Grain counts and XRD mineralogy of turbidite sand and hemipelagic mud from ODP Leg 168 sites

Sequences of late Pliocene to Holocene sediment lap onto juvenile igneous crust within 20 km of the Juan de Fuca Ridge in northwestern Cascadia Basin, Pacific Ocean. The detrital modes of turbidite sands do not vary significantly within or among sites drilled during Leg 168 of the Ocean Drilling Program. Average values of total quartz, total feldspar, and unstable lithic fragments are Q = 35, F = 35, and L = 30. Average values of monocrystalline quartz, plagioclase, and K-feldspar are Qm = 46, P = 49, and K = 5, and the average detrital modes of polycrystalline quartz, volcanic-rock fragments, and sedimentary-rock plus metamorphic-rock fragments are Qp = 16, Lv = 43, and Lsm = 41. Likely source areas include the Olympic Peninsula and Vancouver Island; sediment transport was focused primarily through the Strait of Juan de Fuca, Juan de Fuca Channel, Vancouver Valley, and Nitinat Valley. Relative abundance of clay minerals (<2-µm-size fraction) fluctuate erratically with depth, stratigraphic age, and sediment type (mud vs. turbidite matrix). Mineral abundance in mud samples are 0%-35% smectite (mean = 8%), 18%-59% illite (mean = 40%), and 29%-78% chlorite + kaolinite (mean = 52%). We attribute the relatively low content of smectite to rapid mechanical weathering of polymictic source terrains, with little or no input of volcanic detritus from the Columbia River. The scatter in clay mineralogy probably was caused by converging of surface currents, turbidity currents, and near-bottom nepheloid clouds from several directions, as well as subtle changes in glacial vs. interglacial weathering products.

太平洋西北部卡斯卡迪亚盆地（Cascadia Basin）内，距胡安·德富卡海岭（Juan de Fuca Ridge）20公里范围内，上新世晚期至全新世的沉积序列覆盖于新生火成地壳之上。大洋钻探计划（Ocean Drilling Program）第168航次所钻取的各站位内部及站位间，浊积砂岩（turbidite sands）的碎屑模式无显著差异。总石英、总长石及不稳定岩屑的平均占比分别为Q=35、F=35与L=30；单晶石英、斜长石及钾长石的平均占比为Qm=46、P=49与K=5；多晶石英、火山岩岩屑以及沉积岩与变质岩岩屑的碎屑模式平均占比则为Qp=16、Lv=43与Lsm=41。潜在物源区包括奥林匹克半岛（Olympic Peninsula）与温哥华岛（Vancouver Island）；沉积物主要通过胡安·德富卡海峡（Strait of Juan de Fuca）、胡安·德富卡水道（Juan de Fuca Channel）、温哥华谷（Vancouver Valley）与尼蒂纳特谷（Nitinat Valley）输运。黏土矿物（clay minerals，<2μm粒级组分）的相对丰度随深度、地层年龄及沉积物类型（泥质与浊积基质）呈无规律波动。泥质样品中的矿物丰度为：蒙脱石（smectite）0%~35%（均值8%）、伊利石（illite）18%~59%（均值40%），绿泥石（chlorite）与高岭石（kaolinite）合计29%~78%（均值52%）。研究认为，蒙脱石含量相对偏低源于多物源区的快速机械风化，且几乎无来自哥伦比亚河（Columbia River）的火山碎屑输入。黏土矿物组成的离散性可能由多方向表层流、浊流及近底浊云（nepheloid clouds）的汇聚作用，以及冰期与间冰期风化产物的细微差异共同导致。