The Agadir Slide offshore NW Africa: Morphology, emplacement dynamics, and potential contribution to the Moroccan Turbidite System

Abstract

A newly identified large-scale submarine landslide on the NW African margin (Agadir Slide) is investigated in terms of its morphology, internal architecture, timing and emplacement processes by using high-resolution multibeam bathymetry data, 2D seismic profiles and gravity cores. The Agadir Slide is located south of the Agadir Canyon at a water depth ranging from 500 m to 3500 m, with an estimated affected area of ~5500 km 2. The complex morphology of the Agadir Slide reveals two headwall areas and two slide fairways (Western and Central slide fairways). Volume calculations indicate that approximately 340 km 3 of sediment were accumulated downslope in the slide fairways (~270 km 3) and Agadir Canyon (~70 km 3). Novel stratigraphic correlations based on five gravity cores indicate an age of ~142±1 ka for the emplacement of the Agadir Slide. However, the emplacement dynamics of the Agadir Slide suggests it was developed in two distinct, successive stages. The presence of two weak layers (glide planes) is suggested as the major preconditioning factor for the occurrence of slope instability in the study area, and local seismicity related to fault activity and halokinesis likely triggered the Agadir Slide. Importantly, the Agadir Slide neither disintegrated into sediment blocks nor was transformed into turbidity currents. Its timing of emplacement does not correlate with any turbidites recorded downslope across the Moroccan Turbidite System.

The Baiyun Slide Complex, South China Sea: A modern example of slope instability controlling submarine-channel incision on continental slopes

“…Downslope processes such as landslides and turbidity currents are driven by gravity and lead to the deposition of broad mass-transport deposits or turbidite systems within erosive channels (Moscardelli et al, 2006;Casalbore et al 2010;Bourget et al, 2011;Li et al, 2015b). They can transport large volumes (>100 km 3 ) of sediment sourced from the continental shelf and upper slope areas into the deep ocean (Georgiopoulou et al, 2010;Li et al, 2018), re-shaping the sea floor to influence subsequent sedimentary processes (Casalbore et al 2018). They can also control the distribution of sand in deep-water environments (Haflidason et al, 2004;Mosher et al, 2017).…”

Progress and Prospect of Seafloor Instability Research

“…沉积物输送到深海 [64] ，重新塑造海床并影响后续的 沉积过程，发育广泛的漂积体沉积，可见明显侵蚀 特征，进而控制深水环境中砂粒的分布。因此，海 底滑坡及其诱发的密度流是将泥沙从陆坡运往深海 的重要过程。 高分辨率的地球物理数据是识别海底滑坡的主 要手段。海底滑坡在地震剖面上一般表现为杂乱至 透明的地震相，表明滑塌沉积物混合均匀，具有典 型的碎屑流地震反射特征 [66,67] 。一般将海底滑坡划 分为头部物源区、体部滑移区和趾部挤压区，在不 同部位表现出拉张或挤压的构造特征。由于海洋环 境错综复杂，海底滑坡的成因机制与陆上滑坡存 在众多差异，海底滑坡发育的内在条件有沉积物的 类型、沉积物的饱和度、地形条件(坡度)和软弱 层等 [68] 。除此之外，还有一些触发因素可能会降低 海底陆坡的稳定性，如地震活动、高沉积速率、火 山喷发、削峭作用、侵蚀、差异压实、泥底辟、火 山隆升、人类活动、流体活动、超孔隙压力、天然 气水合物分解、气体泄露和海平面变化等 [69] 。 海底滑坡主要存在两种滑移形式，即旋转式滑 坡和板片式滑坡(见图 2) ，前者通常伴随海底陆坡 的滑塌过程，后者则与地层内部的沿层滑动面息息 相关 [70,71] 。世界上最大的 Storrega 海底滑坡的发育坡 度约为 0.7°，但滑坡体波及范围超过 95 000 km 2 。 已有研究 [64,71] 在西非的 Agadir 和 Sahara 大型海底滑 坡中均识别出了多期次的沿层滑动面，并表现出阶 表 2 海底蠕变的全球分布、地形地貌特征与内部地质结构 海域 土耳其马尔马拉海 [48] 里海 [50] 地中海 [51] 法国阿基坦陆缘 [52] 加拿大波弗特海 [53] 韩国东海 [54] 南海东南部 [55] 南海神狐海域 [49,56] 南海东沙海域 [45] 地形地貌特征与内部地质结构 沟槽近平行于等深线，隆起侧翼坡度可达 40°，其高度随水深增加而增大 经济发展造成巨大危害 [73] 。例如，在加拿大海域 Grand Banks 的海底滑坡(1929 年) [74] 、印度尼西亚…”

The ignition and evolution of a giant submarine gravity flow

“…The source area for the initial failure is interpreted to be from the upper reaches of the Agadir Canyon, along the NW Moroccan continental shelf (Wynn et al, 2010). However, efforts to map and core these areas have found no coincident erosional hiatuses or signi cant landslide scars to explain the ow's large volume (Krastel et al, 2016, Li et al, 2018.…”

“…3B). The exact depth of erosion is not possible to constrain from the cores on the canyon oor because the substrate comprises thick > 50 m remobilized muds derived from the continental slope (Li et al, 2018;Böttner et al, in review). However, cores situated high on the canyon margins show that Bed 5 was capable of eroding ~ 4 m deep ~ 230 m above the Lower Canyon oor (CD166-37; Talling et al, 2007).…”

“…It is thought that such events are born from appropriately large landslides, which initially failed and then disintegrated along their ow path into giant gravity ow events (Mulder & Alexander, 2001;Masson et al, 2006;Paull et al, 2010). However, two of the largest documented submarine gravity ow events, found offshore Morocco and Newfoundland, have no associated landslide scars (Piper et al, 1999;Mosher & Piper, 2007;Krastel et al, 2016;Li et al, 2018). This suggests they must have evolved from small failures into much larger events.…”