Geological and Paleontological Sites of Brazil - 066
THE FERNANDO DE NORONHA ARCHIPELAGO
FERNANDO F.M. DE ALMEIDA
© Almeida,F.F.M. 2000. The Fernando de Noronha archipelago. In: Schobbenhaus,C.; Campos,D.A.; Queiroz,E.T.; Winge,M.; Berbert-Born,M. (Edit.) Sítios Geológicos e Paleontológicos do Brasil. Published 27/1/2000 on Internet at the address http://www.unb.br/ig/sigep/sitio066/sitio066english.htm [Actually http://sigep.cprm.gov.br/sitio066/sitio066english.htm ]
(The above bibliographic reference of author copy rights is required for any use of this article in any media, being forbidden the use for any commercial purpose)
Fernando de Noronha is a small volcanic archipelago in the equatorial South Atlantic, 350 km off the city of Natal, the Rio Grande do Norte State capital, northern Brazil. The archipelago consists mainly of highly alkaline and subsatured volcanic and subvolcanic rocks and are part of a homonymous volcanic mountain developed along an E-W oceanic fracture zone. It encompasses an area of 18.4 km², 90% of which within the main island. There are also 18 minor islands outcropping on a reduced island platform that truncates the volcanic mountain as deep as approximately -100 m. The Remédios and Quixaba formations represent two major volcanic episodes. The former one, dated Upper Miocene, comprises pyroclastics intruded by phonolite and trachyte domes, plugs and dikes, as well as by a number of dikes showing a varied types of alkaline rocks. Erosion processes destroyed the volcanic rocks from this cycle, and was followed by the Quixaba Formation, during the Upper Pliocene, represented by ankaratrite lava flows, pyroclastic rocks and rare nefelinite dikes. The archipelago has a population of no more than 3,000 inhabitants, concentrated in the principal island, and a small floating population of tourists and researchers. It is a Marine National Park accessed by air flights, offering a pleasant weather, beautiful landscapes, wonderful beaches and very clean seawater, with dolphins, turtles and many fish types. Despite of its natural attractions, the tourism has not been stimulated, not only due to the poor available infrastructure, but also to preserve the local ecology.
The study of small oceanic islands has a high scientific interest because it offers the opportunity to gather relevant information on the nature of the oceanic crust and the processes related to its spreading. Nevertheless, they can be pleasant sites for both tourism (Figure 3) and important ecological research.
The archipelago is located in the equatorial South Atlantic at 3º 51' S and 32º 25' WG (Figure 1). The main island, also named Fernando de Noronha, has an area of 16,4 km² representing 91% of the total area of the archipelago, and it is surrounded by twenty other small islands, being Rata Island (Figure 4) the largest of them (81 ha). They occur above an erosion platform, 3 to 4 km wide, which may extend to a depth of approximately -100 m. Beyond this depth the bottoms spread out to configure a conic mountain with a base diameter of 60 km on the ocean floor at -4,000 m. This mountain is part of an alignment of submerged E-W volcanic highs which compose the Fernando de Noronha Chain, along the homonymous fracture zone. At the projection of the alignment to the Ceará coast the Ceará guyot cuts the continental slope. In the continent, near Fortaleza, phonolite intrusives similar to the ones encountered in Fernando de Noronha yielded an age of 30 Ma (Cordani, 1970), suggesting that the fracture zone has also affected the margin of the continental crust. The Rocas atoll is also a volcanic guyot of the chain leveled by the ocean and completely covered by Lithothamnium algae reefs and sands of marine organisms.
Figure 2 - Situation of the Fernando de Noronha Archipelago at the Brazilian coast
Figure 2 - Geological map of Almeida (1955) as simplified by M.C.N. Ullbrich (1994). Abbreviations: B, bay; E, inlet; I, island; M, hill; Pr, beach; Pt, cape
Figure 3 - Leão beach, at the E side of the main island. Photo G. Barreto, rev. Terra, dez 1999, p. 31
Figure 4 - Ilha Rata. Foto G. Barreto, rev. Terra, dez 1999, p. 33
The archipelago was officially discovered by
Américo Vespúcio in 1503 and, in the following year, it acquired the name of a
Portuguese lord, Fernão de Loronha, to whom the Crown donated the archipelago. It was
occupied by the Dutch in 1612 and from 1635 to 1654, and subsequently by the French in
1736, later banned by the Portuguese, who fortified the main island. At the second half of
the XIX century many foreign researchers visited the archipelago, particularly after C.
Darwins observations (1839) on the geology, petrography, and volcanic nature of the
main island during his stay in 1832 in the historical expedition of the Beagle.
