Sedimentary facies of the Rio Claro Formation, Neocenozoic, state of São Paulo

Abstract

The Rio Claro Formation and associated deposits comprise continental Neocenozoic sediments of the Depressão Periférica (inner lowlands) of southeastem Brazil. These sediments are discontinuous and thin, and their interpretation and classification required the use of geomorphic concepts as well as the usual lithostratigraphic ones. Five planation surfaces of regional extent have been recognized; the lowest and youngest three occur within the limits of the Depressão Periférica, whereas the oldest two occur at higher levels in the neighbouring terrams. Coarse deposits and ferricretes associated with these planation surfaces suggest that they were formed during pediplanation cycles under dry climatic conditions. The Rio Claro Formation is more continuous in its type-area (near the city ofRio Claro), and is discontinuous near the eastem border of the Depressão Periférica. It lies upon the two highest planation surfaces which leveled the hills ofthe area, and has about the same age as these surfaces. It contains deposits of meandering fluvial system formed under humid climates, comprising four principallithofacies: sandy and gravelly muds due to gravity processes; gravels and sands of channellag and channel bar deposits; sands of crevasse splay deposits; and flood pIam muds. In the type-area of the Rio Claro Formation accumulation was controlled by the Neogene reactivation of faults with vertical displacements in the Pitanga structural high, which were active mainly in Mesozoic times. On the eastem border of the Depressão Periférica the Neocenozoic sediments show clear association with lithological barriers (diabase sills and dykes)