Along-Shelf Changes in Mixed Carbonate-Siliciclastic Sedimentation Patterns

Abstract Modern mixed carbonate and siliciclastic shelves have been described on continental margins around the world. In general, the mixed shelf sedimentation model shows a cross-shelf gradient from terrigenous to carbonate sediments or a shift from the continental shelf to the basin. Here we use acoustic seabed mapping, surface sediment sampling and high-resolution seabed videos to report the coeval occurrence of biogenic reefs, rhodolith beds and river deltas along the southeast-east Brazilian shelf. This investigation discusses the importance of combined environmental controls, such as shelf width, sediment input sources, mesoscale circulation and sea-level changes, in controlling mixed-sediment distribution patterns. In the study area, shelf width varies from 40 to 200 km, the Rio Doce is the major sediment input source, the Brazil Current plays a significant role along the shelf edge, and upwelling processes are reported within this area. Results show that a transition from terrigenous to carbonate sediments can occur laterally or along-shelf, i.e., terrigenous and carbonate sediments coexist along the same water depth range. In narrow mixed continental shelves, a strong lateral/latitudinal change in the sedimentary domains can be observed due to lateral variation in environmental controls, such as sediment input and carbonate production. Wider continental shelves tend to display a greater mixing transition zone across the depth gradient. The coexistence of reef beds with terrigenous-derived sedimentation is the product of different temporal scales of geological and oceanographic processes. The origin and formation of the main facies are related to the last deglaciation period, but modern sediment input and oceanographic processes (seasonal upwelling) are important drivers to terrigenous and carbonate sedimentation patterns. These findings bring attention to the interpretation of the marine geological record, where a carbonate-siliciclastic facies boundary can represent lateral changes instead of depth gradients.