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32-792: The basin as named after the Parnaíba River , which is approximately 1,400 km (870 mi) long, and runs relatively parallel to the major axis of the basin. It is located between the Amazonian Craton to the west and the São Francisco Craton to the south east. The São Luís Craton sits north of the basin and the Borborema Province is the east. The basin currently covers a Precambrian basement composed of Archean–Early Proterozoic cratonic blocks, Late Proterozoic Brasiliano/Pan-African fold belts and basement inliers. The existence of

64-456: A 'loaded' column that represents a sedimentary unit accumulated over a certain geological time period, and a corresponding 'unloaded' column that represents the position of the underlying basement without the effects of the sediments. In the scenario, the pressure at the base of the loaded column, is given by: where W d {\displaystyle W_{d}} is the water depth of deposition, c {\displaystyle c}

96-568: A Parnaíba block was hypothesized from geophysical evidence, petrography, geochronology of the basement rocks, and from collisional tectonic models. It was regarded as one of the continental fragments inherited by the South American platform after the dispersal of the Rodinia supercontinent to form the Gondwana supercontinent . The São Francisco Craton and São Luis Craton existed before the opening of

128-565: A complex basement comprising several Archaean and Proterozoic terranes that stabilized during the Brasiliano orogeny. Seismic reflection data reveal a remnant basin beneath the major pre-Silurian unconformity 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 km along a north–south trend. The remnant basin

160-437: 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

192-455: Is considered an Important Bird Area and is home to species such as scarlet ibis ( Eudocimus ruber ) and rufous crab hawk ( Buteogallus aequinoctialis ). Two species of aquatic turtles are endemic to the general region, the Maranhão slider ( Trachemys adiutrix ) that is found in the delta and the side-necked turtles Mesoclemmys perplexa found inland, but neither is entirely restricted to

224-549: Is performed by stepwise removal of the top layer at any one stage during the analysis and performing back-stripping as if for a single layer case. For the remaining column, mean densities and thickness must be used at each time, or calculation, step. Equation ( 2 ) then becomes the tectonic amount of subsidence during sedimentation of the top most layer only. In this case L ∗ {\displaystyle L^{*}} and ρ L {\displaystyle \rho _{L}} can be defined as

256-440: Is possible to deduce information on the basin forming mechanisms. The technique developed by Watts & Ryan in 1976 allows for the recovery of the basement subsidence and uplift history in the absence of sediment and water loading and, therefore isolate the contribution from the tectonic forces responsible for the formation of a rift basin. It is a method by which successive layers of basin fill sediment are "stripped off"

288-621: Is termed a cratonic sag basin. Low to moderate thermal subsidence 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. Thermal time constraints of this subsidence range from 70 to 90 million years ago. The underlying subsidence appears largely continuous throughout

320-432: Is the decompacted sediment thickness, ρ s {\displaystyle \rho _{s}} is the mean sediment density, W d {\displaystyle W_{d}} is the average depth at which the sedimentary units were deposited, ρ w {\displaystyle \rho _{w}} and ρ m {\displaystyle \rho _{m}} are

352-570: Is the freshwater stingray Potamotrygon signata . More than 70% of the fish species in the basin are members of either Characiformes or Siluriformes . The fauna in the headwaters has not yet been thoroughly surveyed. In addition to the natives, there are seven introduced fish species in the basin. The river delta is protected by the 313,800 hectares (775,000 acres) Delta do Parnaíba Environmental Protection Area , created in 1996. The river's delta has large stands of mangrove , as well as dunes, beaches, marshes and other habitats. The delta

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384-439: Is the mean thickness of the crust, S {\displaystyle S} is the sediment thickness corrected for compaction, g {\displaystyle g} is the average gravity and ρ w {\displaystyle \rho _{w}} , ρ s {\displaystyle \rho _{s}} and ρ c {\displaystyle \rho _{c}} are

416-435: Is the porosity at the surface and c {\displaystyle c} is a rock specific compaction constant. The fundamental equation in back-stripping corrects the observed stratigraphic record for the effects of sediment and water loading and changes in water depth, and is given by: where Y {\displaystyle Y} is the tectonically driven subsidence, S {\displaystyle S}

448-460: Is thought to have originated as a foreland basin , and is poorly constrained between 574 and 500 million years ago. 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

480-707: The Atlantic Ocean in Mesozoic 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 collision between the Amazonian craton and the Parnaíba block and, to the east, the Transbrasiliano Lineament marks the border with the Borborema Province. Stratigraphy of