Many petrographers studied samples of rocks collected by these expeditions, mainly Smith
and Burri (1933), but Branner (1888, 1889, 1890) produced more specific descriptions about
the geology of the archipelago and the position occupied by many of the collected and
already described rocks. He made interesting observations on the calcarenites, dunes and
beaches of the archipelago. During the first half of the XX century there was no interest
on geological and petrographical studies of the archipelago, but in 1948 the Departamento
Nacional da Produção Mineral addressed there the engineer Pouchain, who published
the results of his investigations on the phosphates and calcarenites of the Rata Island.
The present author, as an engineer of the same Department, had the opportunity of travel
three times to the archipelago, to produce a geological map
(Figure 2) at the 1:15.000 scale and to
study its geomorphology, petrography, geological history, geochemistry and petrogenesis
(Almeida, 1955). More recent investigations have centered mainly on geochemical aspects,
such as the ones by Gunn and Watkins, 1978; Gerlach et al. 1987; Schwab and Block, 1985;
Weaver, 1990; and Ulbrich et al., 1994. Ulbrich (1994) described the petrography and
discussed the diversified nomenclature and classification of the archipelago rocks.
Cordani (1970) dated some of its rocks by the K-Ar method, whereas Maringolo (1994)
produced a thesis on the dikes of the principal island.
DESCRIPTION OF THE SITE
As an oceanic volcanic island Fernando de Noronha shows a modest relief. In its central region, at 30-45 m over sea level, there is a low plateau that was the result of an erosion surface carved on tuffs and volcanic breccia under sub-aerial conditions. Hills of phonolitic rocks crops out above this plateau (Figure 5). The highest point of the island is the Pico mountain (321 m), a curious erosion landscape due to the fall of joint blocks (Figure 6). The airport was constructed on the plain area between the phonolite hills. Two gentle plateaus, 150-200 m high, are composed of an alternation of pyroclastic rocks and nefelinic ultrabasic lava (ankaratrite) flows disposed along steps according to the gentle plunge of the flows, ending at a moderate coastal slope or shore cliffs. The valley traces are influenced by the geological structure; they are usually dry and start at the phonolite hills. The small islands of the archipelago show diverse relief aspects, depending on their geological framework. The phonolitic ones show high peaks (Frade, Sela Gineta and Cabeluda islands), whereas the islands composed by lava flows are tabular features, as Rata and São José, or scarp rocks (Dois Irmãos). The islands formed by calcarenites are low and show a tabular relief (Meio and Rasa islands).
Figure 5 - Aspect of the erosion surface of the central plateau. Mountains on phonolite. Photo F.F.M. Almeida
Figure 6 - Pico Mountain. Residual relief in phonolite. Photo F.F.M. Almeida
At the south and eastern borders of the main island, where the volcanic rocks are more exposed to the shock of the waves formed by trade winds, there are shore cliffs; there are also abundant reefs of Lithothamnium algae and scarce sand beaches. Sand beaches as well as active dunes are well-developed at the northern coast. Evidence of ancient sea levels is presumable or may be recognized in the main island. The highest one, at 30 m above present sea level, served as a base level to the erosion of the central plateau. The other one is confirmed by the presence of marine limestone with Lithophyllum algae at the vicinity of the port, in Santo Antônio peninsula (Figure 7). Beach deposits containing rolled pebbles, fish remnants, corals and other marine organisms occur in this peninsula at 7 m above sea level and as far as 70 m from the coast line, although similar deposits elsewhere can be recognized at 12 m above the present-day sea level. The Caracas sandstone, described below, is younger than this level, because it covers these deposits. The sandstone was formed when the sea level was 6 m lower than the present position. At this time, extensive beaches existed in the south and southeast of the archipelago, together with dunes of carbonate sand which covered the present northeastern islands (Figure 8). Conglomerates and ancient sandstones at the small Chapéu de Nordeste Island, Rata and some others lie under the Caracas eolian sandstone.
Figure 7 - Marine limestone in a terrace 9 m above sea level. Santo Antônio Peninsula. Photo F.F.M. Almeida
Figure 8 - Sueste Bay, a flooded fluvial valley after the elevation of the sea level. In the bar, rests of ancient consolidated dunes (Caracas arenite). Photo F.F.M. Almeida
The archipelago climate, of Awi-type, is similar
to the eastern coast of the Rio Grande do Norte State, with two well-defined seasons,
being the raining one from February to July. Persistent trade winds flow from ENE.
