Puncoviscana Formation () is a

formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondary ...

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

and

metasedimentary rock In geology, metasedimentary rock is a type of metamorphic rock. Such a rock was first formed through the deposition and solidification of sediment Sediment is a solid material that is transported to a new location where it is deposited. It occu ...

s Late Ediacaran and

Lower Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordovici ...

age, estimated at between 700 and 535 Ma, that crop out in the

Argentine Northwest The Argentine Northwest (, NOA) is a geographic and historical region of Argentina comprising the provinces of Catamarca Province, Catamarca, Jujuy Province, Jujuy, La Rioja Province, Argentina, La Rioja, Salta Province, Salta, Santiago del Estero ...

. Most of the formation lies in

Jujuy San Salvador de Jujuy (), commonly known as Jujuy and locally often referred to as San Salvador, is the capital and largest city of Jujuy Province in northwest Argentina. Also, it is the seat of the Doctor Manuel Belgrano Department. It lies near ...

Salta Salta () is the capital and largest city in the Provinces of Argentina, Argentine province of Salta Province, the same name. With a population of 618,375 according to the 2010 census, it is also the List of cities in Argentina, 7th most-populous ...

and

Tucumán Province Tucumán () is the most densely populated, and the second-smallest by land area, of the provinces of Argentina. Located in the northwest of the country, the province has the capital of San Miguel de Tucumán, often shortened to Tucumán. Neighb ...

albeit some authors extend the formation further south to the

Sierras Pampeanas The Sierras Pampeanas (also called Central Sierras or Pampas Sierras) (English: Pampas Mountains) is a geographical region of Argentina. The Sierras Pampeanas are a chain of mountains that rise sharply from the surrounding pampa region of N ...

near

Córdoba Córdoba most commonly refers to: * Córdoba, Spain, a major city in southern Spain and formerly the imperial capital of Islamic Spain * Córdoba, Argentina, the second largest city in Argentina and the capital of Córdoba Province Córdoba or Cord ...

. There are various tectonic interpretations on the origin and type of

sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock They form when long-term subsidence ...

that accumulated Puncoviscana Formations sediments. An early interpretation was that the sediments originated from a

passive margin A passive margin is the transition between Lithosphere#Oceanic lithosphere, oceanic and Lithosphere#Continental lithosphere, continental lithosphere that is not an active plate continental margin, margin. A passive margin forms by sedimentatio ...

al basin of the ancient continent

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

. Others suggested an intra-cratonic

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben ...

aulacogen An aulacogen is a failed arm of a triple junction. Aulacogens are a part of plate tectonics where oceanic and continental crust is continuously being created, destroyed, and rearranged on the Earth’s surface. Rift zones are places where new c ...

basin between Río de la Plata-Pampia Craton and Arequipa Massif. Yet other hypotheses revolve around the idea that the Puncoviscana Formation is related to a

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its d ...

called

Pampia Pampia was an ancient microcontinent or terrane that collided with Río de la Plata Craton and Río Apas Craton during the Pampean orogeny of late Proterozoic and early Cambrian The Cambrian ( ) is the first geological period of the ...

that accreted to Gondwana causing the closure of a sea in the way.

Stratigraphy, lithology and fossils

The formation includes rocks such as

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

s, conglomerates,

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

slate Slate is a fine-grained, foliated, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade, regional metamorphism. It is the finest-grained foliated metamorphic ro ...

s and

schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

Stratigraphically Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostrati ...

, the upper boundary of the Puncoviscana Formation is the Tilcarian

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

, which is overlain by

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

and

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

sedimentary rocks of the

Mesón Group Mesón Group () is a Cambrian to Early Ordovician sedimentary group located in the Argentine Northwest and nearby parts of Bolivia. The group members rest unconformably on top of the Ediacaran–Cambrian Puncoviscana Formation. According to G.F. A ...

that extend across the Argentine Northwest and

Bolivia Bolivia, officially the Plurinational State of Bolivia, is a landlocked country located in central South America. The country features diverse geography, including vast Amazonian plains, tropical lowlands, mountains, the Gran Chaco Province, w ...

