The Parnaíba Basin ( Portuguese: ''Bacia do Parnaíba'') is a large cratonic

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 ...

located in the

North North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating Direction (geometry), direction or geography. Etymology T ...

and

Northeast The points of the compass are a set of horizontal, radially arrayed compass directions (or azimuths) used in navigation and cartography. A '' compass rose'' is primarily composed of four cardinal directions—north, east, south, and west—eac ...

portion of

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

. About 50% of its area is in the state of

Maranhão Maranhão () is a States of Brazil, state in Brazil. Located in the country's Northeast Region, Brazil, Northeast Region, it has a population of about 7 million and an area of and it is divided into 217 municipalities. Clockwise from north, it ...

, and the other 50% is in the states of

Pará Pará () is a Federative units of Brazil, state of Brazil, located in northern Brazil and traversed by the lower Amazon River. It borders the Brazilian states of Amapá, Maranhão, Tocantins (state), Tocantins, Mato Grosso, Amazonas (Brazilian st ...

Piauí Piauí ( ) is one of the states of Brazil, located in the country's Northeast Region, Brazil, Northeast Region. The state has 1.6% of the Brazilian population and produces 0.7% of the Brazilian GDP. Piauí has the shortest coastline of any coas ...

Tocantins Tocantins () is one of the 26 states of Brazil. It is the newest state, formed in 1988 and encompassing what had formerly been the northern two-fifths of the state of Goiás. Tocantins covers and had an estimated population of 1,496,880 in 2014 ...

, and

Ceará Ceará (, ) is one of the 26 states of Brazil, located in the Northeast Region, Brazil, northeastern part of the country, on the Atlantic Ocean, Atlantic coast. It is the List of Brazilian states by population, eighth-largest Brazilian State by ...

. It is one of the largest

Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...

basins in the South American Platform. The basin has a roughly

ellipsoidal An ellipsoid is a surface that can be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface; that is, a surface that may be defined as the ze ...

shape, occupies over 600,000 km², and is composed of ~3.4 km of mainly

sedimentary rock that overlies localized rifts. The basin as named after the

Parnaíba River The Parnaíba River ( ) is a river in Brazil, which forms the border between the states of Maranhão and Piauí. Its main course is long and the Parnaíba River Basin covers .Ramos, T.P.A.; Ramos, R.T.C.; and Ramos, S.A.Q.A. (2014). Ichthyofaun ...

, which is approximately long, and runs relatively parallel to the major axis of the basin.

Regional Setting

It is located between the Amazonian Craton to the west and the

São Francisco Craton SAO or Sao may refer to: Places * Sao civilisation, in Middle Africa from 6th century BC to 16th century AD * Sao, a town in Boussé Department, Burkina Faso * Serb Autonomous Regions (''Srpska autonomna oblast'', SAO), during the breakup of ...

to the south east. The São Luís Craton sits north of the basin and the Borborema Province is to the east. The basin currently covers a Precambrian basement composed of

Archean The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...

to early

Proterozoic The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...

cratonic blocks, Late Proterozoic Brasiliano/Pan-African fold belts and basement inliers.de Castro, D.L., Oliveira, D.C. & Hollanda, M.H.B.M. Geostatistical Interplay Between Geophysical and Geochemical Data: Mapping Litho-Structural Assemblages of Mesozoic Igneous Activities in the Parnaíba Basin (NE Brazil). ''Surv Geophys'' 39, 683–713 (2018). The existence of a Parnaíba block was hypothesized from geophysical evidence,

petrography Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. The clas ...

geochronology Geochronology is the science of Chronological dating, determining the age of rock (geology), rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, ...

of the basement rocks, and from collisional tectonic models. It was regarded as one of the

continental fragment Continental crustal fragments, partly synonymous with microcontinents, are pieces of continents that have broken off from main continental masses to form distinct islands that are often several hundred kilometers from their place of origin. Caus ...

s inherited by the South American platform after the dispersal of the Rodinia supercontinent to form the Gondwana supercontinent. The

and São Luis Craton existed before the opening of the

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

times, were part of larger cratonic landmasses, and probably surrounded a central Parnaíba block presently concealed under the basin's sediments. To the west, the Araguaia suture zone represents the final

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 ...

collision between the Amazonian craton and the Parnaíba block and, to the east, the Transbrasiliano Lineament marks the border with the Borborema Province.Reinhardt A. Fuck, Benjamim Bley Brito Neves, Carlos Schobbenhaus, Rodinia descendants in South America, Precambrian Research, Volume 160, Issues 1–2, 2008, Pages 108-126, ISSN 0301-9268, .

