The Riachuelo Formation is a geologic

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 secondary ...

of the

Early Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Ea ...

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

( Late Aptian to Cenomanian) age in 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 ...

Sergipe-Alagoas Basin The Sergipe-Alagoas Basin is a continental margin basin in the Sergipe and Alagoas states of northeastern Brazil, about 20 to 50 kilometres wide onshore, but with its widest extension offshore, more precisely 13,000 km2 onshore and 40,000&nbs ...

. It is the first Formation of the Basin to contain sediments deposited under fully marine conditions. The formation is subdivided into three members: Angico, Taquari and Maruim. The formation has provided fossils of

ammonites Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttle ...

gastropods The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. ...

bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biv ...

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, ...

s, serpulids, equinoderms, ostracoids,

radiolaria The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The elab ...

ns,

lobster Lobsters are a family (Nephropidae, synonym Homaridae) of marine crustaceans. They have long bodies with muscular tails and live in crevices or burrows on the sea floor. Three of their five pairs of legs have claws, including the first pair, ...

s and Crabs. Several fish species reported from the

Santana Formation The Santana Group is a geologic group, formerly included as the middle part of the Araripe Group, in the Araripe Basin of northeastern Brazil. The group comprises the Crato, Ipubi and Romualdo Formations and is dated to the Aptian to Albian stag ...

also occur in this formation, namely: ''Cladocyclus gardneri'', ''Neoproscinetes penalvai'', ''Notelops brama'', ''Rhacolepis buccalis'', ''Tharrhias araripis'' and ''Vinctifer comptoni''. ''Santanichthys diasii'' is also reported from the Taquari Member of the Riachuelo Formation. Teeth and vertebrae of

elasmobranchii Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including sharks (superorder Selachii), rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of g ...

have also been found in the formation.

Description

The Riachuelo Formation is a 500 metres (1,600 ft) thick mix of Aptian and Albian sediments formed by high-energy shelf carbonate deposits (oncolitic-oolitic-peloidal-bioclastic packstones and grainstones), interbedded with lagoonal mudstones and siltstones, and fan-delta like siliciclastic rocks. It overlies Muribeca Formation and underlies the Cotinguiba Formation. The formation sprouts in a track approximately 20 km wide and it extends itself between the cities of Itaporanga d’Ajuda and Pacatuba. It also has outcrops in the cities of Riachuelo, Santa Rosa de Lima,

Divina Pastora Divina Pastora is a municipality located in the Brazilian state of Sergipe. The Parish Church of Our Lady of the Divine Shepherd (''Igreja Matriz de Nossa Senhora Divina Pastora''), a listed historic structure by the National Institute of Histor ...

Laranjeiras Laranjeiras (, ''orange trees'') is an upper-middle-class neighborhood located in the South Zone of Rio de Janeiro, Brazil. Primarily residential, It is one of the city's oldest neighborhoods, having been founded in the 17th century, with the ...

Rosário do Catete Rosário do Catete is a municipality located in the Brazilian state of Sergipe. Its population was 11,008 (2020) and its area is 105 km². References Municipalities in Sergipe {{Sergipe-geo-stub ...

, Maruim, General Maynard and

Carmópolis Carmópolis is a municipality located in the Brazilian state of Sergipe. Its population was 16,937 (2020). Carmópolis covers and has a population density of 294.15 inhabitants per square kilometer. It is located from the state capital of Sergip ...

. There are three members of this formation: Angico Member, whose environment has been interpreted as costal areas consisting of the basin margins and grabens, deltaic fans formed, and the coarse sediments they carried, the Maruim Member, in the lower sedimentation areas,with carbonate ramps and dolomitized oolite/oncolite banks deposited under fluctuating sea level conditions, and the Taquari Member, corresponding to calcareous mudstone and shale from lagoonal and slope environments. Riachuelo Formation cross section

Ammonites are particularly plentiful and have provided a firm biocronostratigraphic control for the basin. The succession of ammonites is divided in five zones: The Epicheloniceras-Diadochoceras-Eodouvilleiceras zone, from the Aptian, the Douvilleiceras zone, from the lower Albian, the Oxytropidoceras zone, from the middle Albian, and the Elobiceras and Mortoniceras-Neokentroceras zones, both from the late Albian.

History

The name Riachuelo Formation was used for the first time in the geological literature by

Moraes Rego Morais or Moraes (the latter is an archaic spelling in Portugal, but contemporary in Brazil and India — or for both variants) is a Portuguese surname. The link between Morais/Moraes and the Spanish surname Morales is controversial. Notable p ...

in 1929. Then, in 1959, Bender made a new interpretation, separating the upper part of the Riachuelo Formation from the lower one, naming it Maruim. This lower unit described by Bender was considered by Beurlen K. in 1963 as only one facies of the Riachuelo-Maruim Complex. In 1970, Schaller analyzed the unit again and segmenting it into four members: Angico, Taquari, Maruim and Aguilhada. Feijó, in 1994, classified the formation as a siliciclastic-carbonatic platform complex and divided it into the members: Angico, Taquari and Maruim, which is the classification currently used.

Fossil content

Molluscs

Echinoderms

Annelids

Crustaceans

Fishes

References

{{Reflist Geologic formations of Brazil Cretaceous Brazil Aptian Albian Cenomanian