Contrasting Upper Devonian Foreslopes of the Canning Basin: Examples From Retrograding Frasnian and Prograding Famennian Platform Flanks

Playton, Ted E.¹, Charles Kerans², Phillip E. Playford³ (1) The Univeristy of Texas at Austin, Austin, TX (2) The University of Texas at Austin, Austin, TX (3) Geological Survey of Western Australia, Perth, W.A, Australia

Foreslope deposits that flank reef-rimmed carbonate platforms are composed of sediment gravity flow architectural elements that have distinct angles of repose and compositions. The primary elements observed from these systems include 1.) megabreccias (coarse debris deposits) from reef collapse, 2.) grain flows and grainy turbidites (grainstone/ gravel deposits) from platform and slope shedding, and 3.) muddy turbidites and hemipelagites (siltstone/wackestone deposits) indicating slope quiescence. The Canning Basin outcrops, northwest Australia, offer excellent exposures of these foreslope elements in different accommodation settings.

Twenty-one measured sections tied to detailed photomosaics were collected for over 10 km2 and 100-500 meters of seismic-scale middle Frasnian and Famennian foreslope strata. Middle Frasnian foreslopes are dominated by grain flows and grainy turbidites with few thick megabreccias scattered within the overall stratigraphy. In contrast, Famennian foreslopes display cyclic vertical successions consisting of megabreccia, hemipelagite, and grain flows/grainy turbidites.

These differences in sediment gravity flow proportions and frequency reflect 2nd-order accommodation changes from Frasnian retrogradation to Famennian progradation. Upright margins of the Frasnian are inherently more stable than Famennian margins that prograded over unstable substrates related to relict backstepping topography. Thus, reef collapse and megabreccia generation is a common, high frequency process during Famennian slope development. Frasnian megabreccias, however, are not common and represent rare catastrophic events that punctuate typical sand/gravel shedding onto the slope. These observations offer a linkage between accommodation, margin geometry, and slope element distribution, which can aid in the prediction of carbonate foreslope reservoir quality and architecture in the Canning Basin and elsewhere.