Hydro-Erosive Behavior Of Macacu Formation Sedimentary Deposits Located On Ilha Do Governador-Rj

Governador Island, inserted in the context of the Graben da Guanabara, is marked by the presence of paleogenic sedimentary deposits exposures belonging to the Macacu Formation. Currently evident in road cuts and old earth extraction deposits, these exposures show a high degree of degradation caused by water erosion, which is evidenced by the presence of rills and gullies. The present work aimed to analyze the hydro-erosive behavior of Macacu Formation sedimentary deposits exposed in an old loan area located in the patrimonial area of the International Airport of Rio de Janeiro (Brazil), seeking to relate sedimentology, hydraulics and erodibility. After the geological interpretation of the sediment exposures, granulometric and mineralogical analyzes, permeability tests, pore space evaluations and erodibility tests were performed. Five different sedimentary facies were recognized, sandy and clayey mudstones of facies La1 and La2, sandstones with cross-stratification of facies Aca1 and Aca2, and massive sandstones of the Am facies. The organization in decreasing granulometric cycles and the existence of muddy facies laminated to massive and sandstones with channeled cross-stratification, lines of quartz pebbles and concentrations of lateritic concretions, were characteristics very similar to those found both in the adjacent deposits, within Governador Island, as in other deposits of the Macacu Formation. The compositional, textural, permoporous and erodibility characteristics similarity of the Aca1, Aca2 and Am facies and the relative distinction with those of the La1 and La2 facies enabled the definition of two zones of homogeneous hydro-erosive behavior marked by the presence of specific mechanisms, in a similar way to the concept of hydrofacies. Zone I, constituted by mudstones, represents an aquitard, more erodible and less permeable in relation to the sandy facies. Erosion develops through hollows and deep vertical incisions, in some cases following structural plans, lamination, or contraction structures. Zone II, made up of sandstones, represents a poor aquifer with less erodibility compared to Zone I. Lower erosion results from the presence of diagenic cement, and the zone forms plateaus and totems above the mudstones controlling local erosion, which begins its path through narrow and deep rills witch, after reaching the mudstones, expand and advance into greater depths. The two delimited zones showed average permoporous characteristics similar to those obtained for correlated hydrofacies described in the literature, and in terms of erodibility they also showed great agreement with patterns already observed.