The supergiant Olympic Dam Cu-U-Au-Ag ore deposit of South Australia occurs in a tectonic-hydrothermal breccia complex that is surrounded by Mesoproterozoic granite. The breccia is composed mainly of granite clasts and minor amounts of Mesoproterozoic volcanic clasts. Very thick (>350 m) sections of bedded sedimentary facies that occur in the breccia complex include laminated to very thin planar mudstone beds, thin to medium internally graded sandstone beds, and thick conglomerate beds. The bedded sedimentary facies extend continuously across a 1.5 km × 0.9 km area and are not limited to small separate maar craters, as previously thought. Detrital chromite and volcanic quartz in the bedded sedimentary facies cannot be matched with local sources, and imply that the provenance extended beyond the area of Olympic Dam. The lateral continuity, provenance characteristics, great thickness, below-wave-base lithofacies, and intracontinental setting suggest that the bedded sedimentary facies are remnants of a sedimentary basin that was present at Olympic Dam prior to formation of the breccia complex. We conclude that the Olympic Dam hydrothermal system operated beneath and partly within an active sedimentary basin, was not confined to maar craters, and did not vent. This new view of the setting raises the possibility that sedimentary processes were involved in ore genesis.