Devonian reef

Palaeokarst Development in a Lower Frasnian (Devonian) Platform Succession,
Canning Basin, Northwestern Australia
Annette D. George (University of Western Australia) and Nancy Chow (University of Manitoba)
Australian Journal of Earth Sciences (1999), v. 46, p. 905-913
ABSTRACT: 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 (~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.