OFFSHORE STRATIGRAPHIC RELATIONSHIPS IN THE SOUTHERN ALASKA SYNTAXIS AND PHYSICAL PROPERTIES OF THE NEOGENE SEDIMENTARY COVER OFFSHORE ST. ELIAS MOUNTAINS

Abstract:

Southeastern Alaska is home to one of the most actively deforming continental margins in the world, and temperate glaciers covering the area contribute to high rates of erosion and offshore sedimentation, making the Neogene sediment covering the offshore Yakutat Block an ideal location to study interactions between climate and tectonics. In 2008, data that includes ~115 km of multichannel seismic reflection and ocean-bottom seismometer wide-angle reflection and refraction data was collected across the central Yakutat shelf in the Gulf of Alaska. We present a high-resolution two-dimensional P-wave velocity model of the glaciomarine sediment and underlying Yakutat basement in the area and compare to coincident seismic reflection profiles in order to determine lithologic boundaries and correlate with stratigraphic units observed onshore and in offshore boreholes. Detailed subsurface mapping of the offshore sedimentary units in and near Yakutat Bay is essential to understanding the long-term development of the central and northern Yakutat shelf as a depocenter, where more of the facies that record St. Elias orogenesis are preserved than onshore. The depositional environment transitions from a wide shelf incised by sea valleys that deliver sediment to slope aprons in the east to a narrow shelf across which sediment is transported to the Aleutian trench in the west. Our velocity profile used in conjunction with seismic reflection data shows transitions in dominant depositional processes over time in the central portion of the block. In Yakutat Bay, sediment velocities are high (~ 4 km/s) compared to velocities farther offshore (< 3 km/s), possibly indicating greater compaction of the sediments in the bay. In the central portion of the shelf, three distinct refractions from sedimentary layers are observed and are interpreted as the Yakataga, Poul Creek, and Kulthieth Formations. In the north, only two refractions from sedimentary layers are observed. Basement refractions are also observed in the north at close offsets in Yakutat Bay, evidence of shallow basement that could be indicative of uplift in the bay. Our detailed mapping of offshore sedimentary facies near the syntaxis provides a regional framework of the geometries of the sedimentary packages and their spatial relationship to major tectonic features.