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Reefal Platform Development, Devonian of the Canning Basin Western Australia

Phillip E. Playford, Neil F. Hurley, Charles Kerans, Michael F. Middleton

Growth of Devonian reefal limestone platforms on the Canning basin was nearly continuous from the late Givetian to the late Famennian. The earliest (Givetian and early Frasnian) platforms were low-relief banks; later Frasnian and Famennian platforms were normally reef rimmed, with high relief. Upright reef margins predominated in the Frasnian, with intervals of backstepping associated with widespread drowning and the development of pinnacle reefs. A brief emergence occurred at the Frasnian-Famennian boundary, and the succeeding Famennian platforms advanced basinward over equivalent marginal slope and basin facies. Platform extinction in the late Famennian resulted from abrupt drowning.

An important initial control on platform trends was basement topography resulting from Middle Devonian or older faulting. Continued faulting during the Late Devonian influenced the morphology of some platforms. Contemporary seismicity also led to neptunian fracturing and the collapse of platform margins.

The hypothetical sea level curve for the Canning basin Devonian is believed to be a combined result of global eustatism and regional diastrophism; it shows gross resemblance to the equivalent Euramerican curve. Rates of sea level rise (or subsidence) are thought to have largely controlled platform evolution. Banks and advancing platforms were associated with slow rises in sea level; upright reef scarps with moderately rapid rises; and backstepping, pinnacle reefs, and drowning with very rapid rises. Shoaling-upward cycles in back-reef limestones resulted from periodic abrupt rises of a few meters, followed by longer stillstands.

Seismic-stratigraphic modeling together with observed seismic records and well data suggest that the evolutionary model for the Devonian platforms deduced from outcrop studies can also be applied in the subsurface.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.