Vulcanismo explosivo riolítico em ambiente subaquática na Bacia Sedimentar do Espírito Santo e sua importância estrutural na formação de jazidas petrolíferas

The authors have found rhyolitic pyroclastic rock constituting small extrusive rock bodies in the early Cenozoic Espirito Santo Basin, Brazil. The pyroclastic rock occurs forming horizontal tabular bodies, about 5m thick, with maximum horizontal extension of 200m. The rock bodies are intercalated by sandstone layers of the Rio Doce Formation. The rock contains α-quartz phenocrysts and small quartz fragments, indicating explosive volcanic activity, such as vapour explosion. The matrix shows intense hydrothermal sericitisation and silicification, attributed to the reaction between hot glass shard and lacustrine water. This rhyolitic magmatism is a new heat source of the sedimentary basin, and it can be related to the petroleum deposit formation. The crack opening by means of dyke intrusion also can be important for oil migration.