Basalt and minor rhyolite flows and breccias in the northwestern extension of Selwyn Basin are stratigraphically above maroon argillite (Lower Cambrian Hyland Group) and beneath black chert (Middle Ordovician Road River Group). All are intensely folded and repeated by shallow thrust faults. The lower part of the volcanic succession is dominated by subaqueous flows, and the upper part includes shallow and subaerial breccias and limestone pods. The volcanic unit is interpreted as many small overlapping seamounts fed by gabbroic dykes. The basaltic rocks are alkalic and contain high concentrations of TiO2 (1.7 - 3.6%), P2O5 (0.5 - 1.2%) and Zr (140 ppm). They resemble the volcanic Marmot Formation in the northeastern part of Selwyn Basin, and are consistent with extension and thinning of continental crust. The volcanic rocks lack significant sulphide mineralization, except where hornfelsed near Cretaceous intrusions. Local high barium concentrations suggest that volcanism may have contributed to stratabound barite in overlying Devonian shales.
