Carbonate-siliciclastic sedimentation on a high-energy, ocean-facing, tropical ramp, NE Brazil

Abstract

The inner portion of the continental shelf of Rio Grande do Norte State, Brazil has been investigated by Landsat-TM imagery and by scuba diving. The area experiences high-energy, shoreline and shelf-margin parallel currents driven by a combination of oceanic, tidal and wave processes. The high meso-tidal range (<4 m) further distributes the high-energy waters over the entire inner shelf region (0-25 m). This shelf is a high-energy, distally steepened ramp in which the distribution of carbonate and siliciclastic facies belts is controlled by the energy of the environment, the sources of siliciclastic sediment and the Quaternary history of the area. Carbonate sediments are generated throughout the sublittoral zone and are dominated by calcareous red and green algae. SmalL, low-diversity coral patch reefs occur in shallow areas. Quartz-rich sands form beaches and subaerial longitudinal dunes, and an offshore, submarine, sandstone outcrop interpreted as a former Quaternary shoreline. This sandstone has been eroded during the Holocene transgression to generate quartz sands deposited in a train of transverse dunes migrating parallel to the coast. Longitudinal sand ribbons comprising bioclastic gravelly sands attest to high-energy, shore-parallel currents in the inner shelf. Branching coralline algae (maErl) form stabilized sea-floor areas. Preliminary 14C dating indicates a Pleistocene highstand period when coarse algal sands accumulated. Holocene lowstand conditions spread quartz-rich sands over the inner ramp area, which were reworked during the Holocene transgression.Present-day erosion, transport and in situ carbonate production results in mixing of these earlier carbonate and siliciclastic units. The Brazilian ramp has similar, but narrower morphology, when compared with the distally steepened ramps of the Yucatan and west Florida. All three ramps are swept by coastline-parallel currents. In Brazil, the higherenergy, windward-facing, inner ramp has calcareous algal sands and patch reefs compared with the inner ramps of the leeward-facing Gulf of Mexico examples that accumulate molluscan sands. Outer ramp facies in each example are planktonic foraminiferal oozes.

Facies architecture of a Late Jurassic carbonate ramp: the Korallenoolith of the Lower Saxony Basin

“…Measured palaeoflows are towards the S or SW, indicating a longshore transport. Longshore sediment transport is also observed in recent tropical ramps from Yucatan and Florida (Testa and Bosence 1998).…”

“…The occurrence of sandstone beds and high quartz content in the upper Korallenoolith Formation indicates that the Wiehengebirgsquarzit passes laterally into the upper Korallenoolith Formation, as already suggested by, e.g., Klüpfel (1931), Freitag (1969), Rumohr (1973), Lorenz (1972), andStinder (1991). The high siliciclastic input above the sequence boundary probably occurred during a sea-level lowstand (Testa and Bosence 1998) and/or may reflect changes in climate. Humid periods commonly enhance continental weathering and cause high terrigenous runoff and siliciclastic deposition (Pittet and Strasser 1998;Bauer et al 2002), whereas arid periods lead to a reduction of the erosion rate in the hinterland (Hallam 1984).…”

New records of red seaweeds to the coast of Ceará State, northeastern Brazil

“…This northeastern region is unique by the occurrence of oligotrophic waters and abundance of substrata, enabling the development of rich marine flora (Horta et al 2001). The coastline is under the influence of highly energetic waves, subhumid climate, high temperatures throughout the year (~26 °C), strong winds and an intense flux of resuspended sediments, despite the lack of fluvial inputs (Testa & Bosence 1998, Knoppers et al 1999, Vital et al 2008, Vital et al 2010.…”

Aspectos sedimentológicos do talude continental setentrional do Rio Grande do Norte, NE do Brasil

“…A plataforma continental brasileira adjacente à costa norte do Rio Grande do Norte (RN) é caracterizada como uma plataforma mista composta por uma cobertura de sedimentos siliciclásticos, carbonato-siliciclásticos e carbonáticos (Pessoa Neto, 2003;Vital et al, 2008), distribuídos nas porções interna, média e externa (Gomes & Vital, 2010), que abriga diversos compartimentos de relevo como dunas submersas, bancos de recifes de corais, rochas praiais, e proeminentes vales incisos (Testa & Bosence, 1998;Lima & Vital, 2006;Schwarzer et al, 2006;Vital et al, 2008Vital et al, , 2010a.…”