中新世东非高原初始裂谷作用的特殊沉积记录:沉积旋回性和古环境指标的教训

IF 1.9 3区 地球科学 Q1 GEOLOGY
Jens Hornung, Matthias Hinderer, Dennis Brüsch, Rainer Petschick
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引用次数: 0

摘要

关于初始构造和沉积动力学的知识,以及早期裂谷作用对气候和环境演变的影响,在很大程度上仍然是推测性的,因为沉积物通常被深埋。在东非裂谷系内,反转构造将其中一些序列抬升到地表,因此为了解裂谷前沉积历史提供了罕见的窗口。一个这样的例子,一个例外的700 m长,最大60 m高的新路堑为详细研究阿尔伯丁裂谷南部(乌干达西部)的初始裂谷序列提供了机会。这集中在基底和鲜为人知的中新世中晚期,以揭示最初阿尔伯丁裂谷的气候、环境、水文和构造演化。收集了一个庞大而稳健的多代理数据集,包括169 m的地层厚度,根据修订的岩石地层模型,其跨度为14.5至4.9Ma。实地工作包括沉积记录的测井、伽马射线光谱、磁化率和用光成像技术绘制的二维壁图。此外,对这些剖面进行了取样,以进行散装矿物和粘土矿物分析。该序列揭示了一套岩相和建筑元素,详细描述了河湖系统的演化。确定了五种沉积环境,显示出从冲积平原到三角洲平原的总体后退趋势,最后是沼泽/浅湖条件。中尺度基准面周期、建筑元素的保存潜力和堆叠模式显示出有限的容纳空间。然而,它会随着时间的推移而增加。这一总体趋势表明构造沉降增加,这可以通过裂谷前阶段的弯曲下倾角和同裂谷阶段的上部分级为断层控制的地壳伸展来解释,这越来越破坏了大规模的河流系统。从中新世中期到上新世早期,本研究表明,古气候趋势的特点是Th浓度增加且波动更大,长石损失,含水硫酸盐矿物的夹层透镜体,以及从蒙脱石为主的粘土向高岭石为主的粘土转变。除了水合硫酸盐外,这些信号都被解释为碎屑,它们反映了集水区土壤化学风化和剥离强度的增加。湖泊沉积相的增加和湖平面的上升支持了湿度增加的趋势。然而,含水硫酸盐、铁白云石和成土层的夹层证明了持续的季节性和干燥间隔。最后,基于修订的地层模型,提出了露头沉积旋回与盆地尺度旋回的层序地层对比。根据这些旋回,从裂谷前阶段到同裂谷阶段的过渡以不整合和中新世晚期的构造脉冲为标志。然而,河流补给、沉积体系以及气候条件的响应较少间断,其特征是逐渐趋势和时间延迟。长裂谷前阶段(约10 Myr)和向同裂谷阶段的逐渐过渡符合活动裂谷模型,该模型基于软流圈上升流对岩石圈的热减薄。
本文章由计算机程序翻译,如有差异,请以英文原文为准。

An exceptional sedimentary record of initial rifting on the East African Plateau in the Miocene: Lessons from depositional cyclicity and palaeoenvironmental proxies

An exceptional sedimentary record of initial rifting on the East African Plateau in the Miocene: Lessons from depositional cyclicity and palaeoenvironmental proxies

Knowledge about the initial tectonic and depositional dynamics, as well as the influence of early rifting on climate and environmental evolution remains speculative to a large extent, because sediments are usually deeply buried. Within the East African Rift System, inversion tectonics uplifted a few of these successions to the surface hence presenting rare windows into the pre-rift depositional history. One such example, an exceptional 700 m long and up to 60 m high fresh road cut provided the opportunity to study in detail initial rift successions of the southern Albertine Rift (Western Uganda). This focusses on the basal and poorly known Middle to Late Miocene in order to unravel the climatic, environmental, hydrological and tectonic evolution of the initial Albertine Rift. A large and robust multi-proxy dataset was gathered comprising 169 m of stratigraphic thickness, which spans from 14.5 to 4.9 Ma according to a revised lithostratigraphic model. Fieldwork comprised logging of the sedimentary record, spectral gamma ray, magnetic susceptibility and 2D wall mapping with photomosaics. Additionally, the sections were sampled for bulk mineral and clay mineral analysis. The succession exposes a suite of lithofacies and architectural elements detailing the evolution of a fluvio-lacustrine system. Five depositional environments were identified which show an overall back-stepping trend from an alluvial plain to a delta plain and finally palustrine/shallow lacustrine conditions. Mesoscale base-level cycles, preservation potential of architectural elements, and stacking pattern exhibit limited accommodation space. However, it increases over time. This overall trend indicates increasing tectonic subsidence, which can be explained by flexural downwarp within the pre-rift phase and in the upper part grading into fault-controlled crustal extension of the syn-rift phase, which more and more disrupted a large-scale river system. From the Middle Miocene up to the early Pliocene, this study revealed that palaeoclimate trends become marked by increasing and more fluctuating Th concentrations, loss of feldspar, intercalated lenses of hydroxosulphate minerals, and a shift from smectite-dominated to kaolinite-dominated clays. These signals are all interpreted as detrital except for the hydroxosulphates, and they mirror the increasing intensity of chemical weathering and stripping of soils in the catchment. A trend towards increasing humidity is supported by an increase in lacustrine sediment facies and a lake-level rise. Nevertheless, intercalation of hydroxosulphate, ferricretes and pedogenised horizons prove ongoing seasonality and dry intervals. Finally, based on a revised stratigraphic model a sequence stratigraphic correlation of the outcrop's depositional cycles with basin-scale cycles is presented. According to these cycles, transition from the pre-rift to the syn-rift stage is marked by an unconformity and a tectonic pulse in the latest Miocene. However, the response of fluvial supply, the depositional system as well as climate conditions are less punctuated and characterised by gradual trends and temporal delays. The long pre-rift phase (ca 10 Myr) and the gradual transition to the syn-rift phase is in accordance with the active rifting model, which is based on thermal thinning of the lithosphere by asthenospheric upwelling.

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来源期刊
CiteScore
4.10
自引率
16.70%
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42
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