Francisco Hilário Rego Bezerra

Holocene sea-level history on the Rio Grande do Norte State coast, Brazil

Abstract The Rio Grande do Norte State coast, northeastern Brazil, lies in an intraplate region which displays elevated Holocene shorelines, abandoned tidal flats and other intertidal deposits. A field and chronological investigation, which used 48 radiocarbon dates on unaltered mollusk shells, peats, coral reefs, and vermetids in living position and death assemblages encrusted on beachrock, tidal flat and peat deposits, was carried out along two littoral zones, one trending east–west and the other north–south. The sea-level envelope curve for the region indicates at least one sea-level oscillation after the maximum Holocene highstand, which occurred at ∼5000 cal. yr BP. This curve shows notable deviations from the mean sea-level curve for the Central Brazilian coast in the early and mid Holocene. However, in general, this envelope curve fits a glacio-isostatic prediction for the area. A significant deviation from this prediction, related to a small sea-level oscillation, occurred from ∼2100 to ∼1100 cal. yr BP. The comparison between the glacio-isostatic prediction and the sea-level observation enables us to conclude that local events may have disturbed the sea-level record. Minor Holocene sea-level oscillations, which were mainly associated with climate changes or tectonics and superimposed on a major smooth pattern, were observed in South Africa and Australia. But in northeastern Brazil, neotectonic controlled crustal movements or variation in marine and wind currents could provide the answer. The data indicate that local events may have an important role in the history of coastal progradation and retreat.

Late Pleistocene marine terrace deposits in northeastern Brazil: sea-level change and tectonic implications

Abstract Coastal areas of passive margins are among the best places to examine the reality of sea-level changes. Elevation of marine terraces is central to interpretation of their approximate age in this kind of tectonic setting. The recognition of Pleistocene shorelines on the South American eastern continental margin is becoming increasingly common, but data are commonly imprecise in both time and space, and the resulting chronology is based mainly on geomorphological attributes. Nine thermoluminescence dates and four optically stimulated luminescence dates of 220–206 ka and 117–110 ka were obtained for two marine terrace deposits along 340 km of coast in Rio Grande do Norte State, northeastern Brazil. They are correlated with the highstands of marine oxygen-isotope substages 7c and 5c sea level, respectively. The older deposit occurs mainly on the N–S-trending coast and ranges in elevation between 7.5 and 1.3 m. The latter is found along the E–W-trending coast at altitudes that vary between 1 m and 20 m. It indicates relative downfaulting of the 220–206-ka marine terrace and uplift of the 117–110-ka marine deposit, which is locally about 12 m higher than deposits of similar age described 1000 km to the south. It follows that elevation alone is an unreliable tool for establishing a Quaternary sea-level change chronology also on passive margin coastal areas.

How active is a passive margin? Paleoseismicity in northeastern Brazil

The seismicity of intraplate areas reflects both far-field plate-boundary stresses and local effects, including dormant structures. In northeastern Brazil, within the passive margin of the South American plate, a short instrumental seismic record yields a pattern consistent with east-west plate push via events that do not exceed m b = 5.2. Paleoseismic evidence from remote sensing, well records, earthquake-induced liquefaction, and radiocarbon dating of beachrock, coastal peats, and fault-filling sediment indicates the occurrence of larger prehistoric Holocene earthquakes, including some of Ms ≥ 6.8. The finding is of obvious significance for seismic hazard assessment, but because the azimuth of the maximum horizontal stress (S Hmax ) may be a poor guide to the present stress field, intraplate events on reactivated structures are of limited value for testing deformation models.

Holocene coastal tectonics in NE Brazil

Abstract We have identified two beachrock facies in NE Brazil which can be used as reliable sea-level indicators. Facies (a) represents the lower foreshore and the upper shoreface, being mainly characterized by coarse unsorted sandstones with trough cross-stratification, whereas facies (b) corresponds to sediments deposited on the middle to lower foreshore and is usually characterized by medium to coarse, better sorted sandstones which form seaward-dipping, swash cross beds. The age of the beachrock bodies ranges from c. 7000 to 1150 cal. a bp. Although the relative sea-level record is complicated by oscillations which are probably due to minor climatic changes, it was possible to identify a general rise at c. 7000 cal. a bp which reached its maximum of +2 m c. 5000 cal. a bp and then started to fall to its present level some 300 years later. Our sea-level data are consistent with glacio-hydroisostatic models for the region but dates of 4080 cal. a bp on shells in growth position at +5 m above sea level and 2780 cal. a bp on a beachrock at sea level east of the Carnaubais fault point to rapid, possibly coseismic, late Holocene emergence.

