Quartz Cement in the Fontainebleau Sandstone, Paris Basin, France: Crystallography and Implications for Mechanisms of Cement Growth

摘要

Authigenic quartz cement is the most abundant form of diagenetic cement in clastic sedimentary rocks. Despite this, there are many unknowns relating to mechanisms of growth of quartz cement and the crystallography of quartz cement. The key focus of this paper is to investigate further the issues of crystallography and quartz cement growth mechanisms, using the shallowly buried Oligo-Miocene Fontainebleau sandstone, France, as a case study. We address the following points: (1) are authigenic quartz overgrowths really in crystallographic continuity with their substrate grains? (2) What is the crystallographic inter-relationship between zones of quartz cement growth? (3) Are all quartz overgrowths entirely quartz, or do other silica polymorphs exist within overgrowths? The study combines an array of techniques to answer these questions, including transmitted-light optics, cathodoluminescence (CL), and electron backscatter diffraction (EBSD), the latter of the two being performed with the use of a scanning electron microscope (SEM). The use of EBSD to this study is crucial because it provides essential crystallographic information on the grains and their overgrowths. The data revealed: (1) quartz overgrowths comprise several zones visible in optical and CL images as parallel, isopachous, alternating bright and dark bands; (2) these bands represent areas of poorly crystalline silica and fully crystalline quartz; (3) one entire zone consists only of poorly crystalline quartz; (4) the final growth stage occurred as prismatic microcrystalline quartz into the remaining porosity; (5) the crystallographic orientation across most of the overgrowth, as far as the microcrystalline quartz layer, is the same as that of the detrital grain (i.e., it is syntaxial); and (6) the microcrystalline quartz layer has crystals with different and variable orientations relative to the detrital grain. This indicates that part of the quartz cement is not in crystallographic continuity with the substrate grain and displays an epitaxial relationship. Detailed analysis of the orientation data shows that there is a rational crystallographic rotation around a variety of axes, which indicates that the orientation of the final growth stages was not random.