The Cause of Burial Diagenesis in Sandstones Revealed by Authigenic Clay Minerals

Clay mineral diagenesis occurs in all buried siliciclastic sediments, albeit fine-grained and low-permeable or coarse-grained and highly permeable. Because of the limited permeability after compaction, this sheds doubt on the presumption that fluid flow is a causal factor in burial diagenesis besides temperature and effective pressure conditions. To assess the influence of fluid flow on their diagenesis, a number of sandstones have been studied focusing on clay minerals. In the studied sandstones, the authigenic clay minerals show a considerable variation in their chemical composition and mineralogy. In particular, authigenic illite and chlorite are common as authigenic cements in sandstones, often forming grain-rimming cements. Each of these two authigenic clay minerals varies distinctly in chemical composition, mineralogy and crystal habit at a small scale, compared to individual pores and laminae/beds. This indicates that local conditions determined not only which authigenic clay minerals formed, but also their chemical composition. Local conditions include the detrital mineral assemblage, specific textural features including clay grain coatings, and the ratio of the volume of susceptible components to pores. During burial diagenesis, a number of elements involved in clay mineral authigenesis, including Al, K, Mg and Fe, are locally fixed by clay mineral precipitation. In addition, diagenesis is driven by the amount and distribution of susceptible detrital components and the changes in physical conditions including temperature and effective pressure. The physical conditions allow chemical processes to commence and continue. The evidence presented is inconsistent with external fluid flow as a causal agent of burial diagenesis, supporting a largely closed, diagenetic system.