Orientation of microcrystalline quartz in the Fontainebleau Formation, Paris Basin and why it preserves porosity

Abstract

In sedimentary basins, deeply buried sandstones typically have low porosity due to cementation and compaction. There are several known causes of anomalous preservation of porosity in sandstones, one of which is microcrystalline quartz coatings on sand grains that inhibit growth of quartz cement. Here we have used high resolution scanning electron microscopy, electron backscattered diffraction, Fourier transform infrared spectroscopy and transmission electron microscopy to study the microcrystalline quartz-cemented Oligocene Fontainebleau Formation, Paris Basin, France. Where the microcrystalline quartz is absent in the Fontainebleau Formation, the rock is tightly cemented by syntaxial quartz overgrowths. Where microcrystalline quartz is present, a small and variable amount of syntaxial quartz cement formed initially but was followed by a layer of amorphous silica. The microcrystalline quartz sits on top of the amorphous silica. In contrast to the syntaxial quartz overgrowths, the microcrystalline quartz has a different crystallographic orientation than the underlying quartz grain. Instead, the microcrystalline quartz has its c-axes parallel to whatever host surface it is sitting on, implying that its a-axes (better defined as [11–20] directions) are perpendicular to the host surface. This is analogous to chalcedony. The c-axes of individual microcrystalline quartz crystals are not parallel to each other on their growth surfaces, instead having about 30° of rotational scatter on the growth surface. The amorphous silica has insulated the quartz grain and any incipient syntaxial cement and helped to prevent further syntaxial growth. The subsequent growth of polynuclear, chalcedony-like microcrystalline quartz has also prevented further growth of quartz since the fast-growth c-axes do not extend into the neighboring pore but instead compete for space on the host grain surface. The combination of insulating amorphous silica and the orientation of microcrystalline quartz have been effective in preventing any further syntaxial quartz cement and so have preserved porosity.