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42-854: The formation overlies the Chañares Formation (and/or the Ischicuca Formation ), and is overlain by the Ischigualasto Formation . 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. The up to 1,000 metres (3,300 ft) thick formation of claystones , mudstones and sandstones

84-774: A fluvial to lacustrine environment . The formation is most prominently exposed within Talampaya National Park , a UNESCO World Heritage Site within La Rioja Province. The Chañares formation is the lowermost stratigraphic unit of the Agua de la Peña Group , overlying the Tarjados Formation of the Paganzo Group , and underlying the Los Rastros Formation . Though previously considered Ladinian in age, U-Pb dating has determined that most or all of

126-409: A few sandstone and conglomerate beds near the top of the upper member. These indicate that the depositional regime continued to shift towards the deltaic environment of the overlying Los Rastros Formation. The coarser beds also contain a few rare body fossils from fish and tetrapods . A white tuff near the base of the upper member has been dated to 233.7 ± 0.4 Ma, while a zircon cluster from

168-465: A heterogeneous mixture of lithologies, so long as this distinguishes them from adjacent bodies of rock. The concept of a geologic formation goes back to the beginnings of modern scientific geology. The term was used by Abraham Gottlob Werner in his theory of the origin of the Earth, which was developed over the period from 1774 to his death in 1817. The concept became increasingly formalized over time and

210-412: A high concentration of volcanic ash and debris, ranging from glassy shards at the base to weathered bentonites at the top. Nearby volcanic eruptions likely impacted the local climate and river systems, shifting the depositional regime from a stable braided fluvial system to one dominated by shallow floodplains and lahars . Marsicano et al. (2016) obtained a CA-TIMS U-Pb age of 236.1 ± 0.6 Ma from

252-442: A hip height exceeding 75 cm. These prints are found in deltaic sandbars , parallel to the lakeshore. While some sources tentatively interpret these as theropod footprints, the generalized nature of Triassic dinosaurs and their relatives prevent a precise referral. Some of the tridactyl trackways also preserve hand prints, with quadrupedal pseudosuchians or other archosaurs as likely trackmakers. Named reptile ichnotaxa from

294-581: A large portion of the formation was deposited in the early Carnian (237–234 Ma), near the start of the Late Triassic . 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

336-526: A newly designated formation could not be named the Kaibab Formation, since the Kaibab Limestone is already established as a formation name. The first use of a name has precedence over all others, as does the first name applied to a particular formation. As with other stratigraphic units, the formal designation of a formation includes a stratotype which is usually a type section . A type section

378-594: A permanent natural or artificial feature of the geographic area in which they were first described. The name consists of the geographic name plus either "Formation" or a descriptive name. Examples include the Morrison Formation , named for the town of Morrison, Colorado , and the Kaibab Limestone , named after the Kaibab Plateau of Arizona. The names must not duplicate previous formation names, so, for example,

420-402: A sandy tuff at the top of the formation was dated to 233.6 ± 1.1 Ma. Tetrapod burrows, likely produced by small eucynodonts, have been described from the lower section of the formation. A large dicynodont fossil preserves taphonomic markers which help to reconstruct the sequence of burial, decay, and fossilization responsible for articulated fossils in the formation. A diverse insect fauna

462-596: A siltstone bed immediately below the first major fossil layer within the bluish facies. Ezcurra et al. (2017) later studied a slightly older sandstone bed using LA-MC-IMP-MS U-Pb dating. They found a date of 236.2 ± 1.1 Ma from a cluster of the three youngest zircons . The most productive and historically relevant fossil beds of the Chañares Formation lie within these volcaniclastic layers. The layers are replete with massive calcareous concretions , some up to 2 meters in width. They commonly preserve articulated skeletal material, often complete skeletons from multiple taxa in

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504-618: A single concretion. Well-preserved fossils found within concretions typically represent smaller taxa, which were buried rapidly after death. Volcanic catastrophes such as lahars, ash falls, or pyroclastic flows are the preferred cause of these rapid mass mortality events . More fragmentary fossils are also occasionally found outside of concretions, much like fossils of the underlying Tarjadia AZ. Fossils found outside concretions are typically from large animals, and were probably buried more slowly (albeit still fairly rapidly) by gradual processes. The classic Chañares fauna, recently codified as

