Inorganic bedded cherts from the Magadi area, Kenya
Inorganic bedded cherts from the Magadi area, Kenya
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来自肯尼亚马加迪地区的无机层状燧石
DOI：
10.1007/bf00388011
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发表时间：
1969
影响因子：
3.5
通讯作者：
H. Eugster
中科院分区：
文献类型：
--
作者：
H. Eugster
高马加迪层可分为下部和上部层序，由富含尼罗罗非鱼遗骸的层隔开。下部层序包含一个或多个麦羟硅钠石层位。通过挖沟，有可能确定“燧石系列”的燧石在地层学上相当于麦羟硅层位。在许多地方都观察到麦羟硅钠石-燧石的过渡，可能出现在小于50厘米的距离上。这是通过用蒸馏沃茨除去钠来实现的。由此产生的层状硅质岩可能保留麦羟硅钠岩中存在的微细纹层。这种转变伴随着许多纹理和结构特征，如大的干燥多边形，屈曲，网状，挤压，泥裂缝和方解石的铸件。麦羟硅钠石的沉淀被假定为发生在一个分层的湖在盐水-表水层界面的年度增量。最长的持续时间估计为4000 - 6000年。溶解度测量表明，目前大多数马加迪沃茨相对于麦羟硅钠石和水羟硅钠石是过饱和的，尽管没有观察到沉淀。层内砾石中硅质岩碎片的存在表明，在麦羟硅钠石沉淀期间已经开始向硅质岩转化。在马加迪观察到的层状硅质岩形成的无机机制也适用于W.澳大利亚麦羟硅钠石作为燧石前体的证据发现在沉积速率，钠闪石的存在和结构，如斑点和十字豆荚。所提出的模型扩展到包括前寒武纪带状铁的形成一般和限制这些形成地球的早期历史的可能组成的封闭盆地的沃茨解释。
The High Magadi beds can be divided into a lower and an upper sequence, separated by a bed rich in Tilapia nilotica remains. The lower sequence contains one or more magadiite horizons. Through digging of trenches it has been possible to establish that the cherts of the “Chert Series” are stratigraphically equivalent to the magadiite horizons. The transition magadiite-chert has been observed in many places and may occur over a distance of less than 50 cm. It is accomplished by removal of sodium by percolating waters. The resulting bedded chert may preserve the fine lamination present in magadiite. The conversion is accompanied by many textural and structural features such as large desiccation polygons, buckling, reticulation, extrusion, casts of mud-cracks and calcite.The precipitation of magadiite is postulated to have taken place in annual increments in a stratified lake at the brine-epilimnion interface. The maximum duration is estimated at 4,000–6,000 years. Solubility measurements indicate that most Magadi waters at the present time are supersaturated with respect to magadiite and kenyaite, though no precipitation has been observed. The presence of chert chips in intraformational gravels indicates that the conversion to chert was already initiated during the magadiite precipitation interval.The inorganic mechanism for the formation of bedded chert observed at Magadi is suggested to apply also to the Dales Gorge Member of the Precambrian Brockman Iron Formation of W. Australia. Evidence for magadiite as chert precursor is found in the depositional rates, the presence of riebeckite and in textures such as macules and cross-pods. The proposed model is extended to include Precambrian banded iron formations in general and the restriction of these formations to the early history of the earth is explained by the likely compositions of waters from closed basins.