David Lopes de Castro

Assinaturas magnéticas e gravimétricas do arcabouço estrutural da bacia Potiguar emersa, NE do Brasil

Apresenta-se nesse trabalho uma interpretacao qualitativa das assinaturas aeromagnetica e gravimetrica do arcabouco estrutural da Bacia Potiguar emersa, situada na porcao setentrional da Provincia Borborema. O embasamento pre-cambriano consiste da amalgamacao de blocos crustais arqueanos e proterozoicos, formados por diversas sequencias litoestratigraficas de rochas igneas e metamorficas. Estas unidades geologicas sao parcialmente recobertas por um expressivo pacote sedimentar cretaceo, de ate 6000 m de espessura, da Bacia Potiguar e depositos de idades terciaria e quaternaria. O contexto geotectonico regional encontra-se condicionado por extensas zonas de cisalhamento neoproterozoicas, que foram reativadas por esforcos distensionais associados a abertura do Atlântico Sul no Cretaceo Inferior. Neste sentido, o estudo da evolucao tectonica mesozoica da Bacia Potiguar passa pelo conhecimento da trama estrutural pre-cambriana e sua relacao com a arquitetura interna da bacia. A interpretacao integrada dos dados geofisicos permitiu a caracterizacao de seis dominios magneticos e gravimotricos distintos para o arcabouco regional da Bacia Potiguar, que podem ser correlacionados com os principais blocos geotectonicos da regiao. A arquitetura do rifte principal da bacia pode ser bem delineada nos diversos mapas geofisicos, estando esta inserida na configuracao geotectonica regional, cujo trend estrutural tem direcao principal NE-SW. As estruturas internas da bacia, como depocentros e altos estruturais internos, sao tambem orientadas na direcao preferencial NE-SW. Foi possivel identificar descontinuidades nos lineamentos magneticos e gravimetricos, que fornecem novas evidencias para o controle estrutural do embasamento sobre a distribuicao de falhas normais e de transferencia, que afetaram as rochas sedimentares meso-cenozoicas da bacia.

Imageamento digital de paleocavernas colapsadas com ground penetrating radar

In this paper we present the methodological procedures for digital imaging of collapsed paleocaves in carbonate tufas using ground penetrating radar (GPR). These carbonate deposits occur in the Quixere region, Ceara State (NE Brazil), on the western border of the Potiguar Basin. Collapsed paleocaves are exposed along a state road, which were selected to this study. We chose a portion of the called Quixere outcrop for making a photomosaic and caring out a GPR test section to compare and parameterize the karst geometries on the geophysics line. The results were satisfactory and led to the adoption of criteria for the interpretation of others GPR sections acquired in the region of the Quixere outcrop. Two grids of GPR lines were acquired; the first one was wider and more spaced and guided the location of the second grid, denser and located in the southern part of the outcrop. The radargrams of the second grid reveal satisfactorily the collapsed paleocaves geometries. For each grid has been developed a digital solid model of the Quixere outcrop. The first model allows the recognition of the general distribution and location of collapsed paleocaves in tufa deposits, while the second more detailed digital model provides not only the 3D individualization of the major paleocaves, but also the estimation of their respective volumes. The digital solid models are presented here as a new frontier in the study of analog outcrops to reservoirs (for groundwater and hydrocarbon), in which the volumetric parameterization and characterization of geological bodies become essential for composing the databases, which together with petrophysical properties information, are used in more realistic computer simulations for sedimentary reservoirs.

