The Santana Group is a geologic group, formerly included as the middle part of the Araripe Group, in the Araripe Basin of northeastern

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

. The group comprises the Crato, Ipubi and

Romualdo Formation The Romualdo Formation is a geologic Konservat-Lagerstätte in northeastern Brazil's Araripe Basin where the states of Pernambuco, Piauí and Ceará come together. The geological formation, previously designated as the Romualdo Member of the ...

s and is dated to the

Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ag ...

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0 ...

stages of the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145& ...

. The formations of the group were deposited in a

lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

to subtidal shallow marine

environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...

in the Araripe rift basin.Santana Group
at

Fossilworks Fossilworks is a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database, a large relational database assembled by hundreds of paleontologists from around the world. History Fossilworks was crea ...

.org The Santana Group has provided a rich assemblage of fossils; flora, fish, arthropods insects, turtles, snakes, dinosaurs, such as '' Irritator'', and pterosaurs, among which '' Thalassodromeus''. The stratigraphic units of the group contained several feathers of birds, among those the first record of

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

birds in Brazil. The Romualdo and Crato Formations are renowned for their excellent conservation and designated

Lagerstätte A Lagerstätte (, from '' Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These f ...

n. In 2006, the Araripe Basin was designated a UNESCO Global Geopark.

Description

The Santana Group was formerly described as belonging to the Araripe Group.Scherer et al., 2013, p.28 The fossiliferous Santana Formation was previously defined as containing the Crato and Romualdo Members, but redefinition of the stratigraphy led to the establishment of the Santana Group, replacing the middle part of the Araripe Group and the former Crato, Ipubi and Romualdo members were elevated to separate formations.Assine, 1992, p.291Fabin et al., 2018, p.2050

Basin history

The tectono-sedimentary evolution of the Araripe Basin, located in the Borborema Geologic Province, encompasses four stages, with five tectonostratigraphic phases:Fabin et al., 2018, pp.2051-2052
1) Syneclise phase -

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 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 shortest period of the Paleoz ...

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...

- characterized by tectonic quiescence in the Borborema Province. It is represented by the deposits of the Cariri Formation, that include medium to coarse-grained quartz

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 silicat ...

s, locally conglomeratic, deposited in large

braided Braided is a musical group consisting of Casey LeBlanc, Ashley Leitão, and Amber Fleury, who all competed on the third season of ''Canadian Idol'' in 2005. They are the third music group to come from an Idol show in the world, after Young Divas ...

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...

systems
2) Pre-rift phase -

Tithonian In the geological timescale, the Tithonian is the latest age of the Late Jurassic Epoch and the uppermost stage of the Upper Jurassic Series. It spans the time between 152.1 ± 4 Ma and 145.0 ± 4 Ma (million years ago). It is preceded by ...

- characterized by the mechanical subsidence due to lithosphere thinning that preceded the rift. It is represented by the Brejo Santo Formation, that comprises red

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, especiall ...

s and

claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too ...

s, and the Missão Velha Formation, constituted by medium to coarse-grained quartz-feldspathic sandstones, locally conglomeratic, that contains entire trunks and fragments of silicified wood ('' Dadoxilon benderi'') conifer
3) Rift phase -

Berriasian In the geological timescale, the Berriasian is an age/stage of the Early/Lower Cretaceous. It is the oldest subdivision in the entire Cretaceous. It has been taken to span the time between 145.0 ± 4.0 Ma and 139.8 ± 3.0 Ma (million years ag ...

Hauterivian The Hauterivian is, in the geologic timescale, an age in the Early Cretaceous Epoch or a stage in the Lower Cretaceous Series. It spans the time between 132.9 ± 2 Ma and 129.4 ± 1.5 Ma (million years ago). The Hauterivian is preceded by th ...

- characterized by increasing mechanical subsidence that created a system of grabens and half grabens. It is represented by the Abaiara Formation, that includes shales,

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.Blatt ''et al.'' 1980, ...

s, sandstones and conglomerates
4) Post-Rift I phase -

- characterized by thermal subsidence. The lowermost unit Barbalha Formation, represents a fluviolacustrine phase and is composed of red and gray shales, siltstones and claystones. The Santana Group was formed during this stage and comprises three stratigraphic units: * Crato Formation that is composed of six intervals of laminated

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 w ...

s (C1 to C6), interbedded with calciferous siltstones and

marl Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. Marl makes up the lower part ...

s, and is very rich in fossils of vertebrate and invertebrate organisms * Ipubi Formation, that is mainly composed of organic-rich, black greenish bituminous shales, claystones and

algal Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...

limestones that are interbedded with

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywa ...

anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the ...

beds * Romualdo Formation, that represents a calciferous siliciclastic succession composed of fine to medium-grained sandstones, argillaceous siltstones, calciferous shales, and limestones, very rich in fossils. This formation recorded the marine proto-Atlantic incursion that involved the Araripe Basin, and other interior basins and created a large seaway throughout the Borborema Province during the Albian 5) Post-rift II phase - Albian to

Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in ...

- characterized by a major sag phase, and is formed by two stratigraphic units: * Araripina Formation, that occurs in the western region of the basin and is composed by rhythmites and heterolithic layers of reddish, purplish and yellowish fine-grained sandstone and mudstone * Exu Formation, that comprises medium-to coarse grained sandstones, fine grained clayey sandstones and local conglomeratic beds

Fossil content

The upper and lower formations of the group are highly fossiliferous, with mainly insects, amphibians, fish,Fara et al., 2005, p.152Aparecida et al., 2015, p.25 and flora dominating the lower Crato Formation,Jorge de Lima et al., 2015, p.102 shrimps ('' Araripenaeus timidus''),Pinheiro, 2014, p.4 and various genera of dinosaurs and fish in the Romualdo Formation.Weishampel et al., 2004, pp.563-570Cavalcanti Duque & Franca Barreto, 2018, p.9 A rich assemblage of

pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...

fossils are found in both formations,Elgin & Frey, 2011Martill, 2007, p.895 and the Ipubi Formation provided an indeterminate Pelomedusoides turtle.Ribeiro Oliveira et al., 2011, p.62 The Santana Group provided several feathers of birds, among those the first record of

birds in Brazil.Prado et al., 2016, p.35

Gallery

Araripe Basin - depositional environments and sequence stratigraphy.jpg, Schematic development of the depositional environments Araripe Basin - lake level cycles - Crato, Ipubi and Romualdo Formations.jpg, Lake level cyclicity Santana Group - fossilized feathers and Dastilbe fossil.jpg, Fossil feathers and '' Dastilbe'' fossil

References

Bibliography

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