CARBONATE PLATFORM EVOLUTION ACROSS THE FRASNIAN/FAMENNIAN BOUNDARY – NAPIER RANGE, CANNING BASIN, WESTERN AUSTRALIA

The Late Devonian Frasnian/Famennian (F/F) transition represents a period of remarkable global change culminating with one of the most profound mass extinction events of the Phanerozoic. The Canning Basin's continuous record across this boundary provides an unparalleled opportunity to study what effect the F/F crisis had on the evolution and internal architecture of a post extinction reef system.

This study recognizes three phases of Famennian platform evolution within the Barnett Spring area: 1) the development of a series of large antiformal, early Famennian deep-water sponge-stromatolite bioherms; 2) carbonate platform nucleation on the positive antecedent topography created by these marginal-slope bioherms; and 3) rapid platform progradation and the development of an extensive network of syndepositional neptunian dikes. The geometries of these dikes can be used to delineate the dimensions of the underlying bioherm structures. Neptunian dikes striking parallel to the length of the platform define the long axis or crest of the underlying antiformal structure, while dikes paralleling the platform-margin roughly define the basinward extent of the underlying antiformal bioherm. These dikes are thought to have been generated by gravitational instability associated with the early lithified reef front prograding over unlithified basinal sediments deposited off the flanks of the underlying structure, and by the differential compaction of early lithified platformal sediments over the rigid axial crest of the underlying antiformal marginal slope bioherms.

Significant changes in platform architecture across the F/F boundary are recognized within the Napier Range including: the shift to microbially mediated sedimentation throughout the platform and reefal-margin; the appearance of prominent tepee horizons within the back-reef; and the development meter-scale 5th order platform cycles capped with small-scale exposure horizons. These features suggest that the dominant controls on platform architecture throughout the Famennian were the effects of the faunal turnover and elimination of the Frasnian reef-building community associated with the F/F event, and high-frequency sea-level oscillations coupled with decreasing long-term accommodation space.