José Antonio Barbosa

New dyrosaurid crocodylomorph and evidences for faunal turnover at the K–P transition in Brazil

The discovery of a new dyrosaurid crocodylomorph from the well-dated Palaeocene deposits of northeastern Brazil sheds new light on the evolutionary history of this extinct group of marine crocodylomorphs that have survived the Cretaceous–Palaeogene (K–P) extinction crisis. Guarinisuchus munizi, the most complete member of this group collected in South America so far, is closely related to the African forms, and this fact suggests that dyrosaurids had crossed the Atlantic Ocean before the K–P boundary and dispersed from there to North America and other parts of South America. This discovery also suggests that on the coast of northeastern Brazil, dyrosaurids replaced the pre-existing Late Cretaceous fauna of diversified mosasaurs, a group of marine lizards, after the K–P extinction event, becoming the main predators, together with sharks, in shallow marine Palaeocene environments. More detailed stratigraphic records and detailed dating of the deposits with dyrosaurids are necessary to correlate this particular pattern found in the ancient northeastern Brazilian coast within the evolution of the group, especially in Africa.

Carbon isotopes, rare-earth elements and mercury geochemistry across the K–T transition of the Paraíba Basin, northeastern Brazil

Abstract The Paraíba Basin, northeastern Brazil, is divided into three sub-basins: Olinda, Alhandra and Miriri, which encompass the formations Beberibe (Coniacian–Santonian), Itamaracá (Campanian) and Gramame and Maria Farinha (Maastrichtian to Danian, respectively). In the Olinda sub-basin, the Cretaceous–Palaeogene transition (KTB) has been recorded by the carbonates of the Gramame and Maria Farinha formations. This study focus on the behaviour of C and O isotopes, major and rare-earth elements and mercury in carbonates from three drill holes in the Olinda sub-basin. The climate was fairly cold during the marine transgression in which carbonates of the Itamaracá Formation were deposited. A temperature and bioproductivity increase has been registered in the Early Maastrichtian (Gramame Formation), with a gradual fall during the rest of this period. A positive δ13C (+2‰) excursion near the KTB is followed by a drop to values around +1‰ immediately after this transition. In one drill hole, several negative δ13C anomalies predate the KTB, possibly related to either multiple impacts or volcanic activity that preceded this transition. In two of the three drill holes, the total mercury increases immediately after the KTB and, in two of them, mercury spikes (four of them in one case) precede this transition, which has been interpreted as an indication that volcanic activity predated the transition. Rare earth element patterns support a marine origin for the carbonates in the Campanian–Maastrichtian transition and KTB in the Olinda sub-basin. In carbonates from one of the drill holes, absent to weakly positive Ce anomalies (−0.1 and 0.002) in the KTB coincide with a fall in δ13C values, followed by an increase (from 2.3 to 1.8‰ and back to 2.3‰) and in increment in mercury values (from 0.4 to 2.7 ng g−1). The presence of pyrite nodules associated with a weakly negative Eu anomaly point to slightly reducing conditions around the KTB.

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.

Dente de um picnodontiforme (Actinopterygii, Neopterygii) da Formação Estiva, Cenomaniano-Turoniano da Bacia de Pernambuco, NE do Brasil

Ray-finned fishes (Actinopterigii) pycnodontiformes compounds a specialized group, small to medium-sized, with laterally compressed body and a batteries of flattened, peg-like teeth on the vomer, adapted for feeding on hard-shelled prey and by its life in near shore environments and/or characterized by reefs. Here is presented their first record to the carbonate deposits included in Estiva Formation, Maracaipe region, in the south of coastal Pernambuco Basin, considered Cenomanian-Turonian in age, where is also the first evidence of vertebrate rests. The record of fish remains open new perspective to this stratigraphic unit and to the basin in general, until the moment poorly known. Key words: pycnodontiformes, Cretaceous, Pernambuco Basin, Estiva Formation.

Characteristics, distribution and diagenetic stages of chert in the La Silla Formation (Lower Ordovician), Argentine Precordillera

The late Cambrian – late Tremadocian La Silla Formation is a carbonate unit of the eastern Precordillera in Argentina whose facies indicate a shallow platform environment. Until this moment, there were no studies that referred to the diagenetic evolution of these rocks. The present study involves the characteristics and distribution of the silicification that affects this unit and determines its different diagenetic stages through petrographic (with cathodoluminescence support) and stratigraphic analyses. An early diagenetic chert, in a stage previous to the compactation, was observed. This chert is related to silica-rich seawater in contact with permeable and porous sediments. A later pulse of chert, associated with fracturing, also occurs. The knowledge provided by the characteristics and distribution of chert in these carbonates is significant, especially when considering that the La Silla Formation in San Juan province is the most quarried unit for the elaboration of lime.

