Deposits in the Palaeozoic marine basin of the Sarrabus region (SE Sardinia, western Mediterranean, Italy) show the presence of rhyolitic micropillow lithofacies in the uppermost sector of extrusive subaqueous acid domes. These particular volcanic rocks contain quartz phenocrysts affected by curved fractures attributed to thermal shock produced in the magma-water interface. The flanks of subaqueous domes contain proximal epiclastic sandstones and greywackes characterized by the presence of non-mature angular quartz clasts, very similar to the occurrences detected in the rhyolitic micropillows. Therefore, a syngenetic (referred to acid domes) and proximal (unreworked) character is inferred for these epiclastic deposits. A genetic model is proposed to explain the association of lithofacies (volcanic and epiclastic) in direct relationship with the subaqueous growth and erosion of the domes. This process of genesis of texturally unmature epiclastic rocks not related to explosive acid volcanism seems rather new, and the conclusions of the paper may be in sharp contrast with some of the criteria previously described for distinguishing between pyroclastic and epiclastic crystal-rich rocks.