Arcabouço geológico e hidrofácies do Sistema Aquífero Guarani, no município de Ribeirão Preto (SP)

Abstract

Subsurface geology and hydrogeological studies using geophysical well logs allowed to define the geologic framework and to characterize the hydrofacies of Guarani Aquifer System (GAS) in the city of Ribeirão Preto. The GAS is composed by the sediments of Pirambóia and Botucatu formations and rests upon an impermeable pelitic substrate, the Corumbatai Formation, which is considered an aquiclude, and is mostly covered by basic volcanic rocks of Serra Geral Formation. The typical geophysical patterns of Pirambóia and Botucatu formations are well correlated and their complex facies variations are compatible to the stratigraphy of the GAS. The thickness variations of the Pirambóia Formation were not defined because only one of the studied wells penetrated the entire Mesozoic sequence. In the case of Botucatu Formation the paleodune relief preserved by the volcanic lavas and the depressions created by deep aeolian erosion are responsible for the thickness variations. This situation emphasizes the inadequacy of using the Botucatu and Serra Geral formations contact as a datum for structural analysis. The shape of the structural contour of the unconformity at the top of Pirambóia Formation, that is considered to be a good local marker, confirmed the existence of a structural low in the central-southwestern portion of the city. The Ribeirão Preto depression plunges southwest dropping over 100m across 15 Km. Three hydrofacies - A (lower), B (medium) and C (upper) - were identified in the Pirambóia Aquifer. The upper and lower hydrofacies show log signature typical of reduced permeability, whereas the medium one shows log signature of high permeability. The Botucatu Aquifer has a single hydrofacies (D) with log signature indicative of elevated permeability.