Submarine slopes prograde via accretion of sediment to clinoform foresets and degrade in response to channel or canyon incision or to mass-wasting processes. The timescales over which progradation and degradation occur, and the large-scale stratigraphic record of these processes, remain unclear due to poor age constraints in subsurface-based studies and areally limited exposures of exhumed systems. We here integrate 3D seismic reflection and borehole data to study the geometry and origin of ancient slope canyons developed in late Mesozoic strata of the Måløy Slope, offshore Norway. Slope degradation and canyon incision commenced during the late Kimmeridgian, coincident with the latter stages of rifting. Later periods of canyon formation occurred during the Aptian to Albian and the Albian to Cenomanian, during early post-rift subsidence. The canyons are straight, up to 700 m deep, and 10 km wide on the upper slope and die out downdip onto the lower slope. The canyons trend broadly perpendicular to and crosscut most of the rift-related normal faults, although syn-filling fault growth locally helped to preserve thicker canyon-fill successions. The headwalls of the oldest (late Kimmeridgian) canyons are located at a fault-controlled shelf edge, where younger canyons overstep this fault, which was inactive when they formed, extending across the paleo-shelf. Downslope, Aptian to Albian canyons either erode into the older, late Kimmeridgian to Barremian canyon fills, forming a complicated set of unconformities, or in the case of the Albian to Cenomanian canyons, die out into correlative conformities. Boreholes indicate that the canyon bases are defined by sharp, erosional surfaces, across which we observe an abrupt upward shift from shallow- to deep-marine facies (i.e., late Kimmeridgian canyons), or deep marine to deep marine facies (Aptian to Albian and Albian to Cenomanian canyons). Missing biostratigraphic zones indicate the canyons record relatively protracted periods (c. 2–17 Myr) of structurally enhanced slope degradation and sediment bypass, separated by > 10 Myr periods of deposition and slope accretion. The trigger for slope degradation is unclear, but it likely reflects basinward tilting of this tectonically active margin, enhanced by incision of the slope by erosive sediment gravity flows. The results of our study have implications for the timescales over which large-scale slope progradation and degradation may occur on other tectonically active slopes, and the complex geophysical and geological record of these processes. We also show that canyon formation can cause large volumes of margin-derived sediment to bypass proximal sub-basins within rifted terranes, an important process not currently captured by marine rift-basin tectono-stratigraphic models.

中文翻译：

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海底斜坡在地质时间尺度上的反复退化和发展

海底斜坡通过沉积物增加到斜形的前额而前进，并随着河道或峡谷的切割或大量消融的过程而退化。由于基于地下的研究中年龄限制不严格以及发掘出的化石系统的暴露量有限，因此发生降解和降解的时间尺度以及这些过程的大规模地层记录仍不清楚。我们在这里整合3D地震反射和钻孔数据，研究挪威海上Måløy斜坡中生代晚期地层中发育的古代斜坡峡谷的几何形状和成因。斜坡退化和峡谷切开始于基米底纪晚期，与裂谷的后期相吻合。峡谷形成的后期发生在裂陷后早期沉降的Aptian至Albian以及Albian至Cenomanian期间。峡谷是笔直的，最深达700 m，上斜坡宽10 km，然后逐渐下降到下斜坡。峡谷的走向大致垂直于大多数与裂谷有关的正断层，并与之相交，尽管局部局部充填断层的增长有助于保留较厚的峡谷充填序列。最古老的（基米底吉安晚期）峡谷的顶壁位于断层控制的陆架边缘，较年轻的峡谷越过该断层，该断层在形成时就没有作用，并贯穿古陆架。下坡，阿普特峡谷至阿比安峡谷，或侵蚀到较早的基米第吉纪至巴里米亚峡谷填土，形成一系列复杂的不整合面，或者就阿比安至塞诺曼尼亚峡谷而言，消失为相关的整合面。钻孔表明峡谷的底部由尖锐的侵蚀性表面所界定，从中我们可以看到，从浅海相到深海相（即，基米底纪晚期峡谷），或从深海相到深海相（阿普特，阿尔比，阿尔比，塞诺曼尼亚峡谷）突然向上移动。缺少生物地层区表明峡谷记录了相对较长的时期（约2-17 Myr），其结构增强的斜坡退化和沉积物绕开，相隔的沉积和斜坡积聚时间超过10 Myr。斜坡退化的触发因素尚不清楚，但可能反映了该构造活动边界向盆地的倾斜，而侵蚀性沉积物重力流使斜坡切开而加剧了倾斜。我们的研究结果对在其他构造活动斜坡上可能发生大范围斜坡恶化和退化的时间尺度具有影响，以及这些过程的复杂地球物理和地质记录。我们还表明，峡谷的形成会导致大量边缘衍生的沉积物绕过裂陷地层内的近端子盆地，这是海洋裂谷盆地构造-地层模型目前未捕获的重要过程。