The Oltulelei Formation of the southern Kenyan Rift Valley: A chronicle of rapid landscape transformation over the last 500 k.y.
The Oltulelei Formation of the southern Kenyan Rift Valley: A chronicle of rapid landscape transformation over the last 500 k.y.
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DOI：
10.1130/b31853.1
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发表时间：
2018-09-01
影响因子：
4.9
通讯作者：
Deino, Alan
中科院分区：
文献类型：
--
作者：
Behrensmeyer, Anna K.;Potts, Richard;Deino, Alan
肯尼亚南部东非裂谷最近50万年的地质历史由本文新描述的Oltulelei组记录。这个地层中年代久远的沉积旋回揭示了构造和气候是如何塑造裂谷沉积的，以及人类祖先居住的景观。Oltulelei组（约320 ~约36 ka）位于Olorgesailie组（1.2 ~ 0.5 Ma）的侵蚀不整合面上，分为Olkesiteti组、Oltepesi组和Tinga组三个连续单元。该组的层型出露于奥洛格萨列山北部和西北部的奥洛格萨列盆地。互层砂岩的40Ar/39Ar测年为三个主要沉积旋回的时间提供了地质年代学控制。Oltulelei组横向变动性表明，在过去500 ky的时间里，Olorgesailie盆地和Koora地堑北部形成了3个裂谷次盆地，它们周期性地连接和断开。侵蚀阶段将大量沉积物从Olorgesailie盆地中移走，需要向Olorgesailie山西南和南部的较低基底水平进行通流排水。沉积阶段主要是河流沉积，硅质碎屑和火山碎屑沉积物填满山谷，掩埋解剖景观。凝灰岩、浅湖和湿地沉积物也会出现，表明断层控制的污水池或堵塞的排水系统中地下水位升高。我们认为，裂谷段之间的连接和断开是由构造力驱动的，主要是火山基底岩的断裂和火山沉积物的周期性流入，再加上气候条件，使得大量沉积物进入和流出次盆地。Oltulelei组保存了南肯尼亚裂谷物理景观动态变化的记录，在10(4)-10(5)年的时间尺度上，侵蚀与沉积发生了重大变化。这项研究提供了关于活动裂谷环境中沉积的新的、校准良好的信息，以及一个基于露头的、三维的、盆地尺度的地质历史，可以与肯尼亚南部新兴的钻芯古气候记录相结合。保存在Oltulelei组的地层记录促进了人们对构造和气候如何塑造中晚更新世动物更替以及肯尼亚南部从阿舍利时代到中石器时代技术过渡的理解，并提出了新的问题。
The last half-million years of geological history of the East African Rift in southern Kenya is recorded by the Oltulelei Formation, which is newly described in this paper. Well-dated depositional cycles in this formation reveal how tectonics and climate shaped rift valley sedimentation, as well as the landscapes inhabited by human ancestors. The Oltulelei Formation (ca. 320 to ca. 36 ka) overlies the Olorgesailie Formation (1.2-0.5 Ma) on an erosional unconformity and is divided into three successive units, the Olkesiteti, Oltepesi, and Tinga Members. The stratotype for the formation is exposed in the Olorgesailie Basin, north and northwest of Mount Olorgesailie. The 40Ar/39Ar dates on interbedded tephras provide geochronological controls on the timing of three major depositional cycles. Lateral variability in the Oltulelei Formation shows that the Olorgesailie Basin and the northern Koora Graben formed three rift subbasins that were periodically connected and disconnected over the past 500 k.y. Erosional phases removed large volumes of sediment from the Olorgesailie Basin, requiring through-flowing drainage to a lower base level southwest and south of Mount Olorgesailie. Aggradational phases were primarily fluvial, with siliciclastic and volcaniclastic sediment filling valleys and burying dissected landscapes. Tufa, shallow lacustrine, and wetlands deposits also occur and indicate elevated water tables in fault-controlled sumps or blocked drainages. We propose that connections and disconnections between rift segments were driven by tectonic forces, primarily faulting of volcanic basement rocks and periodic influxes of volcanic sediment, combined with climatic conditions that enabled erosion and transport of large volumes of sediment into and out of the subbasins. The Oltulelei Formation preserves a record of a dynamic change in the physical landscape of the southern Kenya Rift, with major shifts in erosion versus deposition on time scales of 10(4)-10(5) yr. This study provides new, well-calibrated information on sedimentation in active rift settings as well as an outcrop-based, three-dimensional, basin-scale geological history that can be integrated with emerging drill-core paleoclim ate records from southern Kenya. The stratigraphic record preserved in the Oltulelei Formation advances understanding and poses new questions about how tectonics and climate shaped middle to late Pleistocene faunal turnover and the transition in southern Kenya from Acheulean to Middle Stone Age technology.