Iran Carlos Stalliviere Correa

Les variations du niveau de la mer durant les derniers 17.500 ans BP: l'exemple de la plate-forme continentale du Rio Grande do Sul-Brésil

Abstract The horizontal coastline translation on the Rio Grande do Sul continental shelf during the post-glacial Wisconsinian period, differed markedly between the northern region where it was about 8.6 m/year, and the southern region where the translation was 16.7 m/year. The northern region with a narrow continental shelf and a relatively strong pendant, suffered a slow transgression characterized by a stronger hydrodynamic regime. In the southern region, the transgression was quite faster and combined with a lower energy regime, caused by the presence of a wider continental shelf and a soft pendant. The vertical coastline translation on the Rio Grande do Sul continental shelf, between 17,500 and 6500 yrs B.P. initiated from −130 m to the current sea level three intermediary stabilizations: the first one at 11,000 yrs B.P. between −60 and −70 m; the second one at 9000 yrs B.P. between −32 and −45 m; and the third at 8000 yrs B.P. between −20 and −25 m. On Rio Grande do Sul continental shelf morphosedimentary marks confirm the sea-level stabilizations. They are characterized by an increasing of the shelf pendant associated to a biodetritical and coarse sand sedimentation at the profile trench as well as the presence of heavy mineral concentrations.

Paleocanais como indicativo de eventos regressivos quaternários do nível do mar no Sul do Brasil

Seismic studies reveals paleochannels that dissected the Rio Grande do Sul coastal zone. Two paleochannels systems were established in the Patos Lagoon area. The channel filling of the younger system is Holocene. It is related to the last marine regression of the Pleistocene, corresponding to the oxygen isotope stage 2. The incision and filling of the older system is related to the previous regressive-transgressive event, corresponding to the oxygen isotope stages 6-5. The paleodrainage paths were connected with those previously recognized on the adjacent continental shelf. Landwards, the paleodrainage lines can be linked with the present courses of the Camaqua and Jacui rivers. The paleodrainage network recognized on the coastal zone represents a river-shelf system, linking the drainage basin to the depositional settings on the marginal basin, bypassing the continental shelf exposed during a forced regression event. The bypassing discharge of the younger system fed delta systems installed on the shelf edge during the sea level lowstand that lasted until the end of the last glaciation. The last transgression drowned the incised drainage, infilling it and closing the inlets formerly existing between the Patos Lagoon and the Atlantic Ocean. The incised paleodrainage herein considered may have played an important role on the basin-margin architecture, facies distribution and accommodation during the Quaternary sea level oscillations.

Holocene Paleoinlet of the Bojuru Region, Lagoa dos Patos, Southern Brazil

Abstract The Holocene environmental evolution of the Bojuru region, Rio Grande do Sul state, southern Brazil, was inferred from seismic, geochronologic, sedimentologic, and paleontologic data. The absolute ages of two samples, determined by the 14C method, are 9400 ± 140 Cal BP and 7370 ±150 Cal BP. On the basis of lithology and palynomorph and diatom data, the six stages of paleoenvironmental development of this region were chosen. Layers of muddy sediments characterized by a significant incidence of marine palynomorphs and marine and estuarine diatoms were deposited during the Early and Middle Holocene. Seismic records (3.5 kHz) revealed a prominent buried paleochannel that can be correlated to the present Barra Falsa channel. The main period of the channel incision was related to the last regressive event of the Late Pleistocene. During the last Holocene transgression, this paleochannel was filled up by fluvial, estuarine, and marine sediments. The integrated analysis of data reveals that the Barra Falsa feature resulted from a former channel connecting the Patos lagoon and the Atlantic Ocean. The closure of this inlet channel is attributed to shifts in the sedimentation milieu related to the transgressive maximum and subsequent regression during the Late Holocene.

Seasonal sediment transport pathways in Lirquen Harbor, Chile, as inferred from grain-size trends

