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

• Applicant: Dr. Eric Salomon
• Subject Area: Palaeontology
• Term: from 2017 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 391950124

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.

• DFG Programme: Research Grants
• International Connection: USA
• Cooperation Partners: Dr. Tobias Häger; Robert Lander, Ph.D.; Professor Dr. Ralf Littke; Dr. Volker Lüders
• Co-Investigator: Professor Dr. Harald Stollhofen