Geological and Paleontological Sites of Brazil - The Fernando de Noronha archipelago. 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 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 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, 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 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. They occur above an erosion platform, 3 to 4 km wide, which may extend to a depth of approximately 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, m. This mountain is part of an alignment of submerged E-W volcanic highs which compose the Fernando de Noronha Chainalong the homonymous fracture zone. In the continent, near Fortaleza, phonolite intrusives similar to the ones encountered in Fernando de Noronha yielded an age of 30 Ma Cordani,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
Nova brasil fm recife online dating Lithothamnium algae reefs and sands of marine organisms.
Terra, dezp. Figure 4 - Ilha Rata. It was occupied by the Dutch in and from toand subsequently by the French inlater 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. Many petrographers studied samples of rocks collected by these expeditions, mainly Smith and Burribut Branner, 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. 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: More recent investigations have centered mainly on geochemical aspects, such as the ones by Gunn and Watkins, ; Gerlach et al.
Ulbrich described the petrography and discussed the diversified nomenclature and classification of the archipelago rocks. Cordani dated some of its rocks by the K-Ar method, whereas Maringolo produced a thesis on the dikes of the principal island.
The archipelago has been subjected to several government administrations, belonging today to the Pernambuco State. As an oceanic volcanic island Fernando de Noronha shows a modest
Nova brasil fm recife online dating. In its central region, at 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 ma curious erosion landscape due Nova brasil fm recife online dating the fall of joint blocks Nova brasil fm recife online dating 6. The airport was constructed on the plain area between the phonolite hills. Two gentle plateaus, 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 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. Residual relief in phonolite. 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. Beach
Nova brasil fm recife online dating 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 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 Nova brasil fm recife online dating covered the present northeastern islands Figure 8.
Figure 7 - Marine limestone in a terrace 9 m above sea level. Nova brasil fm recife online dating 8 - Sueste Bay, a flooded fluvial valley after the elevation of the sea level.
In the bar, rests of ancient consolidated dunes Caracas arenite. 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. The original vegetation was almost completely destroyed and nowadays it is restricted to the occurrence of scarce shrubby vegetation or small arboreous and large macega, herb and graminea areas. Miocene and Pliocene volcanic rocks are exposed in the Fernando de Noronha archipelago.
Sediments occur in the islands as a few coastal deposits: They are intrusive magmatites in pyroclasts that support the highest elevations of the central part of the main island.
It seems that the pyroclastic rocks only crop out in this island, as irregular beds with total thickness that may exceed m.
They are composed of rock fragments of several dimensions, with blocks that may reach a diameter of one meter or more.
Among the fragments occur a variety of phonolites, trachytes and essexites. They result of explosive paroxysms preceding and accompanying the intrusion of the phonolitic and trachytic bodies. In the main island eleven large independent bodies of phonolite Nova brasil fm recife online dating been found.
The one which includes the Pico crops out with a diameter of m and of m at the Boa Vista Dome. Phonolite dikes, with thicknesses of several meters, are frequent in the main island. The phonolite domes and plugs of Fernando de Noronha were the result of endogenous crystallization and were exposed by erosion before the Nova brasil fm recife online dating of the ankaratritic effusions.
Monchiquites, fourchites, camptonites, and other rare types of sodium lamprophyric rocks compose the majority of the dikes which Nova brasil fm recife online dating well observed along the shoreline of the Fernando de Noronha Figure Nova brasil fm recife online dating. There are also dikes of ankaratrite, augitite, alkali-trachyte, limburgite, sanaite, olivine teschenite, porphyry essexite, olivine nefelinite, basanite, gauteite and of other rock types.
The petrographic nomenclature of many of these dikes, as well as of other rock types in the island, is still controversial Ulbrich, ; Maringolo, The dikes strike dominantly to NW and NE. Phonolite, trachytes and porphyritic essexites are supposed to be
Nova brasil fm recife online dating oldest magmatic rocks exposed in the island.
Figure 9 - Monchiquite dikes at the shore cliff of the Atalaia Point. 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. 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, 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 m of exposed thickness above sea level. This formation also occurs in some islands, particularly the Rata Island. The flows are massive in its interior but vesicular or amigdaloidal at the base and mainly at the top Figure 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 11as thin as two centimeters of thickness, were seen in alternation with lapilli-tuff containing Nova brasil fm recife online dating of the same The pyroclastic rocks are tuffs, tuff-breccia, lapilli-tuffs and agglomerates, essentially
Nova brasil fm recife online dating 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. It has a diameter of almost 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. Figure 11 - Thin flows of melilite ankaratrite containing bombs of the same rock.
Shore cliff at the Quixaba beach. It is noticeably in this flow the abundance and dimensions of dark-green colored dunite xenoliths, almost essentially composed of olivine.
Ulbrich 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. They possibly come from a deep and ancient zone of the upper mantle. Schwabe and Block concluded that the basanite shows geochemical relations to the Quixaba Formation.
Ulbrich 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 Ruberti Figure 12 - Basalt flows covered by the Caracas arenite.
The intrusion of the large phonolite bodies occurred at approximately 9 Ma. So, the first volcanic cycle exposed in the island is dated Upper Miocene. live television channels and 45 radio stations.
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