--> ABSTRACT: Deducing Shoreline Trajectories in a Mixed Siliciclastic-Carbonate Depositional Setting, Upper Devonian Imperial Formation, Northwest Territories, Canada, by Acker, Rachael; Hubbard, Stephen ; Hadlari, Thomas ; #90142 (2012)

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Deducing Shoreline Trajectories in a Mixed Siliciclastic-Carbonate Depositional Setting, Upper Devonian Imperial Formation, Northwest Territories, Canada

Acker, Rachael *1; Hubbard, Stephen 1; Hadlari, Thomas 2
(1) Geosciences, University of Calgary, Calgary, AB, Canada.
(2) Geological Survey of Canada, Calgary, AB, Canada.

Shoreline trajectory analysis provides insight into the depositional evolution of ancient basin margins. A 361 m thick, aerially limited outcrop of the Devonian Imperial Formation in the Mackenzie Corridor, Northwest Territories, Canada, comprises 28 upwards coarsening cycles, 3-15 m thick. The formation is a potential conventional reservoir and unconventional gas target in the subsurface within 10 km of the studied outcrop. The coarsening upwards cycles, or parasequences, are arranged into a series of seven parasequence sets; variations in gross parasequence thickness, sandstone:siltstone ratio, and facies are utilized to deduce shoreline trajectories. The majority of parasequences are capped with hummocky cross-stratified sandstone and the consistency in their character suggests that aggradation was dominant. It is hoped that shoreline trajectory interpretations from the thick, yet areally limited outcrop exposure can be used to predict stratal packaging in the prospective reservoir region.

Despite the overall aggadational nature of the depositional system, the lower 17-18 parasequences comprise 4 subtly progradational parasequence sets, inferred to be characterized by rising shoreline trajectories. Each parasequence set is characterized by an upwards increase in gross parasequence thickness, grain-size, and proportion of proximal, sandy facies. The upper 9-10 parasequences form three retrogradational parasequence sets, with each parasequence characterized by an upwards decrease in gross thickness and proportion of sandy lithofacies. These cycles are interpreted to record an overall, slight back-stepping of shorelines through the depositional evolution.

The top of the 18th parasequence is characterized by unique facies attributes, including trough cross-stratification and in-situ fossils (i.e., horn and tabulate corals). In-situ coral growth is interpreted to result from sediment starvation attributed to either: (1) onset of transgression; or (2) autogenically controlled sediment flux (e.g., delta lobe switching). Fossil debris layers characterize the tops of parasequences immediately above and below the in-situ fossil layer. The tops of other parasequences consist of hummocky cross-stratified sandstone beds, suggesting that parasequence 18 preserves the most proximal sandstone beds in the entire stratigraphic package; it marks the transition from progradational to retrogradational shoreline trajectory.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California