512-523: The Portuguese : Boa Esperança Hydroelectric Power Plant dam, but is otherwise navigable. The fish species richness in the Parnaíba River Basin has traditionally been considered impoverished, but this has been disproven by recent surveys, which have recorded about 140 native species (including several that remain undescribed ) and about 40% of these are endemic . One of the basin endemics

544-619: The Parnaíba River Basin. Back-stripping Back-stripping (also back stripping or backstripping) is a geophysical analysis technique used on sedimentary rock sequences. It is used to quantitatively estimate the depth that the basement would be in the absence of sediment and water loading. This depth provides a measure of the unknown tectonic driving forces that are responsible for basin formation (otherwise known as tectonic subsidence or uplift). By comparing backstripped curves to theoretical curves for basin subsidence and uplift it

576-469: 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. There are five distinct and recognizable tectonostratigraphic (TS) units that are separated by basin-wide unconformities that make up

608-555: 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 enciroment changes, and includes shale, siltstone, mudstone, sandstone, conglomerate, breccia, limestone, and more. Two magmatic pulses can be seen in

640-412: The basin's deepening history can be plotted in reverse, leading to clues as to its tectonic or isostatic origin. A more complete analysis uses decompaction of the remaining sequence following each stage of the back-stripping. This takes into account the amount of compaction caused by the loading of the later layers and allows a better estimation of the depositional thickness of the remaining layers and

672-454: The basin's history, there are minor deviations associated with regional disconformities. These variations in subsidence can be interpreted as brief uplift events from epeirogenic movement caused by changing patterns of dynamic topography. Parna%C3%ADba River The Parnaíba River ( Portuguese : Rio Parnaíba [paɾnaˈibɐ] ) is a river in Brazil , which forms the border between

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704-502: The densities of the water and mantle respectively, and Δ S L {\displaystyle \Delta _{SL}} the difference in sea-level height between the Present and the time at which the sediments were deposited. The three independent terms account for the contributions of sediment loading, water depth and sea-level oscillations to the subsidence of the basin. To derive equation ( 2 ) one should first consider

736-408: The densities of water, the sediment and the crust respectively. The pressure at the base of the unloaded column is given by: where Y {\displaystyle Y} is the tectonic or corrected subsidence, ρ m {\displaystyle \rho _{m}} is the density of the mantle, and b {\displaystyle b} is the distance from the base of

768-486: 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. 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

800-394: The porosity must be known. Empirical studies show that the porosity of rocks decreases exponentially with depth. In general we can describe this with the relationship: where ϕ {\displaystyle \phi } is the porosity of the rock at depth z {\displaystyle z} , ϕ 0 {\displaystyle \phi _{0}}

832-699: 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 dykes and sills mainly within the Silurian to Carboniferous and as magmatic flows in the Jurassic and, less commonly, in the Cretaceous Balsas megasequence Caninde megaseqeuence Serra Grande megasequence The Parnaíba basin is located above

864-533: The states of Maranhão and Piauí . Its main course is 1,400 km (870 mi) long and the Parnaíba River Basin covers 330,000 km (130,000 sq mi). The Parnaíba River rises in the Chapada das Mangabeiras range, and flows northeastward to empty into the Atlantic Ocean , being the longest river entirely located within Brazil's Northeast Region . The middle and upper regions of this river are separated by

896-400: The thickness and density of the entire remaining sedimentary column after removal of the top layer l {\displaystyle l} (i.e. the decompacted thickness). The thickness of a sediment pile with l {\displaystyle l} layers is then: The density of the sedimentary column underneath layer l {\displaystyle l} is given by

928-452: 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

960-414: The total stratigraphy during analysis of that basin's history. In a typical scenario, a sedimentary basin deepens away from a marginal flexure , and the accompanying isochronous strata typically thicken basinward. By isolating the isochronous packages one-by-one, these can be "peeled off" or backstripped - and the lower bounding surface rotated upward to a datum. By successively backstripping isochrons,

992-451: The unloaded crust to the depth of compensation (which is assumed to be at the base of the loaded crust) and is given by: Substitution of ( 3 ),( 4 ) and ( 5 ) after simplifying, we obtain ( 2 ). For a multi-layered sedimentary basin, it is necessary to successively back-strip each individually identifiable layer separately to obtain a complete evolution of the tectonic subsidence. Using equation ( 2 ),a complete subsidence analysis

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1024-422: The variation of water depth with time. As a result of their porosity, sedimentary strata are compacted by overlaying sedimentary layers after deposition. Consequently, the thickness of each layer in a sedimentary sequence was larger at the time of its deposition than it is when measured in the field. In order to consider the influence of sediment compaction on the thickness and density of the stratigraphic column,

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