argentina. The formation of claystones and tuffs was deposited in a fluvial to lacustrine environment.

Fossil content

Some localities are well known for their abundance of tetrapod fossils. Theraspids include the kannemeyeriid Dinodontosaurus, and cynodonts such as Probainognathus and Massetognathus, the latter being the most abundant tetrapod taxon in the formation represented by Massetognathus pascuali and M. teruggii.
Other notable tetrapods present from the formation are the archosaurs. Ornithodirans include Lewisuchus, Lagerpeton, and Marasuchus.
Other archosaurs include Gracilisuchus and Luperosuchus. Three proterochampsids are known from the formation, Tropidosuchus, Chanaresuchus, and Gualosuchus. They are very similar in appearance, but differ in size and cranial proportions. Tropidosuchus had an estimate body mass of about, Gualosuchus of approximately, and Chanaresuchus weighed about.
In 2018, plant remains and palynomorphs preserved in the coprolites produced by large dicynodonts, and a study on their implications for inferring the diet of dicynodonts from the formation were described. Also tetrapod burrows, likely produced by small eucynodonts, were described.

Other fossils

Other fossils found in the formation include: