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Linking fracturing and rock mechanic properties to the erosion of a beachrock shore platform
Marine Geology ( IF 2.9 ) Pub Date : 2021-08-27 , DOI: 10.1016/j.margeo.2021.106616
Warwick W. Hastie 1 , Ayanda T. Mthembu 1 , Andrew N. Green 1 , Jonathan van den Bergh 1
Affiliation  

The intertidal zone and raised shore (supratidal) platform at Mission Rocks, northern KwaZulu-Natal coast, South Africa, consists of fracture-bounded Holocene beachrock units, and megagravel (boulder) deposits. This area is microtidal and wave-dominated, with swells approaching from the southeast. The area is also influenced by storm wave events. The seaward edge of the intertidal platform is undercut, leading to the formation of overhangs, which are modelled here as cantilevered beams. This analysis shows that beam volume/weight does not control failure because relatively short, thick beams (l:h < 2) are less susceptible to failure, and the beam length-to-thickness ratio is important. This accords with tensile strength testing which demonstrates that the bedding planes are weak and that saturated beachrock (i.e., at the platform edge) is ~25% weaker than unsaturated rock. Fracture, block and boulder analyses of seven coastal sections, and their geomorphological features (elevation, slope, terrain ruggedness), reveal three key relationships: (1) The number of fracture intersections and fracture density are higher in the raised shore platforms than in the intertidal, which is relatively unfractured, lower in elevation and less rugged than the raised shore platform. These parameters correlate positively with the number of fracture-bounded blocks. (2) Fracture-bounded blocks on the raised shore platform (n = 145) have a mean shape ratio of 2.25 and are 10–20 m2 in size, while the disarticulated boulders (n = 172) have a mean shape ratio of 1.3–1.5 and are 2–3 m2 in size. It is considered that the reduction in shape and size of the boulders is the result of fracture-bounded blocks being quarried and transported from the raised shore platform, and not from the intertidal platform, during high-energy storm events. (3) Coast-perpendicular fractures in the intertidal platform deepen and widen towards the sea, forming gullies in which wave run-up and retreat was observed. A relatively good negative correlation (r2 = 0.7) was found between the number of these gullies and values of maximum terrain ruggedness which suggests that platform lowering occurs when an appropriate fracture network (relatively dense and suitably orientated) receives unimpeded swells and waves. Overall, we find that the evolution of the beachrock platform in terms of ruggedness and downwearing is influenced, at least in part, by the fracture networks present, most notably in terms of their orientation, density, persistence and intersection geometry.



中文翻译:

将压裂和岩石力学特性与海滩岩岸平台的侵蚀联系起来

南非夸祖鲁-纳塔尔海岸北部 Mission Rocks 的潮间带和隆起的海岸(潮上)平台由以裂缝为界的全新世海滩岩单元和巨砾(巨石)沉积物组成。该区域以微潮和波浪为主,海浪从东南方向逼近。该地区还受到风暴波事件的影响。潮间带平台的向海边缘被底切,导致形成悬垂,此处将其建模为悬臂梁。该分析表明,梁体积/重量不能控制失效,因为相对较短、较厚的梁 (l:h < 2) 不太容易发生失效,梁长厚比很重要。这与拉伸强度测试一致,该测试表明层理平面很弱并且饱和的海滩岩石(即,在平台边缘)比不饱和岩石弱约 25%。七个海岸段的裂缝、块体和巨石分析及其地貌特征(高程、坡度、地形崎岖度)揭示了三个关键关系:(1)凸起的岸边平台的裂缝交叉点数量和裂缝密度高于潮间带,相对未破裂,海拔较低,比凸起的岸边平台更不坚固。这些参数与裂缝边界块的数量呈正相关。(2) 抬升岸平台上的裂缝限界块体( (1) 加高岸平台的裂缝交汇点数和裂缝密度高于潮间带,潮间带相对无裂缝,海拔较低,不及凸起岸平台崎岖不平。这些参数与裂缝边界块的数量呈正相关。(2) 抬升岸平台上的裂缝限界块体( (1) 加高岸平台的裂缝交汇点数和裂缝密度高于潮间带,潮间带相对无裂缝,海拔较低,不及凸起岸平台崎岖不平。这些参数与裂缝边界块的数量呈正相关。(2) 抬升岸平台上的裂缝限界块体(n  = 145) 的平均形状比为 2.25,大小为 10–20 m 2,而分离的巨石 ( n  = 172) 的平均形状比为 1.3–1.5 ,大小为 2–3 m 2。据认为,巨石的形状和尺寸的减小是在高能风暴事件期间从升高的海岸平台而不是从潮间带平台采石和运输的结果。(3) 潮间带台地海岸垂直裂缝向海方向加深和扩大,形成有波浪上退的沟壑。一个相对较好的负相关(r 2 = 0.7) 在这些沟壑的数量和最大地形崎岖度值之间发现,这表明当适当的裂缝网络(相对密集和适当定向)接收不受阻碍的涌浪和波浪时,会发生平台下降。总体而言,我们发现海滩岩石平台在坚固性和磨损方面的演变至少部分受到存在的裂缝网络的影响,最显着的是在它们的方向、密度、持久性和交叉几何形状方面。

更新日期:2021-09-03
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