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Evolution of from Oligocene Canyon System from the Eastern Scotian Margin

Brake, Virginia 1; Mosher, David 2; Wach, Grant 1
1 Earth Sciences, Dalhousie University, Halifax, NS, Canada.
2 Geological Survey of Canada, Dartmouth, NS, Canada.

The Cenozoic section of the eastern Scotian margin is a largely progradational sequence of mostly fine-grained clastic sediment. The Stonehouse 3D seismic dataset, spanning outer shelf and upper slope terrain of a portion of the eastern Scotian margin, provides an opportunity to study in detail Cenozoic seismic stratigraphy within this critical transition zone. Close inspection of the shelf to slope hingeline through the Cenozoic shows that the margin is not progradational but rather aggradational, having major implications on dominant sedimentation processes. The modern seafloor is heavily incised by canyons and valleys, providing conduits for off-shelf sediment transport, slope by-pass and deposition on the continental rise and abyssal plain. Canyon incision appears to have been episodic throughout the Cenozoic, involving multiple phases of cut-and-fill with new systems often re-occupying old.

A particularly widespread Oligocene (?) erosional surface demonstrates a canyon system more extensive than other buried canyons along the margin, incising as deeply as Paleocene and undetectable by mid to late Miocene times. The canyon measures >150 km in length, 6-15 km in width and incises as deeply as 1 km along a fairly straight path from the shelf through to the continental slope and abyssal plain. Sediment removed by this canyon system is on the order of 64 km3 within the 3D dataset alone. The canyon system is part of a subregional unconformity consisting of at least two unconformities that locally merge. These two major incision phases are separated by periods of fill and other small scale erosive events. The thalweg of initial incision indicates a braided pattern with high amplitude abandonment loops, while the thalweg of the later canyon system has high amplitude packages of the same size oriented down axis. Canyon fill between incision phases is high amplitude and weakly continuous while subsequent fill is low amplitude and continuous, indicating a switch from somewhat chaotic to continuous fill.

Episodic canyon incision throughout the history of the Scotian margin indicates a limited residence period of sediments on the shelf and slope with canyon incision acting as a major influence on sediment distribution and conduit for sediment bypass. The magnitude of the Oligocene canyon system and migration of the paleoshelf break suggests a significant fall in relative sea level post-Tertiary with a subsequent rise after an Oligocene lowstand.

 

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