Shelf morphology as an indicator of sedimentary regimes: a synthesis from a mixed siliciclastic-carbonate shelf on the eastern Brazilian margin

Abstract

Modern shelf morphology is the result of the interplay between short and long term sedimentary processes. The relation between rates of sediment supply/carbonate growth and accommodation space creation will not only control coastal transgression and regression, but will also define the shelf sedimentary regimes acting to shape the seabed. Herein, shelf morphology and sedimentology are investigated in order to discuss how these characteristics can be representative of distinct sedimentary regimes. The study area is the eastern Brazilian shelf where coastal transgression and regression coexist with the most important coral reef system of the South Atlantic. A compilation of existing published and unpublished data was carried out in order to produce morphological and faciological maps and compare the mapped features with high-resolution seismic and sonographic data. The results show three major regions or morphological compartments: Abrolhos Shelf, Doce River Shelf and the Paleovalleys Shelf. In terms of shelf sedimentary domain, rhodolith beds predominate over the outer shelf along the entire area, coralline reefs are present along the northern Abrolhos inner shelf and a significant terrigenous mud deposit is observed associated to the Doce River adjacent inner shelf beds. The rest of the shelf is composed by bioclastic or terrigenous mud sand and gravel. Terrigenous sedimentation is always restricted to the shoreface or inner shelf shallower areas and carbonate sands and gravels are predominant elsewhere. The Abrolhos shelf shows two distinct sectors; the northern area is a typical mixed sediment environment that has a supply regime along the coast/shoreface, mainly due to longshore transport and a carbonate regime along the inner and outer shelf. The southern shelf morphology and sedimentation are controlled by the antecedent topography and is typically a accommodation regime shelf with associated rhodolith beds. The Doce river shelf is a supply regime environment with the formation of a 5 to 8m thick regressive deposit with downlapping clinoforms. Southward from the Doce river shelf, a significant shift in sedimentary regime is observed as the morphology becomes very irregular with associated hardbottoms and unfilled paleovalleys. This sector of the shelf (Paleovalley shelf) is characterized by an accommodation regime. The interpretation shows that the entire study area can be defined as a mixed sedimentation shelf, showing supply and accommodation regimes. Shelf morphology worked as an indicator of these changes. Carbonate/terrigenous deposition during a highstand/regressive phase coeval along the eastern Brazilian shelf, either laterally and across shelf. This lateral/along coast variation in sediment supply and carbonate production leads to distinct lateral facies and geometry. These spatial changes in morphology and facies, with coexistence of carbonate and siliciclastic sedimentation, are very important for the correlation and interpretation of the geological record, especially stratigraphic surfaces and sequence units.

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