Puncoviscana Formation ( es, Formación Puncoviscana) 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 secondar ...

sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentati ...

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 naturally occurring material that is broken down by processes of weathering and er ...

s Late Ediacaran and

Lower Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ( ...

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

Argentine Northwest The Argentine Northwest (''Noroeste Argentino'') is a geographic and historical region of Argentina composed of the provinces of Catamarca, Jujuy, La Rioja, Salta, Santiago del Estero and Tucumán. Geography The Argentine Northwest comprises v ...

. 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 Argentine province of the same name. With a population of 618,375 according to the 2010 census, it is also the 7th most-populous city in Argentina. The city serves as the cultural and economic ce ...

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. Neigh ...

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 ...

near Córdoba. 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 oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting creat ...

al basin of the ancient continent

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...

. 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-grabe ...

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. Specifically, aulacogens are a ri ...

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 own ...

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. It was one of the first terranes to be amalgamated to the old ...

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 minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

s, conglomerates,

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

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 rock. ...

s and

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...

s. Stratigraphically, 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 o ...

, which is overlain by

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

and

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

sedimentary rocks of the

Mesón Group Mesón Group ( es, Grupo Mesón) is a Cambrian to Early Ordovician sedimentary formation located in the Argentine Northwest and nearby parts of Bolivia , image_flag = Bandera de Bolivia (Estado).svg , flag_alt ...

that extend across the Argentine Northwest and

Bolivia , image_flag = Bandera de Bolivia (Estado).svg , flag_alt = Horizontal tricolor (red, yellow, and green from top to bottom) with the coat of arms of Bolivia in the center , flag_alt2 = 7 × 7 square p ...

Fossil content

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

Beltanelloides ''Beltanelliformis'' is a genus of discoid fossil from the Ediacaran period containing the two species B. brunsae and B. minutae, sometimes ascribed to the Ediacaran Biota. The chemical signature obtained from organically preserved specimens po ...

'' * '' Paliela'' * ''

Selkirkia ''Selkirkia'' is a genus of predatory, tubicolous priapulid worms known from the Middle Cambrian Burgess Shale, Ogygopsis Shale and Puncoviscana Formation. 142 specimens of ''Selkirkia'' are known from the Greater Phyllopod bed, where they compr ...

'' ;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 hos ...

Tectonic interpretations of the Puncoviscana Basin

Positions of ancient continents, 550 million years ago

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 (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...

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

Pampean orogeny The Pampean orogeny ( es, orogenia pampeana) 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 an ...

. 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 (, "river of silver"), 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. It empties into the Atlantic Ocean and fo ...

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 Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the vol ...

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 lo ...

in the western margin of

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

Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is ...

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 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