The depositional record of the Frasnian/Famennian boundary interval in a fore-reef succession, Canning Basin, Western Australia

The effects of the global Frasnian/Famennian (F/F) biotic crisis on the shallow-marine reef-builders and associated biota in a laterally extensive late Frasnian reef complex on the Lennard Shelf, northern Canning Basin, are recorded in the well-exposed fore-reef succession (Napier Formation) of the Napier Range. The conodont-defined F/F boundary lies within a distinctive and mappable stratigraphic interval up to 15 in thick which separates highly fossiliferous Frasnian from markedly less fossiliferous Famennian carbonate and siliciclastic lithofacies. Marked lateral facies variation within the interval means that the actual boundary is not represented by a discrete bed or surface. The interval is characterised by a range of lithofacies that record very low sedimentation rates on the slope and reworking and bioturbation of slope strata indicating a condensed interval with negligible sediment input from contemporaneous carbonate platform production. Siliciclastic lithofacies and abundant sponge debris are locally developed. Macrofossil content is generally very low but there is no evidence for anoxic or hypoxic conditions during deposition at a range of palaeowater depths. Allochthonous blocks of stromatoporoid-bearing limestone are absent from the uppermost Frasnian fore-reef strata and stromatoporoid debris is significantly reduced prior to the end of the Frasnian. This suggests that extinction of the stromatoporoid reef margins occurred before the F/F boundary with microbial communities dominating the platform margins prior to the Famennian. Sedimentological and biostratigraphic evidence suggests that a series of sea-level fluctuations, in response to basin tectonism, generated recurring or ongoing environmental stress in the stromatoporoid-dominated reefal communities because of recurring reductions in living space, influxes of siliciclastic sediment supply from the proximal highlands and quite possibly generation of toxic (too saline or too nutrient-rich) lagoonal water. These conditions were potentially superimposed on a global regime of incipient cooling climate leading to widespread glaciation in the Late Carboniferous. (C) 2002 Elsevier Science B.V. All rights reserved.