Geraldo da Silva Vilas Boas

The Barreiras Group in the Northeastern coast of the State of Bahia, Brazil: depositional mechanisms and processes

The Barreiras Group is a Miocene to Lower Pleistocene continental terrigenous sedimentary deposit exhibiting a large occurrence along the Brazilian coast. In the Conde region, located in the northeastern part of the State of Bahia, the sedimentological characteristics of these sediments are indicative of a deposition as gravelly and sandy bed load in braided fluvial systems, related to alluvial fans, under an arid to semi-arid climate. The basal portion of the group is dominated by a gravelly-sandy lithofacies deposited by debris flows and pseudoplastic debris flows, with lesser occurrences of subaqueous deposits, characterizing a proximal fluvial system deposition. The upper portion is made up of gravelly-sandy sediments that include subaqueous, debris flows and pseudoplastic debris flows deposits. They suggest deposition in a more distal zone as indicated by the larger occurrence of subaqueous deposits and the presence, though rare, of downstream-accretion macroforms. Besides the climate control, deposition of the Barreiras Group was strongly influenced by the intraplate tectonism, which has been affecting the South America Platform since the Middle Miocene, when neotectonism began in Brazil.

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.

High and Low Frequency Erosive and Constructive Cycles in Estuarine Beaches: an example from Garcez Point, Bahia / Brazil

Monitoring of the morphodynamic variations of the beaches associated with an estuary contiguous with Garcez Point, Bahia, Brazil, and the superposition of aerial photographs from the region, show the presence of distinctive erosive and constructive cycles of low and high frequencies. Between 1959 and 1989, one event of shoreline erosion and progradation was recognized on the oceanic beaches just outside the estuary. Inside the estuary, an erosion phase at the southern margin coincides with a constructive phase at the other side, and vice-versa. On the southern estuarine beach, low-frequency cycles of erosion and progradation are also perceived, but with the inverse trend when compared to the contiguous oceanic beach. During the beach monitoring period (February/1991 to July/1992), the oceanic beach showed retreat rates varying from 23.7 m/year, at the channel entrance, to 1.0 m/year, three kilometers away from it. During the same period, the estuarine beach advanced at a rate of 60.3 m/year. The long-term dynamics of the shoreline position in both sides of the estuarine entrance appears to be related to the position of the channel in the ebb-tidal delta.

Sondagem geoelétrica usada para investigar o espaço de acomodação dos sedimentos quaternários no estuário Jacuípe, Bahia, Brasil

O presente trabalho verificou o espaco de acomodacao de sedimentos do estuario Jacuipe, localizado no municipio de Camacari, no litoral norte da Bahia. A tecnica de sondagem eletrica vertical foi usada para interpretar as unidades litoestratigraficas que preenchem a referida area. O ambiente estudado se mostra controlado estruturalmente pela transicao dos dominios tectonicos do embasamento cristalino e da sub-bacia do Reconcavo. Ha indicios, apesar de incipientes, de que esse controle possa afetar, dramaticamente, a reducao do espaco de acomodacao estuarino, cenario opostamente relatado na literatura. As sondagens eletricas verticais revelaram que os pacotes sedimentares quaternarios do estuario recobrem diretamente a sub-bacia do Reconcavo, o maior deles possuindo espessura de 20 m de areia. Foram determinados os volumes dos espacos de acomodacao para diversos cenarios estuarinos, dentre eles o arcabouco total durante o Holoceno, de 6,84 × 107 m3 e o espaco restante, de 6,67 × 106 m3. Este ultimo valor comprovou o estado tipico de um ambiente em avancado estagio de preenchimento, com apenas 9,89% do volume inicial.

FÁCIES SEDIMENTARES E MODELO DE SEDIMENTAÇÃO DO GRUPO PARAGUAÇU NA REGIÃO DE RIO DE CONTAS, BORDA OCIDENTAL DA CHAPADA DIAMANTINA, BAHIA

The Middle Proterozoic metasediments of the Paraguacu Group, outcropping in the Rio de Contas region, on the western border of the Chapada Diamantina (Bahia), consist of three terrigenous sedimentary facies displaying characteristic features of eolian-, coastal-, and shelf-type depositional environments. The facies 1, which occupies the basal portion of the sequence, is made up of quartzites and laminated metasandstones interpreted as transgressive in nature, and deposited over as extensive shallow marine platform sowing storm effects. The intermediate facies 2 is composed of quartizites with large scale high-angle cross-bedding defined as eolian is origin, accumulated on vast coastal dune fields. Facies 3, which constitutes the top of the group starts with transgressive, fine, and laminated metasandstones (sub-facies 3A) deposited in a shoreface environment, under moderate energy conditions. This is overlain by muddy deposits (sub-facies 3B), also transgressive in nature and accumulated in a shelf environment submitted to hydraulic regimes of alternating calm and stormy periods. The uppermost facies of the Paraguacu Group (sub-facies 3C) consists of metasiltstones and metasandstones deposited in a shoreface zone prograding over a muddy shelf.