Middle Miocene to Pleistocene sedimentary record of rift evolution in the southern Albert Rift (Uganda)
Middle Miocene to Pleistocene sedimentary record of rift evolution in the southern Albert Rift (Uganda)
复制标题
阿尔伯特裂谷南部（乌干达）裂谷演化的中中新世至更新世沉积记录
DOI：
10.1007/s00531-010-0560-z
复制
发表时间：
2010
影响因子：
2.3
通讯作者：
I. Ssemmanda
中科院分区：
文献类型：
--
作者：
S. Roller;J. Hornung;M. Hinderer;I. Ssemmanda
本研究提供了一个几乎完整的中中新世至更新世东非裂谷西部分支同生裂谷沉积序列。所研究的继承暴露在几个补丁之间的鲁文佐里块的北方尖和阿尔伯特裂谷的东肩东倾斜块。在这个位置，它达到了600米的最大厚度，其中350米已通过分析岩相和沉积物结构进行了系统的测井。继承的地层细分依赖于出版的生物地层学数据的地方性软体动物协会和它们的相关性在东非。遇到的同生裂谷沉积物是硅质碎屑岩，从粘土到粗砾石，含石膏和铁质夹层或夹层。岩相和结构分析表明，冲积平原，三角洲平原，近岸，三角洲前缘，或湖泊沉积环境。根据沉积环境的垂向叠加模式、沉积环境的逆冲和逆冲趋势以及气候指标（如长石、石膏和/或氢氧化铁沉淀的保存），可划分出四个演化阶段：（1）第一阶段介于约100年至约100年之间; 14.5 10.0 Ma以推移质为主的河流环境为特征，具有块状桑迪-砾质床形、富菱镁矿砂、罕见的铁矿物和相对低的可容纳空间。这一阶段被解释为半干旱气候下的前和早期同裂谷沉积。(ii)从约10.0到4.5马主要是细粒硅质碎屑沉积在一个远端河流平原湖泊环境，其特点是有限的容纳空间。薄层的波动，粘土的优势和频繁的铁沉淀表明气候更加潮湿，具有季节性和微弱的构造活动。(iii)在第三阶段，4.5和2.0 Ma之间的三角洲平原和近岸沉积频繁ferrugineous numerations和丰富的软体动物协会发生，表明一个潮湿的时期，湖平面高点和加速沉降。(iv)在2.0和1.5马之间的最后沉积间隔砾石单元重新出现较少的铁，但更多的碳酸盐和石膏的结晶，长石砂岩。这一阶段记录了一个普遍的干旱化趋势，最有可能是由于即将到来的鲁文佐里山脉的雨障，以及加速裂谷侧翼隆起和裂谷底部的强烈沉降。这项研究的结果是特别重要的划定在阿尔伯特裂谷沉积的关键控制。
This study presents an almost complete Middle Miocene to Pleistocene sequence of synrift sediments in the western branch of the East African Rift. The studied succession is exposed in several patches on an eastward tilted block between the northern tip of the Rwenzori Block and the eastern shoulder of the Albert Rift. In this position, it reaches a maximum thickness of 600 m of which 350 m have been logged systematically by analysing lithofacies and sediment architecture. Stratigraphic subdivision of the succession relies on published biostratigraphic data of endemic mollusc associations and their correlation across East Africa. The synrift sediments encountered are siliciclastics ranging from clay to coarse gravel with gypsum and ferrugineous interlayers or impregnations. Lithofacies and architectural analysis indicate alluvial plain, delta plain, nearshore, delta front, or lacustrine depositional environments. Based on the vertical stacking pattern, prograding and retrograding trends of the depositional environments, and climatic indicators (e.g. conservation of feldspar, gypsum, and/or iron hydroxide precipitation), four evolutionary phases can be distinguished: (i) a first phase between ca. 14.5 and 10.0 Ma is characterised by bedload-dominated fluvial environment with massive sandy to gravelly bedforms, feldspar-rich sands, rare iron impregnations and relatively low accommodation space. This phase is interpreted as pre- and early synrift sedimentation under a semiarid climate. (ii) From ca. 10.0 to 4.5 Ma predominantly fine-grained siliciclastics were deposited in a distal fluvial plain to lacustrine setting characterised by limited accommodation space. Fluctuation of thin beds, dominance of clay and frequent iron impregnations point to a more humid climate with seasonality and weak tectonic activity. (iii) During the third phase between 4.5 and 2.0 Ma delta plain and nearshore deposits with frequent ferrugineous impregnations and rich mollusc associations occurred, indicating a humid period with lake-level highstands and accelerated subsidence. (iv) During the final sedimentary interval between 2.0 and 1.5 Ma gravel units reoccurred with less iron- but more carbonate and gypsum impregnations, and arkosic sandstones. This phase recorded a general aridisation trend most probably caused by the upcoming rain barrier of the Rwenzori Mountains together with accelerated rift-flank uplift and strong subsidence of the rift floor. The results of this study are of particular importance for delineating key controls on sedimentation in the Albert Rift.