Rapid burial of dry sandstone - the unique compaction-dominated diagenesis of the Twyfelfontein Formation, NW Namibia

该项目将研究纳米比亚西北部风成岩的成岩作用，该地层被133 Ma前的Etendeka溢流玄武岩迅速埋藏，经历了强烈的石英颗粒破裂、变形带的形成和压力溶解。在这项研究中，我们将重点研究这种快速埋藏对该砂岩的机械压实和化学作用的影响和控制参数，以及岩石中流体的运移。我们推测，TwyFelfontein砂岩的成岩作用演化过程与其他砂岩截然不同，因为前者在埋藏过程中大部分是干燥的，而通常沉积物的孔隙空间充满了流体。特别是，我们假设，机械压实作用主导了成岩作用，这是由于早期埋藏过程中缺乏孔隙流体，上覆玄武岩密度较高，它们迅速侵位，因此作用于刚性颗粒的有效应力较高。此外，我们推测，由于最初缺乏流体，化学压实作用被推迟，仅在成岩晚期才开始。最后，我们假设玄武岩的侵位和同沉积的伸展构造对砂岩内部的局部变形有贡献。TwyFelfontein组良好的露头条件，以及由于其沙漠环境而缺乏蚀变影响，使得能够定性和定量地研究这些影响。我们的分析将首先对砂岩的埋藏深度进行关键评估，使用认可的地温计和气压计相结合的方法，对位于TwyFelfontein地层正下方的含煤地层和页岩进行评估。为了研究成岩作用，我们将进行野外、显微构造和地球化学分析。显微结构分析将利用光学显微镜、二次电子显微镜和阴极发光成像进行，其中阴极发光成像是分析颗粒破裂的有力工具。化学分析将通过电子探针和激光-电感耦合等离子体质谱进行，这将允许表征连续的水泥生长阶段。此外，我们将分析捕获在砂岩胶结物中的流体包裹体，这些包裹体提供了关于流体在捕获时的化学成分和温度的有价值的信息。最后，我们将把我们的结果结合到数值模拟中，这将有助于理解其他特征，如化学压实的时间。这次对唯一暴露良好的TwyFelfontein地层的全面研究将填补在压实控制的砂岩成岩作用的一般理解方面的一个重要空白。因此，它将成为暴露程度较低的环境的理想模拟，例如荷兰的二叠纪风成斯洛希特伦砂岩地层，该地层面临持续的压实诱发地震活动，并将为储层建模提供关键信息。

The project will deal with the diagenesis of the aeolian Twyfelfontein Formation in NW Namibia which was buried rapidly by a thick pile of 133 Ma-old Etendeka flood basalts and experienced intense quartz grain fracturing, formation of deformation bands, and pressure solution. In this study, we will focus on the effects and control parameters of this quick burial on this sandstone with respect to mechanical compaction and chemical alteration, as well as on fluid migration within the rock. We hypothesize that the diagenesis of the Twyfelfontein sandstone evolved distinctively different to other sandstones, as the former was largely dry during burial, whereas usually the pore space of sediments is filled with fluids. In particular, we hypothesize that mechanical compaction dominated the diagenesis due to the absence of pore fluids during early burial, the higher density of overlying basalts, their rapid emplacement and a therefore high effective stress that acted on the rigid grains. Further, we postulate that because of initially lacking fluids, chemical compaction was delayed and started only at a late diagenetic stage. Finally, we hypothesize that the emplacement of basalts and syndepositional extensional tectonics contributed to localized deformation within the sandstone. The excellent outcrop conditions of the Twyfelfontein Formation and the lack of alteration effects due to its desert setting, allow to qualitatively and quantitatively studying these effects. Our analysis will begin with a critical assessment of the burial depth of the sandstone using a combination of approved geothermometers and -barometers on coal-bearing strata and shales that occur directly underneath the Twyfelfontein formation. For studying the diagenetic effects, we will conduct field, microstructural, and geochemical analyses. Microstructural analysis will be done using optical microscopy, secondary electron microscopy, and cathodoluminescence imaging of which especially the latter is a powerful tool to analyze grain fracturing. Chemical analysis will be conducted via electron microprobe and Laser-ICP-MS, which will allow characterization of successive cement growth phases. Further, we will analyze fluid inclusions trapped in sandstone cements, which yield valuable information on chemical compositions and temperatures of fluids at the time of entrapment. Finally, we will incorporate our results into numerical modeling, which will help to understand additional characteristics, such as the timing of chemical compaction. This comprehensive study of the uniquely well-exposed Twyfelfontein formation will close an important gap in the general understanding of compaction controlled sandstone diagenesis. It will therefore serve as an ideal analogue for less well exposed settings such as the Permian aeolian Slochteren sandstone Formation in the Netherlands which faces continued compaction-induced seismic activity, and will provide key information for reservoir modelling.