Island beaches morphological and sedimentary short term variations – the case of SE Fernando de Noronha Island, South Atlantic, Brazil

Abstract

RESUMOSintetizam-se conhecimentos modernos sobre as ilhas e arquipé-lagos oceânicos brasileiros, cuja origem e localização geográfica sofreu decisiva influência de estruturas da litosfera oceânica. Reúnem-se informações sobre a geologia das ilhas e a nomenclatura recente, química e mineralógica, de suas rochas. É sugestiva a influência de hotspots na evolução de zonas de fratura, originando cadeias vulcânicas e ilhas. No Ceará a atuação de hotspot suposto astenosférico, na extensão ao continente de zona de fratura reativada, desenvolveu estruturas, magmatismo alcalino, alto fluxo térmico e sismicidade. Penetrando nas fraturas reativadas dessa zona no oceano, com o deslizamento da placa formou-se uma cadeia vulcânica, em cuja extremidade está o Arquipélago de Fernando de Noronha. Essa evolução é similar à da Ilha de Trindade e Arquipélago Martin Vaz, que constituem o topo emerso de grandes montes vulcânicos do extremo oriental das duas cadeias, situadas em zona de fratura que penetra na borda do continente. O estranho Aarquipélago de São Pedro e São Paulo possui origem inteiramente diversa, pois emerge no trecho transformante ativo de uma cadeia de zona de fratura onde esta secciona o rift-valley axial da Dorsal Meso-Atlântica, expondo-se uma protrusão de rochas mantélicas sem vulcânicas emersas aflorantes, embora as existam submersas.PALAVRAS-CHAVE zona de fratura, hotspot, Placa Sul-Americana, vulcanismo. ABSTRACTThe Brazilian oceanic islands and relationships with the Atlantic tectonics. A synthesis of modern knowledge on the Brazilian oceanic islands and archipelagoes shows that the oceanic lithosphere have influenced their origin and geographic situation. Distribution, allignment and age of the volcanic chains evidence a relationship between the origin of islands and archipelagoes and the activity of hotspots along fracture zones. At the Ceará coast, in the extension to the continent of a reactivated zone of oceanic breaking, typical structures, alkaline magmatism, high thermal flow and sismicity result from the influence of a hotspot, presumed to be astenospheric. A volcanic chain is formed as the plate is displaced and the reactivated breakings penetrate the ocean. The Fernando de Noronha Archipelago is at the end of the fracture zone. A similar evolution would have originated the Island of Trindade and the Archipelago Martin Vaz, that constitute the eastern emerged top of two chains of great volcanic mountains. The strange Archipelago of São Pedro and São Paulo is of a quite different origin, because it emerges on an active transforming segment of a fracture zone chain where it cuts the axial rift-valley of the Meso-Atlantic Ridge, exposing a protrusion of volcanic mantle rocks without exposed volcanic rocks, but they exist submerged.

Spatial sedimentary distribution, seasonality and the characteristics of organic matter on Fernando de Noronha insular shelf

“…However, in the results for the platform adjacent to FN the 2 φ fraction has a greater quantity of biogenic particles due to the bioclastic nature of the sediments. This bioclastic composition is similar to that observed by Barcellos et al (2011Barcellos et al ( , 2016a for FN's southern coastal region. Whereas the sediments of the bay and the beach were composed of carbonate materials in all seasons, the MBI had a greater 0.250 mm (2 φ) fraction.…”

“…The exception occurs in the SW samples (STs: 1, 6, 11, 17, 18) that, due to the higher gravel content, (≈ 28.6%) were classified as poorly sorted. Similarly, in a seasonal sedimentary study on the Sueste beach and bay, located on the island's southern coast (Mar de Fora), Barcellos et al (2011) and Barcellos et al (2016a) observed that in all the periods analyzed, the grains were composed of sand, with a predominance of well to moderately sorted fine sands.…”

“…The insular shelf is 10 km in diameter and has an asymmetrical ellipsoid shape, where the archipelago extends northwards. Thus FN's southern shelf is twice as wide (6-7 km) as its northern shelf, which extends for 3 km out to the shelf break (Teixeira et al, 2003;DHN, 2005;Almeida, 2006;Manso et al, 2011;Barcellos et al, 2011;Barcellos et al, 2016a).…”

“…In the period from January to March, NNE currents occur, giving rise, in the northern part of the main island, to waves with amplitudes of up to 5m (16-20s), characterizing a high-energy environment (Manso et al, 2011). In winter, the prevailing SE winds (70% of the frequency) alternate with easterly winds (25% of the frequency), the prevailing waves coming from the SE and having a mean height of 1.6m at the Sueste beach (Höflich, 1984;Barcellos et al, 2011).…”

“…In recent decades an increase has been observed in the number of studies conducted on calcareous beach systems on oceanic islands, such as those by Kench (1997), Norcross et al (2002), Smith and Cheung (2002), Eversole and Fletcher (2003), Genz et al (2007), Pessenda et al (2008), Pereira et al (2010); Manso et al (2011), and Barcellos et al (2011Barcellos et al ( , 2016a. However, studies on sedimentology, with a multi-proxy approach, on oceanic island shelves are still few.…”