Geologia e geomorfologia do quaternário costeiro do estado do Rio Grande do Norte

O objetivo deste trabalho e a apresentacao de um mapa geologico-geomorfologico preliminar para o Quaternario costeiro do Estado do Rio Grande do Norte, com destaque para os depositos eolicos. O mapa foi elaborado a partir de fotografias aereas, escala 1:70.000, imagens de satelite (P/B, banda 4), escala 1:100.000, mapas topograficos e trabalhos de campo. Foram feitas datacoes 14C em rochas praiais e depositos de paleomangues e datacoes por termoluminescencia em sedimentos eolicos e marinhos. Nos depositos eolicos foram identificados tres dominios geomorfologicos de dunas inativas e um de dunas ativas. Eles formaram-se no Quaternario a partir de retrabalhamento de areias da propria planicie costeira e de sedimentos da Formacao Barreiras. As idades TL de 36 amostras indicam que os depositos eolicos foram formados durante seis intervalos principais: 390.000 - 326.000, 270.000 - 240.000, 210.000 - 150.000, 63.000. - 24.000, 11.000 - 9.000 e, 6.500 anos - presente. Essas idades sugerem sua deposicao tanto durante estagios glaciais como interglaciais, relacionados a niveis relativos de mar baixos e altos respectivamente. Dois terracos marinhos foram identificados, com idades de 210.000 a 214.000 anos e 110.000 a 120.000 anos. O primeiro ocorre entre Natal e Baia Formosa, onde a costa se orienta na direcao N - S; o segundo encontra-se no trecho da costa entre Sao Bento e Zumbi, com orientacao E - W. Extensos depositos de intermare holocenicos, principalmente de arenitos praiais, estao localizados na costa orientada N - S e apresentam idades entre 7.000 - 6.000 anos AP. e 5.500 - 4.500 anos AP.

Scale-dependent influence of pre-existing basement shear zones on rift faulting: a case study from NE Brazil

Rifting of continental crust initiates faults that are commonly influenced by pre-existing structures. We document newly identified faults cutting Precambrian units in the interior of the NE Brazilian margin to assess the effects of structural inheritance on both rift geometry and fault architecture. Stratigraphic and structural data indicate that the faults were active in the main phase of rifting of Gondwana. The influence of pre-existing structures on the Mesozoic rift faulting is scale dependent. Regionally, the faults trend parallel to subvertical, crustal-scale Brasiliano (c. 750–540 Ma) shear zones. Mylonitic foliations and broadly distributed low strain in the lower crust indicated by shear-wave splitting controlled the overall orientation and kinematics of the rift faults. However, outcrop observations of the faults show that at scales up to hundreds of metres, mylonitic foliations have little influence on fault architectures. Faults cross-cut shear zones and do not commonly utilize foliation planes as shear fractures. Instead, slip zones and fractures have a range of orientations that form acute angles to the local foliation orientation. This observation explains the range of focal mechanisms associated with seismicity that coincides with ancient shear zones in intra-continental areas.

Deformação Rúptil em Depósitos da Formação Barreiras na Porção Leste da Bacia Potiguar

This work identifies and quantifies the geometry and kinematics of faults affecting the Barreiras Formation in the eastern part of the Potiguar basin, Brazil. Focus is on the tectonic evolution of the study area and the quantification of fault offsets. Sedimentary rocks along the eastern coast of the State of Rio Grande do Norte are affected by faults that trend mainly NE-SW and NW-SE. Analyses of morphotectonic features, together with borehole, gravimetric, geoelectric, and structural data, indicate that strike-slip movements were associated with uplift and subsidence of fault blocks. The integration of these data suggests Neogene transtensional tectonics related to early N-S and late E-W compression.

