The Late Proterozoic Rock Harbour Group, as exposed on the Flat Islands, Placentia Bay, consists of a 700 m thick fining-upward sequence of deep-water conglomerates, pebbly sandstones, and thinly-bedded siltstones, capped by a chaotic olistostrome. This sequence rests unconformably on sediments and pillowed tholeiitic basalts of the Burin Group, which were deformed, eroded, and then exposed on the sea floor before Rock Harbour deposition. The conglomerates, sandstones, and siltstones of the Rock Harbour Group may have been deposited within a deep slope canyon cut into the Burin Group, or in a perched slope basin at the margin of the rift system, which was initiated immediately before extrusion of the oceanic tholeiites that characterize the Burin Group. Remarkably good analogues for these two depositional settings can be found in the modern Gulf of California.Although the precise age of the Rock Harbour Group is uncertain, it may be a deep-water equivalent of mixtites found within the Conception Group in the eastern Avalon Zone. However, lithologically similar deep-water sandstones and conglomerates spanning the entire period of development of the Burin Rift Zone may be present in the vicinity of Placentia Bay. If so, then it is reasonable to expect a considerable range in the ages of these units from place to place.

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