Jair Weschenfelder

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.

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.

Morfodinâmica da desembocadura da Lagoa de Tramandaí (RS, Brasil)

The link of the lagoon bodies with the ocean occurs through of the inlets, which are also important ways to sediments exchange. The morphodynamics of this environment is controlled by a series of variables, such as tidal prism and channel geometry, amongst others. The interactions between these variables can cause a modification of the inlet geographical position. The knowledge of these parameters and interaction processes is extremely relevant for preservation and management of lagoon systems. This work analyzed the stability of the mouth of Tramandai Lagoon, based on hydrodynamic, sedimentological and morphological data, integrated in a spatial and temporal analysis. The mouth of the lagoon is located between the beaches of Tramandai and Imbe (north coast of Rio Grande do Sul) and is fixed by a rip-rap structure along its left margin. The methodology applied in this analysis consisted of bathymetry measurements, geoprocessing techniques and calculation of the empirical stability relations. It was possible to note that prior to fixing the left margin of the channel, the mouth migrated to the NE following of the predominant littoral drift. The analysis of the relations of physical stability allowed to conclude that the channel presented strong instability before the work of fixing the mouth. After the construction of the rip-rap, the minimum transversal section of the inlet has been fixed and there are no silting records.

Incised valley paleoenvironments interpreted by seismic stratigraphic approach in Patos Lagoon, Southern Brazil

The Rio Grande do Sul (RS) coastal plain area (33,000 km 2 ) had its physiography modified several times through the Quaternary, responding to allogenic and autogenic forcings. The Patos Lagoon covers a significant area of RS coastal plain (10,000 km 2 ), where incised valleys were identified in previous works. About 1,000 km of high resolution (3.5 kHz) seismic profiles, radiocarbon datings, Standard Penetration Test (SPT) and gravity cores were analyzed to interpret the paleoenvironmental evolution as preserved in incised valley infills. Seismic facies were recognized by seismic parameters. The sediment cores were used to ground-truth the seismic interpretations and help in the paleoenvironmental identification. Key surfaces were established to detail the stratigraphical framework, and seismic facies were grouped into four seismic units, which one classified in respective system tracts within three depositional sequences. The oldest preserved deposits are predominantly fluvial and estuarine facies, representing the falling stage and lowstand system tracts. The Holocene transgressive records are dominated by muddy material, mainly represented by estuarine facies with local variations. The transgression culminated in Late Holocene deposits of Patos Lagoon, representing the highstand system tract. The depositional pattern of the vertical succession was controlled by eustatic variations, while the autogenic forcing (paleogeography and sediment supply) modulated the local facies variation.