Araripe Basin Research Articles

The microbial nature of laminated limestones: Lessons from the Upper Aptian, Araripe Basin, Brazil

The Araripe Basin, located in northeastern Brazil, originated during the Gondwana continental break-up responsible for the opening of the South Atlantic during the Early Cretaceous. In the Araripe Basin, the post-rift Aptian sequence corresponds to the Santana Group, which is composed, in upward succession, of mostly clastic continental and rare carbonate layers of the Barbalha, Crato, Ipubi and Romualdo Formations. The laminated limestones of the Crato Formation were deposited in a lacustrine environment preceding the deposition of the Ipubi Formation evaporites. They are age-equivalent to the limestones of the pre-salt interval of the east coast of Brazil, which contains large petroleum reserves. The excellent preservation of its macrofossils has made the Crato Formation known worldwide as a Fossil Lagerstätte. The limestones are macroscopically homogeneous, and their deposition has been previously attributed to chemical precipitation. Although the carbonate laminites are macroscopically undifferentiated, mineralogical variations, microscopic texture and distinctive biotic aspects supported the characterization of four microfacies: planar laminated, crustiform, nodular and rhythmic. The microfacies analysis indicated a strong and pervasive biological activity in the Crato limestone morphogenesis. Organominerals precipitated by the metabolic action of cyanobacteria and/or sulfate-reducing bacteria and methanogenic-oxidizing archea are represented by calcite and pyrite. Calcified coccoid and filaments are common, furthermore, the presence of calcified biofilms composed of exopolymeric substances (EPS) is ubiquitous. The presence of amorphous organic matter (AOM) and gypsum, particularly in the rhythmic microfacies, indicates anoxic/dysoxic conditions and stressful environments during periods of drought and low lake levels which favored the development and preservation of microbial biofilms. Phytoclasts and miospores when present in the succession indicate an extrabasinal contribution during wetter periods, although the environment remained of very low energy. The evidence of microbial influence in the formation of the laminated limestones of the Crato Formation is of great importance for understanding the excellent preservation of the unit's fossils and for modeling the evolution of the Aptian carbonate sequences in Brazil.

Mawsoniid remains (Sarcopterygii: Actinistia) from the lacustrine Missão Velha Formation (Lower Cretaceous) of the Araripe Basin, North-East Brazil

Mawsoniids are a lineage of extinct fresh/brackish water coelacanth fishes, common in Cretaceous Godwanan deposits of South America, North and West Africa and Madagascar. Here we formally describe mawsoniid remains from the fluvio-lacustrine Missão Velha Formation (Lower Cretaceous) of the Araripe Basin, North-East Brazil. The examples from the Missão Velha Formation are here described as Mawsonia cf. gigas, based mainly on elements of the lower jaw and opercular series. The occurrence of M. cf. gigas in the Missão Velha Formation expands the stratigraphic and geographical ranges of the type species.

Inventário Taxonômico das Coleções Paleoentomológicas do Museu Dom José e da Universidade Estadual Vale do Acaraú, Sobral-Ce

A reality yet observed in institutions that has paleontological collections, such as museums and universities, is not aware of the specimens that make up their collections, and his study could bring many answers to taxonomy and phylogeny. This work present a general taxonomic classification of the fossils insects specimens belonging to the collections of the Museu Dom Jose and the Universidade Estadual Vale do Acarau, in Sobral city, Ceara State and, in this context, displays specimens not yet described in the literature, from the Santana Formation of Araripe Basin. Was performed an extensive literature search and the specimens were observed through comparative anatomy in the literature. In all, 39 samples analyzed, with representatives of seven orders (Blattoptera, Coleoptera, Diptera, Ephemeroptera, Hemiptera, Orthoptera and Odonatoptera). Registered taxa were: Araripegryllus femininus,Araripegryllus sp, Cratoelcana damianii, Cratolocustopsis sp., Hallex xestocephalus (?), Hallex brevipes (?), Mesoblattina sp., Nothomacromia sensibilis, Paracearagryllus poliacanthus, Protoligoneuria limai, Vulcanoia sp., and one Staphylinidae beetle, one unheard diptero Ceratopogonidae, one Progonomicidae Coleorrhyncha, one heteroptero Cimicomorpha and indeterminates cicadomorphas. Collections of studies are always welcome because they generate knowledge and questions to be answered with new works.

