Geology, geochemistry, and stratigraphy of Lemudongo formation, Kenya Rift Valley
Geology, geochemistry, and stratigraphy of Lemudongo formation, Kenya Rift Valley
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肯尼亚裂谷 Lemudongo 地层的地质、地球化学和地层学
DOI：
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发表时间：
2007
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通讯作者：
Martin Williams
中科院分区：
文献类型：
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作者：
S. Ambrose;C. Nyamai;E. Mathu;Martin Williams
勒穆东奥组在这里被定义为晚中新世至晚更新世层状熔岩、空降和瀑布凝灰岩、湖泊、冲积和河流沉积以及古土壤序列的一部分，出露在肯尼亚南部裂谷西缘约25350公里的区域，在内罗毕以西约100公里处。研究区被三个主要的永久性河流系统深深切割，暴露了三个晚第三纪湖盆的沉积物。勒木洞组为第二古湖盆地沉积，形成于中新世晚期。本文对几个地区的地层剖面进行了描述和对比，确定了勒木东组，并提出了盆地沉积历史和环境重建。勒木东澳组有三期沉积，总厚度135m，一期沉积主要为湖相和湖缘粉砂岩、泥岩和砂岩。第二期包括盆地中心的古土壤和东部盆地边缘的河流和冲积沉积。第三阶段主要由水系凝灰岩和粉砂岩组成，顶部是焊接凝灰岩。第二阶段可能反映了更干旱的气候，或较低的盆地溢流海拔。勒木洞沟上一期泥岩中的4个凝灰岩的年龄为6.12~6.08 Ma。勒穆东奥峡谷1号遗址的主要化石产层位于古老的凝灰岩之间，紧靠其上。化石与海滩和/或三角洲砂岩和细砾石以及代表间歇性泛滥湖缘的粉砂和砂质粘土岩有关。
The Lemudong’o Formation is defined here as part of a late Miocene to Late Pleistocene sequence of stratified lavas, air-fall and waterlain tuffs, lacustrine, alluvial, and fluvial sediments, and paleosols, that crop out over an approximately 25 3 50 km area on the western margin of the southern Kenyan Rift Valley, approximately 100 km west of Nairobi. The study area is deeply incised by three major permanent river systems that expose sediments of three late Neogene lake basins. The Lemudong’o Formation comprises deposits of the second paleolake basin, which formed during the late Miocene. Stratigraphic sections in several localities are described and correlated in this report, the Lemudong’o Formation is defined, and a basin sedimentary history and environmental reconstruction is proposed. The Lemudong’o Formation has three main phases of sedimentation with a total thickness of 135 m. Phase 1 is represented by predominantly lacustrine and lake-margin siltstones, mudstones, and sandstones. Phase 2 comprises paleosols in the basin center, and fluvial and alluvial sediments on the eastern basin margin. Phase 3 comprises mainly waterlain tuffs and silts, capped by a welded tuff. Phase 2 may reflect a more arid climate, or a lower basin-overflow elevation. Four tuffs in upper phase-1 mudstones in Lemudong’o Gorge are dated to 6.12–6.08 Ma. The main fossil-bearing horizons at Lemudong’o Gorge Locality 1 lie between, and immediately above, the dated tuffs. Fossils are associated with beach and/or deltaic sands and fine gravels, and silty and sandy claystones representative of an intermittently flooded lake margin.