Neptunian
Fractures and Carbonate-Platform-Margin Escarpments: Lessons from Frasnian (Upper Devonian),
Ward, W. Bruce1 (1) Earthworks
LLC and Theoretical & Applied Geology,
Frasnian backstepped
platforms in the southern
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