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The Intrinsic Effect of Shape from Retrogradation Motif and Timing of Drowning: From Example from a Frasnian Carbonate Pinnacle Reef System, Bugle Gap, Canning Basin, Western Australia

Adams, Erwin W. 1; Hasler, Claude-Alain 2
1 Shell International E&P, Rijswijk, Netherlands.
2 Department of Earth Sciences, University of Geneva, Geneva, Switzerland.

Attic oil accumulations associated with small buildups developing under the influence of a progressive increase in accommodation space on top of larger flat-topped carbonate platforms have been previously discovered. While high-resolution seismic data are necessary to detect the presence of these small buildups or pinnacle reefs, quantitative data from an analog outcrop setting can provide input to numerically assess the geometric and volumetric evolution of these drowned carbonate platforms and pinnacle reefs. Outcrops of the Devonian reef complexes of the Canning Basin of Western Australia reveal textbook examples of carbonate platform margins developing during high rates of subsidence. Several Frasnian outcrops in the Bugle Gap area of the Canning Basin are well exposed, show minor postdepositional tectonic deformation, have an exhumed topography, and are recognized by a set of retrograding and backstepping pinnacle reefs forming the southern tip of a carbonate platform. The evolution and stratigraphic architecture of these pinnacle reefs was evaluated, spatially recorded using digital surveying tools, and the quantified data assembled and visualized in a digital outcrop model. Subsequently, 2D surface models and 3D volumetric models were built reconstructing pinnacle reef development, allowing quantification of volumetric and geometric parameters. An intrinsic cause of the demise of isolated carbonate systems is related to the shift - as a consequence of increasing slope height - of the depositional regime from accretion to erosion, and hence from aggradation to retrogradation. This is because retrogradation reduces the production area at the platform top eventually becoming nil, and as a result, the time of drowning depends on the size of the production area. Nevertheless, those Bugle Gap pinnacle reefs that do have similar sizes seem to have different times for their termination. It could be demonstrated and quantified timing of drowning is not only depending on the size of the production area. Also shape and shape parameters of the production area are important constraints determining the timing of the demise of platform systems. More generally, wider implications can be evoked to highstand systems tracts and prograding carbonate systems.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009