当前位置: X-MOL 学术Earth Sci. Rev. › 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.)
Provenance of Kalahari Sand: Paleoweathering and recycling in a linked fluvial-aeolian system
Earth-Science Reviews ( IF 12.1 ) Pub Date : 2021-11-14 , DOI: 10.1016/j.earscirev.2021.103867
Eduardo Garzanti 1 , Guido Pastore 1 , Abi Stone 2 , Shlomy Vainer 3 , Pieter Vermeesch 4 , Alberto Resentini 1
Affiliation  

We here review what is known about the dunefields and fluvial systems of the Kalahari Basin in terms of geological setting and Quaternary dynamics and set out what has been hypothesized about the provenance of Kalahari sand so far. Previous work has tackled this problem by applying a limited number of techniques (mostly sediment textures and heavy minerals) to parts of the large dryland. The generally highly quartzose mineralogy of aeolian dunes and their compositional variability have been only broadly evaluated and several sedimentological issues have thus remained controversial, including the relative role played by fluvial processes versus aeolian reworking of older sediments and weathering controls. This reveals a need for a systematic provenance study that considers the entire basin. For this reason, here we combine original petrographic, heavy-mineral, and detrital-zircon geochronology data with previously published clay-mineral, geochemical, and geochronological information to present the first comprehensive provenance study of the vast Kalahari sand sea.

Our multi-proxy dataset comprises 100 samples, collected across the Kalahari Basin from 11°S (NW Zambia) to 28°S (NW South Africa) and from 15°E (Angola) to 28°30′W (Zimbabwe). Kalahari aeolian-dune sand mostly consists of monocrystalline quartz associated with durable heavy minerals and thus drastically differs from coastal dunefields of Namibia and Angola, which are notably richer in feldspar, lithic grains, and chemically labile clinopyroxene. The western Kalahari dunefield of southeastern Namibia is distinguished by its quartz-rich feldspatho-quartzose sand, indicating partly first-cycle provenance from the Damara Belt and Mesoproterozoic terranes. Within the basin, supply from Proterozoic outcrops is documented locally. Composition varies notably at the western and eastern edges of the sand sea, reflecting partly first-cycle fluvial supply from crystalline basements of Cambrian to Archean age in central Namibia and western Zimbabwe. Basaltic detritus from Jurassic Karoo lavas is dominant in aeolian dunes near Victoria Falls.

Bulk-sediment petrography and geochemistry of northern and central Kalahari pure quartzose sand, together with heavy-mineral and clay-mineral assemblages, indicate extensive recycling via aeolian and ephemeral-fluvial processes in arid climate of sediment strongly weathered during previous humid climatic stages in subequatorial Africa. Distilled homogenized composition of aeolian-dune sand thus reverberates the echo of paleo-weathering passed on to the present landscape through multiple episodes of fluvial and aeolian recycling.

Intracratonic sag basins such as the Kalahari contain vast amounts of quartz-rich polycyclic sand that may be tapped by rivers eroding backwards during rejuvenation stages associated with rift propagation. Such an event may considerably increase the sediment flux to the ocean, fostering the progradation of river-fed continental-embankments, as documented by augmented accumulation rates coupled with upward increasing mineralogical durability in the post-Tortonian subsurface succession of the Zambezi Delta.

The Central Kalahari is not a true desert. It has none of the naked, shifting sand dunes that typify the Sahara and other great deserts of the world. In some years the rains may exceed twenty — once even forty — inches, awakening a magic green paradise.” Mark Owens, Cry of the Kalahari.



中文翻译:

喀拉哈里沙的来源:相关的河流-风成系统中的古风化和再循环

我们在此回顾了在地质背景和第四纪动力学方面对喀拉哈里盆地沙丘和河流系统的了解,并阐述了迄今为止对喀拉哈里沙源的假设。以前的工作通过将有限数量的技术(主要是沉积物质地和重矿物)应用于部分大型旱地来解决这个问题。风成沙丘的通常高度石英化的矿物学及其成分变异性仅被广泛评估,因此一些沉积学问题仍然存在争议,包括河流过程旧沉积物的风成再加工和风化控制。这表明需要对整个盆地进行系统的来源研究。为此,我们将原始岩石学、重矿物和碎屑锆石年代学数据与先前发表的粘土矿物、地球化学和地质年代学信息相结合,首次对广阔的喀拉哈里砂海进行全面的物源研究。

我们的多代理数据集包含 100 个样本,从 11°S(赞比亚西北部)到 28°S(南非西北部)以及从 15°E(安哥拉)到 28°30′W(津巴布韦)的整个喀拉哈里盆地收集。喀拉哈里风成沙丘砂主要由与耐用重矿物相关的单晶石英组成,因此与纳米比亚和安哥拉的沿海沙丘截然不同,后者尤其富含长石、岩屑颗粒和化学不稳定的单斜辉石。纳米比亚东南部的西部喀拉哈里沙丘以富含石英的长石石英砂为特色,表明部分第一旋回物源来自达马拉带和中元古代地体。在盆地内,当地记录了来自元古代露头的供应。在沙海的西部和东部边缘,成分差异显着,部分反映了从寒武纪结晶基底到纳米比亚中部和津巴布韦西部太古代时代的第一循环河流供应。来自侏罗纪卡鲁熔岩的玄武岩碎屑在维多利亚瀑布附近的风成沙丘中占主导地位。

喀拉哈里北部和中部纯石英砂的大块沉积物岩相学和地球化学,连同重矿物和粘土矿物组合,表明在赤道以北以前潮湿气候阶段强烈风化的干旱气候中,沉积物通过风成和短暂-河流过程进行了广泛的再循环非洲。因此,风积沙丘的蒸馏均质成分通过多次河流和风积循环作用传递到当前景观中的古风化的回声产生回响。

克拉通内凹陷盆地,如喀拉哈里盆地,含有大量富含石英的多环砂,在与裂谷传播相关的复兴阶段,河流向后侵蚀可能会挖掘这些砂。这样的事件可能会显着增加流入海洋的沉积物通量,促进以河流为源的大陆堤防的进积,正如赞比西河三角洲后托尔顿期地下层序中积累率增加以及矿物耐久性向上增加所证明的那样。

中部喀拉哈里沙漠并不是真正的沙漠。它没有典型的撒哈拉沙漠和世界其他大沙漠的裸露、移动的沙丘。在某些年份,降雨量可能会超过二十——甚至是四十——英寸,唤醒一个神奇的绿色天堂。” 马克欧文斯,卡拉哈里的哭泣。

更新日期:2021-12-02
down
wechat
bug