ABSTRACT

Semi-quantitative X-ray fluorescence analyses were made of core samples from concealed unconformities from England and Iraq in order to detect SiO₂, Al₂O₃, K₂O, Na₂O, MgO, CaO, Fe₂O₃, P₂O₅, and Sr. These can be used as diagenetic indicators reflecting mineralogical changes in the vicinity of unconformities. The unconformities consisted of several age gaps and included Cretaceous/Devonian: Lower Lias/Rhaeto-Liassic/Beds of unknown age/Carboniferous Limestone Series; Triassic/Lower Carboniferous; Triassic/Upper Coal Measures; Triassic/Permo-Trias/Permian; and, finally, multiple unconformities from the Middle East, Western Desert-Iraq.

The investigation was coupled with detailed petrographic, scanning electron microscopy, and X-ray diffraction analyses. and results have shown that increased amounts of SiO₂ are indicative of increased silicification. Al₂O₃ and K₂O, as well as the concentration of kaolinite, suggest disintegration of alumino-silicates. Fe₂O, is very good proof of an increase in redoxomorphic processes, which is coupled with increased reddening of sediments as a result of prolonged exposures. MgO indicates dolomitization at unconformities. CaO reflects the introduction of new cements in sandstones, and when coupled with MgO profiles, can be an indicator of dedolomitized zones in the subsurface adjacent to unconformities. Na₂O results are inconclusive due to the high matrix absorption ratio of Na. P₂O₅ is a good indicator of phosphatization in carbonate rocks and requires the presence of organic matter as well as the availability of reducing conditions. Conversely, the lack of P₂O₅ in the vicinity of unconformities suggests prevailing oxidizing conditions at the time of exposure. Finally, Sr is a very important diagenetic indicator at unconformities, where increased values suggest newly introduced cements in sandstones plugging pore spaces; a loss of Sr at unconformities (especially in carbonate rocks) would suggest various stages of dolomitization and dedolomitization, thus leading to the complete elimination of Sr through the phenomena of dissolution and precipitation, rather han solid state diffusion.