Aptian / Albian ( Early Cretaceous ) paleogeography of the South Atlantic : a paleontological perspective

Manuscrito ID 30126. Recebido em: 10/06/2014. Aprovado em: 10/06/2014. ABSTRACT: Paleontological data obtained in recent years reinforce the hypothesis that Aptian marine sedimentation in the sedimentary basins of the Brazilian continental margin – except the Pelotas basin, the southernmost Brazilian basin – took place under the domain of waters coming from the north through the Tethys Sea (Central Atlantic). Tethyan waters could reach the basins of the Brazilian continental margin via the seaway then existing in the present-day region of northeastern Brazil. Here there are records in several basins, notably in the São Luís (Codó Formation), Parnaíba (Codó Formation), Araripe (Santana Formation), Tucano (Marizal Formation), Sergipe (Riachuelo Formation) and Camamu (Algodões Formation) basins. Despite irrefutable marine evidence — e.g., dinoflagellates, echinoids, foraminifera, molluscs and fishes, conspicuously present in the Araripe Basin — there are very few paleogeographic reconstructions that include the seaway which is totally ignored in the international literature. The skepticism is even greater in relation to the Tethyan affinity although the evidence has been well documented by molluscs and dinoflagellates, together with ammonoids in the Sergipe Basin. That skepticism may be due to the fact that, in tectonic and geodynamic terms, the opening of the South Atlantic indeed proceeded from south to north, at least in the part that extends from Argentina to the northeastern Brazilian state of Paraíba.


INTRODUCTION
Contrary to the paleogeographic reconstruction based on the confi guration of tectonic plates, the initial Mesozoic entrance of sea water into the BCMBs (Brazilian Continental Margin Basins, Fig. 1) resulted from the incursion of oceanic waters from the northern hemisphere, i.e., from the Tethys Sea.Th e traditional model, advocating the progressive advance of the primitive South Atlantic from south to north, is disbelieved, having been induced by the fact that the tectonic opening of the South Atlantic had occurred from south to north.However, the majority of BCMBs contain marine biotas of Aptian/Albian age -the timing of the earlier marine incursion -which indicate their Tethyan origin.Th e evidence for this, accrued over recent decades, eff ectively renders irrefutable the early Tethyan infl uence model.
For the Albian (113 -100 Ma), a similar model, proposed by Dias-Brito (1992, 2000), was based on the geographic distribution of pithonellids (pelagic calcispheres); he considered the Tethyan Realm as having a greater extension, reaching all the BCMBs, excepting the southernmost one (the Pelotas Basin) (Fig. 1).Arai (2005Arai ( , 2007Arai ( , 2011)), based on studies of organic-walled dinofl agellate cysts, recognized the presence of  Milani et al., 2007).The Brazilian Continental Margin Basins (BCMBs) are indicated by italics, and the basins discussed in this paper are indicated by bold characters.
the Tethyan Realm in the BCMBs even in the Aptian  and confirmed that Tethyan waters emanating from the north reached as far as the Santos Basin, which represents the southernmost Tethyan-influenced site in the South Atlantic.
The present paper aimed to substantiate that model by means of an inventory of fossil biotic elements that confirm Tethyan characters in the Aptian -Albian interval (125 -100 Ma).

