A combined study of radar facies, lithofacies and three‐dimensional architecture of an alpine alluvial fan (Illgraben fan, Switzerland)
A combined study of radar facies, lithofacies and three‐dimensional architecture of an alpine alluvial fan (Illgraben fan, Switzerland)
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DOI：
10.1111/sed.12139
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发表时间：
2015-01
期刊：
影响因子：
3.5
通讯作者：
D. Franke;J. Hornung;M. Hinderer
中科院分区：
文献类型：
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作者：
D. Franke;J. Hornung;M. Hinderer
冲积扇是记录山区沉积和侵蚀过程历史的有用档案，因此可以揭示影响其发展的环境控制因素。从经济上讲，它们作为地下水储集层以及深埋系统中烃类的寄主岩石发挥着重要作用。然而，这些档案的解释和储层结构的评价是有问题的，因为沉积相分布的显著非均质性使得对比困难。这个问题是复杂的，因为现代松散扇系统的累积沉积物往往暴露不良，很少有这样的系统是使用高分辨率地下分析技术调查的重点。为了克服标准露头模拟研究的这一局限性，使用探地雷达对高山冲积扇进行了地球物理调查，以设计一个缩放的三维地下模型。对雷达相进行分类，并将其校准为提供露头壁的扇系统内的岩相，这些岩相用于推导沉积结构的三维模型并识别演化扇阶段。瑞士阿尔卑斯山的伊尔格拉本扇被选为案例研究，并对一个约60公里的探地雷达网络进行了调查。雷达相可划分为7种类型，分为泥石流沉积和河流沉积。这些雷达相类型的组合显示了三个沉积单元，它们被连续的扇宽反射体分开;它们被解释为与影响整个扇的沉积物匮乏事件相对应的古表面。泥石流沉积物的比例总体上从约50%下降到15%，而溪流沉积物的比例相应增加。最上面的沉积单元在其底部以一个宽达700米的重要切割面为界，随后大部分被水流沉积物填满。明显的古表面和沉积趋势表明，异旋回控制的伊尔格拉本扇的演变，使这个风扇的一个有价值的档案，重建过去的沉积物通量和阿尔卑斯山的环境变化。综合露头地球物理方法的结果鼓励类似的未来研究扇检索其沉积历史以及其潜在的储层特性。
Alluvial fans serve as useful archives that record the history of depositional and erosional processes in mountainous regions and thus can reveal the environmental controls that influenced their development. Economically, they play an important role as groundwater reservoirs as well as host rocks for hydrocarbons in deeply buried systems. The interpretation of these archives and the evaluation of their reservoir architecture, however, are problematic because marked heterogeneity in the distribution of sedimentary facies makes correlation difficult. This problem is compounded because the accumulated sedimentary deposits of modern unconsolidated fan systems tend to be poorly exposed and few such systems have been the focus of investigation using high‐resolution subsurface analytical techniques. To overcome this limitation of standard outcrop–analogue studies, a geophysical survey of an alpine alluvial fan was performed using ground‐penetrating radar to devise a scaled three‐dimensional subsurface model. Radar facies were classified and calibrated to lithofacies within a fan system that provided outcropping walls and these were used to derive a three‐dimensional model of the sedimentary architecture and identify evolutionary fan stages. The Illgraben fan in the Swiss Alps was selected as a case study and a network of ca 60 km sections of ground‐penetrating radar was surveyed. Seven radar facies types could be distinguished, which were grouped into debris‐flow deposits and stream‐flow deposits. Assemblages of these radar facies types show three depositional units, which are separated by continuous, fan‐wide reflectors; they were interpreted as palaeo‐surfaces corresponding to episodes of sediment starvation that affected the entire fan. An overall upward decline in the proportion of debris‐flow deposits from ca 50% to 15% and a corresponding increase in stream‐flow deposits were identified. The uppermost depositional unit is bounded at its base by a significant incision surface up to 700 m wide, which was subsequently filled up mostly by stream‐flow deposits. The pronounced palaeo‐surfaces and depositional trends suggest that allocyclic controls governed the evolution of the Illgraben fan, making this fan a valuable archive from which to reconstruct past sediment fluxes and environmental change in the Alps. The results of the integrated outcrop–geophysical approach encourage similar future studies on fans to retrieve their depositional history as well as their potential reservoir properties.