(Table 1) Heavy minerals in silt and sand samples of IODP Site 301-U1301

The northwestern Cascadia Basin of western North America accumulated high-sedimentation-rate sequences during the Pleistocene sea-level low-stands. The continental shelf was largely exposed at that time, and rivers and estuaries delivered large sediment fluxes directly to the deep ocean. The IODP EXP1301 core, which was taken from the middle portion of the Cascadia Basin, is well preserved and exhibits the deeper and - more distal sedimentary facies. The lithology in this location is composed of two units, 1) hemipelagic mud with a thin sand layer and 2) thick, coarsening upward silt-sand turbidites with a small proportion of granules at the top. We will focus on the detailed sand-grain proportions in order to understand the origin of these sediments. We determined the modal proportions of the heavy minerals, and the chemical composition of olivine and orthopyroxene in fourteen samples. These are characterized by an abundance of amphibole, pyroxenes and epidote, and the presence of minerals derived from peridotite. There is no drastic change in the modal and mineral compositions of the sands and silts between the turbidite and hemipelagic sequences. There were two probable drainage systems on the continent, the Frazer and Columbia rivers, which shed turbidites into the Cascadia Basin after 1.6 Ma, especially at 0.46-0.76 Ma. Based on a comparison of the modal and mineral compositions, the Northern Cascadia Basin has been supplied with sediments, mainly from the Frazer River, through the Straits of Juan de Fuca, by Pleistocene to Holocene turbidites.北美西部喀斯喀特盆地（Cascadia Basin）西北部在更新世（Pleistocene）海平面低位期沉积了高沉积速率层序。当时大陆架大面积出露，河流与河口将大量沉积物通量直接输送至深海。取自喀斯喀特盆地中部的国际大洋发现计划（International Ocean Discovery Program，IODP）1301航次钻孔岩芯保存完好，展现了更深、更远端的沉积相。该区域的岩性分为两个单元：1）夹薄层砂的半远洋泥（hemipelagic mud）；2）顶部含少量颗粒、向上粒度逐渐变粗的厚层粉砂-砂质浊积岩（turbidite）。本研究聚焦于详细的砂粒粒度占比，以厘清这套沉积物的成因。研究人员对14个样品中的重矿物模态占比，以及橄榄石（olivine）与斜方辉石（orthopyroxene）的化学成分进行了测定。这些矿物组合以广泛产出角闪石、辉石与绿帘石为特征，同时含有源自橄榄岩（peridotite）的矿物。浊积岩与半远洋沉积序列中的砂质与粉砂质组分，其矿物模态与化学成分均未出现显著变化。北美大陆存在两套潜在的物源水系：弗雷泽河与哥伦比亚河，二者在距今1.6 Ma之后将浊积物输送至喀斯喀特盆地，尤以0.46~0.76 Ma期间最为显著。通过对比矿物模态与化学成分，研究认为更新世至全新世（Holocene）的浊积作用通过胡安·德富卡海峡（Straits of Juan de Fuca）将沉积物主要从弗雷泽河输送至北喀斯喀特盆地。