A gestão ambiental nas paisagens da bacia do Araripe no Estado do Ceará

Considerada a maior bacia sedimentar do interior do Nordeste brasileiro, a bacia do Araripe apresenta uma relevante importância para o estado do Ceará, no que tange às formas de uso e ocupação, que estão diretamente associadas às suas características naturais singulares. Os depósitos sedimentares dessa bacia apresentam um grande potencial de recursos minerais e uma vasta riqueza fossilífera. Dentre as diversas formas de uso e ocupação, destacam-se a agropecuária, o turismo, a mineração e a expansão urbana. Tais formas de uso têm sido responsáveis por vários danos ambientais nessa área com repercussões diretas sobre o equilíbrio da dinâmica natural. Nessa perspectiva, o poder público tem tomado diversas medidas coercitivas tais como a criação de unidades de conservação e o estabelecimento de um complexo sistema de procedimentos de licenciamento ambiental. Apesar de tais medidas, constata-se que tanto as unidades de conservação como os procedimentos de licenciamento ambiental apresentam sérios problemas estruturais, limitando a eficiência da gestão ambiental como um todo. Nessa perspectiva, a presente análise está diretamente focada numa avaliação crítica acerca das medidas públicas adotadas no âmbito da gestão ambiental nas paisagens da bacia do Araripe no estado do Ceará.

Spectroscopic studies of the fish fossils (Cladocyclus gardneri and Vinctifer comptoni) from the Ipubi Formation of the Cretaceous Period

Fossils are mineralized remains or traces from animals, plants and other organisms aged to about 108years. The chemical processes of fossilization are dated back from old geological periods on Earth. The understanding of these processes and the structure of the fossils are one of the goals of paleontology and geology in the sedimentary environments. Many researches have tried to unveil details about special kinds of biological samples; however, a lack of data is noticed for various other specimens. This study reports the investigations through infrared spectroscopy, X-ray fluorescence and X-ray diffraction measurements for two types of fish fossils from the Cretaceous Period. The sample of Cladocyclus gardneri and Vinctifer comptoni fossils were collected from the Ipubi Formation, being one of the less studied, among the formations that constitute the important Santana group in the Araripe Basin, Brazil. The results obtained through different techniques, showed that the C. gardneri fish fossil contains hydroxyapatite and calcite as constituents whereas its rock matrix was formed by calcite, quartz and pyrite. Regarding the V. comptoni, the measurements confirmed the presence of hydroxyapatite in the fossil and its rock matrix gypsum, pyrite, quartz and calcite. The above scientific data contributed to the understanding the fossil formation in the Ipubi Formation, an important environment of the Cretaceous Period, which is rich in well-preserved fossils from different species.

Discovery of †Obaichthyidae gars (Holostei, Ginglymodi, Lepisosteiformes) in the Aptian Codó Formation of the Parnaíba Basin: Remarks on paleobiogeographical and temporal range

Here we report the first †obaichthyid gar from the Lower Cretaceous (Aptian) Codó Formation of the Parnaíba Basin, Northeastern Brazil. It shows the following obaichthyid characters: numerous odontods firmly attached to the outer surface of the dermal bones, free and mobile maxilla, presence of interopercle, lack of contact between the metapterygoid and ectopterygoid, absence of lacrimomaxillary bones, and a prominent spine at the posterior margin of the scales. Due to the presence of scales bearing a prominent ventral posterior spine and a number of additional posterior marginal spines, the fish from the Codó Formation is noticeably the same species found in the Albian Santana Formation of the Araripe Basin, Dentilepisosteus laevis. Although probably restricted to fresh or brackish water, the new discovery adds one more taxon to the assemblages found in the Parnaíba and the Araripe basins. The new record extends the temporal range of this species down into the Aptian (about ∼10 myr older than the previous occurrence).

Characterization of natural fracture systems: Analysis of uncertainty effects in linear scanline results

The exploitation of hydrocarbon reserves in naturally fractured reservoirs composed of different types of rocks has drawn considerable attention from the fracture characterization research community because of the importance of fractures to the prediction of fluid flow. One of the most common methods for rapidly analyzing fracture features is the scanline technique, which provides an estimate of fracture density and frequency. Despite the confidence provided by the systematic use of this method, errors and uncertainties caused by sampling biases exist. The problems caused by these uncertainties can detrimentally affect the construction of a computational model due to misleading trends. This study evaluated the uncertainty caused by sampling biases in the scanline data of opening-mode fractures in outcrops of naturally fractured Aptian laminated limestone from the Crato Formation, Araripe Basin, northeastern Brazil. The Monte Carlo method was chosen to introduce random values into the sampled values, which enabled us to verify the importance of errors in the accuracy of the method of representing the fracture network. In this study, errors and uncertainties were grouped into one parameter, termed the coefficient of uncertainty, which was defined as the ratio between the uncertainties, created by the errors and artifacts introduced artificially, and the original scanline data. The propagation of errors and uncertainties in the scanline data to the coefficients of the corresponding power law were determined. This evaluation can be applied in the construction of more reliable geomechanical models using analog geological models for naturally fractured reservoirs.

