Riachuelo Formation

The Riachuelo Formation is a 500 metres (1,600 ft){{Cite journal |last1=Bengtson |first1=P. |last2=Zucon |first2=M. H. |last3=Sobral |first3=A. da C. S. |date=2018 |title=Cretaceous ammonite zonation of the Sergipe Basin, northeastern Brazil |url=https://www.sciencedirect.com/science/article/abs/pii/S019566711730215X |journal=Cretaceous Research |volume=88 |pages=111–122 |doi=10.1016/j.cretres.2017.07.021 |bibcode=2018CrRes..88..111B |s2cid=134100757 |via=}} 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.{{Cite journal |last1=Turbay |first1=Caio V.G. |last2=Cesero |first2=Pedro |last3=Koutsoukos |first3=Eduardo A. |title=On a fossil lobster (Decapoda, Malacostraca, Crustacea) from the Riachuelo Formation, Albian of the Sergipe Basin, Brazil |date=2002 |journal=An. Acad. Bras. Ciênc. |volume=74 |issue=2|page=363 |doi=10.1590/S0001-37652002000200013 |doi-access=free }} It overlies Muribeca Formation and underlies the Cotinguiba Formation.{{Cite journal |last1=MANSO |first1=C. L. de C. |last2=SOUZA-LIMA |first2=Wagner |date=2012 |title=NOVOS EQUINÓIDES PARA O INTERVALO APTIANO-ALBIANO (CRETÁCEO INFERIOR) DA SUB-BACIA DE SERGIPE, NORDESTE DO BRASIL |journal=Geociências |volume=31 |issue=4 |pages=584–605}} 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, Laranjeiras, Rosário do Catete, Maruim, General Maynard and Carmópolis.{{Cite journal |last1=da Silva |first1=C. R. M. |last2=Barros |first2=S. D. S. |date=2017 |title=AVALIAÇÃO DO POTENCIAL DE FOSFATO NO BRASIL – PARTE III: BACIA SERGIPE-ALAGOAS, ÁREA SUB-BACIA SERGIPE ESTADO DE SERGIPE |journal=Serviço Geológico do Brasil |issue=20}}

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.{{Cite journal |last1=Antonietto |first1=Lucas S. |last2=do Carmo |first2=Dermeval A. |title=Biostratigraphic and paleozoogeographic review of the upper Aptian-Albian ostracods of Riachuelo Formation, Sergipe-Alagoas Basin, Northeastern Brazil |date=2015 |url=https://www.researchgate.net/publication/289556930 |journal=Sociedade Brasileira de Paleontologia |volume=18 |issue=3 |pages=355–368|doi=10.4072/rbp.2015.3.02 |doi-access=free }}

File:Riachuelo Formation cross section.png

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.

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

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Category:Geologic formations of Brazil

Category:Cretaceous Brazil

Category:Aptian

Category:Albian

Category:Cenomanian