Contribuição à Geomorfologia da área da folha Paulo Afonso

Resumo

The author, prof. Alfredo José Pôrto Domingues, geographer of the National Council of Geography, presents a geomorphological study of the region covered by the Paulo Afonso sheet (preparatory edition for the Chart of Brazil, scale 1:1 000 000) of the Chart of Brasil.
This paper, based in research in the field, is accompanied by a geomorphological map which corresponds to the above mentioned sheet, in the scale of 1 : 250 000.
The author divides the region in four different areas - different from the physical standpoint:
I - Eastern crystalline plains;
II - sedimentary plateaus;
III - Fossile surfaces and mountains ranges ("sertanejas") ;
IV - Western crystalline plains.
The first areas are subdivided in three zones:
A - Zone of Glória-Delmiro-Riacho;
B - Zone of the plateaus; and
C - Zone of Piranhas.
I) The first zone is characterized by its uniformity, the altitude oscillating between 230 and 280 meters. It is formed by a series of elongated hills with gentle slopes and wide valleys where brooks meander among the sediments.
Dominating this surface, there appear, here and there, some elevations which are remnants of higher levels.
The zone of the plateaus (Delmiro-Piranhas) is made up of short plateaus which were thus formed due to the exiguity of the sedimentary deposits.
The surface of Piranhas is lower than the plateaus and it is, in some places, intermixed with the fossil surface, while, in others, that surface cuts the last.
Due to greater humidity, the drainage is somewhat more concentrated, a fact that show the transition to the littoral, where humidity is higher.
II) The area of the sedimentary plateaus occupies the western part of the map. The topography is almost flat, leveled, and vast plateaus occur; the escarpments of these plateaus represent the more conspicuous accidents. Though, at first sight, one may think of only one surface of erosion, a more detailed study will reveal structural terraces as well as other formations of cyclical origin. In these terraces, disturbed beds are found as well as faults which cause differences of altitude of hundreds of meters. Some of these faults disturb even the crystalline substratum.
III) The fossil surfaces are, in this area, almost coincident with the horizontal plane. In some areas, though, after the Cretaceous period, tectonic movements came to affect the crystalline shield: the sinking of some areas which became “grabens” as, for instance, the one found at Santa Rita.
In some other zones, titled fault blocks were formed.
In what concerns to the crystallin mountains, some of them do not present the sedimentary cover anymore, but display an asymmetric profile.
IV) The crystalline region appears again to the west; the same happens with the plains and the semi-aridness is accentuated.
After a long period of post-Cretaceous peneplanization, the deposition of sediments tool place within the region covered by the map.
During the last phase of the deposition, tectonic movements took place, causing a series of faults, elevation of blocks, etc.
Erosion started, then, to work actively, partially removing the Tertiary and Cretaceous sediments. The pre-Cretaceous surface, examined at several places, shows that it was cut, in some points, by surfaces of erosion.
Erosion worked in several stages.  In the first, an elevated surface of one hundred to one hundred fifty meters was cut. In sequence, the surfaces of three hundred to three hundred seventy meters, two hundred thirty to two hundred eighty meters and one hundred forty to one hundred eighty meters appeared.
The leveling work is not only due to stream erosion and the author emphasizes the importance of physical and chemical disaggregation, sheet erosion and the action of inferior plant life.