Moab Praxedes Gomes

Characterization of the Brazilian continental shelf adjacent to Rio Grande do Norte state, NE Brazil

B S T R A C T This study focuses on the analysis of high-resolution seismic profiles, integrated with sedimentological, echosounder, SRTM and satellite image datasets, of the Brazilian continental shelf adjacent to the Rio Grande do Norte State, NE Brazil. Located in the northeast of Brazil, the State of Rio Grande do Norte is bounded by two main coastal and shelf systems: the eastern coastal-shelf, from the Sagi River to the Touros High, and the northern coastal-shelf, extending from Touros High to Tibau. This shelf represents a modern, highly dynamic mixed carbonate-siliciclastic system characterized by reduced width and shallow depths as compared with other parts of the Brazilian shelf. It has an average width of 40 km, the shelf-break lying at a depth of ~ 60 m. This shelf is subject to the full strength of the westerly South Equatorial current combined with high winds and moderate to high tides and waves. A sharply defined stratigraphic boundary, probably between the Pleistocene and Holocene deposits, is clearly to be observed in the seismic record. Incised-valleys extending from the main river mouths (e.g.the Potengi, Acu, and Apodi) to the shelf break dominate the area investigated and may indicate periods of lower sea level.

The investigation of a mixed carbonate-siliciclastic shelf, NE Brazil: side-scan sonar imagery, underwater photography, and surface-sediment data

Side-scan sonar imagery and underwater photography, coupled with bathymetric and sedimentologic data, revealed noteworthy features and the interplay of multiple processes on the north-eastern Brazilian continental shelf. The high-resolution data cover a 500 km2 area of the shallow (~70 m) and narrow (~40 km) shelf, which comprises the submerged portion of the Potiguar Basin; the area includes the partially filled incised valley of the ancient Acu River.The seafloor imagery was integrated with 673 surface sediment samples to distinguish boundaries within the mixed siliciclastic-carbonate region. The sediment grain size increases from nearshore, muddy, siliciclastic sediments to biogenic, coarse sand and gravel on the outer shelf. Carbonate-rich sediment predominates, except on the eastern margin of the Acu Incised Valley (AIV), the nearshore zone and estuary mouths. The AIV is filled mainly with muddy sediments reworked from the latest transgressive deposits. Bedform configurations indicate bottom currents associated with tidal and wave-driven currents.Mixing processes are controlled by the interplay of high hydrodynamic energy regime and shelf physiography. Special attention was paid to the middle and outer shelves, where a large field of newly found reefs occurs between 20 and 50 m depths. The reef substrate is composed of limestones, probably belonging to the Guamare Formation.SCUBA dive inspections revealed benthic colonies (corals: Montastraea cavernosa, Porites astreoides, Siderastrea stellata; sponges: Scopalina ruetzleri, Callyspongia vaginalis, Ectyoplasia ferox, Ircinia sp., Spirastrella sp., Aiolochroia sp.) over fossil reefs. A preliminary examination of the foraminiferal species showed a Caribbean-type reef community, including Amphisorus hemprichii, Amphistegina gibbosa, Archaias angulatus, Heterostegina antillarum, Homotrema rubra, Peneroplis carinatus and P. proteus.

Fluxo de processamento aplicado a dados de sísmica de alta resolução em ambiente de Plataforma Continental: exemplo: Macau-RN

This paper presents a proposal for a processing flow applied to high-resolution seismic data. It is organized in the implementation steps of operations and takes in account different factors in the flow organization. Usually used as raw data, the high resolution seismic processing is little discussed in literature. It differentiates in many aspects of processing used in conventional seismic data. The application of the processing flow to shallow seismic data was based on key criteria such as: behavior of a wave of high frequency, properties of the environmental, operation of the filters, gain, which allowed to obtaining a result with a better quality than the raw data. The flow defaults a logical sequence of procedures (header edition, filter, gain, deconvolution) for this kind of data, with basic steps, necessary for a general flow, and some additional processes, revealing potential problems and their solutions.

Beaches of Rio Grande do Norte

The Rio Grande do Norte coast extends 410 km, and consists of sandy beaches (72 %), active sea cliffs carved into Cenozoic sediments of the Barreiras and Tibau formations (26 %), and transgressive dune fields and beachrock. It comprises two different sectors: the northern (equatorial) coast trends east for 244 km, while the eastern (oriental) coast trends south for 166 km. The eastern sector is characterized by wave-dominated and some tide-modified beaches that are mainly reflective to intermediate states. In contrast, the northern sector has resulted in tide-modified and tide-dominated beaches that range from reflective (the dominant state) to intermediate. In general the Refletive + Low tide terrace (R + LTT) is present along the entire coast for most of the year, while wave-dominated Longshore bar and trough (LBT), Rhythmic bar and beach (RBB), Transverse bar and rip (TBR), Low tide terrace (LTT) and reflective (R) occur along the eastern sector, and tide-dominated Beach + tidal sand flats (B + TSF) occurs along parts of the northern sector. R + rock flats and coral reef flats are present in both sectors, where bedrock and beachrock reefs are prevalent. Beachrock reefs are very common along the Rio Grande do Norte shore, occurring in both the offshore and onshore zones. Beach morphodynamics is modified due the presence of the beachrocks. Sea level variability is dominated by tides (up 98 % of the energy spectra). The sea level subtidal component is well correlated with the winds, but demonstrate very low amplitudes. Longshore currents are wind modulated, while cross-shore currents are primarily modulated by tides and secondarily by winds. Erosional hotspots and, both natural and anthropogenic hazards, are present along the Rio Grande do Norte coast.