Upper Cretaceous volcaniclastic sedimentation of the Campos Basin, Southeastern Brazil

Abstract

During the Late Cretaceous, the presence of one or more highly explosive volcanoes (plinians) located in the surroundings of the Campos Basin significantly affected the sedimentary record of this basin. Their influence extended in intermittently from the Late Coniacian to the Maastrichtian in two main ways: by the contribution of volcaniclasts deposited intercalated with the coeval marine sediments; and by the action of seismicity and associated earthquakes; which favored the onset of turbidity currents and caused deformations in strata of already deposited sediments. The volcaniclasts ejected by these volcanoes contained volcanic glass of which composition was equivalent to trachyte and showed enrichment in both incompatible trace elements and light rare earth elements. Thus they can be easily differentiated from the materials produced by the stratovolcanoes from the Cabiúnas Formation (Lower Cretaceous), which are composed by basic tholeiitic igneous rocks. Deposition of the tra-chytic volcaniclasts originated two stratigraphic marker beds of great importance in the Campos Basin: "3-Dedos" and "3B" marker beds. The mineralogical, chemical and petrographic study of these horizons has shown that, in spite of representing different deposicional facies (ash fall and reworking, respectively), their rocks were formed by volcaniclasts produced by the same type of volcanoes; therefore they represent features of the same geological process.