The Quilalar and Surprise Creek Formations are new Proterozoic units in the Western Succession of the Mount Isa Inlier; they replace the Surprise Creek Beds of earlier workers. They occur stratigraphically between the Haslingden Group and Mount Isa Group, and are themselves separated by a major regional unconformity, and, in places, by the Fiery Creek Volcanics. The Quilalar Formation is a sandstone-dolomite-dolomitic siltstone sequence unconformable on basement Tewinga Group rocks and Kalkadoon Granite, but conformable or disconformable on Haslingden Group rocks. It is a regressive to ?transgressive shoreline deposit which contains possible dune, beach, lagoonal and offshore sandbar and carbonate shelf palaeoenvironments. The Surprise Creek Formation is a sequence fining upwards from conglomerate and sandstone to siltstone, the latter containing abundant load-casting and water-escape structures. Fluvial, lagoonal, near-shore and delta-slope or delta-front palaeoenvironments are indicated. Regionally the Surprise Creek Formation is host to numerous small copper deposits. Age of the Quilalar Formation is between 1780 and 1680 m.y., although it is probably older than 1740 m.y.; the Surprise Creek Formation is from 1680 to about 1670 m.y. old. Definitive correlations are proposed between the Western and Eastern Succession: the Quilalar Formation west of, and Mary Kathleen Group east of, the Kalkadoon-Leichhardt basement block are correlated because of marked congruence of stratigraphy, lithology and photogeology after fault reconstruction. The Surprise Creek Formation similarly is correlated with the Mount Albert Group. All units were probably continuous across the basement block and formed a single depositional entity before folding and faulting. From these correlations we propose that Mount Isa Group equivalents are either absent from or have not yet been recognised in the Eastern Succession, and that the Pb-Zn deposit at Dugald River is older than the Mount Isa Pb-Zn deposit. At least two separate periods of major stratabound Pb-Zn accumulation are therefore present in the Mount Isa region; this may be of significance in regional exploration strategy.
