Early Palaeozoic strata of the northeastern Lachlan Fold Belt, Australia, consist of an Early Ordovician sequence (Adaminaby Group) of quartz-rich turbidites and chert, and the conformably overlying Late Ordovician-?Early Silurian Coomber Formation, a mafic volcaniclastic sequence of lithic sandstone and mudstone with mafic volcanic and intrusive rocks. Analysis of detrital modes from the Adaminaby Group indicate a uniform quartzose petrofacies (Q91F2L7) composed of deep-water quartz and sublithic sandstones derived from the west. By contrast, the lithic sandstone of the Coomber Formation represents a volcanolithic petrofacies (Q1F34L65) of mainly intermediate volcanic detritus with a tendency to become more silicic up-section. These sandstones are considered to be derived from a calc-alkaline andesitic-basaltic volcanic centre in the eastern Lachlan Fold Belt. The change from a quartz-rich petrofacies to a volcanolithic petrofacies was sharp with no evidence of mixing. This change primarily reflects a rapid outpouring of mafic volcanic material in the Late Ordovician, together with an approximately concurrent eustatic rise in sea level.