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首页> 外文期刊>South African Journal of Geology: Being the Transaction of the Geological Society of South African: Syndie die Verhandelinge van die Geologiese Vereniging van Suid-Afrika >Lithology and depositional environments of the Lower Jurassic Clarens Formation in the eastern Cape, South Africa
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Lithology and depositional environments of the Lower Jurassic Clarens Formation in the eastern Cape, South Africa

机译：南非东开普省下侏罗统克拉伦斯组的岩性和沉积环境

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摘要

The Lower Jurassic Clarens Formation of the main Karoo Basin, South Africa, is regarded a climatically controlled fluvioaeolian to aeolian lithostratigraphic interval. It reveals a tectonically quiet period of orogenic unloading of the Cape Fold Belt and initiation of the rifting along the Indian Ocean rift axis. In the Clarens Formation outcrop area west of 27°E its lithofacies spectrum differs significantly from that known from the major part of the basin. Abundant pyroclastic and lacustrine facies elements characterise relatively small-scale local depocenters reaching below the regional base of the Clarens Formation. Such depositories have subcircular outlines and are surrounded by massive structureless sandstones being the product of phreatomagmatic crater formation. They separate the basin-wide aeolian facies succession from one illustrating the shallowing-upward crater lakes infilled due to gravitational and aeolian sediment supply. The existence of the crater lakes evidence shallow groundwater levels and reflect increased precipitation towards the end of Clarens Formation deposition. Low-frequency severe storms are suggested by hummocky cross-stratified beds in the lake-fill sequences. The strong impact of phreatomagmatic volcanic activity on the sediment supply and facies development of parts of the Clarens Formation illustrate a new facet of increasing rifting activity prior to the Lower Jurassic Karoo flood volcanism.
机译：南非主要卡鲁盆地的下侏罗统克拉伦斯组被认为是受气候控制的潮风到风化岩石地层学间隔。它揭示了海角褶皱带的造山带卸载和沿印度洋裂谷轴的裂谷开始的构造上安静的时期。在27°E以西的Clarens地层露头地区，其岩相谱与该盆地大部分地区的岩相谱显着不同。大量的火山碎屑相和湖相相元素表征了相对较小规模的局部沉积中心，达到了克拉伦斯组区域基底以下。这些储层具有亚圆形的轮廓，并被大量的无结构砂岩包围，这是深部岩浆火山口形成的产物。他们将整个盆地范围的风相相与一幅说明重力和风积物供应导致的浅向上的火山口湖相分离。火山口湖的存在表明地下水位很浅，并且反映出克拉伦斯组沉积末期的降水增加。湖泊填土层序中的高架交叉层状床暗示了低频强风暴。岩浆火山活动对克拉伦斯组部分沉积物供应和相发育的强烈影响，说明了下侏罗世卡鲁洪水洪水之前裂谷活动增加的一个新方面。

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