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The Yakutat Group, part of a belt of highly deformed late Mesozoic rocks deposited in deep water and extending for more than 1,600 km along the Gulf of Alaska margin, underlies an area 200 km long by 15-30 km wide of the Saint Elias Mountains foothills and adjacent coastal plain. Bedded rocks of the Yakutat Group are dominantly Cretaceous graywacke and pelite with local pebble-cobble conglomerate and sparse pods of oolitic chert-nodule limestone. Interspersed with the bedded rocks are melanges composed of large blocks of competent rocks up to several kilometers in size engulfed in a pervasively sheared matrix of pelite or tuffaceous pelite. These clasts include externally derived or exotic greenstone, marble, metagraywacke, metachert, and diorite that are at least in part of Triassic(?) and Middle Jurassic age. Following deposition, the Yakutat was subjected to (1) pre-late Eocene compressive folding and thrusting, (2) regional zeolite to low greenschist-facies metamorphism with emplacement of early Tertiary granitic plutons, and (3) disruption by large-scale dextral shearing.
Available data suggest that the Yakutat was deposited in an oceanic trench associated with a volcano-plutonic arc. Large exotic clasts were presumably incorporated in the melange by tectonic disruption and submarine sliding of older rocks exposed along the inner wall of the trench. A source for the sediments and exotic clasts does not exist in contiguous parts of the Saint Elias Mountains. The most probable provenance lies roughly 240 km southeast in the Chicago-Baranof-Admiralty Islands area of southeastern Alaska. Large-scale post-middle Eocene dextral slip on the Fairweather fault is postulated to explain separation of the Yakutat Group from its source area.
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