Platform-Margin Trajectory as a Control on Neptunian Dike Distributions, Devonian Reef Complexes, Canning Basin, Western Australia
Ned Frost
Jackson School of Geosciences, University of Texas at Austin
Abstract:
Syndepositional opening-mode fractures known as
neptunian dikes are a ubiquitous feature of early-cemented platform-margins and
are recognized in many carbonate systems throughout the geologic record. The
Canning Basin’s Devonian reef complexes preserve perhaps some the world’s best
examples of these features. Neptunian dikes exert a strong influence on
diagenetic patterns and permeability distributions; understanding the controls
on their distribution is critical to accurate reservoir
prediction and
characterization.
Mechanisms
proposed for the generation of neptunian
dikes include: differential compaction of underlying sediments, compaction over
basement structure, slope bedding-plane slippage, coeval faulting, gravitational
instability, and seismicity. Which
mechanisms
control syndepositional fracturing
depends largely on platform-margin trajectory, with fundamental differences in
dike distribution, orientation, and extension observed between retrograding and
prograding platform-margins.
In the retrograding platform-margins of the Frasnian southwest Oscar Range syndepositional fracturing is most intense in the reef/reef-flat, and decreases only subtly into the platform interior. Dike orientations run parallel and orthogonal to regional structural features, often intersecting the platform-margin at high angles, suggesting that compaction over basement topography and Devonian fault movement (e.g. Djowi Fault) were the dominant controls on syndepositional fracturing.
Fracture patterns observed in the prograding Famennian Barnett Spring platform are fundamentally different than those of the southwest Oscar Range. In Barnett Spring neptunian dikes primarily run parallel and orthogonal to the platform-margin (not regional structure) and are best developed in the reef/reef-flat, with fracture intensity decreasing dramatically towards the platform interior. These prograding margins consistently show double the amount of extension observed in the retrograding Frasnian margins.
AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid