Morphology and sedimentology of the shelf-upper slope transition in the Abrolhos continental shelf (east Brazilian margin)

Abstract

In mixed terrigenous-carbonate shelves, terrigenous sediment input may influence changes in slope morphology and sedimentation. Moreover, sea level changes and antecedent geology can also play an important role in defining depositional patterns for the shelf-slope system. This work investigates contrasting depositional and geomorphological patterns in the Abrolhos shelf-slope system, eastern Brazilian margin, South Atlantic. We have combined existing bathymetric datasets with bottom imagery and samples collected at the Johnson-Sea-Link manned submersible along two Abrolhos slope sites, which are separated by almost 2° of latitude. Our dataset includes seafloor sediments, single-beam echosounder, and 20 h of video recording. Dives ranged from 90 to 620 m water depth. Results showed that the north and south Abrolhos shelf-slope systems are distinct in terms of their morphology and sedimentology. The depositional pattern in the northern upper slope is typically carbonate dominated, with the occurrence of carbonate mounds around 500 m deep, small biogenic structures around 200 m deep, interspersed with wackstones and packstones. The upper slope is characterized by a steep limestone wall at 120 m deep that resembles a shelf-edge reef, followed by extensive rhodolith beds at the shelf edge and outer shelf. The southern upper slope shows a sigmoidal profile with a gentle gradient. Mixed siliciclastic–carbonate fine sedimentation dominates from 300 to 700 m deep. On the upper slope, fine carbonate sedimentation predominates, and a biogenic reef structure is observed in a water depth around 120–130 m. The shelf edge and outer shelf are also characterized by extensive rhodolith beds. Main findings indicate that a volcanic rock complex is an important antecedent geology that may influence the setting up of distinct sedimentary regimes and sediment input to the slope. The geometry of the antecedent geology controls the paleotopography forming a gradient towards the south. This, combined with sea-level changes and Rio Doce riverine sediment input, can explain the contrasts between the northern and southern shelf-upper slope depositional settings. Here, for the first time, the presence of possible Last Glacial Maximum or early deglaciation stage shelf-edge reef systems is reported for the Abrolhos Bank.