The Mesoproterozoic (~1590–1586 Ma) Gawler Range Volcanics (GRV) comprise a sequence of A-type dominantly felsic volcanic rocks which were extruded in the central part of the Gawler Craton as part of a volcano-plutonic Silicic Large Igneous Province which also extended into the Curnamona Province. The GRV magmatism was broadly synchronous with greenschist to granulite facies metamorphism, folding and thrusting associated with the Olarian Orogeny (~1620–1570 Ma), and is interpreted to have occurred in a back-arc or foreland basin setting. Unlike other parts of the GRV which at their formation were dominated by felsic volcanism and generally had no or only minor mafic volcanism and sedimentation, the GRV sequence at Roopena provides a rare example of the products of a dominantly mafic volcanic event which was occurring synchronous with significant sedimentation within a N-S-trending half-graben. The lower GRV in the Roopena area consists of three formations which are penecontemporaneous in age; the Roopena Basalt, the Fresh Well Formation and the Angle Dam Volcanics. The Roopena Basalt is a sequence of basaltic lavas with altered, brecciated and amygdaloidal flow tops and local minor hyaloclastite and peperite, with the thickest succession of lava flows proximal to the graben-bounding fault. The Roopena Basalt is conformably interlayered with the Fresh Well Formation, a flat-lying fluvio- lacustrine volcaniclastic sedimentary sequence comprising mudstone, siltstone, fine-grained to coarse-grained lithic sandstone, pebble conglomerate, and three major ash fall tuff intervals which preserve evidence of phases of extrabasinal explosive felsic volcanism synchronous with sedimentation. These sediments consist of stacked upward-coarsening prograding depositional sequences with sharp contacts, providing evidence of rapid flooding events likely to have been driven by extensional tectonism. The Fresh Well Formation contains a dacite-mudstone-siltstone-sandstone peperitic breccia containing juvenile clasts with both blocky and fluidal morphologies. The juvenile volcanic clasts within the peperite are correlated with the rhyolitic to dacitic Angle Dam Volcanics. These rocks originate from high level subvolcanic magmas that were intruded along a N-S-trending Palaeoproterozoic basement fault, and may have been locally extrusive. Detailed lithological logging of the volcanosedimentary succession at Roopena, and the correlation of time-markers across the basin sequence, demonstrate how the basin evolved over time and reveal the effects of faulting on volcanism and sedimentation. The occurrence of sediments in restricted fault-bound grabens such as that observed at Roopena is more suggestive of crustal extension in a tectonic back-arc setting, rather than in a foreland basin setting, where deposition on a broader regional scale would be expected. Furthermore, the bulk composition of the Roopena Basalt, which has a NMORB to island arc or calc-alkaline geochemistry, enrichment of LILE over HFSE, and relatively primitive isotopic values, has closer affinities to that of continental back-arc basin basalts rather than that of basalts which formed in foreland basins.