The origin of silica cements revealed by spatially resolved oxygen isotope microanalysis and electron-beam microscopy; Heidelberg Formation, Germany

Abstract

The origin and growth mechanisms of microcrystalline quartz cement in sandstones and its relationship to quartz overgrowths remain puzzling even though it has an important effect on rock properties in deeply buried sandstones; microcrystalline quartz cement inhibits the growth of quartz cement and so leaves sandstones with anomalously high porosity deep in sedimentary basins. Here we have examined the relationships between quartz cement, and duplicate layers of chalcedony/amorphous silica and microcrystalline quartz from outcrops of the Upper Cretaceous Heidelberg Formation, Germany. We have used an array of techniques including: NanoSIMS for spatially-resolved stable oxygen isotope analysis, and high-resolution chemical analysis of the silica cements using electron microprobe analysis, as well as crystallographic studies using TEM and EBSD. Based on burial history modeling and the low degree of compaction of the Heidelberg Formation sandstones, we conclude that maximum temperature due to conductive heating was about 42 °C; quartz overgrowths with δ18O of 18.8 ‰ developed from meteoric-derived formation water with δ18O of about −12 ‰ V-SMOW. Based on published precedents and an awareness that silica solubility decreases with decreasing temperature, we assume that chalcedony/amorphous silica and then microcrystalline quartz developed during cooling of flowing formation water. Chalcedony with δ18O of 27.4 ‰ developed from meteoric-derived formation water with δ18O of between −1 and −3 ‰ V-SMOW depending on whether the initial temperature of the cooling formation water was 45 or 55 °C. The interpreted higher δ18O of the warmer water responsible for chalcedony and microcrystalline quartz growth is typical of basin-center waters that have undergone considerable water–rock interaction. It is likely that the fluid movement was caused by thrust tectonics in the Coniacian to Campanian. The duplicate layers of chalcedony/amorphous silica and then microcrystalline quartz suggests that there were two thrust-induced fluid expulsion events.