Phytogeographic, stratigraphic, and paleoclimatic significance of Pseudofrenelopsis capillata sp. nov. from the Lower Cretaceous Crato Formation, Brazil

Coniferales are represented during the Early Cretaceous in northern South America both by macrofossils and palynomorphs of the families Araucariaceae and Cheirolepidiaceae. Fossils of Cheirolepidiaceae are often abundant in coastal deposits; plants of this family are considered to have grown under semiarid to arid climate conditions because of characteristic anatomical features which include several adaptations to aridity and/or salinity. Here histologically preserved specimens of Pseudofrenelopsis from laminated limestones of the Crato Formation (Araripe Basin, NE Brazil) were studied by light and scanning electron microscopy. A new species Pseudofrenelopsis capillata sp. nov. is described which seems to have grown exclusively at the Early Cretaceous (sub)paleoequatorial area of South America and may be therefore endemic. The new taxon may have grown in a riparian environment along the borders of a large lake as a minor constituent of the surrounding vegetation.

A new genus and species of enantiornithine bird from the Early Cretaceous of Brazil (FAST TRACK)

ABSTRACT: The fossil record of birds in Gondwana is almost restricted to the Late Cretaceous. Herein we describe a new fossil from the Araripe Basin, Cratoavis cearensis nov. gen et sp., composed of an articulated skeleton with feathers attached to the wings and surrounding the body. The present discovery considerably extends the temporal record of the Enantiornithes birds at South America to the Early Cretaceous. For the first time, an almost complete and articulated skeleton of an Early Cretaceous bird from South America is documented.

Taphonomic and paleoenvironmental considerations for the concentrations of macroinvertibrate fossils in the Romualdo Member, Santana Formation, Late Aptian – Early Albian, Araripe Basin, Araripina, NE, Brazil

Benthic macroinvertebrate fossils can be seen towards to the top of the Romualdo Member of the Santana Formation, in the Araripe Basin, Northeast Brazil, and can provide paleoenvironmental and paleobiogeographical information regarding the Cretaceous marine transgression which reached the interior basins in Northeast Brazil. We analyse taphonomic characteristics of macroinvertebrate concentrations of two outcrops (Torrinha and Torre Grande) within the municipality Araripina, Pernambuco, in order to enhance our understanding of the Cretaceous paleoenvironment in the western portion of the Araripe Basin. At the outcrop Torrinha, proximal tempestitic taphofacies were identified. These predominantly consist of ceritid, cassiopid, and later, naticid gastropods as well as undetermined bivalves. Given this lack of variability it can be deduced that there were no significant paleoenvironmental changes during the successive stages tempestitic sedimentation. In the Torre Grande outcrop distal to proximal tempestitic taphofacies were identified from the base to the top respectively pointing to a decrease in paleodepth. Asides from the macroinvertebrates present in Torrinha, there are also echinoids – unequivocal evidence for marine conditions. These occurrences appear to be restricted to Romualdo Member outcrops in the Araripina municipality (the Southeast portion of the Araripe Basin) confirming a previously published hypothesis suggesting that the Cretaceous marine transgression originated from the neighbouring Parnaíba Basin to the west. This study identified marine molluscs of a similar age to those in the Romualdo Member's equivalent rock units in the Parnaíba and Sergipe-Alagoas (SE-AL) basins suggesting a marine connection between these basins and the Araripe Basin during the Early Cretaceous.