546-449: Is not a valid lithological basis for defining a formation. The contrast in lithology between formations required to justify their establishment varies with the complexity of the geology of a region. Formations must be able to be delineated at the scale of geologic mapping normally practiced in the region; the thickness of formations may range from less than a meter to several thousand meters. Geologic formations are typically named after

588-560: Is central to the geologic discipline of stratigraphy , and the formation is the fundamental unit of stratigraphy. Formations may be combined into groups of strata or divided into members . Members differ from formations in that they need not be mappable at the same scale as formations, though they must be lithologically distinctive where present. The definition and recognition of formations allow geologists to correlate geologic strata across wide distances between outcrops and exposures of rock strata . Formations were at first described as

630-432: Is generally similar to the bluish facies in appearance, with wide beds of fine-grained volcaniclastic sediments with a pale grey color. Unlike the bluish facies, concretions and body fossils are absent, having been replaced by Taenidium worm burrows. These burrows likely indicate that the environment had transitioned into a lacustrine (lake) ecosystem by the time that the upper member was deposited. Some outcrops preserve

672-455: Is ideally a good exposure of the formation that shows its entire thickness. If the formation is nowhere entirely exposed, or if it shows considerably lateral variation, additional reference sections may be defined. Long-established formations dating to before the modern codification of stratigraphy, or which lack tabular form (such as volcanic formations), may substitute a type locality for a type section as their stratotype. The geologist defining

714-746: Is known from the Ischichuca Formation, which is sometimes considered equivalent to the Chañares Formation. 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

756-495: Is now codified in such works as the North American Stratigraphic Code and its counterparts in other regions. Geologic maps showing where various formations are exposed at the surface are fundamental to such fields as structural geology , allowing geologists to infer the tectonic history of a region or predict likely locations for buried mineral resources. The boundaries of a formation are chosen to give it

798-637: Is rare due to the historical significance of the Chañares Formation. As a result, some authors restrict the Ischichuca Formation to a few layers of lake and delta sediments between the Chañares and Los Rastros formations, while others reject using it in the first place. The Chañares formation was originally thought to be deposited during the Ladinian stage of the Middle Triassic . However, Uranium-Lead radiometric dating by Marsicano et al. (2016) later found that

840-622: 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,

882-564: 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. Plant macrofossils are absent from the Chañares Formation (in contrast to the Los Rastros and Ischigualasto formations). Nevertheless, fossil wood, foliage, and reproductive structures have been described from

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924-563: The Massetognathus - Chanaresuchus Assemblage Zone, is characteristic of this section of the formation. The largest components of the ecosystem include dicynodonts (namely Dinodontosaurus ) and indeterminate carnivorous paracrocodylomorphs , similar to taxa from the underlying Tarjadia AZ. By far the most common fossils belong to three species of cynodonts : Massetognathus pascuali , Probainognathus jenseni , and Chiniquodon theotonicus . These three species comprise nearly 3/4ths of

966-555: The Paganzo Group , and is conformably overlain by greenish lake and delta sediments of the Los Rastros Formation . The Chañares Formation has also had an interesting history with the lower part of the Ischichuca Formation . This sequence of sediments on the western edge of the basin has occasionally been considered to take priority over the Chañares Formation and the lower part of the Los Rastros Formation. However, this

1008-507: The Chañares Formation dates to the early Carnian stage of the Late Triassic . The Chañares Formation has provided a diverse and well-preserved faunal assemblage which has been studied intensively since the 1960s. The most common reptiles were proterochampsids ( Chanaresuchus , Tropidosuchus , and Gualosuchus ) , which lived alongside true archosaurs such as Lewisuchus , Lagerpeton , Marasuchus , Gracilisuchus , and Luperosuchus . Cynodonts were abundant, represented by

1050-418: The Ischichuca Formation. Geologic formation A geological formation , or simply formation , is a body of rock having a consistent set of physical characteristics ( lithology ) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exposed in a geographical region (the stratigraphic column ). It is the fundamental unit of lithostratigraphy ,

1092-707: The Los Rastros Formation include Rhynchosauroides sp. ( sphenodonts or other small reptiles) and Rigalites ischigualastianus (large quadrupedal pseudosuchians). 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. 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). Among Triassic formations of South America,

1134-531: The Los Rastros Formation is a poorly-preserved temnospondyl amphibian. 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. 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 , with

1176-567: 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). Indeterminate corixids (water boatmen) are also known from the Gualo locality. A wide variety of plant fossils are known from

