A Evolução geomorfológica da Baía de Guanabara e das regiões vizinhas

Abstract

The author of this article is Professor Francis Ruellan, Director of Courses of the School. For Advanced Studies of the Institute of Geography of the University of Paris, professor of the National Faculty of Philosophy of the University of Brazil, and Technical Adviser of the Cultural Meetings of the National Council of Geography, He reminds us that Guanabara Bay, closed in by a narrow neck, was named Rio de Janeiro by the Portuguese and by Americo Vespucci, doubtless by analogy with the forms around the estuary of the Tage. The bay originated in an angle of fault depression, between two groups of faulted blocks, namely the Serra dos Órgãos which is part of the Serra do Mar, and the small littoral massifs. The faulted blocks, inclined toward the North-Northwest, are drained by rivers which flow towards the interior and which have incised themselves in several cycles aided by the deep, disintegrating erosion due to the warm, humid climate (fig 1, 2 e 3).

            The fragment of the Serra do Mar which limits Guanabara Bay to the north, is a faulted block, essentially formed of gneiss of the Brazilian Archean complex, folded in a SW-NE direction. This cliff has been subjected to the long erosion which led to the formation of the Campos level, then to other erosions which molded the intermediary hills, and finally reached the present level of the valleys of Petrópolis and Teresópolis (an altitude of about 750 to 900 meters). Each of these erosion levels still continues to evolve upward from these breaks m the gradient. (fig 4)

            The Paraíba valley, formed in a large angle of depression, at first was at a higher level than at present. But in proportion as river incisions were made, new erosions attacked that former level and cut it down to form the low hills in dome or ·half-orange shapes. Which flank the main river.

            These new upward erosions have caused elaborate Appalachian-type adaptation in the folded structure, have accentuated the subdivision of the relief into alveolars filled with alluvium torn from the slopes of decomposed rocks which have set them apart. and have isolated peaks with rocky, escaped, bate walls of the same type as canine-form rocks called hump or sugar loaves on the littoral

            On the southern escarpment of the faulted block of the Serra dos Órgãos, where fault steps can be observed, short rivers have caused sharp erosion and formed a dissected front of faulted mass. It is young enough to make captures rare there, in spite of the facilitating rock separation and decomposition.
            With the help of this powerful decomposition, the rivets of the slope have modeled out U-shaped valleys dominated by abrupt rocks. However, there are traces of erosion levels with cones of former aluvions which indicate variation in the base-level.

            Almost without transition after this mountainous course of active erosion, there is, downstream, a plain course where the rivet meanders without a well-defined course, as though the Guanabara depression, situated at the foot of the faulted block of the Serra dos Órgãos were of recent origin and the drainage unorganized.

            On the other hand, the presence of a small basin of interior sedimentation in Itaboraí, between the massifs of Niterói and the Serra dos Órgãos which date from the end of the Niocene or at the beginning of the Pliocene, with numerous heavy; classic elements, places a little earlier the faults which have cut the southern edge of the Serra dos Órgãos massif. The presence of flesh water fossils in the calcareous formations of Itaboraí, shows furthermore, that the movement of faulted blocks has not been sufficient to cause a marine invasion. Finally, the common level of the gneiss formations and the sediment of the old basin, proves that after the deposit of the latter - that is, at the end of the Pliocene or in the Pleistocene - erosion was still taking place on a base level higher than the present one (fig 5).
            These facts lead one to think that although the origin of the Guanabara depression may have been linked up with vertical movements, the marine invasion and the present topography of the flat have another cause.
            The small littoral masses, although situated at a much lower altitude than the Serra dos Órgãos, have evolved in a similar manner. They consist of faulted block, which fell toward the north, and which have a dissected escarpment toward the south. They form two groups, the entrance of Guanabara Bay approximately indicating the dividing point. The western group is higher and has mote divisions. The eastern group is of lower altitude, and is more continuous. The depressed group to the east of the Bay, has more fully conserved the forms made by a series of fluvial erosion cycles. The latter have left valleys at different levels in alluvial troughs, and have cut the relief into small alveoles occupied by farms. It is however remarkable that the erosion levels have the same altitude in both groups, which obviously proves that the soil movements which caused the differences in the height of the groups, occurred before these erosions The erosion levels ate conserved better in the depressed Nitetói group (fig 6)
            The new, deep, vertical erosions have accentuated an adaptation of the Appalachian type in the direction of the former Laurentian folds, which is isolated from the cliffs with rocky, denuded walls.
            The movements which cut and inclined the blocks of the small littoral massifs, are undoubtedly recent, for the short, swift rivets with a ve1y abrupt profile on the dissected front of the faulted block have not sensibly eroded with captures, the slight slope tuning northward and drained by long rivers which me slower and with much slighter slope (fig 7).

            After the filling of the interior basin of Itaboraí, fluvial erosion first modeled the thick bed of decomposition clay which coveted the Guanabara depression, starting at base-level steps at 80-100 meters, 50-65 meters, 25-35 meters and 15-20 meters. These levels, posterior to the formation of the tertiary basin, ate from the end of the Pliocene or the beginning of the Pleistocene (fig 8).

            On the outer coast, to the west and to the east of the Bay's entrance, variations in relative level of the land masses and of the waters have, on these same levels, modeled littoral platforms and sea-cliffs

            Then by an important negative movement, fluvial erosion strongly dissect€d the thick bed of decomposition and the alluvial or colluvial clay covering it. This dissection has been especially sharp along the river which went through the present neck of the bay. By mounting erosion, from a minimum of - 56 meters, it moved upward, cutting the old erosion plain of the Guanabara depression in to numerous hills which conserve traces of incised levels (fig 12). These strong fluvial erosions have been followed by a positive movement which has caused a twisting exterior coastline and has formed the Guanabara Rio in the space furrowed by fluvial erosion. This positive movement has been followed by quick stabilization through the construction of bats, and by fluvial-marine aluvions which at first filled the bottom of the small rias, and then barred the entrance to them, while small littoral plains were developing from the lagoons thus formed large, more recent bars isolated the lagoons running parallel to the general direction of the coast. In the regions at the bottom of the bay, less directly subject to the action of the waves, marshes have helped the filling, and the rivers, the flow of which is hindered by the tide, have formed vast swamps which presented a difficult drainage problem In the bay, the former fluvial courses have been obliterated where the. widenings weaken the tide. Numerous bars have been built up at the foot of the mountainous region at the opening of the bay, where the waves encounter material prepared by weathering and brought there by run-off and by the rivers. They have, to some extent, reshaped the relief by reuniting numerous islands to the continent with tombolos. They have also transformed the exterior coast into a succession of large dune-surmounted beaches supported by points of rocky islands. Finally, they have begun to close the Bay's opening.

            The abundance of materials prepared by weathering in this humid, tropical climate is equally important to explain these rapid marine constructions and the forms which ate definitely peculiar to fluvial erosion and accumulation. Man has completed this work o f Nature by artificial filled in land ( fig 9 and 12).

            The marvelous bay, which one cannot til e of admiring, is not a river as was undoubtedly thought when the name Rio de Janeiro was given to it. But, inasmuch as its most original forms are due to fluvial erosion and the later chief remodeling was brought by marine invasion and accumulation, it warrants at least the name, Guanabara Ria (fig 12).