Portuguese contourite depositional systems derived from Natural Gamma Ray records analysed during IODP Exp339@en

The Contourite Depositional Systems (CDS) in the Gulf of Cádiz and on the West Iberian margin preserve a unique archive of Mediterranean Outflow Water (MOW) variability over the past 5.3 Ma. These CDS have been recently drilled in several places during the IODP Expedition 339. These drill sites now offer a new window to the internal Pliocene and Quaternary architecture of the CDS. In this study, we use downhole and core Gamma Ray (GR) data acquired from 5 sites drilled in the CDS along the middle slope and 1 site drilled in the deeper setting of the lower slope, out of the MOW path. The GR data primarily tracks the clay content in the sediment and is the expression of sediment supply and, for sites drilled in the CDS, of the bottom current processes. Both appear astronomically controlled as shown by spectral analysis performed on the GR data. Results also reveal that the GR log patterns correlate well across the sites over the last 1.4 My. Several GR horizons corresponding to drops in GR values were identified, most of which fit with coarse-grained deposits observed in cores and interpreted as contourite beds. The GR horizons are interpreted as isochronous horizons, providing a regional scale chronostratigraphic framework for the CDS depositional records with an accuracy of ~ 20 ky. We further assess the spatial and temporal variability of the CDS hiatuses at the regional scale.加的斯湾与伊比利亚西部陆缘的等深流沉积体系（Contourite Depositional Systems, CDS）保存了过去530万年以来地中海流出水（Mediterranean Outflow Water, MOW）演化的独特沉积记录。此前，国际大洋发现计划（International Ocean Discovery Program, IODP）339航次已在多处该类沉积体系布设钻井，本次钻探点位为研究等深流沉积体系的上新世与第四纪内部结构提供了全新视角。本研究利用沿中陆坡布设的5个等深流沉积体系钻井，以及1个位于地中海流出水路径之外的下陆坡深水区钻井所获取的井下与岩心伽马射线（Gamma Ray, GR）测井数据。伽马射线测井数据主要反映沉积物中的黏土含量，其变化体现了沉积物供给特征；而针对等深流沉积体系布设的钻井数据，还可反映底流活动过程。频谱分析结果显示，上述两类过程均受天文周期调控。研究还发现，过去140万年以来，各钻井的伽马射线测井曲线形态具有良好的区域可对比性。本研究识别出多处伽马射线值下降的测井界面，其中绝大多数与岩心中观察到的粗粒等深流沉积层对应。上述伽马射线测井界面被解释为等时界面，可为等深流沉积体系的沉积记录提供精度约2万年的区域性年代地层框架。本研究进一步评估了区域尺度上等深流沉积体系沉积间断的时空演化特征。