Palaeokarst development in a lower Frasnian (Devonian) platform succession, Canning Basin, northwestern Australia

Recognition of palaeokarst in the oldest exposed Devonian (Givetian-lower Frasnian) platform successions of the Canning Basin reef complexes has eluded investigators for over forty years. The first evidence for palaeokarst, developed on microbial mud-mounds in a single stratigraphic horizon, is documented and records an episode of exposure during early carbonate platform development. Surface palaeokarst features are scalloped surfaces, solution pits and a pipe, underlain by fenestral limestone with sediment-filled fossil moulds and vugs. The platform succession has variably developed metre-scale cycles which are composed predominantly of shallowing-upward subtidal facies, with some cycles having fenestral peloidal mudstone caps. Changes in facies type and stratigraphic arrangement up the succession define two deepening-upward units (similar to 70 and 180 m thick), with the palaeokarst surface representing emergence following rapid shallowing at the top of the lower unit. The stratigraphic position of the palaeokarst between these two units suggests it may represent a sequence boundary. This may have been caused by a low-magnitude eustatic fall or footwall-uplift event superimposed on a rapidly subsiding basin margin.