A new genus and species of enantiornithine bird from the Early Cretaceous of Brazil

<p>The fossil record of birds in Gondwana is almost restricted to the Late Cretaceous. Herein we describe a new fossil from the Araripe Basin, <italic>Cratoavis cearensis</italic> nov. gen et sp., composed of an articulated skeleton with feathers attached to the wings and surrounding the body. The present discovery considerably extends the temporal record of the Enantiornithes birds at South America to the Early Cretaceous. For the first time, an almost complete and articulated skeleton of an Early Cretaceous bird from South America is documented.</p>

Facies architecture and sequence stratigraphy of an early post-rift fluvial succession, Aptian Barbalha Formation, Araripe Basin, northeastern Brazil

The Barbalha Formation (Aptian) records deposition in a fluvial and lacustrine environment accumulated in an early post-rift sag basin. Characterization of the facies architecture and sequence stratigraphic framework of the alluvial succession was carried out through detailed description and interpretation of outcrops and cored wells. The development of depositional sequences in this unit reflects variation in the accommodation-to-sediment supply (A/S) ratio. Two depositional sequences, showing an overall fining-upward trend, are preserved within the succession. The sequences are bounded by regional subaerial unconformities formed during negative A/S ratio, and may be subdivided in Low-accommodation Systems Tracts (LAST) (positive A/S ratio close to zero) and High accommodation Systems Tracts (HAST) (A/S ratio between 0.5 and 1). Sequence 1, with a minimum thickness of 100m, is characterized by amalgamated, multi-storey, braided fluvial channel sand bodies, defining a LAST. These are interlayered with crevasse splay and floodplain deposits toward the top, passing to open lacustrine deposits, defining a HAST. Sequence 2, with minimum thickness ranging from 50 to 90m, overlies the organic-rich lacustrine deposits. At the base, this sequence is composed of amalgamated, multistorey braided fluvial channel sand bodies (LAST), similar to Sequence 1, overlain by well-drained floodplain with fixed fluvial channel deposits, interpreted as an anastomosed fluvial system, which are in turn capped by lacustrine deposits, both grouped in a HAST. Paleocurrent data on fluvial deposits of sequences 1 and 2 show a consistent paleoflow to the SE. Sedimentological evidence indicates humid to sub-humid climatic conditions during deposition of sequences 1 and 2. Accumulation of fluvial sequences 1 and 2 was mainly controlled by tectonics. Variation in A/S ratios must be related to tectonic subsidence and uplift of the basin.

Evaluation of the petroleum composition and quality with increasing thermal maturity as simulated by hydrous pyrolysis: A case study using a Brazilian source rock with Type I kerogen

Hydrous pyrolysis (HP) experiments were used to investigate the petroleum composition and quality of petroleum generated from a Brazilian lacustrine source rock containing Type I kerogen with increasing thermal maturity. The tested sample was of Aptian age from the Araripe Basin (NE-Brazil). The temperatures (280–360°C) and times (12–132h) employed in the experiments simulated petroleum generation and expulsion (i.e., oil window) prior to secondary gas generation from the cracking of oil. Results show that similar to other oil prone source rocks, kerogen initially decomposes in part to a polar rich bitumen, which decomposes in part to hydrocarbon rich oil. These two overall reactions overlap with one another and have been recognized in oil shale retorting and natural petroleum generation. During bitumen decomposition to oil, some of the bitumen is converted to pyrobitumen, which results in an increase in the apparent kerogen (i.e., insoluble carbon) content with increasing maturation.The petroleum composition and its quality (i.e., API gravity, gas/oil ratio, C15+ fractions, alkane distribution, and sulfur content) are affected by thermal maturation within the oil window. API gravity, C15+ fractions and gas/oil ratios generated by HP are similar to those of natural petroleum considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous age. API gravity of the HP expelled oils shows a complex relationship with increasing thermal maturation that is most influenced by the expulsion of asphaltenes. C15+ fractions (i.e., saturates, aromatics, resins and asphaltenes) show that expelled oils and bitumen are compositionally separate organic phases with no overlap in composition. Gas/oil ratios (GOR) initially decrease from 508–131m3/m3 during bitumen generation and remain essentially constant (81–84m3/m3) to the end of oil generation. This constancy in GOR is different from the continuous increase through the oil window observed in anhydrous pyrolysis experiments. Alkane distributions of the HP expelled oils are similar to those of natural crude oils considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous age. Isoprenoid and n-alkane ratios (i.e., pristane/n-C17 and phytane/n-C18) decrease with increasing thermal maturity as observed in natural crude oils. Pristane/phytane ratios remain constant with increasing thermal maturity through the oil window, with ratios being slightly higher in the expelled oils relative to those in the bitumen. Generated hydrocarbon gases are similar to natural gases associated with crude oils considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous, with the exception of elevated ethane contents. The general overall agreement in composition of natural and hydrous pyrolysis petroleum of lacustrine source rocks observed in this study supports the utility of HP to better characterize petroleum systems and the effects of maturation and expulsion on petroleum composition and quality.