Geological context
The dinoflagellates, which comprise the main basis of the present work, appeared worldwide in the Middle Triassic (ca.247 -237 Ma).However, Western Gondwana, of which present-day Brazil occupies a part, was undergoing an erosional phase during Triassic-Jurassic time (252 -145 Ma).The sediments deposited in these periods were accordingly of continental (non-marine) derivation, some even subaerial (non-aqueous, mostly eolian).Thus, the pre-rift stage is represented mainly by oxidized, reddish sediments (Souza- Lima & Hamsi Jr. 2003).
Through the Early Cretaceous, most of the BCMBs experienced a rift phase.Along the extensional continental margin and in the Recôncavo, Tucano, Jatobá, Araripe and Potiguar basins, rift lakes were formed during the Neocomian.In the equatorial-marginal basins west of the Potiguar Basin, rift sedimentation occurred mostly during the Aptian-Albian.In the Aptian, independently of their tectonic constitution, all of these basins received synchronous sedimentation due to an eustatic rise, which was responsible for the earliest conspicuous occurrence of Mesozoic marine organisms in Brazilian sedimentary basins.
At the maximum of the Aptian transgression, the Brazilian interior sea connected temporarily the North Atlantic and the South Atlantic that were hitherto discrete (Arai 1999(Arai , 2000) ) and resulted in the deposition of an extensive megasequence (Megasequence Zuni).In several Brazilian interior basins (e.g., Parecis, Parnaíba and São Francisco basins), Zuni represents their only Cretaceous succession (Arai 2002).These basins were affected by uplifting after the Cretaceous deposition and became "chapadas" (mesa-type elevated terrains).Contrastingly, the BCMBs were not uplifted, due to continued subsidence; all BCBMs received open marine sediments from the Albian and that process continued thereafter (to the present day).
In the Aptian, except for the Pelotas Basin, the waters of the southern South Atlantic had still not effectively entered the BCMBs.However, due to the great Aptian transgression, which caused intermittent entrance of water from the Central Atlantic, a generalized deposition took place under marine influence in the BCMB and interior basins.The intermittent introduction of sea water was responsible for the deposition of the evaporites of the Alagoas local stage (equivalent, grosso modo, to Aptian) in most of the basins.The Aptian transgression was sufficiently extensive to produce evaporites even in basins situated in the interior of the continent (e.g., Codó Formation, Parnaíba Basin; and Ipubi Member of the Santana Formation, Araripe Basin), including those toward Africa (Avocat et al. 1992).In the Pelotas Basin, evaporites are practically absent, because it was open-marine in the Aptian, when the drift phase had already commenced (Dias et al. 1995, Souza-Lima & Hamsi Jr. 2003).

MATERIALS AND METHODOLOGY
The basic paleobiogeographic model for this paper is based on a comprehensive survey of fossil dinoflagellate assemblages (i.e., protistan algae belonging to the Division Dinoflagellata, Class Dinophyceae) from Cretaceous strata of Brazil.These reveal a consistent paleobiogeographic pattern for the primitive South Atlantic (Arai 2007(Arai , 2011)).Moreover, the model has been strengthened by the inventory of other marine organisms which confirm Tethyan influence during Aptian/Albian time (see Appendix).

Marine fossils in the interior basins
The following marine organisms have proven efficacious in the study of the Brazilian interior basins: dinoflagellates, foraminifera, radiolarians, molluscs, echinoids, crustaceans, fishes and reptiles (turtles).

Dinoflagellates
There is no doubt that the Subtilisphaera Ecozone, which represents a phytoplanktonic bloom, provides the most persuasive testimony of Tethyan affinity.
This ecozone, originally defined by Regali (1989), is characterized by an assemblage having high frequency and low diversity, and is constituted overwhelmingly by species belonging to the genus Subtilisphaera.Its occurrences can be diachronic on a worldwide scale, but in Brazil it is confined to strata of Aptian age.Its distribution is evidently restricted between the 20° paleolatitudes in both hemispheres (N and S).Its northernmost occurrence is in Morocco (Below 1981) and the southernmost one is in the Almada Basin, Brazil (Lana & Pedrão 2000a, b;Pedrão & Lana 2000).In addition to these localities, typical occurrences of the Ecozone Subtilisphaera have been recorded in the following: Senegal (Jain & Millepied 1975); Maracaibo Basin, Venezuela (Colmenares 1994); São Luís Basin (Arai et al. 1994); Ceará Basin (locus typicus, Regali 1989); Potiguar Basin (Arai et al. 1994); Parnaíba Basin (Antonioli 2001, Antonioli & Arai 2002); and Araripe Basin (Arai et al. 1994).
This geographic distribution signifies the ecozone's Tethyan character.The ecozone is less conspicuous in the Potiguar Basin, thereby suggesting that the main route of the marine ingression was through the São Luís, Parnaíba and Araripe basins, where the ecozone is more prominent.
Paleoecologically, the Subtilisphaera Ecozone constitutes a fossil record of phytoplanktonic blooms, which were clearly favoured by an epicontinental marine environment (Arai et al. 1994).
In contrast to the virtually monospecific Subtilisphaera Ecozone, palynological analyses performed by Arai et al. (2006) in the lower Aptian of the Pelotas Basin have revealed a rich and highly diversified assemblage comprising spores, pollen grains, dinoflagellate cysts, acritarchs and palynoforaminifera.The following dinoflagellate-cyst (dinocyst) taxa were identified (Arai 2011)

Radiolarians
The only records of radiolarians in Brazilian interior basins come from the Areado Formation (Aptian, São Francisco Basin).The first discovery was by Kattah (1991), followed by Dias-Brito et al. (1999).

