Chañares Formation

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 Ischigualasto-Villa Unión Basin. It unconformably overlies red fluvial (river) sediments of the Tarjados Formation of the Paganzo Group, and is conformably overlain by greenish lake and delta sediments of the Los Rastros Formation. 2020 U-Pb dating of the overlying lower Los Rastros Formation yielded an age of 234.47 ± 0.44 Ma, making the vast majority of the Chañares Formation lowermost Carnian. Nevertheless, the Ladinian-Carnian boundary may still lie within the first few meters of the formation, despite the primary fossiliferous sections being well-supported as early Carnian in age. Fluvial olive-grey facies Above this unconformity lies the oldest section of the Chañares Formation, a package of olive-grey fluvial sediments. As one goes up the section, increasingly finer beds of sandstone and siltstone are interlayered with coarser lenses, corresponding to periodic sheet floods along braided rivers. Weakly-developed palaeosols can be found within this section, filled with root traces, pebbles and small brown calcareous nodules. Winding systems of burrows have also been found in this section, likely created by small cynodonts. A white tuff near the base of the upper member has been dated to 233.7 ± 0.4 Ma, while a zircon cluster from a sandy tuff at the top of the formation was dated to 233.6 ± 1.1 Ma. == Paleobiota ==

Synapsids Tetrapod burrows, likely produced by small eucynodonts, have been described from the lower section of the formation. ; Cynodonts ; Dicynodonts Reptiles Fish Invertebrates A diverse insect fauna is known from the Ischichuca Formation, which is sometimes considered equivalent to the Chañares Formation. Plants Plant remains and palynomorphs preserved in the dicynodont coprolites were described in 2018. Though it is difficult to determine the affinities of the larger plant fragments, the palynomorphs are more conclusive. They belong to a broad range of plants, most abundantly pollen from umkomasiales (a type of seed fern), and in smaller portions from podocarpacean and voltzialean conifers. Spores from humid-loving groups such as bryophytes, lycopsids, true ferns, and algae were also present but rare. The palynomorph taxa generally resemble those of the Dicroidium flora which is common in other late Ladinian-early Carnian units. More precisely, the flora is intermediate between the temperate Ipswich flora of far southern Gondwana, and the hot, subtropical Onslow flora which developed along the southern shore of the Neotethys. This transitional character is also observed in the flora of the Ischigualasto Formation, as well as the Flagstone Bench Formation of Antarctica. Curiously, the Los Rastros Formation, which was deposited between the Chañares and Ischigualasto Formations, preserves a typical Ipswich flora. This likely indicates that all three formations lie at a latitude which allows them to quickly shift between the different floras during small climatic changes. == See also ==