Permo‐Triassic Lacustrine Deposits in the Eastern Karoo Basin, Natal, South Africa
Permo‐Triassic Lacustrine Deposits in the Eastern Karoo Basin, Natal, South Africa
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DOI：
10.1002/9781444303698.ch12
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发表时间：
2009-06
期刊：
影响因子：
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通讯作者：
D. V. Dijk;D. Hobday;A. Tankard
中科院分区：
文献类型：
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作者：
D. V. Dijk;D. Hobday;A. Tankard
卡鲁盆地东部的博福特地层含有淡水鱼类、两栖动物、爬行动物、兽形目动物、甲壳类动物、昆虫和植物的典型湖相组合。该盆地受到来自南部和东部的沉积物流入，北部的贡献较小。最大水深约150米。细粒沉积物沉积在湖底的大面积区域，并被生物改造。一个主要的块状湖相粉砂岩相偶尔包含爬行动物的完整骨骼。河流-三角洲侵蚀的复杂、变化的模式产生了一条高度不规则的海岸线，其特征是宽阔、开放的海湾，沿海平原上有较小的孤立池塘。湖海岸侵蚀产生了向上变粗的层序，淤泥质下部有波浪波纹、粪化石和鱼迹，砂岩上部有牵引流结构。粗粒沉积物集中为河口砂坝、分流河道充填物和漫滩扇。薄的、向上变粗的海湾充填序列包含了各种各样的化石。识别出三种湾填子类型。其中一个显示粒度逐渐向上增加，形成细粒粉砂质砂岩，这归因于漫滩洪水。第二个湖泊在顶部附近含有侵蚀性的透镜状决口扇砂岩，而第三个湖泊缺乏沉积物，有机物含量高。这些湖泊最终被向北延伸的河流系统填充，河流沉积最终随着干旱条件的出现而停止。干旱条件也是风暴堡后期沙丘、干盐湖和短暂溪流环境的特征。典型的干盐湖沉积物显示浅水和出露构造，以及插入的钙长石。这些湖泊是恐龙的有利栖息地，并存在十种不同类型的脊椎动物足迹，包括四足和双足行走轨迹和双足跳跃足迹。卡鲁沉积作用被广泛的侏罗纪火山活动终止。
Beaufort strata of the eastern Karoo Basin contain a classic lacustrine assemblage of freshwater fish, amphibians, reptiles, therapsids, crustaceans, insects and plants. The basin was subject to sediment influx from the south and east, with a subordinate contribution from the north. Maximum water depth was about 150 m. Fine‐grained sediments settled over wide areas of the lake floor and were reworked by organisms. A predominantly massive lacustrine siltstone facies contains occasional complete skeletons of reptiles. A complex, shifting pattern of fluvio‐deltaic progradation produced a highly irregular shoreline characterized by broad, open embayments, with smaller isolated ponds on the coastal plain. Lake shore encroachment generated upward‐coarsening sequences with wave ripples, coprolites and fish trails in the silty lower part and traction‐current structures in the sandstone above. Coarse‐grained sediments were concentrated as mouth bars, distributary channel fills and overbank splays. Thin, upward‐coarsening bay‐fill sequences contain a great variety of fossils. Three bay‐fill subtypes are recognized. One shows a gradual upward increase in grain size into fine‐grained silty sandstones and is attributed to overbank flooding. The second contains erosively‐based, lenticular crevasse‐splay sandstones near the top, while the third was sediment‐starved and has a high organic content.The lakes were eventually infilled by northward‐prograding fluvial systems, and fluvial sedimentation finally ceased with the onset of arid conditions. Arid conditions also characterized the later Stormberg environment of dunefields, playa lakes and ephemeral streams. Typically the playa lake deposits show shallow water and emergence structures, together with intercalated evaporites. These lakes were a favourable habitat for dinosaurs, and ten different types of vertebrate footprints are present, including quadrupedal and bipedal walking tracks and bipedal hopping prints. Karoo sedimentation was terminated by widespread Jurassic volcanism.