Molluscs
The gastropod species, Craginia araripensis and Gymnentone romualdoi, instituted by Beurlen (1964) from the Santana Formation, are considered to be unquestionably marine.

Crustaceans
Codoisopus brejensis belongs to the crustacean Family Archaeoniscidae (Order Isopoda) and has been recorded from the Codó Formation.Although the species was endemic, it is considered to indicate Tethyan influence, because the Family Archaeoniscidae is exclusively Tethyan (Lindoso et al. 2013a).
The Santana Formation also contains copepods (Subclass Copepoda).Well-preserved copepods, belonging to the Superfamily Dichelesthioidea, have been found in gills of the fish Cladocyclus gardneri (Cressey & Patterson 1973).On the other hand, it is possible that neither the copepod nor its host (Cladocyclus gardneri) is definitively marine in the Santana Formation (John G. Maisey, personal communication).

Fishes
Among classic fish species of the Santana Formation, Araripichthys castilhoi (Family Araripichthyidae) is surely Tethyan (Maisey & Moody 2001, Alvarado-Ortega & Brito 2011).Cratoamia gondwanica is a marine fish, of the Tribe Vidalamiini (Subfamily Vidalamiinae, Family Amiidae), described from the Crato Member (Santana Formation).Although instituted as a new species, it is considered indicative of Tethyan influence, because the Tribe Vidalamiini is unknown beyond the Tethyan Realm (Brito et al. 2008).
Nanaichthys longipinnus -a marine fish attributed to the Subfamily Rubiesichthyinae (Family Amiidae) -has been reported from the Marizal Formation (Tucano Basin).It was described as a new species, but is considered to connote Tethyan influence in view of the fact that the Subfamily Rubiesichthyinae is evidently exclusive to the Tethyan Realm (Amaral & Brito 2012).

Reptiles (Turtles)
Among fossil reptiles of the Santana Formation, there are two species of marine turtles: Araripemys barretoi and Santanachelys gaffneyi.The former was reported by Price (1973) from the Santana do Cariri region and considered to be representative of a group hitherto unrecorded outside the northern hemisphere.Araripemys barretoi was registered also from the Parnaíba Basin, in the Albian interval of the Itapecuru Formation (Kischlat & Carvalho 2000, Batista & Carvalho 2007).These authors have considered Araripemys barretoi as freshwater turtle, but Oliveira (2007) admitted that it could inhabit also marine environments.

Tethyan fossils in the Sergipe Basin
Whereas in the interior basins marine deposition was interrupted in the Albian-Cenomanian, the continental-marginal basins continued to receive marine sediments.The situation of the Sergipe Basin is particularly notable for two reasons: (1) even after the disappearance of the seaway of northeastern Brazil, it continued to receive marine sedimentation due to the opening of Pernambuco-Paraíba gateway; (2) because of the regression and uplift, which began in Late Cretaceous, the basin has several exposures whence megafossils can be collected.

Radiolarians
Radiolarians were recorded by Koutsoukos & Hart (1990) from the Albian (Riachuelo Formation) and the Cenomanian-Turonian (Cotinguiba Formation).Although the authors did not comment vis-à-vis the Tethyan character of these microfossils, they alluded to taxonomic and morphotypic affinities with coeval assemblages reported from the Caribbean region (Puerto Rico) and the North American western interior (Colorado, Kansas, Wyoming, Alberta).

Molluscs
Bivalves are the most traditional focus of paleontological studies of Aptian/Albian strata in the Sergipe Basin.One of the classic species is Myophorella coqueiroensis, which is marine (Hessel 2005).Andrade & Seeling (2000) and Seeling & Andrade (2000) emphasized the richness of marine bivalve faunas in the Sergipe Basin.Among bivalves of the Riachuelo Formation (Aptian-Albian), the genus Neithea is very important in containing such Tethyan species as: Neithea alpina, N. coquandi and N. hispanica.The species Neithea alpina and N. coquandi are also present in the Algodões Formation (Albian-Cenomanian, Camamu Basin) (Andrade et al. 2004).
Gastropods are also very abundant in the Sergipe Basin; according to Cassab (2000), most of them have Tethyan affinity.