Caracterização palinoflorística e paleoambiental da Formação Santana (Cretáceo Inferior), Poço 4-BO-1-PE, bacia do Araripe, Nordeste do Brasil

A paleoenvironmental characterization of the Santana Formation sedimentary rocks (4-BO-1-PE well), Araripe Basin, Late Aptian-Early Albian, was accomplished through qualitative and quantitative analyses of the palynofl ora. Palynological and lithological evidence indicate a predominantly continental depositional environment for this section, composed by a fl uvial, lacustrine deltaic system in a semi-arid climate. The detection of a single dinofl agellate cyst suggests that small sea ingressions took place during Aptian-Albian times in the region, as also verifi ed in other coeval strata of marginal Brazilian Cretaceous basins. Comparisons with palynofl oras from other continents support the hypothesis that this assemblage belongs to the North-Gondwana microfl oristic province.

A new angiosperm from the Crato Formation (Araripe Basin, Brazil) and comments on the Early Cretaceous monocotyledons.

The Crato Formation paleoflora is one of the few equatorial floras of the Early Cretaceous. It is diverse, with many angiosperms, especially representatives of the clades magnoliids, monocotyledons and eudicots, which confirms the assumption that angiosperm diversity during the last part of the Early Cretaceous was reasonably high. The morphology of a new fossil monocot is studied and compared to all other Smilacaceae genus, especially in the venation. Cratosmilax jacksoni gen. et sp. nov. can be related to the Smilacaceae family, becoming the oldest record of the family so far. Cratosmilax jacksoni is a single mesophilic leaf with entire margins, ovate shape, with acute apex and base, four venation orders and main acrodromous veins. It is the first terrestrial monocot described for the Crato Formation, monocots were previously described for the same formation, and are considered aquatics. Cratosmilax jacksoni is the first fossil record of Smilacaceae in Brazil, and the oldest record of this family.

Diagênese dos arenitos da Tectonossequência Rifte na Bacia do Araripe, NE do Brasil

A Bacia do Araripe é a mais extensa das bacias interiores do Nordeste do Brasil, apresentando história geológica mais complexa. O intervalo estudado corresponde às formações Missão Velha e Abaiara, aflorantes na porção centro-oeste do Vale do Cariri. Visando subsidiar os estudos de diagênese, foi realizada uma análise faciológica que permitiu identificar dez fácies sedimentares, individualizadas, consoantes às diferentes litologias e principais estruturas sedimentares. O estudo petrográfico permitiu classificar os litótipos como quartzarenitos, que apresentam arcabouço rico em grãos de quartzo, feldspatos e fragmentos líticos e, subordinadamente, minerais pesados. A história diagenética mostrou-se bastante complexa, sendo caracterizada por uma grande variedade de fases que se sucederam durante os estágios de eo, meso e telodiagênese. O estágio eodiagenético é marcado pela infiltração mecânica de argilas e pelo ínicio da compactação mecânica. O estágio mesodiagenético é caracterizado pelo final da compactação mecânica, compactação química, crescimentos secundários de quartzo e feldspato, cimentação de calcita, dissolução, formação de caulinita autigênica, alteração dos grãos para clorita e illita e precipitação de minerais opacos. O estágio telodiagenético é evidenciado pela oxidação de grãos, matriz e cimentos presentes. Tais informações permitiram realizar um estudo comparativo das formações estudadas.

A history of basin inversion, scarp retreat and shallow denudation: The Araripe basin as a keystone for understanding long-term landscape evolution in NE Brazil

