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.

太平洋卡斯卡迪亚盆地西北部胡安德富卡海岭周边20千米范围内，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航次所钻探的各站位内部及站位间的浊积砂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、L=30。单晶石英、斜长石、钾长石（K-feldspar）的平均占比分别为Qm=46、P=49、K=5；多晶石英、火山岩岩屑、沉积岩+变质岩岩屑的平均碎屑占比分别为Qp=16、Lv=43、Lsm=41。潜在物源区包括奥林匹克半岛与温哥华岛；沉积物主要经胡安德富卡海峡、胡安德富卡水道、温哥华谷及尼蒂纳特谷输运。黏土矿物（粒径<2微米组分）的相对丰度随深度、地层时代及沉积物类型（泥质与浊积岩基质）呈无规则波动。泥质样品中的矿物占比为：蒙脱石（smectite）0%~35%（平均值8%）、伊利石（illite）18%~59%（平均值40%）、绿泥石（chlorite）与高岭石（kaolinite）29%~78%（平均值52%）。我们将蒙脱石含量偏低归因于多物源源区的快速机械风化，且哥伦比亚河几乎未输入火山碎屑物质。黏土矿物组成的离散性可能源于多方向表层流、浊流及近底悬浮云团的汇聚，以及冰期与间冰期风化产物的细微变化。