Echinoids
Echinoids (typically marine) are very common in the Aptian-Coniacian interval of the Sergipe Basin (Seeling et al. 2000, Manso 2003); and Manso & Souza-Lima ( 2012) described new taxa from the basin's Aptian-Albian strata.The last-mentioned authors did not affirm a Tethyan affinity, but previous records of taxa cited in their paper suggest that all of them are indeed Tethyan.

DISCUSSION
As demonstrated above, the paleontological data, involving several fossil groups, indicate the Tethyan origin of earlier marine waters that entered the primitive South Atlantic, even before the deposition of Aptian evaporites.The Tethyan influence persisted in all BCMBs (excepting the Pelotas Basin) until the latest Albian (Vraconian).For the Albian, the Tethyan influence is further evidenced by geochemical data (Azevedo 2001(Azevedo , 2004)).
In northeastern Brazil, non-marine conditions were established in the interior basins in the Cenomanian, according to dating of the estuarine deposits of the Itapecuru Formation (Klein & Fereira 1979) in the São Luís Basin.These represent the latest marine sedimentation on the NW-SE trend formed by the São Luís, Parnaíba and Araripe basins.In contrast to the interior basins, the basins of the continental margin of northeastern Brazil continued to receive marine sediments, and the Tethyan influence persisted in the Potiguar and Sergipe basins until the Late Cretaceous.
Recent geological data confirm the model proposed herein; i.e., the Tethyan origin of earlier marine waters emanating from the north.Of particular relevance are the following factors: (1) recognition of Neocomian (ca.140 Ma) marine sediments on the Romanche Fracture Zone, between northeastern Brazil and West Africa (Gasperini et al. 2001); and (2) recognition of granitic basement in the Rio Grande Rise (Carvalho 2013, Lisboa 2013a, b).The former point shows that the Tethyan waters reached the Brazilian equatorial margin as early as the beginning of the Cretaceous; the latter indicates that the physical barrier constituted by the "Microcontinent Rio Grande" and the Walvis Ridge effectively restrained free interchange of marine waters between the Pelotas and Santos basins.A similar model was proposed by Dingle (1999), who referred to the "proto-Walvis Ridge barrier".

CONCLUSIONS
■ Aptian marine sedimentation took place under the influence of Tethyan waters in all the BCMBs (Brazilian Continental-Margin Basins), except the Pelotas Basin, which was separated from them by the barrier constituted by the "Rio Grande Microcontinent".■ The most probable route for the water entering the Northeastern Brazil seaway is through the São Luís, Parnaíba, Araripe and Sergipe basins, thus permitting the ingress of Tethyan waters into the basins of eastern and southeastern Brazil.■ Considering the occurrences of marine fossils in the Areado Formation (São Francisco Basin), the total extent of the Aptian marine transgression into the continental interior could be much greater than has traditionally been postulated.That could be confirmed in the future by the possible discovery of Aptian marine strata in the Parecis Basin (Arai 1999).■ The classic modeling of the Barremian-Aptian marine transgression (e.g., Dias 1991Dias , 2005Dias , 2008) ) needs to be reviewed.■ Paleontological studies can provide important data in support of a paradigm revolution.For example, the continental-drift hypothesis, proposed by Wegener in the early 20 th century, was originally underpinned by detailed paleontological observations and the fitting of the contours of South America and Africa.

Figure 1 .
Figure 1.Brazilian sedimentary basins (modifi ed from Milani et al., 2007).The Brazilian Continental Margin Basins (BCMBs) are indicated by italics, and the basins discussed in this paper are indicated by bold characters.

Figure 2 .
Figure 2. Aptian paleogeographic map as proposed byArai (2005Arai ( , 2007Arai ( , 2011) ) and modified from Scotese's (2001) map for 120 Ma.Northeasternmost Brazil and West Africa were still joined, but waters coming from the Central Atlantic could reach the northern South Atlantic (NSA) via the seaway connecting the São Luís, Parnaíba, Araripe, Sergipe and Almada basins.The areas in violet color represent major evaporitic basins; the red line represents the barrier related to the structural high comprising the Rio Grande Rise and the Walvis Ridge (RGRWRB); the red circles (A -J) are occurrences of the Subtilisphaera Ecozone that suggest Tethyan influence; the yellow circles (L -M) are occurrences of typically Austral marine palynofloras; and the blue circle (K) is the radiolarian occurrence of Areado Formation.Note that Aptian marine strata are as yet unconfirmed in the Parecis Basin (N).