Miocene and Pliocene volcanic rocks are exposed in
the Fernando de Noronha archipelago. The basement composed of pyroclastic rocks is
intruded by a great variety of alkaline magmatic rocks covered after a long hiatus
by flows of nefelinic ultrabasic rocks (ankaratrites) and their equivalent
pyroclasts. Sediments occur in the islands as a few coastal deposits: the marine ones
belong to the present cycle and the eolian sediments to the Quaternary.
Figure 9 - Monchiquite dikes at the shore cliff of the Atalaia Point. Photo F.F.M. Almeida
The strongly melanocratic rocks, with scarce, if
not absent, feldspars, are the youngest of the Remédios Formation. Effusive equivalents
of some of these rocks may exist at the top of the volcanic edifice, but they were
destroyed by erosion prior to the ankaratritic flows.
Special emphasis is given to the xenoliths and ejectolites of the Remédios Formation since they may represent very deep sourced rocks of the volcanic edifice. The clasts are dominated by rock types described in outcrops of the main island, except clasts of nefelinic syenites and hornblende gabbros which seem to be absent in the main island.
The Quixaba Formation (Almeida, 1955) is a sequence of black flows of alternating ankaratrite lava and pyroclasts containing components of the same lava. This formation rests on two step-like plateaus of the main island; the eastern one may reach 180 m of exposed thickness above sea level. This formation also occurs in some islands, particularly the Rata Island. The flows are generally horizontal, but may show dips as steep as 30º to the south, concordant to the flow direction of the lava. The flows are massive in its interior but vesicular or amigdaloidal at the base and mainly at the top (Figure 10). Their thickness may exceed 40 m at the shore cliff of the Sancho Bay, but it is generally just a few meters thick. Flows of melilite ankaratrite (Figure 11), as thin as two centimeters of thickness, were seen in alternation with lapilli-tuff containing bombs of the same material. The pyroclastic rocks are tuffs, tuff-breccia, lapilli-tuffs and agglomerates, essentially containing components of the flows, including isolated crystals. Angular blocks of lava may reach 2 m. Perfect bombs and fragments of rope-like lava are not rare. They may form a chaotic group of fragments of several dimensions, or exhibit coarse stratification, locally with fine well-stratified beds. A striking structural feature is the Tamandaré chimney, at the northeast edge of the main island. It has a diameter of almost 130 m and truncates the lower flow. Its walls are of ankaratrite, with remnants of tuff and chimney agglomerate, and crosscut by dikes. The low tide discovers the escoriaceae lava of its interior. It was a place for escape of solutions, where calcite precipitated as perfect scalenohedral crystals, filling fractures on breccia and agglomerates. Dikes of coarse nefelinite were observed in a few points of the main island.
Figure 10 - Three flows of ankaratritic lava at Caracas Point. Photo F.F.M. Almeida
Figure 11 - Thin flows of melilite ankaratrite containing bombs of the same rock. Shore cliff at the Quixaba beach. Photo F.F.M. Almeida
The small São José, Cuscuz and de Fora islands, adjoining the northern end of the Santo Antônio peninsula, are composed of the same horizontal basanite flow showing an original thickness in excess of 25 m (Figure 12). The ankaratrites of the point, as well as the flow at the base of the small Chapéu de Nordeste Island, discovered during low tide, are inclined to NE. This fact led the author to suppose the basanite to be younger than the Quixaba Formation, the reason for the name Formation São José. It is noticeably in this flow the abundance and dimensions of dark-green colored dunite xenoliths, almost essentially composed of olivine. Ulbrich (1994) also refers to lherzolite and harzburgite xenoliths. Their shapes are angular to subrounded (Figure 13) and, for some of them, the author has seen diameters up to 35 cm. They are fragments of a voluminous rock body engulfed and transported towards the surface by the magma. At the São José Island it can be noticed that the number and size of the xenoliths are the highest in the lower part of the flow, as a result of a decantation process. They possibly come from a deep and ancient zone of the upper mantle. Although the ages obtained by Cordani (1970) for the basanite of the São José Island are older than the rocks of the Quixaba Formation, these data may be treated with some care, because the rock is contaminated by xenoliths of mantle olivinite. Schwabe and Block (1985) concluded that the basanite shows geochemical relations to the Quixaba Formation. Ulbrich (1994) also indicates that this rock should not be considered a formation, but probably represents the climax of the Quixaba volcanism and may be petrographically correlated to basanites of a chimney that cuts ankaratrites at the Sancho Bay, identified by Ulbrich and Ruberti (1992).
