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Facies Analysis and Stratigraphic Relationships of Avulsion Splay Successions in the Paleogene Wilcox Formation, Deep Water Gulf of Mexico

Power, Bruce; Clark, Julian; Covault, Jacob; Fildani, Andrea; Sullivan, Morgan; Carson, Brooke; Zarra, Larry; and Romans, Brian
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The Wilcox Formation (Paleocene-Eocene) in the deepwater Gulf of Mexico comprises a thick (2000-6000 feet), sandstone-rich succession of deep water sedimentary rocks interpreted to have been deposited in channelized and unconfined lobe/sheet systems in slope and basin floor environments. Over the last decade, the Wilcox Formation has emerged as a major hydrocarbon reservoir, and is the focus of significant exploration and development activity within the petroleum industry.

Argillaceous sandstone beds are a common occurrence in sediments interpreted to have been deposited in medial to distal unconfined lobe/sheet environments in the Wilcox Formation and other deep water systems. They are interpreted to be deposited by gravity driven flows that are transitional between laminar and turbulent flow, and are classified as hybrid event beds. The argillaceous character of hybrid event beds is interpreted to reflect longitudinal flow evolution of the turbidity current event, in which the silt/clay to sand ratio is interpreted to increase with increasing runout length. This change in grain size ratio of suspended sediment dampens turbulence, causing a transition from turbulent to laminar flow. The resulting deposits contain significantly greater amounts of argillaceous silt and clay than would be found in a sand bed deposited as a turbidite. The Wilcox Formation also contains intervals with abundant hybrid event beds that are interstratified with strata interpreted as to have been deposited in channel and overbank environments. Interpretation of these hybrid event–dominated intervals as distal lobe/sheet sediments is challenging, as it requires repeated large magnitude shifts of depositional environment from proximal to distal. Equally challenging would be interpreting these intervals as channel margin deposits. Channelized environments are interpreted to be dominated by deposits that reflect higher energy turbidity current processes and through-going flow to more distal environments.

These argillaceous intervals are interpreted to represent the initial deposits of channel avulsion. The mixture of hybrid event beds, debrites, and turbidite sands and shales is interpreted to be deposited by the initial flows of a channel that has broken through its confining levee, and is forming an avulsion splay in what was previously an unconfined environment. A distinctive aspect of the interpreted avulsion splay intervals is that they commonly underlie confined channel or levee/overbank intervals, and are interpreted to have a genetic relationship to these overlying channelized strata. Similar deposits of argillaceous sandstones in the Neoproteropzoic Isaac Formation of the Windermere Supergroup (east-central British Columbia, Canada) have also been interpreted as hybrid event beds in avulsion-related crevasse splay deposits.


AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013