At the border between the states of Ceará and Pernambuco (northeast Brazil), the Chapada do Araripe is a high plateau (800–1000ma.s.l.) formed by a slab of Cenomanian fluvial sandstone. This caprock is underlain by lacustrine or marine Albian layers and older rift deposits. During the Cretaceous, the Araripe basin lay at a palaeoelevation close to sea-level. Through a presentation and discussion of original field and cartographic data we analyse the mechanisms of topographic inversion in this sedimentary basin in relation to local or regional crustal upwarp. The contrast between the plateau — a weakly dissected structural surface — and the surrounding lowlands is explained through a study of the erosional scarps — cuesta-like landforms and their outliers — that fringe the Chapada. No evidence of local tectonic inversion is found. River incision, spring sapping, landslides and other forms of mass movement are listed as efficient processes of topographic inversion and scarp retreat, the rates and patterns of which appear to be controlled by lithological contrasts and conditions of exhumation of the basement. Geometric relationships with regional stepped surfaces (e.g., the low-elevation Sertaneja Surface), exhumed palaeosurfaces and regional drainage systems are analysed. Our estimation of the amplitude of denudation and topographic inversion (0.6–0.7km) differs significantly from apatite fission-track-derived estimates reported in recent literature, which would imply burial by considerable thicknesses of younger sediments followed by 1.5km or more of post-rift denudation — not just in the study area, but also in the Tucano–Jatoba basin to the south. The exhumation and reworking of surrounding basement surfaces probably began during the early Cenozoic, as shown to the northwest of the Chapada by the presence of widespread laterites of probable Palaeogene age. A second stage of topographic inversion occurred during the Oligocene or later. This would correspond to the major stages of river incision, partial planation and basin inversion.

A new toothed pterosaur (Pterodactyloidea: Anhangueridae) from the Early Cretaceous Romualdo Formation, NE Brazil

A new species of pterosaur, Maaradactylus kellneri gen. nov., sp. nov. (Archosauria: Pterosauria) from the Romualdo Formation (Aptian/Albian), is herein described. The specimen (MPSC R 2357) was found at Sítio São Gonçalo, Santana do Cariri city (State of Ceará, northeast Brazil) and consists of the skull, atlas and axis, and represents one of the largest skulls of the Anhangueridae from the Araripe Basin described. The autapomorphies of the new pterosaur include the following characters: a premaxillary sagittal crest that is relatively long and high, beginning at the anterior part of the skull (rostrum) and extending to the 22nd pair of alveoli, not covering the nasoantorbital fenestra or the choanaes, and also the presence of 35 pairs of alveoli; smooth palatal ridge, which starts on the 5th pair of alveoli and ends on the 13th pair; palate is convex shaped in the anterior region; choanae not extending laterally; small and convex palatal elevation; the 5th, 6th and 7th alveoli smaller than the 4th and 8th; the alveoli decreasing in size from the 9th to the 12th and increasing from the 13th to 18th, and from the 18th to the 35th they are arranged in triplets. Furthermore, the lateral surface of the premaxillary crest shows grooves and tridimensional structures that may have housed blood vessels.

The Araripe Basin in NE Brazil: An intracontinental graben inverted to a high-standing horst

The Mesozoic sediments of the Araripe Basin peak at ca. 1000m, more than 500m above the host Precambrian basement. This means that an intracontinental graben has been inverted to a topographically high-standing horst, which raises four fundamental questions: (1) Where are the inversion structures? (2) What is the age of the inversion? (3) How can inversion occur in a thick lithosphere? (4) What forces are responsible for the inversion? Here we show that displacement on inverted normal faults increases towards the core of the horst, and new reverse faults thrust basement over basin. Field observations indicate that inversion has occurred on faults striking between NNE–SSW and E–W, following the structure of the host basement. Inversion is reverse/dextral in the NNE–SSW system, and reverse/sinistral in the ENE–WSW system. This is consistent with the Mid-Atlantic Ridge and Andean pushes. From the tectonic analysis of topography and gravity anomaly, the NW–SE faults seem critical to the inversion, although not clearly recognized in the field. The inversion of high-angle normal faults occurred by oblique slip and injection of clay-rich breccias, which apparently reduced friction sufficiently to allow for major inversion. Fault scarps with a prominent topographic expression and faults cutting through colluvium indicate that the latest stage of inversion is Quaternary, although older inversion episodes are likely. The new tectonic data, the timing, and the stress field altogether indicate that inversion was due to plate-wide compression, which preferentially concentrated along inherited shear zones and reactivated them to form the main inversion faults of the Araripe Basin. The high degree of inversion in the Araripe Basin is most likely due to its position at a bend in the major Patos shear zone, which worked as a restraining bend under the maximum compressive stress acting in NE Brazil.

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

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 Sao Luis (Codo Formation), Parnaiba (Codo Formation), Araripe (Santana Formation), Tucano (Marizal Formation), Sergipe (Riachuelo Formation) and Camamu (Algodoes 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 Paraiba.