--> Abstract: Neptunian Fractures and Carbonate-Platform-Margin Escarpments: Lessons from Frasnian (Upper Devonian), Napier Range, Canning B; #90063 (2007)

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Neptunian Fractures and Carbonate-Platform-Margin Escarpments: Lessons from Frasnian (Upper Devonian), Napier Range, Canning Basin, Western Australia

 

Ward, W. Bruce1 (1) Earthworks LLC and Theoretical & Applied Geology, Newtown, CT

 

Frasnian backstepped platforms in the southern Napier Range suggest models for interpreting margin escarpments and synsedimentary fractures and for predicting alignments of steep-margin carbonate platforms. Map-view geometries of Frasnian platform-margin escarpments up to 100-m high consist of linear segments connected at angular corners (including Dingo Gap's zigzag margin and Windjana Gorge's "Classic Face").

 

In his pioneering studies, Playford surmised that platform-margin escarpments were formed by fracturing and collapse. Frasnian platforms demonstrate that collapse not only modified platform-margin profiles, but also their plan-view geometry. Linear margins and angular corners resulted from removal of reef-margin and slope facies along pre-existing neptunian fractures perpendicular, parallel, and oblique to regional trends. Oblique fractures include opened structural joint sets associated with syndepositional faulting. Younger Frasnian reef and slope facies are laterally juxtaposed to pre-collapse platform facies. Escarpments are encrusted by cyanobacterial limestones (up to 2-m thick) that define time lines.

 

As Playford noted, fault blocks control large-scale orientations of the platforms. Frasnian exposures also suggest that margins away from fault-block edges (e.g., backstepped platforms) are allowed to develop smaller-scale (0.1 - 1 km) geometries oblique to larger-scale trends. Progradation of latest Frasnian and Famennian platforms smoothed out the margin irregularities. Dominant controls on collapse and margin and fracture orientations changed from being tectonic structure and gravity during the Frasnian to slope geometries and gravity during the Famennian.

 

Tectonic subsidence, faulting, sea-level changes, platform style, collapse, fracturing, marine cementation, and seawater circulation are part of feedback mechanisms that perpetuated upright escarpments and collapses along margins.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California