Consistent classification and concise naming of rocks and sediments are essential for effective communication throughout the international scientific community. An ideal classification scheme combines objective, quantifiable description of readily observable features that are grouped into named categories. At the same time, it is desirable to have groupings that incorporate a maximum level of genetic or interpretive significance (groupings that reflect mechanisms of formation, environments of deposition, and the like). Although many classifications have been proposed for carbonate rocks and sediments, only two — the Folk (1959,/62,) and Dunham (1962) classifications — have successfully met the test of time (along with two others that are variants of the Dunham scheme). All four schemes are based on the distinction of three fundamental components: grains (skeletal fragments, ooids, pellets/peloids, intraclasts, and non-carbonate detritus), matrix or carbonate mud, and open pores or sparry-calcite-filled primary interparticle porosity (see diagram on previous page). The differences between the classifications are mainly that Folk uses the relative percentages of grains and matrix, Dunham as well as Embry and Klovan use mud- versus grain-supported fabrics, and Wright uses a more genetic division into biological, diagenetic, and depositional fabrics. This chapter summarizes the features of each classification and provides petrographic examples of carbonate rocks with their Folk and Dunham names; dolostone classifications and examples are covered in the chapter on dolomites.
The Folk classification uses multiple descriptive terms. The fundamental name is based on the four grain types and the relative abundances of grains (allochems), matrix, and cement or pore space. Eleven basic terms are generated (top diagram on facing page), including ones for pure mud rocks (micrites), muddy rocks with spar patches (dismicrites) and organically-bound rocks (biolithites). Because of their special environmental significance, intraclasts and ooids are favored in the naming process (see top diagram caption).
To describe the features of carbonate rocks that reflect the degree of sorting and rounding, Folkʼs terminology includes textural modifiers (middle diagram on facing page). In general, deposits classed on the left side of the diagram were formed in “low-energy” settings; rock types farther to the right represent deposition in increasingly high-energy depositional settings.
A third component of a full Folk name relates to the average grain or crystal size of the rock. That terminology is summarized in the bottom diagram on the facing page.
A carbonate rock named under the Folk classification can include any or all of the terms generated in these three categories, plus any additional descriptive terms the user desires. Thus, for example, the same rock could be termed a “biosparite” or a “rounded biosparite” or a “coarse calcarenite: rounded biosparite” or a “coarse calcarenite: rounded rudist-coral biosparite” or a “slightly dolomitized coarse calcarenite: rounded rudistcoral biosparite”, depending on the level of detail desired.
Quantifiable, descriptive (objective) terminology.
Although primarily descriptive, rock terms convey considerable genetic (environmental) information.
Multiple optional terms — for grain size, faunal composition, alteration, non-carbonate constituents, and other features — allow informative names at any desired level of detail.
Used worldwide, especially by petrographers in academic settings.
Figures & Tables
A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis
This volume expands and improves the AAPG 1978 classic, A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities(AAPG Memoir 27). Carbonate petrography can be quite complicated. Changing assemblages of organisms through time, coupled with the randomness of thin-section cuts through complex shell forms, add to the difficulty of identifying skeletal grains. Furthermore, because many primary carbonate grains are composed of unstable minerals (especially aragonite and high-Mg calcite), diagenetic alteration commonly is quite extensive in carbonate rocks. The variability of inorganic and biogenic carbonate mineralogy through time, however, complicates prediction of patterns of diagenetic alteration. This book is designed to help deal with such challenges. It includes a wide variety of examples of commonly encountered skeletal and nonskeletal grains, cements, fabrics, and porosity types. It includes extensive new tables of age distributions, mineralogy, morphologic characteristics, environmental implications and keys to grain identification. It also encompasses a number of noncarbonate grains, that occur as accessory minerals in carbonate rocks or that may provide important biostratigraphic or paleoenvironmental information in carbonate strata. With this guide, students and other workers with little formal petrographic training should be able to examine thin sections or acetate peels under the microscope and interpret the main rock constituents and their depositional and diagenetic history.