Some Concepts of Cementation and Lithification of Sandstones

Abstract Investigation of the processes involved in lithifying sands made up of grains of silicate minerals indicates that crystallization of mineral matter in intergranular pores is only one mechanism which can be classified as cementation. Together with other methods binding grains, there appear to be such intergradational relations that the term “cementation” involves mineral authigenesis, recrystallization, and intergranular welding. In effect, cementation and lithification appear to be synonymous terms. Variation in the aspect of cementation is dependent strongly upon the framework of the sand mixture which ranges from sand-grain supported (arenite) to clay-silt matrix supported (wacke). Frameworks intergradational between the two are classified as subwacke. A cement is recognized as compatible if its composition is similar to that of the detrital grain to which it becomes welded. An incompatible cement differs in composition from the grain to which it becomes attached, and adhesion is achieved through surface effects or local grain replacement. A different variety of cementation involves clay minerals attached principally to quartz grains. The attachment is accomplished through coalescence of lattices along a common junction. As diagenetic processes are intensified, each variety of cementation is believed to progress through a series of textural modifications which also may involve compositional changes. Such alterations are part of a sequence for each rock and develop textures which attain culminating stages characteristic of the sandstone framework. Arenite frameworks tend to induce grain overgrowth, which may be followed by recrystallization to produce granular interlocks of bilateral and triple-grain junctions. Wacke frameworks pass through stages of cementation involving reconstitution of clay minerals and recrystallization with authigenic chert into an equilibrium-mineral assemblage. Such assemblages show culminating textures characterized by neoformation of micas and intergrowths with detrital monocrystalline quartz along margins of such grains. Subwacke frameworks are affected by intense diagenetic processes to develop cementation textures distributed locally in the rock. There may be culminating textures of both arenites and wackes in the same thin section.