Bottom sediment samples were collected in June and December 1997 at 166 stations on a rectangular grid in a small port (Lirquen Harbor, BioBio Region, Central Southern Chile) facing siltation problems with a view to determining local seasonal sediment transport pathways through the application of three different methodologies based on grain-size trends. Measured winds and near-bottom currents together with hindcast wave allowed to establishing the influence of the prevailing meteorological and hydrodynamic conditions on local sediment circulation. The results suggest that transport of deep and shallow water sediments occur rarely under high-energy conditions. The current data suggest that tidal asymmetry would be an important agent of sediment transport in Lirquen Harbor. The sediment transport pathways inferred from grain-size trends are compared with the measured water circulation of the study area and sediment dispersal patterns on aerial photographs. The results yielded by the grain-size trend methodologies of Gao-Collins (1992) and Le Roux (1994b) indicate a possible seasonal variation in transport pathways agreeing with the prevailing meteo-hydrodynamic conditions, whereas the McLaren-Bowles (1985) approach does not show a significant difference. It is suggested that the latter methodology may represent transport on a spatial macroscale as opposed to the mesoscale patterns yielded by the other two techniques. Based on the results of the three grain-size trend methodologies, the observed current and wind data, the hindcast wave data and aerial photographs a conceptual model of net annual sediment transport is proposed for Lirquen Harbour.

Paleochannels related to late quaternary sea-level changes in Southern Brazil

A evolucao geologica da plataforma continental e zona costeira do sul do Brasil no periodo Quaternario e aqui enfocada. Registros sismicos de alta resolucao da Lagoa dos Patos revelaram canais que dissecaram profundamente a regiao antes da formacao da planicie costeira atual. Dois periodos de incisao de canal sao inferidos, baseados principalmente em registros sismicos. Os cursos fluviais mapeados por sismica podem ser conectados aqueles reconhecidos na plataforma continental adjacente por estudos anteriores. a montante, a incisao da drenagem ocorreu principalmente no prisma sedimentar costeiro depositado durante eventos preteritos de nivel do mar alto. A rede de paleodrenagem, reconhecida na planicie costeira e na plataforma continental, representa um sistema de ligacao da bacia de drenagem continental aos depocentros da bacia marginal, sobrepassando a plataforma continental exposta durante uma regressao forcada. A incisao da drenagem na planicie costeira e plataforma continental do Rio Grande do Sul desempenhou um papel importante na arquitetura marginal, distribuicao de facies e no espaco de acomodacao durante as flutuacoes do nivel do mar no Quaternario.

Estruturas rasas na margem equatorial ao largo do nordeste brasileiro (Estado do Ceará): análise de relevo e anomalias gravimétricas residuais

A area pesquisada localiza-se na plataforma continental da margem equatorial atlântica, ao largo do Estado do Ceara, formada durante a abertura do oceano Atlântico Equatorial (Aptiano). O mapa gravimetrico residual evidencia estruturas do arcabouco crustal continental ao largo da area estudada, em especial o Lineamento Transbrasiliano e sua continuacao offshore (Lineamento Transversal). As bacias da margem continental (Ceara e Potiguar submersa) sao bem marcadas por minimos gravimetricos. O mapa de anomalias de relevo demonstra que estruturas pre-cambrianas de trend NE-SW controlam feicoes superficiais, principalmente ao largo de Acarau. Ja na plataforma externa ao largo de Camocim, o controle e exercido por feicoes NW-SE e E-W, possivelmente relacionadas a deformacao transformante na Sub-bacia Piaui-Camocim. No setor central, ha a influencia de trends E-W, possivelmente relacionados a fraturas oceânicas, e trends NW-SE, provavelmente relacionados a estruturas pre-cambrianas. Ja no setor sudeste, trends subparalelos a direcao da plataforma aproximam-se do contato entre embasamento raso e Bacia Potiguar submersa. O estado de esforcos neotectonico e inferido como transcorrente com esforco maximo horizontal subparalelo a direcao da margem, atuando principalmente na plataforma interna, e extensional com esforco maximo horizontal perpendicular a direcao da margem, na plataforma externa.

Processus dynamiques dans la distribution des sédiments de la plate-forme continentale du Rio Grande do Sul (Brésil)

ResumeCe travail presente les processus qui ont contribue a la distribution des sediments superficiels de la plateforme continentale du Rio Grande do Sul-Bresil. Cette plateforme est recouverte par quatre populations granulometriques : sable tres grossier a moyen, sable fin a tres fin, silt et argile. Un regime sedimentaire terrigene predomine sur la plate-forme et le talus continental de la region etudiee, avec la presence considerable de sediments reliques et palimpsestes. Les sediments modernes sont restreints et lies a l’influence du fleuve de la Plata et de la lagune des Patos. La presence de sediments grossiers au bord externe de la plate-forme continentale peut etre expliquee par la conjonction de l’effet de seuil et des ondes internes qui ne permettent pas, dans les regions Nord et Sud, le depot de sediments fins sur les sediments plus grossiers. Dans la region centrale, ces effets sont reduits par la structure morphologique denommee « Cone du Rio Grande », ce qui explique la presence de sediments...