Faciology of the Acampamento Velho Formation volcanic rocks (Camaqua Basin) in the region of Serra de Santa Barbara, Cerro do Perau and Cerro do Bugio (municipality of Cacapava do Sul-RS)

Abstract

In the Santa Barbara Sub-Basin (Camaqua Basin), Acampamento Velho Formation (Late Vendian to Early Cambrian) presents two associations of volcanic facies, one at the base and another at the top. The basal association is constituted by (1) andesites, andesitic basalts and (2) basaltic to andesitic breccias. The upper association, with a rhyolitic composition, includes (3) lapillites, (4) tuff, (5) welded tuffs and (6) rhyolitic flows. Facies 1 is interpreted as flows and Facies 2 as basal or frontal clastic accumulations associated with the flows of Facies 1. Facies 3, 4 and 5 are associated with pyroclastic flows, a product of the collapse of the eruptive column, and the lateral variation occurs as a result of the grain size and degree of welding. The rhyolitic flows (Facies 6) cover the previously mentioned facies. The geochemical results (REE and immobile traces) suggest that the rocks are cogenetic, characterizing bimodal volcanism. In the stratigraphic view, the lithological discontinuity existing between the base and the top of Acampamento Velho Formacion allows the postulation of the presence of a hiatus between the two facies associations.