Physical controls on sand composition and relative durability of detrital minerals during ultra‐long distance littoral and aeolian transport (Namibia and southern Angola)
Physical controls on sand composition and relative durability of detrital minerals during ultra‐long distance littoral and aeolian transport (Namibia and southern Angola)
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DOI：
10.1111/sed.12169
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发表时间：
2015-06
期刊：
影响因子：
3.5
通讯作者：
E. Garzanti;A. Resentini;S. Andó;G. Vezzoli;A. Pereira;P. Vermeesch
中科院分区：
文献类型：
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作者：
E. Garzanti;A. Resentini;S. Andó;G. Vezzoli;A. Pereira;P. Vermeesch
纳米比亚沿海至安哥拉南部的沉积物主要来自橙子河，有少量额外的河流输入，化学过程的改变可以忽略不计，这使得这成为调查砂质和成分物理控制的一个很好的测试案例。本研究通过使用一套综合技术，包括图像分析、激光粒度测定、光学显微镜、拉曼光谱和散装沉积物地球化学，监测了非洲南部大西洋海岸约1750 km长沿着海滩和风成沙丘砂的结构、矿物学和地球化学变化。这些结果与以前的报告形成对比，这些报告认为，长石和火山碎屑在运输过程中会分解，沙粒在浅海环境中迅速变圆，石英砂可能是通过物理过程产生的。机械磨损无法改变碎屑成分的相对丰度，包括辉石和镁铁质火山岩碎片，传统上认为这些碎片会迅速被破坏。唯一的例外是在过渡到海洋环境时容易丢失的低石化或裂解的沉积/变质沉积岩碎片，以及在海上筛选和沉积的缓慢沉降的云母。沿海沉积物中相对移动的角闪石趋于贫化，优先夹带在海上或重新沉积在遮蔽的海滩中，而移动的较少的石榴石保留在陆上。在从橙子口到纳米布尔格南部的高能沿岸的环境中，经过300至350 km的沿岸运输后，没有碎屑矿物显示出颗粒圆度的显著增加，但所有矿物在进入沙丘区后迅速变圆。辉石和不透明比较硬的石英和石榴石更快变圆，但砂的矿物学保持不变。除了强烈的瞬时选择性卷吸效应外，物理过程无法显著改变砂的成分。在砂和砂岩的定量物源分析中，即使在由强持续风浪主导的高能环境中进行超长距离运输的情况下，选择性机械破裂也可以在很大程度上被忽略。
Sediment in coastal Namibia to southern Angola is supplied dominantly from the Orange River with minor additional fluvial input and negligible modifications by chemical processes, which makes this a great test case for investigating physical controls on sand texture and composition. This study monitored textural, mineralogical and geochemical variability in beach and aeolian‐dune sands along a ca 1750 km stretch of the Atlantic coast of southern Africa by using an integrated set of techniques, including image analysis, laser granulometry, optical microscopy, Raman spectroscopy and bulk‐sediment geochemistry. These results contrast with previous reports that feldspars and volcanic detritus break down during transport, that sand grains are rounded rapidly in shallow‐marine environments, and that quartzose sands may be produced by physical processes. Mechanical wear is unable to modify the relative abundance of detrital components, including pyroxene and mafic volcanic rock fragments traditionally believed to be destroyed rapidly. The sole exceptions are poorly lithified or cleaved sedimentary/metasedimentary rock fragments, readily lost at the transition to the marine environment, and slow‐settling flaky micas, winnowed and deposited offshore. Coastal sediments tend to be depleted in relatively mobile amphibole, preferentially entrained offshore or re‐deposited in sheltered beaches, while less mobile garnet is retained onshore. No detrital mineral displays a significant increase in grain roundness after 300 to 350 km of longshore transport in high‐energy littoral environments from the Orange mouth to south of the Namib Erg, but all minerals get rapidly rounded after passing into the dunefield. Pyroxene and opaques get rounded faster than harder quartz and garnet, but sand mineralogy remains unchanged. Excepting strong transient selective‐entrainment effects, physical processes are unable to modify sand composition significantly. Selective mechanical breakdown can be largely neglected in quantitative provenance analysis of sand and sandstone even in the case of ultra‐long‐distance transport in high‐energy environments dominated by strong persistent winds and waves.