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Westward view through Atigun Gorge. Atigun River flows over north-verging, thrust-fault contact between Lower Cretaceous (Neocomian) Okpikruak Formation on left side of river and Lower Cretaceous (Aptian-Albian) Fortress Mountain Formation on right side of river. Okpikruak Formation comprises a mélange of turbidites and exotic blocks of chert in a silty mudstone matrix. The Fortress Mountain Formation comprises marine slope faces (gray silty mudstone on lower half of slope to right of river) overlain by shoreface through nonmarine facies (lighter colored sandstone and conglomerate on upper slope at right). In distance, Paleozoic rocks including Lisburne Group carbonates (highest peaks) are thrust over the Okpikruak Formation to form the tectonic front of the Brooks Range. Location about 125 miles south of Prudhoe Bay.
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Westward view through Atigun Gorge. Atigun River flows over north-verging, thrust-fault contact between Lower Cretaceous (Neocomian) Okpikruak Formation on left side of river and Lower Cretaceous (Aptian-Albian) Fortress Mountain Formation on right side of river. Okpikruak Formation comprises a mélange of turbidites and exotic blocks of chert in a silty mudstone matrix. The Fortress Mountain Formation comprises marine slope faces (gray silty mudstone on lower half of slope to right of river) overlain by shoreface through nonmarine facies (lighter colored sandstone and conglomerate on upper slope at right). In distance, Paleozoic rocks including Lisburne Group carbonates (highest peaks) are thrust over the Okpikruak Formation to form the tectonic front of the Brooks Range. Location about 125 miles south of Prudhoe Bay.