Contribuição de uma seção gravimétrica transversal ao estudo da estruturação litosférica na porção setentrional da Província Borborema, NE do Brasil

This paper presents a 2.5D gravimetric model generated from 315 new gravity stations obtained along a NW-SE 750 km geophysical transect in the northeastern portion of the Borborema Province, NE Brazil. The regional and residual components of the gravity field were modeled separately. Modeling of the regional anomalies revealed a crustal thickness variation from 28 to 32 km, considering the average densities of about 2.8 g/cm3 for the continental crust and 3.3 g/cm3 for lithospheric mantle. High frequency residual anomalies were interpreted on the density contrasts between a 10 km thick upper crust and a broad assembly of supracrustal covers and basement units, which show density values between 2.55 to 2.9 g/cm3. The current configuration of the Borborema Province lithosphere is clearly the result of the Mesozoic South American - African breakup of West Gondwana, which overprinted the Brasiliano-Pan African orogenic structures.

ERT and GPR survey of collapsed paleocave systems at the western border of the Potiguar Basin in northeast Brazil

Collapsed paleocave systems are carbonate reservoirs with high internal spatial complexity that are the result of several stages of karst processes. Paleocave-related reservoirs can be spread over large areas with significant thicknesses that are favourable for hydrocarbon exploration. Nevertheless, few studies have provided a detailed understanding of the strong lateral heterogeneity of these reservoirs and their complex karst-controlled development using modern karst terrain analogues. To elucidate this issue, the internal architecture of a collapsed paleocave system has been mapped accurately at the western border of the Potiguar Basin in Northeastern Brazil. The collapsed paleocaves outcrop in an escarpment that delimits the carbonate platform from the transgressive phase of the basin. Electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) sections were acquired parallel and perpendicular to a road cut and served to parameterize the geophysical signatures of the collapsed paleocaves and the host rocks. The collapsed paleocaves were mapped by identifying high-resistivity zones and high-amplitude ground-penetrating radar reflectors. In contrast, the host rocks are marked by low to intermediate resistivity and ground-penetrating radar reflections that range from low amplitude to almost transparent. The resistivity data and the ground-penetrating radar attribute of the rootsquare energy enabled the mapping of the complex spatial distribution of the collapsed paleocaves system. At depths of approximately 20 m, the paleocaves are more spread out and eventually become isolated while sometimes being connected vertically through shafts. However, at shallower levels, the paleocaves are interconnected by ducts or coalesce into a system of paleocaves that are hundreds of metres long over an area of 12000 m2. The results of the study show the detailed internal geometry of this paleocave system at a subseismic scale, which enables the identification of the connectivity pattern among these karst features and the porosity and total volume of the reservoir. This system could serve as an outcrop analogue for other collapsed paleocave carbonate reservoirs worldwide.

Evidence of An Early Cretaceous Giant Dyke Swarm in Northeast Brazil (South America): A Geodynamic Overview

