ABSTRACT

Quartz overgrowths are among the most volumetrically significant diagenetic phases in sandstones. Many combined petrographic and geochemical studies of quartz overgrowths rely heavily on the long-held underlying assumption of their stability. This study, of SEM-cathodoluminescence (SEM-CL) imaging of quartz overgrowths from the Jurassic Khatatba Formation of Egypt, calls this assumption into question. These overgrowths preserve spectacular SEM-CL textures that can be interpreted as recrystallization fabrics. There are three major growth zones in the overgrowths, Q1 to Q3. In the first (Q1), fine SEM-CL zoning is preserved, some with relict fibrous textures and smooth or bulging growth surfaces. These textures could indicate initial precipitation as opaline silica, chalcedony, microquartz, or some phase other than coarse quartz overgrowth. In Q1 there are discordant areas, locally showing smooth and scalloped boundaries, and filled with quartz with the SEM-CL character of later growth zones. Such textures suggest that earlier zones were crosscut by later phases of quartz. The smooth and scalloped boundaries, and their position in the overall framework of cements and grains, are in places inconsistent with simple fracturing, but are consistent with dissolution of earlier growth zones, followed by filling with later quartz. These features suggest that earlier growth zones were unstable and were replaced by coarsely crystalline quartz overgrowth. Such observations, and other empirical and chemical considerations, suggest that, in some cases, quartz overgrowths may result from recrystallization of less stable quartz and silica phases. Thus, any study of sandstone diagenesis, focused on quartz overgrowths, should take this possibility into account.