Figure 12 - Basalt flows covered by the Caracas arenite. São José Island. Photo F.F.M. Almeida
Figure 13 - Dunite xenoliths in the basanite of the São José Island. Photo F.F.M. Al
Cordani (1970) dated by the K-Ar method rocks of the Fernando de Noronha and Rata islands. For the Remédios Formation the oldest dated rock is an alkali-basalt of 12.3 Ma, a minimum age for the host pyroclasts. The intrusion of the large phonolite bodies occurred at approximately 9 Ma. For the basanite of the São José Formation the more probable result is 9.5 Ma, obtained in two samples, however other two indicated 8,1 and 21.9 Ma; a possible Argon loss by heating is considered to explain the latter result. This rock is therefore prior to the Quixaba volcanism, possibly belonging to the Remédios Formation. So, the first volcanic cycle exposed in the island is dated Upper Miocene. The Quixaba volcanism has been processed approximately between 3.3 and 1.7 Ma, during Upper Pliocene.
The volcanism represented by the exposed rocks in the archipelago is characterized by its strongly sodium-alkaline nature, subsaturation in silica (there is no quartz in the rocks, the most abundant mineral of the Earths crust), high degree of differentiation. The silica content varies between 34,4% (melilite ankaratrite) and 60,8% (alkali-trachyte). The magmatic province of Fernando de Noronha is one of the most alkaline among the oceanic volcanic islands of the world. The lighter fractions of the magmatic differentiates were produced earlier, originating the Remédios Formation. After an hiatus in the erosion, which may have lasted 5 to 6 Ma, the ultrabasic volcanism of the Quixaba Formation initiated.
After the volcanism ceased at the end of the Pliocene, an erosion cycle destroyed the external volcanic complexes and carved the island platform. During sea level oscillations in the Pleistocene, the platform was covered by sand and beach pebbles deposits, reefs of limestone algae and marine sand.
During the Pleistocene, when the sea level was six meters lower than the present level, extensive sand beaches existed at the south and southeast of the archipelago. Trade winds from SE, similar to the ones nowadays, dislocated these sands forming dune fields that could be as thick as 20 m, linking the area of the present Rata Island to Santo Antônio peninsula at the Fernando de Noronha Island. Shorter dune areas, as at the entrance of the Sueste Bay, were also locally formed, in the southeast coast of the principal island. After the recent elevation of the sea level, a part of these sands was submerged, and the emerged ones formed the Caracas calcarenite. This sediment, of light cream color, is almost entirely composed by limestone grains, mainly from Corallinaceae algae. The remaining detritus are of minerals from the magmatic rocks, except for a contamination the 25% of femic minerals where the arenite covers an ankaratritic area. SE-blowing winds formed typical eolian cross stratification, as Branner (1889, 1890) had already observed. The consistency is a function of the degree of calcite cementation. At the Rata Island the Caracas arenite has its upper part partially replaced by calcium phosphate originated from excrements of marine birds.
PROTECTION OF THE SITE
The Fernando de Noronha Island, in the homonym
Marine National Park established in 1988, is considered by many of the visitors as the
Brazils most beautiful island. The population of no more than 3.000 inhabitants has
become stable and the number of tourists subjected to limits. The tourism started in the
seventies, existing nowadays daily flights to the island from Recife and Natal. However,
there are restrictive factors to the tourism. The island is far; there is an increasing
cost for permanence in the island (Maciel Filho, 1999); there is no tourism
infrastructure, just an adapted hotel using a North-American Airforce base installations
in operation during the World War II. There are a few Guest Houses in the vicinity of the
Remédios Village, at the eastern region of the island. There are some accessible roads
for vehicles, but just a seven-kilometers paved road.
The beauty of the landscape, the pleasant climate, the morning show of the dolphins in the beach, the marine turtles, the beautiful beaches and the sea of very clean water and plenty of fishes altogether constitute in formidable tourism attractions. The present limitations imposed to tourism and the vigilance exerted by the guards of the Instituto Brasileiro do Meio Ambiente e dos Recursos Naturais Renováveis (IBAMA), of the Rotator Dolphins Project, (research on these cetacean) and of Tamar Project (protection to the turtles' eggs) seem to be sufficient guaranties for the adequate preservation of the ecology of the archipelago, that was strongly damaged in the past, when the original forests were destroyed.
ACKNOWLEDGEMENTS - The author is grateful to Prof. Dr. C.D.R. Carneiro for the evaluation of the text and figure production; to Prof. Dr. R.P. Xavier for the suggestions on the English text; to Mabel H.C. Ulbrich for the simplified map of the archipelago and to M. Reginato, for drafting the figure 2.
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