Riachuelo Formation

The Riachuelo Formation is a geologic formation of the Early to Late Cretaceous (Late Aptian to Cenomanian) age in northeastern Brazil's Sergipe-Alagoas Basin. 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.^[1]

Riachuelo Formation
Stratigraphic range: Late Aptian-Early Cenomanian ~115–97 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Reconstruction of the fauna from the Riachuelo Formation
Type	Geological formation
Sub-units	Angico Member Taquari Member Maruim Member
Underlies	Cotinguiba Formation
Overlies	Muribeca Formation
Thickness	500 m (1,600 ft)
Lithology
Primary	Packstone, Grainstone
Other	Mudstone, siltstone, siliciclastic
Location
Region	Sergipe-Alagoas Basin of Northeastern Brazil
Country	Brazil
Extent	20 kilometres (12 mi)
Type section
Named for	Riachuelo, Sergipe
Named by	Moraes Rego (1929)

The formation has provided fossils of ammonites, gastropods, bivalves, brachiopods, serpulids, equinoderms, ostracoids, radiolarians, lobsters and Crabs. Several fish species reported from the Santana Formation also occur in this formation, namely: Cladocyclus gardneri, Neoproscinetes penalvai, Notelops brama, Rhacolepis buccalis, Tharrhias araripis and Vinctifer comptoni.^[2][3][4][5] Santanichthys diasii is also reported from the Taquari Member of the Riachuelo Formation.^[6] Teeth and vertebrae of elasmobranchii have also been found in the formation.^[7]

Description

The Riachuelo Formation is a 500 metres (1,600 ft)^[8] 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.^[9] It overlies Muribeca Formation and underlies the Cotinguiba Formation.^[10] The formation sprouts in a track approximately 20 km wide and it extends itself between the cities of Itaporanga d’Ajuda and Pacatuba.^[10] It also has outcrops in the cities of Riachuelo, Santa Rosa de Lima, Divina Pastora, Laranjeiras, Rosário do Catete, Maruim, General Maynard and Carmópolis.^[11]

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.^[5][11]

Cross section of riachuelo formation's shelf and sea.

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.^[1]

History

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.^[4]

Fossil content

Molluscs

Molluscs of the Riachuelo Formation
Genus	Species	Presence	Family	Images
Douvilleiceras[12]	D. sergipensis D. euzebioi		Douvilleiceratidae
Puzosia[12]	P. rosarica P. brasiliana P. garajuana		Desmoceratidae
Neokentroceras[12]	N. tectorium		Brancoceratidae
Elobiceras[4]	E. lobitoense E. raymondi		Brancoceratidae
Oxytropidoceras[4]	O. buarquianum O. sergipensis O. involutum O. mauryae		Brancoceratidae
Mortoniceras[4]	M. sergipensis M. rostratum		Brancoceratidae
Vectisites[13]	V. simplex
Eodouvilleiceras[13]			Douvilleiceratidae
Cheloniceras[14]			Douvilleiceratidae
Diadochoceras[14]			Parahoplitidae
Deshayesites[14]			Deshayesitidae
Anisoceras[14]			Anisoceratidae
Protanisoceras[14]			Anisoceratidae
Inoceramus[12]	Indeterminate		Inoceramidae
Neocomiceramus[4]	N. anglicus		Inoceramidae
Neithea[2]	N. alpina N. coquandi N. hispanica		Pectinidae
Linotrigonia[13]			Trigoniidae
Pleuromya[13]			Pleuromyidae
Pinna[13]			Pinnidae
Lopha[13]			Ostreidae
Turritella[13]			Turritellidae
Tylostoma[13]			Tylostomatidae
Natica[13]			Naticidae

Echinoderms

Echinoderms of the Riachuelo Formation
Genus	Species	Presence	Family	Images
Stereocidaris[15]	S. branneri		Cidaridae
Rhabdocidaris[15]	R. brasiliensis		Rhabdocidaridae
Salenia[15]	S. sergipensis		Saleniidae
Phymosoma[15]	P. binexilis P. braziliensis		Phymosomatidae
Orthopsis[15]	O. australis		Orthopsidae
Coenholectypus[15]	C. pennanus		Holectypidae
Conolypus[15]	C. nertoanus
Parapygus[15]	P. aequalis		Echinolampadidae	6 to 9 is Parapygus antillarum.
Hemiaster[15]	H. cranium		Hemiasteridae
Temnocidaris[13]			Cidaridae
Douvillaster[13]			Toxasteridae
Leptosalenia[13]			Saleniidae
Phyllobrissus[16]	P. humilis		Nucleolitidae

Annelids

Annelids of the Riachuelo Formation
Genus	Species	Presence	Family	Images
Diploconcha[17]	D. riachueloi D. scalata		Serpulidae
Serpula[17]	S. sergipensis		Serpulidae

Crustaceans

Crustaceans of the Riachuelo Formation
Genus	Species	Presence	Family	Notes	Images
Palinura[3]	Indeterminate	Angico Member	Lobster	Only posterior part of the body preserved
Brazilomunida[18]	B. brasiliensis		Galatheidae (squat lobster)
Archaeopus[17]	A. rathbunae		Ocypodidae
Cyclothyreus[17]	C. sergipensis		Dromioidea
Maurimia[18]	M. sergipensis		Dynomenidae
Galalheites[17]	G. brasiliensis	Maruim Member	Decapoda
Reticulocosta[5]	R. edrianae		Cytherettidae (ostracod)
Aracajuia[5]	A. benderi A. antiqua		An ostracod
Harbinia[5]	H. sinuata		An ostracod
Sergipella[5][19]	S. viviersae S. transatlantica S. grosdidieri	Taquari Member	An ostracod
Praebythoceratina[5]	P. amsittenensi		An ostracod

Fishes

Cartilaginous fish

Cartilaginous fish of the Riachuelo Formation
Genus	Species	Presence	Description	Notes	Images

Ray-finned fish

Ray-finned fish of the Riachuelo Formation
Genus	Species	Presence	Description	Notes	Images
Cladocyclus[2]	C. gardneri		Ichthyodectidae
Neoproscinetes[2]	N. penalvai		Pycnodontid
Mercediella[20]	M. riachuelensis		Pycnodontid fish	Originally described as Camposichthys riachuelensis[7]
Notelops[2]	N. brama		Elopiform fish
Rhacolepis[2]	R. buccalis		Crossognathiform fish
Tharrhias[2]	T. araripis		Gonorynchiform fish
Vinctifer[2]	V. comptoni		Aspidorhynchiform fish
Santanichthys[6]	S. diasii	Taquari Member	Characiform fish
Beurlenichthys[7]	B. ouricuriensis		Clupeomorph fish
Nolfia[21]	N. riachuelensis	Taquari Member	Clupeoid fish