Depositional History

Stratigraphy of the Parnaíba basin contains depositional sequences, varying in age from Silurian to Cretaceous, marked by a progressive change in depositional environment from marine to continental. Like many other cratonic basins, the Parnaíba Basin displays a polyphase sedimentary history.Tozer, B., A. B. Watts, and M. C. Daly (2017), Crustal structure, gravity anomalies, and subsidence history of the Parnaíba cratonic basin, Northeast Brazil, J. Geophys. Res. Solid Earth, 122, 5591–5621, There are five distinct and recognizable tectonostratigraphic (TS) units that are separated by basin-wide

unconformities An unconformity is a buried erosion surface, erosional or non-depositional surface separating two Rock (geology), rock masses or Stratum, strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer ...

that make up the area. These TS units include: the Riachão unit (TS-1), the Jaibaras unit (TS-2), the Parnaíba unit (TS-3), the Mearim unit (TS-4), and the Grajau unit (TS-5). The sedimentary rocks vary greatly throughout the stratigraphy of the basin and depsostional environment changes, and includes

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 ...

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

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 ...

, conglomerate,

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

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) ...

, and more. Two magmatic pulses can be seen in the rock record, and are known as the Mosquito Formation and the Sardinha Formation. Most of the magmatic rocks are sub-alkaline tholeiitic basalts, and occur as

dikes Dyke or dike may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), formations of magma or sediment that cut through and across the layering of adjacent rocks * Dike (mythology), ''Dikē'', the Greek goddess ...

and sills mainly within the

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

and as magmatic flows in the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

and, less commonly, in the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

.Oliveira, A. L. (2017). Geologia e Petrologia dos basaltos das formações Mosquito e Sardinha, Bacia do Parnaíba.

Grajau Unit (TS-5)

* Cretaceous (deposition between ~120 and 95 million years ago) *Comprises the Corda, Grajau, Codo, and Itapecuru Formations * Deposition of near-shore, shallow marine, and fluviolacustrine clastic sediments has been attributed to subsidence associated with early opening of the South Atlantic

Sardinha Formation

* Middle Cretaceous (emergence between ~130 and 125 million years ago) *Occur mainly as

diabase Diabase (), also called dolerite () or microgabbro, is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-gra ...

and sills and as minor basaltic flows with higher alkali content when compared to the other magmatic pulse in the basin, the Mosquito Formation * Interpreted as a second phase of basaltic magmatism manifesting itself as extensive sills interlocking basin strata that is attributed to the early opening of the South Atlantic Ocean

Mearim Unit (TS-4)

* Late Jurassic (deposition between ~165 and 155 million years ago) *Comprises the

Pastos Bons Formation The Pastos Bons Formation is a Late Jurassic ( Oxfordian to Kimmeridgian) geologic formation of the Parnaíba Basin in Maranhão, northeastern Brazil. The formation forms part of the sag phase of the basin. It overlies the Sardinha Formation an ...

* Deposition of sandstone in a sabkha environment that is restricted to the center of the basin and undergoes of central "sag"Góes, A. M. O., W. A. S. Travassos, and K. C. Nunes (1993), Projeto Parnaiba: Reavaliacao e perspectivas exploratorias, paper presented at Relatorio Petrobras, DEXNOR-DINTER.

Mosquito Formation

* Early Jurassic (emergence between ~205 and 185 million years ago) *Formed mainly by

lava flows Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or unde ...

that are occasionally interbedded with sandstones with lower alkali content when compared to the other magmatic pulse in the basin, the Sardinha Formation * Interpreted as a phase of extrusive volcanism with the deposition of basaltic lava flows resulting from the

Central Atlantic magmatic province The Central Atlantic magmatic province (CAMP) is the Earth's largest continental large igneous province, covering an area of roughly 11 million km2. It is composed mainly of basalt that formed before Pangaea broke up in the Mesozoic Era, near the ...

and the opening of the central Atlantic Ocean

Parnaíba Unit (TS-3)

* Late Ordovician/Silurian to Early Triassic (deposition between ~445 and 220 million years ago) * Interpreted as the product of basinal "sag" or cratonic basin subsidence * Consists of three megasequences, separated by regional unconformities and comprising shallow marine, fluviolacustrine, and terrestrial siliciclastic sediments Balsas megasequence * Late Carboniferous to Middle Triassic (deposition between ~310 and 220 million years ago) * Comprises the Piaui, Pedra de fogo, Motuca, and Sambaiba Formations * Consists of clastic sandstone and

evaporites An evaporite () is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean ...

deposited in subaerial environments Caninde megaseqeuence * Lower Devonian to Carboniferous (deposition between ~400 and 330 million years ago) * Comprises the Itaim, Pimenteiras, Cabecas, Longa, and Poti Formations * Consists of interbedded shallow marine and deltaic depositional environments Serra Grande megasequence * Early Silurian to Lower Devonian (deposition between ~440 and 405 million years ago) * Comprises the Ipu, Tiangua, and Jaicos Formations * Consists of quartz arenites of fluvioglacial, glacial marine, and shallow marine sediments that account for a complete transgressive/regressive cycle

