Ocorrência de rochas vulcânicas ignimbríticas na porção norte da Bacia do Espírito Santo: evolução do modelo tectono-sedimentar

Occurence of ignimbrite volcanics in the northern Espírito Santo Basin, Brazil: an advance in the model of tectonic-sedimentary evolution of the basin

Abstract

The intention of this work is to provide information on ignimbrite volcanoclastics, outcropping in the northern onshore part of the Es-pírito Santo Basin, and to examine their role in the tectonic–sedimentary evolution of the basin. We identified ignimbrites, pyroclastics of rhyolitic to dacitic composition, along the NNW-SSE to NW-SE-trending transcurrent fault system named here São Mateus Alignment or São Mateus Arch. We followed the mostly horizontally layered ignimbrites, at least 50m thick, for about 10km along and close to the margins of the São Mateus River. These ignimbrite bodies had been mapped before as fluvio-lacustrine sandstones of the Tertiary Rio Doce / Barreiras Formation, without recognizing their partially volcanoclastic character. Microscopic examination suggests idiomorphic and fragmented α-quartz phenocrysts, contained in a hydrothermally altered matrix of clay minerals and zeolites. The fragmented form of the phenocrysts points to explosive volcanic activity; grain size tends to increase westward, probably indicating the principal place of volcanic centers. The outcropping sequence of ignimbritic rocks provides a major advance in the understanding of the basin’s Cenozoic tectonic-structural history contributting to a revision of its stratigraphy.