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, 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 ...

. The group comprises the Crato, Ipubi and Romualdo Formations and is dated to the

Aptian The Aptian is an age (geology), age in the geologic timescale or a stage (stratigraphy), stage in the stratigraphic column. It is a subdivision of the Early Cretaceous, Early or Lower Cretaceous epoch (geology), Epoch or series (stratigraphy), S ...

Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...

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 143.1 ...

. The formations of the group were deposited in a lacustrine to subtidal shallow marine environment in the Araripe rift basin.Santana Group
at Fossilworks.org The Santana Group has provided a rich assemblage of fossils; flora, fish, arthropods insects, turtles, snakes, dinosaurs including '' Irritator'', and pterosaurs such as '' Thalassodromeus''. The stratigraphic units of the group contained several feathers of birds, among those the first record of

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 ...

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

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

- 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 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, locally conglomeratic, deposited in large braided fluvial systems
2) Pre-rift phase -

Tithonian In the geological timescale, the Tithonian is the latest age (geology), age of the Late Jurassic Epoch and the uppermost stage (stratigraphy), stage of the Upper Jurassic Series. It spans the time between 149.2 ±0.7 annum, Ma and 143.1 ±0.6 (mi ...

- characterized by the mechanical subsidence due to

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...

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 mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

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

s (C1 to C6), interbedded with calciferous siltstones and marls, 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 limestones that are interbedded with gypsum- anhydrite 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 International Commission on Stratigraphy's (ICS) geological timescale, the oldest or earliest age (geology), age of the Late Cretaceous epoch (geology), Epoch or the lowest stage (stratigraphy), stage of the Upper Cretace ...

- 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 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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