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Seismic geometries in the deep marine Torok Formation illustrate that catastrophic slope failure involving both slope and basin-plain sediments occurred during Early Cretaceous time on the North Slope of Alaska. The magnitude of the failure emphasizes the importance of slumping and sliding as processes of mass transport of sediment in the deep marine environment.
Torok sandstones and shales were deposited on continental slopes, basin plains, and submarine fans. Fluvial-deltaic sands and shales of the Nanushuk Group are the time-equivalent shelf deposits. The Nanushuk-Torok relationship is expressed seismically as offlapping reflectors that record shelf-edge progradation. Slumps and slides are common on Torok slopes where gradients of up to 10° are documented.
The largest such feature, located near Harrison Bay, is 1,500 mi2 in area and 2,000 ft thick. The disturbed zone is lobate in plan view, wedge shaped in cross section, and thins basinward from a dramatic scarp deeply incised into Torok foreset beds. Seismically, the slide is expressed as a series of remnants of undisturbed or rotated glide blocks that strike parallel with the slump scarp and are encased in chaotically bedded slump debris. Geometric similarities to the Turnagain Heights slide (Anchorage, 1964) suggest block gliding as the mechanism of slope failure.
Because the Torok was initially sand-poor, wells drilled through glide blocks and slump debris encountered predominantly shale. Understanding similar seismic geometries in other slope systems will aid in their evaluation as hydrocarbon traps. Favorable reservoir and trap scenarios include turbidite sands in remnant blocks trapped against slump fill and younger turbidite sands ponded behind remnant topography.
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