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Paleokarst in an Upper Devonian Reef Complex of the Canning Basin, Western Australia
Annette D. George, C. McA. Powell
Evidence for subaerial exposure of an early Famennian platform is recorded within the fore-reef slope succession in the Napier Range reef complex of the northern Canning Basin. This observation supports recent sequence-stratigraphic interpretations of fluctuations in relative sea level throughout reef evolution in this basin, and contrasts with previous outcrop-based interpretations that there were no sea-level falls except at the Frasnian-Famennian boundary.
Remnants of the paleokarst surface are preserved in allochthonous reef-flat blocks that are temporally and spatially related to lenticular siliciclastic units. Subaerial exposure of the platform margin during a third-order sea-level fall in the early Famennian led to the formation of karst cavities on the reef flat and collapse of parts of the reef margin. During this time, siliciclastic sands were fed to the slope, where they were deposited as a series of aprons along the margin. In the early stages of the ensuing transgression, renewed marine conditions enabled cyanobacteria to colonize the cavities. As sea level continued to rise, siliciclastic sands were reworked across the platform, filling the encrusted cavities. The reef-flat blocks were dislodged during the late transgression n response to instability that may have been caused by triggers such as rising pore-fluid pressures and/or tectonic activity. They were emplaced on the siliciclastic slope aprons towards the end of sand deposition as sea level on the platform continued to rise and carbonate production resumed.
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