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Large allochthonous blocks up to several tens of meters across are adjacent to reef-fringed isolated carbonate buildups and platforms in the Canning basin of Western Australia and in western Canada. Distinctive criteria such as geopetal fabrics, stratification, lack of facies changes within the blocks, enclosure within basin sediments, and occurrence several kilometers from the nearest buildups indicate the blocks and breccias are allochthonous, although often misinterpreted as in-place bioherms. Framebuilding reef-core facies is the predominant block type, hence most were derived from the margins of carbonate buildups.
In Australia they occur as isolated blocks and contain abundant stromatoporoids and Renalcis, interpreted as reef-core facies, increasing in number toward the carbonate platforms. Some of the blocks have disrupted the underlying basin sediments. A few debris beds of finer breccia beds (fragments floating in carbonate mud matrix) wedge out laterally and presumably formed shallow "channels" perpendicular to the carbonate platforms.
In Canada isolated blocks are unknown. Locally megabreccia beds flank some of the buildups and extend several kilometers into the basin. More common are finer breccias in beds and channels. Both types consist of disoriented, angular fragments of reef-margin stromatoporoid and coral facies, external lagoon facies, and basin mud in a pervasive dark, interstitial dolomitized micrite of basin origin.
The isolated blocks represent material tumbled into the adjacent basin, apparently during times of active slumping of the platform margins. The Canadian deposits apparently were transported by submarine debris flows analogous to subaerial mudflows from buildup margin environments when relief and slope at the margins were greatest.
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