| Summary: | In southeastern Alaska, the Chugach-St. Elias Mountains exhibit extreme topography due to the collision and subduction of the Yakutat microplate beneath the North American plate. The St. Elias orogeny has occurred during a period of enhanced glacial erosion in the Pleistocene when erosive ice streams delivered sediment into the Gulf of Alaska. Sediment cores from Sites U1420, U1421, and U1417 Integrated Ocean Drilling Program Expedition 341 were taken on the continental shelf, within the Bering Trough and Surveyor Channel in the deep marine basin. Analysis of seismic profiles demonstrates an evolution from tectonically to depositionally controlled continental margin strata formation. Despite temporally evolving glacimarine strata formation, XRD data for the silt-sized sediment fraction (15-63 m) from all sites demonstrates that the mineralogy is consistent downhole with minor variations in relative diffraction peak intensities. Elemental data on the same sediment fraction reveal concentrations of both major and trace elements also have very minor variance downhole. All major (e.g., Al, Ca) and trace (e.g., Co, Nd) elemental data vary by only few ppm. Both the consistent downhole mineralogy and elemental data suggest that the provenance of the silt-sized sediment deposited offshore has not changed over the length of the boreholes. Comparison with onshore bedrock geochemistry and surface samples from the modern Gulf of Alaska indicate that the recovered silt is similar in composition to modern regional sediment sources and is a mixture of the different bedrock lithologies within the modern Bering and Malaspina Glacier drainage.
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