Introduction and Quartz and Silica Cements
2015. "Introduction and Quartz and Silica Cements", A Color Guide to the Petrography of Sandstones, Siltstones, Shales and Associated Rocks, Dana S. Ulmer-Scholle, Peter A. Scholle, Juergen Schieber, Robert J. Raine
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Cementation, the authigenic precipitation of minerals in pore spaces within rocks, is one of the most important processes in the lithification of clastic terrigenous deposits (we also include displacive, authigenic mineral precipitates within this general term). Cements can have a wide range of crystal sizes (terminology shown in Table 11.1) and fabrics. They can form throughout the history of sedimentary deposits, starting with surficial (eogenetic) processes in marine and nonmarine settings and continuing through all stages of burial (mesogenetic) diagenesis as well as uplift-associated (telogenetic) diagenesis. Hundreds of different minerals are found as cements in the panoply of different sandstones and mudrocks. However, most typical sandstones and mudrocks contain perhaps one to five cementing minerals, making identification far less complex than it might appear. All cements form by precipitation of materials from aqueous solution, and variations in subsurface fluid temperatures, pressures and chemistries (pH, salinity, specific ionic abundances, etc.) are the major controls on which minerals are precipitated or dissolved. Solutes can be derived from many sources. Some may come directly from seawater or via influx of meteoric waters; others may come from reflux of evaporitic brines. Additional solutes come from circulating basinal fluids, through chemical dissolution of soluble minerals, pressure solution along stylolites and solution seams, maturation of organic matter or dehydration of gypsum beds. These subsurface fluids can be moved through the sedimentary section at basinal scales via compactional dewatering or thermal convection, commonly aided by permeability “highways” created by fractures and faults. In all settings, cements may form through local dissolution or alteration of unstable minerals, relatively small-scale diffusive transport and nearby reprecipitation of more stable minerals. In very low-permeability settings, that may be the only viable mechanism for cementation. On the other hand, the near-complete exclusion of water (or water contact with grains), as in some hydrocarbon reservoirs, may inhibit both water movement and cement formation.
Figures & Tables
AAPG Memoir 109 is designed as a practical guide for students and professionals to learn the fundamentals of microscopic examination of sandstones, mudrocks, and associated rocks. With more than 1100 color illustrations, it covers the identification of grains, textures, and structures of clastic terrigenous rocks as well as their diagenetic alteration (compaction, cementation, dissolution, and replacement) and porosity reduction or enhancement. It also provides classification diagrams for formal description of those rocks and their porosity. Although the majority of the outcrop and subsurface examples come from the United States (35 states and Puerto Rico), there are representative photographs from 32 other countries, including many from the offshore areas. The foldout birefringence chart and an included DVD with Powerpoint files of all of the petrographic images provide additional aids for instructors and students.