Fossil content

Among the fossils found in the formation are: * '' Archaeonassa fossulata'' * ''Asaphoidichnus'' isp. * '' Beltanelloides'' * '' Paliela'' * ''

Selkirkia ''Selkirkia'' is a genus of predatory, tubicolous priapulid worms known from the Middle Cambrian Burgess Shale, Ogygopsis Shale, Puncoviscana Formation and the Early Ordovician Fezouata Formation. 142 specimens of ''Selkirkia'' are known from th ...

'' ;Ichnofossils * ''Diplichnites'' isp. * '' Helminthopsis abeli'' * '' Helminthopsis tenuis'' * ''

Planolites ''Planolites'' is an ichnogenus found throughout the Ediacaran and the Phanerozoic that is made during the feeding process of worm-like animals. The traces are generally small, , unlined, and rarely branched, with fill that differs from the host ...

Tectonic interpretations of the Puncoviscana Basin

The Puncoviscana Basin was the

where the sediments of the Puncosviscana Formation were deposited. There have been differing views among geologists on the

tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

and

paleogeographic Palaeogeography (or paleogeography) is the study of historical geography, generally physical landscapes. Palaeogeography can also include the study of human or cultural environments. When the focus is specifically on landforms, the term paleo ...

position of the Puncoviscana Basin in relation to the events of the

Pampean orogeny The Pampean orogeny () was an orogeny active in the Cambrian in the western margin of the ancient landmass of Gondwana. The orogen's remains can now be observed in central Argentina, in particular at the Sierras de Córdoba and other parts of the e ...

. The Pampean orogeny is believed by some geologists to be associated with the

accretion Accretion may refer to: Science * Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity * Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucl ...

of a Pampia Terrane to the

Río de la Plata Craton The Rio de la Plata Craton (RPC) is a medium-sized continental block found in Uruguay, eastern Argentina and southern Brazil. During its complex and protracted history it interacted with a series other blocks and is therefore considered importan ...

, causing in the way the closure of a sea that existed in-between. This closed seabed would have contained the sediments of the Puncoviscana Basin. Víctor Ramos proposes instead that the Puncoviscana Basin was a

foreland basin A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...

located west of a "Pampia block" that collided with Río de la Plata Craton. Contrasting to this view, Aceñolaza and Toselli contend instead that the Puncuviscana Basin originated from an

splitting the Arequipa-Antofalla Craton from the

Río de la Plata The Río de la Plata (; ), also called the River Plate or La Plata River in English, is the estuary formed by the confluence of the Uruguay River and the Paraná River at Punta Gorda, Colonia, Punta Gorda. It empties into the Atlantic Ocean and ...

and Guaporé cratons. Following this interpretation the aulacogen would have closed during the Pampean orogeny. A 2011 study argues that the Puncoviscana Formation deposited in either a

forearc basin A forearc is a region in a subduction zone between an oceanic trench and the associated volcanic arc. Forearc regions are present along convergent margins and eponymously form 'in front of' the volcanic arcs that are characteristic of convergent ...

or a trench-slope basin associated with a

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc ...

in the western margin of

. The same study suggest that a Puncoviscana Ocean formed in the

Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

as the

Arequipa-Antofalla Arequipa-Antofalla is a basement unit underlying the central Andes in northwestern Argentina, western Bolivia, northern Chile and southern Peru. Geologically, it corresponds to a craton, terrane or block of continental crust. Arequipa-Antofalla ...

drifted away from the Amazonian continent. The opening of the Puncoviscana Ocean probably preceded the opening of the

Iapetus Ocean The Iapetus Ocean (; ) existed in the late Neoproterozoic and early Paleozoic eras of the geologic timescale (between 600 and 400 million years ago). It was in the southern hemisphere, between the paleocontinents of Laurentia, Baltica and Avalon ...

with the Iapetus Ocean being separated from the Puncoviscana Ocean by the Arequipa-Antofalla terrane. By the time of the opening of Iapetus Ocean Puncoviscana ocean was likely being closed.

Stratigraphy, lithology and fossils

Fossil content

Tectonic interpretations of the Puncoviscana Basin

See also

References

Further reading