The Fortress Mountain Formation (Albian) is a sequence of shale, sandstone, and conglomerate exposed as isolated synclinal outcrops in the southern foothills province of the Brooks Range. The formation overlies a variety of severely deformed older rocks, particularly the Okpikruak Formation of Neocomian age. The unit records a significant change in the depositional architecture of the North Slope Cretaceous and is closely tied to the evolution of the Brooks Range orogenic belt.

Detailed stratigraphic and sedimentologic studies of the Fortress Mountain Formation within the type region and at easternmost exposures have demonstrated a systematic change in lithology and bedding style. The lower 40% of the Fortress Mountain is transitional with the Torok Formation and is composed of thick (up to 2,000-m) intervals of gray to black shale with thin, rhythmically interbedded, fine-grained sandstone beds. This succession is interrupted periodically by allochthonous blocks of conglomerate and conglomeratic sandstone. The sequences record a transition from basin plain to slope sedimentation. The middle 30-40% of the formation is composed of thick conglomerate and conglomeratic sandstone lenses that thin toward the north and are arranged in multiple upward-coarsening se uences and megasequences.

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Regional dispersal patterns indicate these beds accumulated within the midfan environment of a northward-prograding submarine fan complex. The upper 20-30% of the Fortress Mountain rests above a conspicuous angular discordance and is composed of upward-fining channel sequences of conglomerate, sandstone, and shale. This phase of deposition records progradation of fan-delta and fluvial environments. The regional depositional architecture of the Fortress Mountain records the buildup and sedimentologic evolution of the Cretaceous shelf, which ultimately allowed progradation of overlying deltaic and interdeltaic complexes of the Nanushuk Group and related strata.

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