Facies Model of a Semiarid Freshwater Wetland, Olduvai Gorge, Tanzania
Facies Model of a Semiarid Freshwater Wetland, Olduvai Gorge, Tanzania
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坦桑尼亚奥杜瓦伊峡谷半干旱淡水湿地相模型
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发表时间：
2003
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通讯作者：
G. Ashley
中科院分区：
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作者：
C. M. Liutkus;G. Ashley
通过对坦桑尼亚Olduvai峡谷早更新世淡水湿地（1.75 Ma）火山碎屑沉积物的结构、矿物学、微化石和地层描述，对其进行了沉积学和地层学研究，建立了沉积相模型。这片淡水湿地位于半干旱封闭盆地的边缘，盆地内有一个盐湖。湖缘沉积物主要由两种不同的岩性组成：蜡质粘土岩和土质沉积物。蜡质沉积物是橄榄绿的，致密的粘土岩，含有自生矿物，如trona，代表了一个波动的盐碱湖（Hay 1976）。土质沉积物是米色、易碎的粉砂岩，含有硅质微化石、骨碎片和小鹅卵石；它们代表着淡水湿地。土壤沉积物和硅藻土的微化石表明淡水沼泽生物群中有一些罕见的耐盐物种。伴生岩相有：硅藻土、碳酸盐和砂岩。2 km 2湿地纵向和横向岩相变化的解释通过50次发掘，确定了7种不同的亚环境：春季沉积物、多年生沼泽、池塘、短暂湿地、边缘湿地、湿地河道和短暂溪流。地层层序，估计代表一个；40-50 kyr的长间隔表明，当湖泊扩张并淹没湿地时，在厚蜡质粘土沉积期间，持续的（10 - 3年）湿润期占主导地位，而同样长的中间干燥期导致湖泊收缩，使湿地繁荣和土质沉积物增加。Olduvai古湿地的岩相组合为淡水湿地的沉积环境模型的发展提供了重要的第一步，该模型可以在其他地方进行测试，特别是在干旱和半干旱环境中。
The sedimentology and stratigraphy of a freshwater wet- land in early Pleistocene (; 1.75 Ma) volcaniclastic deposits, Olduvai Gorge, Tanzania, are characterized using texture, mineralogy, micro- fossils, and stratigraphic description to create a depositional facies model. The freshwater wetland was located on the margin of a semi- arid, closed basin containing a playa lake. The lake-margin deposits are dominated by two distinct lithologies: waxy claystones and earthy sediments. Waxy sediments are olive green, dense claystones that con- tain authigenic minerals such as trona, and represent a fluctuating saline, alkaline lake (Hay 1976). Earthy sediments are beige, friable siltstones that contain siliceous microfossils, bone fragments, and minor pebbles; they represent the freshwater wetland. Microfossils from the earthy sediments and diatomites indicate freshwater marsh biota with some rare salt-tolerant species. Associated lithofacies are: diatomite, carbonate, and sandstone. Interpretation of vertical and lateral lithofacies variations in the 2 km 2 wetland, based on ; 50 excavations, identified seven distinct sub- environments: spring deposits, perennial marsh, pool, ephemeral wet- land, fringing wetland, wetland channel, and ephemeral stream. The stratigraphic sequence, estimated to represent an ; 40-50 kyr long interval, indicates that persistent (; 10 3 yr) wet periods prevailed dur- ing the deposition of thick waxy clays when the lake expanded and flooded the wetlands, and equally long intervening dry periods caused the lake to contract, allowing the flourishing of wetlands and accretion of earthy sediment. The lithofacies associations in the Olduvai paleo- wetland provides an important first step in the development of a de- positional environmental model for freshwater wetlands that can be tested elsewhere, particularly in arid and semiarid settings.