Jaibaras Unit (TS-2)

* Cambrian (deposition between ~530 and 485 million years ago) *Comprises the Jaibras Group, which is the Massapé, Pacujá, Parapuí and Aprazível Formations * Interpreted as a rift infill resulting from a prevalent Cambro-Ordovician rifting event or pull-apart infill resulting from dextral

strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

motion on the Transbrasiliano shear zone during the Cambrian/Early Ordovician

Riachão Unit (TS-1)

* Late Neoproterozoic to Cambrian (deposition between ~570 and 515 million years ago) *Comprises the Riachão Package, which is Riachão I, II and III Sequences * Interpreted as a Neoproterozoic rift, or a remnant

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 ...

, bounded by thick skinned thrust faults of Cambro-Ordovician age

Tectonic Evolution

The Parnaíba basin is located above a complex basement comprising several Archaean and Proterozoic terranes that stabilized during the

Brasiliano orogeny Brasiliano orogeny or Brasiliano cycle ( and ''Ciclo Brasiliano'') refers to a series of orogenies from the Neoproterozoic era, exposed chiefly in Brazil but also in other parts of South America. The Brasiliano orogeny is a regional name for the l ...

Seismic reflection Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismi ...

data reveal a remnant basin beneath the major pre-Silurian

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 ...

and the base of the sediments of the Parnaíba cratonic basin. This pre-Silurian basin, known as the Riachão basin, covers an area of at least 35,000 km² along a north–south trend. The remnant basin is thought to have originated as a

, and is poorly constrained between 574 and 500 million years ago.

The Riachão basin

The Riachão basin has sedimentation that reaches a thickness of 4 km at its maximum, and is composed of three seismostratigraphic sequences (Riachão I, II and III). These sequences are defined by onlaps and erosive unconformities. The Riachão III sequence is a wedge-shaped package of low-amplitude continuous reflectors that onlap the top of the Riachão II sequence to the west. These clastic sediments were deposited in the second foreland phase to the east. The Riachão II sequence is a wedge-shaped package of very low-amplitude discontinuous reflectors that onlap the top of the Riachão I sequence to the east. These sediments were deposited in the first foreland phase, centered in the west. The Riachão I sequence is a thin, banded package of high-amplitude reflections that extend across most of the basin area and define a broad arch. This is interpreted as a carbonate marine platform sequence that took place before any foreland subsidence or clastic sediment input. The Phanerozic Parnaíba basin sits on top of the remnant Riachão basin sequences, and shows no signs of deformation from thrust faulting. An intense erosional event took place after the deposition of the Riachão basin sequences that ultimately created a profound pre-Silurian unconformity. The subsidence and deposition of the Parnaíba basin sediments occurred after this event and have little to do with the previous development and deformation of the Riachão basin.

Modern Parnaíba basin

The current Parnaíba basin has maximum thickness of 3.5 km in its center, covers a Precambrian basin composed of Archean-Early Proterozoic cratonic blocks, consists of phanerozoic sediment, and is termed a cratonic sag basin. Low-to-moderate

thermal subsidence In geology and geophysics, thermal subsidence is a mechanism of subsidence in which conductive cooling of the mantle thickens the lithosphere and causes it to decrease in elevation. This is because of thermal expansion: as mantle material cools ...

caused the tectonostratigraphic units to become increasingly thicker in the central portion of the basin, which produces a saucer shape typical of cratonic sag basins. Tectonic subsidence curves through the Parnaíba Unit, done by backstripping data from 21 different wells, show an exponentially decreasing subsidence profile over 300 million years.Tribaldos, V. R., & White, N. (2018). Implications of preliminary subsidence analyses for the Parnaíba cratonic basin. ''Geological Society, London, Special Publications'', ''472''(1), 147-156. Thermal time constraints of this subsidence range from 70 to 90 million years ago. The underlying subsidence appears largely continuous throughout the basin's history, though there are minor deviations associated with regional unconformities. These variations in subsidence can be interpreted as brief uplift events from

epeirogenic movement In geology, epeirogenic movement (from Greek ''epeiros'', land, and ''genesis'', birth) is upheavals or depressions of land exhibiting long wavelengths and little folding apart from broad undulations. The broad central parts of continents ar ...

caused by changing patterns of dynamic topography.Daly, M. C., Fuck, R. A., Julià, J., Macdonald, D. I., & Watts, A. B. (2018). Cratonic basin formation: a case study of the Parnaíba Basin of Brazil. ''Geological Society, London, Special Publications'', ''472''(1), 1-15.

References

{{reflist Sedimentary basins of South America Parnaíba Geology of Brazil