Faciologia e análise tectônica preliminar da Formação Barreiras no Litoral Sul do estado da Bahia, Brasil

Muitos pesquisadores tem interpretado os sedimentos da Formacao Barreiras como um resultado de fluxos gravitacionais, pela quase completa ausencia de estruturas sedimentares hidrodinâmicas. Propoem tambem pouca ou nenhuma deformacao nesta unidade. O presente trabalho trata da associacao de litofacies, como tambem dos aspectos estruturais da Formacao Barreiras na costa sul do Estado da Bahia, Brasil. A observacao acurada de muitos afloramentos revelou que as facies da Formacao Barreiras na regiao sao tipicamente fluviais, de canais entrelacados. As imaturidades textural e mineralogica dos sedimentos mostram que rios pequenos e temporarios, fluxos hiperconcentrados e lagos efemeros predominaram nas fases iniciais de deposicao. Canais entrelacados bem desenvolvidos, com rios maiores, compoem o ambiente principal de transporte e deposicao durante as fases intermediaria e final de sedimentacao. Os trabalhos de campo evidenciaram grande numero de juntas com orientacao NW-SE e quantidade menor de juntas com direcao NE-SW. A determinacao preliminar da direcao dos esforcos, atraves de sistemas de juntas, indica um campo de tensoes neotectonico caracterizado por uma compressao horizontal maxima com orientacao NW-SE, compativel com o campo regional determinado por breakouts em pocos de sondagens. A analise da rede de drenagem revela a existencia de blocos estruturais limitados por grandes zonas de falhas e basculados em diferentes direcoes.

Luminescence dated Late Pleistocene wave-built terraces in northeastern Brazil

This study presents field and chronological investigations along the coast of northeastern Brazil from � 4 ◦ S to 9 ◦ S latitude, which corresponds to � 700 km of coastline under a semi-diurnal mesotidal regime. We investigated wave-built terrace deposits and dated sediments using the optically stimulated luminescence and thermoluminescence methods on quartz grains. The wave-built terraces yielded two main age groups: 200-230 ka and 100-130 ka, which we interpreted as depositional ages. We correlated these age groups with oxygen-isotope stages 7c and 5e, respectively. These events correspond to the antepenultimate and penultimate transgressions along the Brazilian coast. The deposits occur mainly in patches on low-lying flat plateaus along the littoral zone and incision valleys that cut across coastal tablelands. The altitude of the base of the 200-230 ka terraces ranges from 10 m above mean sea level (asl) to –2m asl, whereas the base of the 100-130 ka varies from 12 m asl to –2 m asl. Both terraces were deposited in the foreshore and upper shoreface zones. We noted a coincidence between sea-level highstand chronologies in northeastern Brazil and those in the Bahamas and Bermuda.

Landforms and Neotectonics in the Equatorial Passive Margin of Brazil

Neotectonic movements have been proposed in the literature in order to explain some landforms of the equatorial passive margin of northeastern Brazil. Its seismo-tectonic activity is concentrated in a few sectors located on or near the coast. Active or recently active structures are mainly identified in Neogene deposits. Identifying a contribution of neotectonics to the morphogeny is difficult in a context where most major morphostructural patterns are explained by Cretaceous tectonics related to oceanic opening and by differential erosion induced by Tertiary epeirogenic uplift. We aim to assess the nature of features considered as possibly neotectonic in origin. Seismogenic faults are not related to significant topographic breaks, except on the coast, where they usually reach only a few meters in height. A study of landforms located near zones of seismo-tectonic activity indicates a possible, probably weak, contribution of neotectonics to the formation of a few high scarps. These scarps occur along or near fault zones reactivated in Cretaceous times. We conclude that neotectonic movements are the result of ongoing deformation along predominantly strike-slip fault zones, with long term deformation rates similar to those recorded by dated landmarks (0.01 mm.yr -1). Despite reported deformation rates that can amount in places to 0.4 mm.yr -1, neotectonic rates are lower than erosion rates. The consequence is that major structural landforms in the region mainly originated in Cretaceous to early Tertiary events.