Sedimentary and Tectonic History of Cretaceous Flysch in Southwestern Alaska: ABSTRACT

Abstract

Cretaceous deep-water sedimentary rocks are discontinuously exposed or have been dredged, along 1,700 km of the outer continental margin of the Alaska Peninsula-Bering Sea shelf. In the Shumagin and Sanak Islands, on the continental shelf near the southwestern end of the Alaska Peninsula, the deep-sea sediments are comprised of monotonous sections of thin (4 cm) to thick (10 m) bedded sandstone and mudstone, showing grading, convolute lamination, groove and flute casts. The sandstone beds are lithic arenite with more than 40% volcanic-derived framework grains. Over 500 measurements of sole marking in the Shumagin and Sanak Islands show maxima End_Page 795------------------------------ toward the southwest and west-northwest respectively, with minor lateral feed from the north. This flysch sequence was deposited primarily by turbidity currents in an elongate trough supplied from a northerly volcanic source area. In the absence of a confining basement seaward of the flysch deposits, the original depositional basin is interpreted as an oceanic trench. These trench deposits were deformed initially in a semilithified state with the development of axial-plane salty cleavage. Fold axes parallel the existing continental shelf edge, trending northeast and west-northwest in the outer Shumagin and Sanak Islands, respectively. Folds are overturned seaward predominantly, axial surfaces dipping landward. Locally units may be described as broken formations, though no melanges are observed. The style of this early folding is consistent with, but not diagnostic of, gravity liding. Alternatively, the rocks may have been deformed by underthrusting at the trench inner wall. At rates of 10-13 to 10-14/sec (calculated assuming underthrusting), the trench sediments may have undergone strain hardening from increasing internal grain friction and cementation during dewatering. End_of_Article - Last_Page 796------------