Atlantic margin in the northeast Brazil (South America) is characterized by a modest magmatic activity compared with the Southern Atlantic margin, where the ParanáEtendeka Province is accounted for eruption of millions of cubic kilometers of mafic and acid magmas (flow basalts, dyke swarms). The most significant magmatic activity in the Equatorial Atlantic margin is the Rio Ceará Mirim Dyke Swarm (CMDS), which marks the early stages of rifting in the Cretaceous. The CMDS consists of an arcuate, nearly 800 km-long mafic dykes that crosscut rocks and structures of the Precambrian basement. The swarm can be divided into two laterally continuous segments, one of E-W strike that is parallel to the southern border of the Mesozoic Potiguar basin, and one barely studied NE-SW segment which, after recent high-resolution aeromagnetic anomalies, can be traced for 300 km along the east border of the Paleozoic Parnaíba basin up to the northern boundary of the São Francisco craton. The E-W-trending segment encompasses essentially tholeiitic basalts including plagioclase, clinopyroxene (±olivine), Fe-Ti oxides and pigeonite in their groundmass. The dykes show oneto 150-meters in width and of up to one kilometer in length; a few dykes exceeding 100 kilometers have been inferred from the aeromagnetic anomalies. The tholeiites have been subdivided into three groups: high-Ti olivine tholeiites, evolved high-Ti tholeiites (TiO2≥1.5 wt.%; Ti/Y >360), and low-Ti tholeiites (TiO2≤1.5 wt%; Ti/Y≤360), with all exhibiting distinct degrees of enrichment in incompatible elements relative to Primitive Mantle. Negative Pb anomalies are found in all three groups, while Nb-Ta abundances similar to those of OIB-type magmas are found in the olivine tholeiites, with moderate to high depletions being observed, respectively, in the evolved high-Ti and low-Ti tholeiites. The latter exhibit some contamination with crustal (felsic) materials. The initial isotopic compositions of the olivine tholeiites show uniform and unradiogenic 87Sr/86Sr (~0.7035–0.7039) combined with (in part) radiogenic 143Nd/144Nd and 206Pb/204Pb (>19.1) ratios, which together reveal a contribution of FOZO (FOcalZOne) component in their genesis. The other tholeiite groups show quite variable Sr-Nd initial ratios with relatively consistent 206Pb/204Pb ratios clustering toward an isotopically enriched mantle (EM1) component. Taken in conjunction with the Nb-Ta anomalies, this enriched signature reflects the involvement of a subduction-modified lithospheric mantle in the source of the evolved high-Ti and low-Ti tholeiites. Hence, FOZO and EMI components might to be coexisted and contributed in varying extents to the generation of the CMDS primary melts. Plagioclase dating of one evolved high-Ti tholeiite dyke provided two plateau ages of 127.1 ± 0.2 Ma and 128.2 ± 1.3 Ma, with an integrated mean age of 127.7 ± 0.1 Ma. Plagioclase multigrain fractions from one low-Ti tholeiite dyke provided two plateau ages of 131.6 ±0.7 Ma and 131.0 ± 0.4 Ma, with mean age of 131.2 ± 0.1 Ma. These 40Ar/39Ar ages clearly reveal that the low-Ti and high-Ti magmas encompassing the E-W segment of the CMDS were emplaced as two pulses during the Early Cretaceous. AMS investigations of the E-W-trending segment evidenced (at least) two main feeder zones located at the intersection of the dykes with Cenozoic N-trending volcanic centers referred in NE Brazil as the Macau magmatism. Such feeder zones were characterized by vertical magnetic fabrics (via steep-plunging magnetic Maria Helena B. M. HOLLANDA, Carlos J. ARCHANJO, Paul R. RENNE, Donald E. NGONGE, David L. CASTRO, Diógenes C. OLIVEIRA and Antomat A. MACÊDO FILHO, 2016. Evidence of An Early Cretaceous Giant Dyke Swarm in Northeast Brazil (South America): A Geodynamic Overview. Acta Geologica Sinica (English Edition), 90(supp. 1): 109-110.

Comportamento geoelétrico da sequência drifte da bacia potiguar (RN/CE)

A 70 km long geoelectric section, formed by twenty-nine vertical electric soundings (VES), reveals the geoelectrical configuration of the drift sequence of Potiguar basin, NE Brazil. This tectono-stratigraphic unit preserves structural records of tectonic activities after the end of the northeastern Brazilian rifting process, since the Albian to present day, and encompasses the main hydrocarbon reservoirs in the onshore portion of the basin. The VES were carried out every 2.0 or 3.0 km along a geophysical cross section to the major axis of the rift architecture. The VES were performed with maximum apertures of the current electrodes, which allow us to investigate depth of 500 to 1000 m. The geological interpretative model was obtained by inversion of the soundings, constrained to a priori information derived from geological mapping, seismic reflection and well logging. The subsurface distribution of geoelectric resistivities reflects the strong structural conditioning of the drift sequence stratigraphic units. At the faulted border of the rift, a set of subvertical, normal faults causes a NW stepping increase in the thickness of the basin infill. While to southeast, the sedimentary cover of the marginal platform has a thickness relatively constant and is little affected by faulting. This model suggests that probable tectonic reactivation of brittle structures of the rift phase determines the structural setting of the drift tectono-sedimentary deposits in the onshore portion of Potiguar basin.