Stratigraphic Relations of the Ipubi Formation: Siliciclastic-Evaporitic Succession of the Araripe Basin

The Ipubi Formation represents the Aptian-Albian siliciclastic-evaporitic succession of Araripe Basin, NE Brazil. This succession comprises siliciclastic rocks (bituminous shales and claystones) and evaporites (gypsum and secondary anhydrite) and represents part of the lacustrine-shallow marine post-rift phase I. This study used sequence stratigraphy concepts to define the relations between changes in the relative lake level and the formation of Ipubi deposits. Results show that the organic-rich shales of the Ipubi Formation formed during a transgressive pulse that covered large areas of the proximal domains. These deposits overlie a regional unconformity that marks the end of the deposition of the underlying Crato Formation. A High Stand stage that followed the transgression influenced the formation of evaporitic deposits. Climate conditions played a major role in influencing the triggering and stopping of evaporite deposition. Thus, a new relative lake level fall event caused the exposure of the Ipubi Formation deposits, and created another regional subaerial unconformity accompanied by widespread karstification of evaporite beds. A posterior transgression caused the deposition of siliciclastic rocks of the Romualdo Formation over the Ipubi Formation strata, and also promoted a new event of karstification of the Ipubi upper evaporite beds.

Carbonate Buildups in the Pernambuco Basin, NE Brazil

The Pernambuco Basin represents one of the most prominent regions for deep water oil and gas exploration off the Brazilian coast. This study aims to identify and describe the occurrence of carbonate buildups in the offshore regions of the basin. The study was based on an analysis of a set of 143 2D time-migrated seismic sections that cover the offshore region of the Pernambuco Basin. An interpretation of the seismic dataset was used to define the main seismic sequences related to the main regional pulses of deposition and to identify three main groups of carbonate buildups: 1) shelf margin reefs, 2) patch reefs, and 3) isolated carbonate buildups. The carbonate buildups formed in two main intervals during post-rift sequences of the Santonian-Maastrichtian and Paleocene-Middle Miocene, which extend the known periods of carbonate deposit formation in the basin. The formation of carbonate buildups was controlled by the tectonic evolution of the Pernambuco Plateau, which created a series of paleotopographic highs that enabled the establishment of oceanographically and climatically favorable conditions for carbonate formation and deposition. These findings are important for understanding the evolution of the basin and for future evaluations of its petroleum potential.

Natural fracture characterization in aptian carbonates, Araripe Basin, NE Brazil

This paper addresses fracture characterization in an Aptian laminated limestone, the Crato Formation, cropping out in the Araripe Basin (NE, Brazil) using a scanline technique. This unit has been used as a geological analogue of buried naturally fractured carbonate reservoirs. In recent years, studies of fractured reservoirs have drawn considerable attention due to their significance for oil production and enhanced recovery. The study was based on the use of traditional scanline surveys, and the recording of fracture orientation, morphology, crosscutting relationships, composition, texture of fracture fill, fracture aperture-size distribution (frequency), spatial distribution (coefficient of variation), and strain for each fracture set. The main fractures identified in the Crato Formation were shear- and opening-mode fractures (veins) and with stylolites also present. In this study we focus on opening-mode fractures, which strike in two main directions, NNW-SSW (set 1) and NE-SW (set 2), and are filled by recrystallized calcite. Fractures of set 2 have a wider kinematic aperture and spacing range and are more likely to be clustered than are fractures of set 1 (NE). These results have been used to populate computational models that consider the widespread fracture system in the geomechanical modeling of carbonate reservoirs.

PALEOAMBIENTES E ICNOFÁCEIS DA SEQÜÊNCIA CARBONÁTICA DA BACIA DA PARAÍBA (CRETÁCEO-PALEOGENO), NORDESTE DO BRASIL

This work deals an investigation of the stratigraphic, sedimentological and paleontological aspects, mainly ichnologic, in outcrops of carbonate deposits of the Gramame and Maria Farinha formations in the Paraiba Basin, northeast Brazil. The objective of the investigation was to obtain detailed information on the depositional environments of these units. The Gramame Formation was deposited in a regime of highstandstand system tract, belowstand the base level of fair weather waves where prevailed conditions of soft bottom and lowstand energy, but often reached by storm events, correlated to the Cruziana ichnofacies. The Maria Farinha Formation was deposited during a regressive event, with the increase of energy and siliciclastic input, important lateral facies variation and Cruziana-Skolithos ichnofacies . The eocenic carbonate rocks, that occur in the south coast of Paraiba state, present s expressive difference from the deposits of the typical Maria Farinha Formation, and it could represent another unit. These reefal carbonates was deposited in reef-lagoon s complexes and arc se pa rated from the Maria Farinha Formation by an unconformity and, probably indicate the establishment of a lowstand system tract . The ichnofacies interpreted for them was Trypanites?. The data integration suggests a paleobathymetry of middle to inner platform (150-80m), for the Gramame Formation, and inner platform to infralitoral (60-20m), including proximity of estuaries, for the Maria Farinha Formation.