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

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.

本数据集涵盖太平洋西北部卡斯卡迪亚盆地（Cascadia Basin）胡安德富卡海岭（Juan de Fuca Ridge）周边20千米范围内，覆于新生火成地壳之上的上新世晚期至全新世沉积序列。大洋钻探计划（Ocean Drilling Program）第168航次所钻各站位内部及站位间的浊积砂（turbidite sands）碎屑模式并无显著差异。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、火山岩岩屑及沉积岩+变质岩岩屑的平均碎屑模式则为Qp = 16、Lv = 43与Lsm = 41。该沉积的潜在物源区包括奥林匹克半岛（Olympic Peninsula）与温哥华岛（Vancouver Island），沉积物搬运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）进行。粒径<2μm的黏土矿物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）呈不规则波动。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%），以及绿泥石+高岭石（chlorite + kaolinite）29%~78%（平均值52%）。研究认为，蒙脱石含量相对偏低源于多岩性源区的快速机械风化，且哥伦比亚河（Columbia River）几乎未输入火山碎屑物质。黏土矿物学的分散性可能由多方向表层流、浊流与近底悬浮浊云的汇聚，以及冰期与间冰期风化产物的细微变化共同导致。