TRACING OF THE FRASER RIVER THROUGH GEOCHEMISTRY OF CASCADIA BASIN TERRIGENOUS SEDIMENTS

Oceanic sediments deposited at high rates close to continents are dominated by terrigenous material and their chemical compositions reflect those of nearby continental masses. This study focuses on oceanic sediments (Pleistocene to Quaternary) transported by the Fraser River and derived from the juvenile Canadian Cordillera. We report major and trace element concentrations (n=68) and Sr-Nd-Hf-Pb isotopic compositions (n=57) for sediments from the northernmost part of the Cascade forearc, drilled at ODP Sites 888 (off-shore Vancouver Island) and 1027 (east flank of the Juan de Fuca Ridge) [1,2]. The two sites have similar compositions, but Site 888, closer to the continent, has higher sandy turbidite contents and higher bulk SiO₂/Al₂O₃ with lower bulk Nb/Zr, attributed to the presence of zircon in the coarse sands. These juvenile sediments also have lower Th/Nb, Th/U, Nb/U and Th/Rb compared to sediments derived from mature continental areas, reflecting U and Rb losses through time (leaching by rain and river water). The radiogenic isotopic compositions suggest the involvement of two dominant end-members, both from the nearby Canadian Cordillera, and they plot on the same mixing array in Sr-Nd plot as Fraser River suspended matter. Erosion products of the depleted, western part of the Cordillera dominate the detrital input, whereas the eastern enriched terranes of the Cordillera contribute only 10-28% of the input. There is no marked change of provenance of sediment during the last 3.5 Ma, in either site, suggesting that the relative proportion of eroded material from the western and eastern terranes was not drastically modified by climate conditions. Sediment compositions appear unaffected by glacial–interglacial climate cycles. The average isotopic compositions of the two sites are both dominated by a continental signature and are significantly different from what is observed further south in the Astoria fan where sediments are deposited from the Columbia River [3]. The average values determined for each site can be used in future studies to identify potential subducted sediment contributions to volcanism in the Garibaldi Volcanic Belt and the High Cascades.

[1] Carpentier et al (2013) Chem Geol 340, 91-104; [2] Carpentier et al (2014) Chem Geol 382, 67-82; [3] Prytulak et al (2006) Chem Geol 233, 276-292.