The surface morphology of the transgressive dunefield barrier extending from Itapeva to Tramandai along the northern littoral of Rio Grande do Sul, is examined and an attempt is presented to link morphological changes across the barrier to Holocene sea level changes. The 4·5 km wide Holocene barrier displays two typical morphologies: an inner part dominated by large-scale, continuous alongshore, overlapping dunefield phases comprising sand sheets, dunes, deflation plains and precipitation ridges; and an outer part dominated by discontinuous, medium-to small-scale, triangular to lobate transgressive dunefield phases, cut by both active and relict (palaeo-) creeks or washouts. Holocene sea level in the region rose to c. + + + + +1 to + + + + +3 m above present reaching a maximum around 5100 years BP and then progressively fell to the present level. We argue that the effect on barrier development was to suppress the development of a drainage system during the rising and maximum stages, and encourage the development of an organized drainage system in the form of regularly spaced washouts during the falling period, and that this change in sea level from rising to falling therefore produced the large-scale differences in barrier morphology.
Regional SettingRio Grande do Sul has a low-relief coastal plain formed during the Quaternary by juxtaposition of sedimentary deposits of four barrier/lagoon systems designated I (oldest) to IV (youngest) by Villwock et al. (1986). The coastal Figure 1. Holocene barrier types along the southern Brazilian coast from Torres to Chui. The first map (I) displays the northernmost sector and II, III and IV the progressively more southern sectors. This re-interpretation of Dillenburg et al.'s (2000) figure indicates that the Holocene barrier system comprises relict (vegetated) transgressive dunefields, active transgressive dunefields, or a complex barrier type comprising both foredune ridges and transgressive dunefields.plain ranges from 20 to 80 km wide and is bordered landward by bedrock highlands. Climate is humid temperate with generally warm to hot temperatures in summer (mean 25°C), and cool temperatures in winter (mean 17°C). Rainfall ranges from 1000 to 1500 mm and is evenly distributed throughout the year.Rio Grande do Sul (RS) has a uniform and gently undulating barrier coast 625 km long, oriented NE-SW and subject to dominant swell waves generated in southern latitudes and wind-generated waves produced by strong spring-summer sea breezes from the northeast. The average significant wave height is 1·5 m measured in water depths of 15-20 m (Motta, 1969). During autumn and winter storms (April to July) wave height may frequently exceed 2·0 m Morphology of transgressive dunefield 409 Figure 2. Sea level envelope and palaeo-sea level reconstructions for the Brazilian coast north of 28° (solid line) and south of 28°( dashed line), based on vermetid samples and with outliers removed (data from Angulo et al., 2006).410 P. A. Hesp et al.Figure 3. Vertical aerial photograph of the Curumim b...
RESUMOA barreira costeira de idade holocênica (sistema barreira-laguna IV), no Rio Grande do Sul, não evoluiu de um modo uniforme nos últimos 6 -5 ka, ao longo dos seus 620km de extensão. O segmento da barreira investigado neste trabalho situa-se entre dois outros segmentos, um ao norte e outro ao sul, onde a barreira holocênica apresenta um caráter regressivo e transgressivo, respectivamente. Com base em dados morfológicos, litológicos de sub-superfície, datações por 14 C e simulações de evolução costeira, foi definida uma natureza transgressiva para este segmento da barreira, porém não uniforme ao longo de seus 30 km de extensão, e estabelecida a sua cronologia evolutiva. Em Tramandaí, a barreira permaneceu relativamente estável nos últimos 6 -5ka, enquanto que em Jardim do Éden e Cidreira a barreira apresentou um comportamento dominantemente transgressivo. Estas diferenças evolutivas são atribuídas a fatores locais que influenciaram no balanço de sedimentos. Como principal fator local é destacada a declividade diferenciada dos substratos da barreira em Tramandaí (0,062°), Jardim do Éden (0,067°) e Cidreira (0,069°), a qual, por sua vez, influenciou nas taxas de translação da barreira durante a elevação do nível do mar, e na energia das ondas incidentes no sistema praial.Palavras-chave: estratigrafia costeira; barreiras costeiras; Shoreface Translation-Barrier Model.
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