Syn- and post-eruptive volcaniclastic sedimentation in Late Archaean subaqueous depositional systems of the Black Flag Group, Eight Mile Dam, Kalgoorlie, Western Australia

The Late Archaean Black Flag Group is widespread but poorly exposed throughout the Kalgoorlie Terrane of the Eastern Goldfields Province in the Yilgarn Craton. From drill core analysis, five distinct facies can be defined within the succession: massive to laminated mudstone, diffusely bedded sandstone, polymictic conglomerate, massive sandstone and rhyolitic breccia. Each of the coarser-grained facies is associated with the mudstone facies, indicating a consistent, below wave base depositional setting in which mass flow processes were responsible for depositing the majority of detritus. Three distinct depositional systems are identified within the Eight Mile Dam succession, each of which is separated by periods of tranquil water sedimentation. Depositional system 1 is defined by an upward-coarsening trend, consisting of vertically stacked mudstone and sandstone, overlain by polymictic conglomerate. This facies arrangement is characteristic of vertically stacked outer lobe, proximal lobe and channel deposits, defining a progradational submarine fan system. This progradational submarine fan consists of polymictic debris and has a maximum depositional age of 2666 ± 6 Ma. Depositional system 2 consists of amalgamated massive sandstone beds with erosive basal contacts, indicative of a proximal channelized submarine fan setting. The debris is juvenile and dacitic in origin and appears to have been derived from contemporaneous pyroclastic volcanism from a single dacitic centre. Depositional system 3 consists of unchannelized rhyolitic breccias that have characteristics indicative of deposition on a slope-apron environment, likely to be closely associated with a subaqueous rhyolitic centre. The rhyolitic slope apron system has a maximum depositional age, and likely volcanic eruption age of 2669 ± 8 Ma. The SHRIMP U/Pb detrital zircon geochronology constrains the age of the Eight Mile Dam Black Flag Group to 2677–2660 Ma.