1218-560: The Los Rastros Formation, including both microfossils and macrofossils . Most species from the formation have been listed by Lutz et al. (2011) and Pedernera et al. (2020). Cha%C3%B1ares Formation The Chañares Formation is a Carnian -age geologic formation of the Ischigualasto-Villa Unión Basin , located in La Rioja Province , Argentina . It is characterized by drab-colored fine-grained volcaniclastic claystones , siltstones , and sandstones which were deposited in

1260-408: The ancestry of dinosaurs and pterosaurs. Several communal latrines are known from the bluish facies, preserving dicynodont coprolites filled with plant remains. The olive-grey fluvial section and the bluish-grey volcaniclastic section collectively form the lower member of the Chañares Formation. They are overlain by an upper member, which is practically devoid of body fossils. The upper member

1302-480: The best sources of Carnian-age tetrapods in South America, along with the slightly younger Ischigualasto Formation which lies above the Los Rastros 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

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1344-550: The essential geologic time markers, based on their relative ages and the law of superposition . The divisions of the geological time scale were described and put in chronological order by the geologists and stratigraphers of the 18th and 19th centuries. Geologic formations can be usefully defined for sedimentary rock layers, low-grade metamorphic rocks , and volcanic rocks . Intrusive igneous rocks and highly metamorphosed rocks are generally not considered to be formations, but are described instead as lithodemes . "Formation"

1386-522: The formation is expected to describe the stratotype in sufficient detail that other geologists can unequivocally recognize the formation. Although formations should not be defined by any criteria other than primary lithology, it is often useful to define biostratigraphic units on paleontological criteria, chronostratigraphic units on the age of the rocks, and chemostratigraphic units on geochemical criteria, and these are included in stratigraphic codes. The concept of formally defined layers or strata

1428-410: The fossils found in the formation, with almost half of the total fossils in the formation referred to Massetognathus alone. Reptile fossils are less prevalent but more diverse, with the most common belonging to the proterochampsid archosauriform Chanaresuchus . Other notable taxa include Lagerpeton , Lagosuchus , and Lewisuchus , which were among the oldest ornithodirans , elaborating on

1470-400: The greatest practical lithological consistency. Formations should not be defined by any criteria other than lithology. The lithology of a formation includes characteristics such as chemical and mineralogical composition, texture, color, primary depositional structures , fossils regarded as rock-forming particles, or other organic materials such as coal or kerogen . The taxonomy of fossils

1512-528: The lower fluvial beds of the Chañares Formation, they are taxonomically and taphonomically distinct from those of succeeding layers. The most common remains are from large dicynodonts (possibly referable to Dinodontosaurus ), the large erpetosuchid Tarjadia , and small non- massetognathine cynodonts closely related to Aleodon and Scalenodon . This ecosystem has been termed the Tarjadia Assemblage Zone, in order to distinguish it from

1554-405: The medium-sized traversodontid Massetognathus , as well as smaller carnivores such as Chiniquodon and Probainognathus . The largest animal in the ecosystem was the giant dicynodont Dinodontosaurus . An older faunal assemblage, distinguished by the large erpetosuchid Tarjadia , has been discovered in the earliest part of the formation. The formation as a whole is considered one of

1596-409: The primary fossiliferous sections being well-supported as early Carnian in age. The formation is primarily sandstone , siltstone , and claystone , arranged in a specific sequence of facies . A distinct and uneven unconformity separates the base of the Chañares Formation from the underlying Tarjados Formation . In the field, this unconformity can be identified by a high-relief layer of chert at

1638-447: The slightly younger classic Chañares assemblage. No radiometric dating has been done on this section, but it may contain the Ladinian-Carnian boundary based on dates obtained within the younger facies. Above the olive-grey fluvial beds is the most fossiliferous and well-studied portion of the formation. This section is characterized by wide and massive layers of very fine bluish-grey sandstone, siltstone, and claystone. These layers have

1680-408: The study of strata or rock layers. A formation must be large enough that it can be mapped at the surface or traced in the subsurface. Formations are otherwise not defined by the thickness of their rock strata, which can vary widely. They are usually, but not universally, tabular in form. They may consist of a single lithology (rock type), or of alternating beds of two or more lithologies, or even

1722-688: The top of the Tarjados Formation. 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 . Though fossils are relatively uncommon in

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1764-483: Was deposited in a deltaic to lacustrine environment during the first post-rift sequence in the basin. 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). The only tetrapod body fossil reported from

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