Sand & Sandstone Textures
2015. "Sand & Sandstone Textures", 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
Download citation file:
The term “texture” encompasses a wide range of attributes of sediments/rocks and their constituent grains, including grain size and sorting, particle morphology (form and sphericity, rounding, and surface texture), grain orientation, imbrication and packing. All of those properties have significance in interpreting transport processes and depositional settings of sedimentary rocks, but they also have economic importance in a wide variety of fields ranging from engineering of construction and road materials to understanding and predicting the porosity and permeability relationships of such materials in petroleum exploration/production or hydrologic contexts. Most textural properties mentioned above are best measured in unconsolidated or easily disaggregateable materials where grains can be size-sorted by sieving, settling, laser particle analysis or other techniques or where individual grains can be viewed in three dimensions. These properties generally are far more difficult to measure accurately in thin sections of consolidated rocks. To use just one example, the size of a grain in thin section can never exceed the longest axis of the grain but it can easily be shorter because most cuts through grains are tangential or oblique to that axis. So, in most cases, the basic size-, sorting- and shape-related properties of grains are merely estimated in thin sections through the use of visual comparators. Even there, the most useful comparators are ones that have been specifically corrected for thin-section use. Detailed direct measurements on grain size and shape can, of course, be done using thin-section microscopy, especially through point-counting large numbers of grains, but there too, correction factors must be used to overcome, to the degree possible, the two-dimensional (2-D) view of three-dimensional (3-D) grains afforded in thin sections (see Harrell and Eriksson, 1979; Johnson, 1994). Advances in computerized photomicrographic image analysis can make the process of measurement and measurement correction both more accurate and far less time consuming (see, for example, Schäfer and Teyssen, 1987; Seelos and Sirocko, 2005; Syvitski, 2007). An enormous amount of effort was made in the period from the 1930s to the 1970s to perfect textural measurements and to find reliable statistical measures that could be used to identify specific environments of deposition. That work still finds application in soft and unconsolidated sediments, but it will not be discussed in detail in this book, because it is of lesser applicability to petrographic studies of hard rocks. A number of papers in the bibliography at the close of this section can be used to follow up on unconsolidated sediment studies (especially the excellent overview of statistical measures provided in Folk, 1980) and almost all textbooks on sands and sandstones include discussions of these topics.
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.