当前位置: X-MOL 学术Mar. Geol. › 论文详情
Our official English website, www.x-mol.net, welcomes your feedback! (Note: you will need to create a separate account there.)
Relict and contemporary influences on the postglacial geomorphology and evolution of a current swept shelf: The Eastern Cape Coast, South Africa
Marine Geology ( IF 2.9 ) Pub Date : 2020-05-15 , DOI: 10.1016/j.margeo.2020.106230
A.N. Green , J.A.G. Cooper , N.P. Dlamini , N.N. Dladla , D. Parker , S.E. Kerwath

Few stratigraphic models of continental shelves incorporate the process of geostrophic current-sweeping, consequently its role in the stratigraphic record is often overlooked. We examine the narrow, current-swept Eastern Cape shelf of South Africa using a combination of geophysical techniques, seafloor sampling and video observations and interpret the role of current action on the transgressive stratigraphy of this steep subtropical shelf. During the Last Glacial Maximum, fluvial valleys incised the acoustic basement rocks. During the subsequent transgression, two distinct shorelines were formed and preserved at −105 m and − 60 m. Their development and preservation is linked to (i) high sediment supply from adjacent fluvial sources, (ii) early diagenesis and (iii) alternating sea-level stillstands and periods of rapid sea-level rise during melt water pulses 1A and 1B, respectively. The deeper shoreline formed in a sandy, wide coastal plain setting with limited bedrock influence, whereas the shallower shoreline comprised alternating rock headlands and embayments like the contemporary coast. Differences in antecedent topography and geology are responsible for the temporal variability in shoreline type; coastal squeeze is exacerbated with increasing bedrock control as the shoreline migrates landward

Between the two shoreline complexes, in the mid-shelf, the transgressive stratigraphy records initial valley infill by progradation of coast-parallel sandy spits. These are capped by a stiff lagoonal mud deposited as ongoing sea-level rise overspilled the valley interfluves, onlapping the adjacent aeolianites. The uppermost stratigraphy comprises mounds of rhodoliths which interfinger with a sandy inner to middle shelf highstand wedge.

After sea-level reached its present position ca 7.4 ka yr BP, the shelf became subject to reworking by the high-energy geostrophic Agulhas Current. This has had the following major effects on the shelf stratigraphy: 1. The topographic relief of the cemented palaeo-shorelines has been emphasised by removal of the post-transgressive cover; and 2. The shelf no longer acts as a depocenter; instead, the seabed consists of rhodoliths, gravel streamers, bedrock or gravel hash of the wave ravinement surface. Given the necessary antecedent conditions such as accommodation, sediment supply and favourable diagenetic climate, prominent shorelines can form and be preserved on the shelf. Strong current sweeping emphasises these morphological features on subtropical shelves.



中文翻译:

遗迹和当代影响对冰川后地貌和当前席卷陆架的演化的影响:南非东开普海岸

大陆架很少有地层模型包含地转流的扫掠过程,因此,它在地层记录中的作用经常被忽略。我们结合地球物理技术,海底采样和视频观测资料,研究了南非目前狭窄的东开普省陆架,并解释了当前作用在该陡峭的亚热带陆架海侵地层中的作用。在最后一次冰河期末期,河谷切开了声学基底岩层。在随后的海侵期间,形成了两条截然不同的海岸线,并保留在-105 m和-60 m处。它们的发展和保存与(i)来自邻近河流源的大量泥沙供应有关,(ii)早期成岩作用和(iii)融水脉冲1A和1B期间交替的海平面静止状态和海平面快速上升的时期。较深的海岸线形成在沙质,宽阔的沿海平原环境中,基岩影响有限,而较浅的海岸线则包括交替的岩石岬角和浮突,如当代海岸。前期地形和地质的差异是海岸线类型随时间变化的原因。随着海岸线向陆地的迁移,随着基岩控制的增加,沿海地区的挤压加剧 前期地形和地质的差异是海岸线类型随时间变化的原因。随着海岸线向陆地的迁移,随着基岩控制的增加,沿海地区的挤压加剧 前期地形和地质的差异是海岸线类型随时间变化的原因。随着海岸线向陆地的迁移,随着基岩控制的增加,沿海地区的挤压加剧

在两个海岸线复合体之间,在陆架中部,海侵地层记录了通过沿海岸平行的砂质喷口的堆积而形成的初始河谷填充物。随着不断上升的海平面使山谷间的缝隙溢出,相邻的风成岩重叠,沉积了坚硬的泻湖泥浆。最上层的地层由数以千计的菱形岩丘组成,它们与内到中层的沙质高架楔形物相交。

在海平面达到约7.4 ka yr BP的当前位置后,该架子变得受到高能地转Agulhas Current的影响。这对陆架地层有以下主要影响:1.去除海侵后盖层,强调了胶合古海岸线的地形起伏;2.架子不再充当中心。取而代之的是,海床由波浪纹沟表面的菱形岩,砾石流,基岩或砾石哈希组成。考虑到必要的先决条件,例如住宿,沉积物供应和有利的成岩气候,可以形成明显的海岸线并将其保存在架子上。强电流横扫强调了亚热带大陆架上的这些形态特征。

更新日期:2020-05-15
down
wechat
bug