International audienceA combined analysis of seismic and morphological features identified in a set of high-resolution seismic reflection and bathymetric data, shows a systematic relationship between major modern seafloor morphological traces and the basinward migration of Late Pleistocene coastlines along the continental shelf of the Santos basin (Rio de Janeiro State, SE Brazil). Observed fairly continuous and sinuous mid-outer shelf escarpments are related to the sea-level variations and shelf exposure during the Last Glacial cycle. A bathymetric step at - 110 m is an erosional remnant of offlapping detached forced-regressive wedges that spread over 50 km in the shelf-dip direction, probably developed during periods of falling sea level between MIS 3 and 2. A second major escarpment at - 130 m was interpreted as the shoreline during the LGM, at the time of most extensive subaerial exposure of the continental shelf. However, a distal escarpment at - 150 m is expressed as a straight contour feature along the two main shelf-edge embayments that characterize the shelf break. This escarpment is coupled with a basal seaward-inclined and highly eroded ramp, and was interpreted as the erosional action of bottom currents during the last transgression due to the displacement of the southward flowing Brazil Current towards the present-day outer shelf. Previously published articles have regarded the morphological features observed on the modern shelf as indicators of stillstands during the post-LGM transgression. We conclude that, on the contrary, most of these features are actually from earlier parts of the Late Pleistocene and were formed in a regressive scenario under oscillating and relative slow sea-level fall

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A B S T R A C TSeismic analysis of sparker lines of GEOMAR cruises allowed us to address a first stratigraphic scenario for the shallow sedimentary record (~300 msec) of the continental shelf off Rio de Janeiro State, northern Santos basin. Two sets of seismic sequences were identified and interpreted as a succession of depositional sequences induced by repeated glacioeustatic cycles. Depositional sequences composing Set I (SqA-SqC) are dominantly sigmoidal, reflecting periods of increasing accommodation space that favoured the preservation of both aggradational and progradational units; sequences of Set II (Sq1-Sq5) are essentially seaward-thickening stacks of forced-regression wedges, implying periods of declining accommodation space. Comparison between seismic lines and chronostratigraphic data allowed the mapped sequences to be placed within the Plio-Quaternary. Correlations also suggest that most of Set I (SqA and lower portion of SqB) was deposited during the Pliocene (undifferentiated Pliocene), while the upper portion of sequence SqC and sequences of Set II (Sq1-Sq5) have been placed within the Quaternary. Correlation of chronostratigraphic data with δ 18 O isotopic "sea level curves" also supports the hypothesis that sequences Sq1-Sq4 are fourth-order forced-regression sequences that record 100-120 kyr glacioeustatic cycles for the last 440-500 kyr, while sedimentary units labeled Sq5 would represent the transgressive and highstand deposition during the Holocene. R E S U M OA análise sísmica de dados sparker das Operações GEOMAR permitiu a elaboração de um primeiro arcabouço estratigráfico da seção rasa (~300 msec) da plataforma continental do Estado do Rio de Janeiro, norte da bacia de Santos. Dois conjuntos de seqüências sísmicas foram interpretados como seqüências deposicionais induzidas por oscilações glacio-eustáticas. O Conjunto I (SqA-SqC), composto por seqüências dominantemente sigmoidais, reflete condições de geração de espaço de acomodação sedimentar capaz de preservar seus componentes agradacionais-progradacionais; o Conjunto II (Sq1-Sq5), composto principalmente por prismas de regressão forçada, indica diminuição relativa de espaço de acomodação. Dados cronoestratigráficos de poços permitiram posicionar a seção sísmica investigada na janela plio-quaternária: à maior parte do Conjunto I (SqA e parte inferior da SqB) foi atribuída uma idade Plioceno (indiferenciado); à seção estratigráfica que se estende da porção superior da SqC até o Conjunto II foi atribuída uma idade quaternária. A correlação entre a base de dados e curvas globais de variações isotópicas de δ 18 O permitiu ainda sugerir que as seqüências Sq1-Sq4 registram sequências regressivas de quarta ordem (ciclos glacio-eustáticos de cerca de 100-120 ka) durante os últimos 440-500 ka. A sequência Sq5 representaria a deposição holocênica, constituída por depósitos transgressivos e de sistemas de mar alto.

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